Symposium Organizers
Gang Li University of California, Los Angeles
Thuc-Quyen Nguyen University of California-Santa Barbara
Dana C. Olson National Renewable Energy Laboratory
Moritz Riede Technische Universitaet Dresden
H1: New Materials and Processing
Session Chairs
Monday PM, November 28, 2011
Grand Ballroom (Sheraton)
9:00 AM - **H1.1
An Integrated Approach for Enhancing Polymer Solar Cells Performance through Material, Interfacial, and Device Engineering.
Alex Jen 1
1 Materials Science & Engineering, University of Washington, Seattle, Washington, United States
Show AbstractAn integrated approach combining materials design and synthesis, interfacial modifications, patterning and optical engineering, and device engineering have been adapted to develop low-cost, efficient and stable polymer solar cells. In this talk, we will discuss our recent development on polymer solar cells with inverted structure. Through the process optimization and interfacial modifications, inverted cells with power conversion efficiency of >6.5% and improved ambient stability can be achieved. We have implemented and characterized models for light propagation in polymer solar cell structures and coupled them to simulations of carrier dissociation and transport. These simulations were used to guide optimization of both standard and inverted device structures to maximize power conversion efficiency. Optical simulations have been used to determine optimum layer thicknesses to maximize optical generation in active device regions and thereby enhance short circuit current. Finally, new synthetic methods to develop low-bandgap polymers with cross-conjugated structures and novel fullerene derivatives for efficient and thermally stable polymer solar cells will be presented. These new materials will be further studied based on our integrated engineering approaches to fully harness their best potentials as PV materials.
9:30 AM - H1.2
Low Band Gap Polymers Utilizing Quinoid Resonance Structure Stabilization: Benzodithiophene and Imide-Based Copolymers for Photovoltaic Applications.
Wade Braunecker 1 , Zbyslaw Owczarczyk 1 , Ross Larsen 1 , Andres Garcia 1 , Nikos Kopidakis 1 , Scott Hammond 1 , David Ginley 1 , Dana Olson 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractConjugated alternating copolymers were designed to have low band gaps for organic photovoltaic (OPV) applications by considering quinoid resonance stabilization. Copolymers of thienoisoindoledione (TID) and benzodithiophene (BDT) had appreciably lower band gaps than copolymers of thienopyrroledione (TPD) and BDT (lower by ~0.3 eV). In addition to intramolecular charge transfer stabilization (i.e., the “push-pull” effect), the former copolymer’s quinoid resonance structure is stabilized by a gain in aromatic resonance energy in the isoindole unit. Additionally, the HOMO levels of the copolymers could be tuned with chemical modifications to the BDT monomer, resulting in open circuit voltages of greater than 1 V in photovoltaic devices. In addition to advancing OPV polymer design rules with optical and electrochemical studies, we illustrate with a novel computational approach employing time-dependent density functional theory and a simple model for monomer-monomer coupling how future synthetic efforts can be guided in this field.
9:45 AM - H1.3
Bulk Hetero-Junction Solar Cells Utilizing Liquid Crystalline Phthalocyanine Derivatives.
Akihiko Fujii 1 , Tetsuro Hori 1 , Tetsuya Masuda 1 , Dao Duy 1 , Takeshi Hayashi 1 , Hiroyuki Yoshida 1 , Yo Shimizu 2 , Masanori Ozaki 1
1 , Osaka University, Suita, Osaka, Japan, 2 Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, Japan
Show AbstractOrganic thin-film solar cells based on a bulk hetero-junction utilizing a non-peripherally alkyl-substituted phthalocyanine, such as, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2), have been studied. C6PcH2 is a low-molecular-weight organic semiconductor and has high solubility for typical organic solvents due to the long substituents. C6PcH2 exhibits liquid crystalline phase, such as, hexagonal disordered columnar mesophase between 161 and 170 °C. In the time-of flight measurement, the high drift mobilities, for example, 1.4 and 0.5 cm2/Vs for hole and electron at -15 °C, were observed in the crystal phase as well as liquid crystal phase [1]. The blend uniform thin film with the C6PcH2 and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM) could be fabricated by a spin-coating method. Solar cells with C6PcH2:PCBM bulk hetero-junction active layer have demonstrated a high external quantum efficiency above 70% in the Q-band absorption region of C6PcH2 and a high energy conversion efficiency of 3.1% [2]. The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH2 and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH2 and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO3 structure, and a high Voc of 1.27 V has been achieved [3].C6PcH2 is available as a dopant for conventional organic thin-film solar cells with an bulk hetero-junction active layer composed of poly(3-hexylthiophene) (P3HT) and PCBM. The improvement of long-wavelength sensitivity in P3HT:PCBM bulk hetero-junction solar cells by doping C6PcH2 has been succeeded.[1] Y. Miyake et al., Appl. Phys. Express, 2011, 4, 021604. [2] T. Hori et al., Appl. Phys. Express, 2010, 3, 101602. [3] T. Hori et al., to be published in Solar Energy Materials and Solar Cells.
10:00 AM - H1.4
Thieno[3,2-b]thieno bis(silolothiophene) Based Polymers for High Performance FETs and OPVs.
Bob Schroeder 1 , Raja Ashraf 1 , Hugo Bronstein 1 , Weimin Zhang 1 , Thomas Anthopoulos 2 , Iain McCulloch 1
1 Chemistry, Imperial College London, London United Kingdom, 2 Physics, Imperial College London, London United Kingdom
Show AbstractExtended π-conjugated ladder type monomers are of great interest for opto-electric applications because the backbone rigidification leads to an improved conjugation and prevents chain folding, which has been reported to hinder the charge carrier mobility. Carbon bridged thiophene-phenylene-thiophene (TPT) and the silicon bridged analogue (SiTPT) have shown very good performances in BHJ solar cells with photocurrent efficiencies above 6%, as well as in OFET with hole mobilities above1 cm2/Vs.[1-4]Herein we present an extension of the work on indacenodithiophene (TPT) and silaindacenodithiophene (SiTPT) monomers by replacing the central benzene ring by a thieno(3,2-b) thiophene unit. This newly developed thieno[3,2-b]thieno bis(silolothiophene) (SiTTTT) donor moiety was synthesized from commercially available reagents and incorporated into a series of donor-acceptor polymers. We will discuss the properties of these novel polymers with an emphasis on their performance in field effect transistors and photovoltaic devices compared to the TPT analogues.[1]Y.-C. Chen, C.-Y. Yu, Y.-L. Fan, L.-I. Hung, C.-P. Chen, C. Ting, Chemical Communications 2010, 46, 6503-6505.[2]W. Zhang, J. Smith, S. E. Watkins, R. Gysel, M. McGehee, A. Salleo, J. Kirkpatrick, S. Ashraf, T. Anthopoulos, M. Heeney, I. McCulloch, Journal of the American Chemical Society 2010, 132, 11437-11439.[3]J. Wang, S. K. Hau, H. Yip, J. A. Davies, K. Chen, Y. Zhang, Y. Sun, A. K.-Y. Jen, Chemistry of Materials 2011, 23, 765-767.[4]R. S. Ashraf, Z. Chen, D. Seok Leem, H. Bronstein, W. Zhang, B. Schroeder, Y. Geerts, J. Smith, S. Watkins, T. W. Anthopoulos, H. Sirringhaus, J. C. de Mello, M. Heeney, I. McCulloch, Chemistry of Materials 2011, 23, 768-770.
10:15 AM - H1.5
A Closer Look at the Effects of Fluorine Substitution in Conjugated Polymers for Solar Cells.
Andrew Stuart 1 , Huaxing Zhou 1 , Liqiang Yang 3 , Wei You 1 3 , Rene Lopez 2 , John Tumbleston 2 , Sam Price 1
1 Chemistry, UNC-Chapel Hill, Chapel Hill, North Carolina, United States, 3 Curriculum in Applied Science and Engineering, UNC-Chapel Hill, Chapel Hill, North Carolina, United States, 2 Physics and Astronomy, UNC-Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractIn previous work we have shown that substitution of Fluorine atoms into the backbone of conjugated polymers increases efficiency from 4 to 7%. However we could only surmise the increase was caused by enhanced charge generation or collection. In this work, we have designed and synthesized two different polymer series to study how the concentration of fluorine substitution affects the photovoltaic response of conjugated polymers. We have carefully examined the response, and found that fluorine substitution enhances both fill factor and short circuit current through suppression of charge recombination processes.
11:00 AM - H1.6
Isoindigo, a Versatile Electron-Deficient Building Block inp- and n-Type Conjugated Systems for Organic Solar Cells.
Romain Stalder 1 , Jianguo Mei 1 3 , Kenneth Graham 1 , Jegadesan Subbiah 2 , Caroline Grand 1 , Leandro Estrada 1 , Franky So 2 , John Reynolds 1
1 Chemistry, University of Florida, Gainesville, Florida, United States, 3 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show Abstract We have first introduced isoindigo (iI), a structural isomer of the well-known indigo dye, as a new electron-deficient building block for conjugated organic systems. Following on this, researchers in the field joined the effort in the development of isoindigo-based materials: conjugated polymers have been used in solar cells reaching 3%, and p-type field-effect transistors with high mobilities. Isoindigo can be easily obtained from the high yielding condensation of two commercially available products. It is thus a new electron-acceptor monomer only two highly scalable steps away from commercial sources, which is important for low-cost organic electronic applications. In our first reports of iI-based conjugated oligomers and polymers, synthesized by Stille or Suzuki couplings under the donor-acceptor (D-A) approach, we demonstrated their efficiency as p-type materials in molecular bulk heterojunctions (BHJ) with fullerenes, while the polymers showed broad tunable light absorption across the visible up to 750 nm. All iI-based materials have low-lying LUMO energy levels (high electron affinities), around -3.7 to -3.9 eV. With bandgaps between 1.5 and 1.9 eV, the corresponding HOMO levels are also low-lying (high ionization potentials), which translates into high open-circuit voltages when these p-type materials are used in BHJ with fullerene derivatives.Based on our recent understanding of the effects of solvent additives on the output characteristics of molecular BHJ solar cells using iI-containing oligothiophenes, we were able to select a combination of additives to increase the original efficiency of 1.7% to over 3.5% under AM 1.5 illumination. We also designed new D-A conjugated polymers incorporating iI and dithienosilole (DTS) moieties, so far reaching 4% efficiency in BHJ with PC70BM. Synthetic tailoring of the isoindigo monomer allowed us to synthesize broadly absorbing conjugated polymers with backbones exclusively composed of electron-accepting units. With low-lying HOMO and LUMO energy levels, these readily reducible polymers are strong candidates as n-type materials, for which we will show the preliminary results in all-polymer solar cells.
11:15 AM - H1.7
High Efficiency Polymer Semiconductors for Organic Photovoltaics.
Steven Tierney 1 , Nicolas Blouin 1 , William Mitchell 1 , Amy Topley 1 , Frank Meyer 1 , Miguel Carrasco-Orozco 1 , Toby Cull 1 , Priti Tiwana 1 , Stephane Berny 1 , Mathis Muth 2 , Andrea Maurano 3
1 Performance Materials Division, Advanced Technologies (PM-AC), Merck Chemicals Ltd, Southampton United Kingdom, 2 Macromolecular Chemistry I, University of Bayreuth, Bayreuth Germany, 3 Department of Chemistry, Imperial College, London United Kingdom
Show AbstractThe development of polymer semiconductors for bulk-heterojunction organic photovoltaics has strongly intensified in the last half-decade, with significant attention on so called “donor-acceptor” or “push-pull” polymer backbone structures. This polymer design approach has enabled the synthesis of low bandgap polymers with the potential to absorb more photons in the polymer:fullerene layer. Nevertheless, there is an equal polymer design requirement to maximize the device’s open-circuit voltage via a deepened HOMO energy level, which can be achieved by incorporation of a weak electron-donating unit into the polymer backbone. Herein, we report polymer structures containing weak electron-donors as a means to meet this dual design requirement. This will include a family of polymers containing the benzo[1,2-b:4,5-b’]dithiophene unit where power conversion efficiencies of over 6% with PCBM[60] have been achieved in polymer:fullerene layers of 200 nm and above. We also report the polymers’ optical and redox properties and some basic morphological characterization of the polymers and the polymer:fullerene blends. We additionally report progress in the application of such polymer:fullerene blends within an inverted device geometry as a demonstration of compatibility with processes for mass-production.
11:30 AM - H1.8
Designing Improved Solar Cells with Molecule-Scale Control: 3-D Microstructure of Self-Assembled Organic Heterojunctions for Next Generation Photovoltaics.
Theanne Schiros 1 , Chien-yang Chiu 2 , Bumjung Kim 2 , Kevin Yager 3 , James Ciston 3 , Stefan Mannsfeld 4 , Noah Tremblay 5 , Dean Delongchamp 6 , Alon Gorodetsky 2 , Kim Kisslinger 3 , Marshall Cox 7 , Ioannis Kymissis 7 , Michael Toney 4 , Colin Nuckolls 2
1 Energy Frontier Research Center, Columbia University, New York, New York, United States, 2 Chemistry, Columbia University, New York, New York, United States, 3 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States, 4 Stanford Synchrotron Radiation Laboratory, Stanford University, Menlo Park, California, United States, 5 Chemistry, Johns Hopkins University, Baltimore, Maryland, United States, 6 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 7 Electrical Engineering, Columbia University, New York, New York, United States
Show AbstractWe demonstrate a new direction in organic photovoltaic device (OPV) design, which exploits molecular shape-complementarity and self-assembly to create controlled nanostructures with improved functional photovoltaic performance. We focus on a new class of doubly concave p-type molecular semiconductors based on a family of contorted benzocoronene derivatives, complementary in shape and electronic form to fullerene acceptors. Utilizing a complementary suite of advanced characterization techniques, including x-ray and electron diffraction and core-level spectroscopy, we build a detailed picture of the OPV active layer, from the level of the molecular interaction between donor and acceptor partners to the long-range crystallinity and morphology of the organic-heterojunction. We show how supramolecular assembly can facilitate 3-D architectures with the simplicity and control of bilayer fabrication -- from “ball-and socket” photovoltaic joints to reticulated heterojunctions to vertical crystal growth on graphene. The insights presented here provide fundamental understanding to guide materials synthesis and direct device architectures towards the molecule-scale control that can enable new technology for breakthrough efficiency OPV.
11:45 AM - H1.9
A Porphyrin-Fullerene Dyad with a Supramolecular ``Double-Cable” Structure as a Novel Electron Acceptor for Bulk Heterojunction Polymer Solar Cells.
Xiong Gong 1 , Chien Lung Wang 1 , Wen Bin Zhang 1 , Stephen Cheng 1
1 Polymer Eng., The University of Akron, Akron, Ohio, United States
Show AbstractA Porphyrin-Fullerene Dyad with a Supramolecular “Double-Cable” Structure as a Novel Electron Acceptor for Bulk Heterojunction Polymer Solar CellsChien-Lung Wang, Wen-Bin Zhang, Ryan M. Van Horn, Yingfeng Tu,Xiong Gong, Stephen Z. D. ChengCollege of Polymer Science and Polymer Engineering, The University of Akron, Akron, OH 44325 (USA)Bulk heterojunction (BHJ) polymer solar cells (PSCs) offer a promising, low-cost, large-area, flexible, light-weight, clean, and quiet alternative energy source for both indoor and outdoor applications. Power conversion efficiencies (PCEs) (in response to solar AM1.5 radiation) as high as 6 - 8% have been reported for BHJ PSCs. In order to achive PCEs over 10%, BHJ materials capable of generating higher short circuit current (Jsc) and larger open circuit voltage (Voc) are required. One approach to increase Jsc and Voc is to develop low-band-gap semiconducting polymers with deeper HOMO (Highest Occupied Molecular Orbital) energies. An alternative approach is to develop new electron acceptors with higher LUMO (Lowest Unoccupied Molecular Orbital) energies. The pathway to low-band-gap semiconducting polymers with deeper HOMOs is now well established, and BHJ PSCs fabricated using these novel semiconducting polymers have demonstrated high PCEs. On the other hand, the development of novel electron acceptors is behind the pace of the progress of development of PSCs. The synthesis of novel electronic acceptors with controlled molecular electronic structures and solid state supramolecular structures is urgently required, among which a supramolecular “double-cable” structure consisiting of two separated channels for charge transport is particularly desirable in generating high Jsc and thus, higher PCEs. In this presentation, we report the design and synthesis of a porphyrin-fullerene dyad (PFD1) and unambiguously demonstrate that it self-assembles into a well-defined 3D structure with alternating arrangements of separate domains of porphyrin and C60 in solid state, which can be deemed as a prototype supramolecular “double-cable” structure in solid state. Ultrafast charge transfer and charge generation were observed from the blending of semiconducting polymers with PFD1, which indicates that PFD1 is an excellent electron acceptor for PSCs. More importantly, the high Jsc were observed from BHJ PSCs, suggesting that a controlled ambipolar transporting property within the electron acceptor domain can lead to improved photovoltaic performances.
12:00 PM - H1.10
Harvard Clean Energy Project: In Silico Study of Three Million Molecular Candidates for Donor Polymers in Organic Photovoltaics.
Carlos Amador-Bedolla 1 , Roberto Olivares-Amaya 2 , Sule Atahan-Evrenk 2 , Johannes Hachmann 2 , Alan Aspuru-Guzik 2
1 Facultad de Química, Universidad Nacional Autónoma de México, Mexico DF, D. F. , Mexico, 2 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States
Show AbstractOrganic photovoltaic devices have recently reached 9% efficiency and lifetimes close to 9 years; the search for the best (co)monomers for donor polymers being based on exacting experimental synthesis. We present initial results from the Harvard Clean Energy Project (http://cleanenergy.harvard.edu), a large scale in-silico framework to design and screen organic photovoltaics by means of computational quantum chemistry including modern density functional theory and ideas from chemoinformatics. The design, automation and calculation of 3 million organic molecules allow the screening of the best candidates for further study.We present results from our initial screening of the best candidates based on chemoinformatics quantitative structure-property relationships, their comparison with results from density functional theory calculations and finally focus on how our first-principles calculations relate with the hypothesis about the effect of donor-acceptor architectures on the efficiency of the photovoltaic device. We finally address promising molecular motifs which have emerged from these studies.
H2: Interfaces and Contacts
Session Chairs
Jinsong Huang
Matthew Lloyd
Monday PM, November 28, 2011
Grand Ballroom (Sheraton)
2:30 PM - **H2.1
Interface Science of Interlayer Materials in Organic Solar Cells: Modeling, C-AFM and OPV Device Performance.
Neal Armstrong 1
1 Chemistry & Biochemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractThin (10-50 nm) interlayer materials are now widely used in organic solar cell (OPV) technologies to provide work function control, control of surface free energy, and in some instances, charge selectivity. The more highly mixed the donor and acceptor phases are in bulk heterojunction OPVs the more important these selective interlayers become, mitigating the problems that arise in surface recombination when donors or acceptors can undergo charge transfer at the wrong contact. This talk will focus first on our recent modeling studies, at nanometer length scales, which demonstrate how critical the electrical heterogeneity of the contacts can be, in controlling the concentration gradients of holes and electrons throughout the OPV. These models show, and experimental device data confirms, how effectively interlayer materials can be in a) creating charge selectivity and b) how effective they can be in mitigating problems of contact heterogeneity, provided that the interlayer material has a high charge mobility for the desired charge carrier. We emphasize the use of photoemission spectroscopies to characterize the energetics of these interlayer materials, and conducting tip AFM experiments to probe the type and distribution of electrical “hot spots” and contact uniformity.
3:00 PM - H2.2
High Performance Inverted Polymer Solar Cells with an Excellent Hole and Electron Collection Layer Optimized for Various Donor Polymer Systems.
Seiichiro Murase 1 2 , Yang Yang 1
1 Material Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States, 2 Electronic & Imaging Materials Reserch Labs, Toray Industries, Inc., Otsu Japan
Show AbstractWe demonstrated high efficiency inverted solar cells based on various donor polymers. A solution-processed ZnO nanonarticle layer was employed to an electron collection layer, while an optimal material was chosen among PEDOT:PSS, MoO3, V2O5 and their combination as a hole collection layer. The optimal material for the hole collection layer varies depending on the donor materials and might be related to their HOMO levels. The amount of surfactant added in a PEDOT:PSS solution was found to be a key factor to achieve a good balance between moderate wettability of the PEDOT:PSS solution on a hydrophobic active layer and excellent performance. With the best combination of a hole and an electron collection layer, the performance of inverted cells exceeded 6% for various donor polymer systems and is comparable with or better than that of conventional cells.
3:15 PM - H2.3
Graphene Based Interconnect for Solution-Processed Tandem Solar Cells.
Vincent Tung 1 , Jaemyung Kim 1 , Jiaxing Huang 1
1 , Northwestern, Evanston, Illinois, United States
Show AbstractGraphene oxide (GO) can be viewed as a two-dimensional, random diblock copolymer with distributed nanosize graphitic patches and highly oxidized domains, thus capable of guiding the assembly of other materials through both π–π stacking and hydrogen bonding. Upon mixing GO and conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in water, a dispersion with dramatically increased viscosity is obtained, which turns into sticky thin films upon casting. Surprisingly, the insulating GO makes PEDOT much more conductive by altering its chain conformation and morphology. The GO/PEDOT gel can function as a metal-free solder for creating mechanical and electrical connections in organic optoelectronic devices. As a proof-of-concept, polymer tandem solar cells have been fabricated by a direct adhesive lamination process enabled by the sticky GO/PEDOT film. The sticky interconnect can greatly simplify the fabrication of organic tandem architectures, which has been quite challenging via solution processing. Thus, it could facilitate the construction of high-efficiency tandem solar cells with different combinations of solution-processable materials.
3:30 PM - **H2.4
Achieving High Performance Polymer Solar Cells via Incorporating Alcohol/Water-Soluble Conjugated Polyelectrolytes as Cathode Interlayer.
Hongbin Wu 1 , Zhicai He 1 , Chengmei Zhong 1 , Xun Huang 2 , Wai-Yeung Wong 3 , Liwei Chen 2 , Shijian Su 1 , Yong Cao 1
1 , Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Functional Optoelectronics Materials and Devices, South China University of Technology, Guangzhou China, 2 , Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences, Suzhou, Jiangsu, China, 3 , Institute of Molecular Functional Materials and Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Hong Kong China
Show AbstractBulk heterojunction solar cells/polymer solar cells (PSCs) have received intense research interests due to a great demand for clean and renewable source of electricity. To date, improvements in PCE of PSCs are limited in one or two of its decisive parameters, such as short-circuit current density (JSC), open-circuit voltage (VOC), or fill factor (FF). Here we report simultaneous enhancement of VOC, JSC and FF in highly efficient PSCs by incorporating an alcohol/water-soluble conjugated polymer as a novel cathode interlayer. Mechanistic investigations with various techniques find that the incorporation of the PFN interlayer leads to significantly improved built-in voltage and charge carrier mobility in the devices. As a result, charge transport properties are improved and recombination loss is reduced. Furthermore, an added benefit of the PFN interlayer is presented in this study is that it can be processed from solvents such as methanol and water, in which most conjugated polymers/fullerene active layers are insoluble. This feature avoids mixing between the active layer and subsequent interlayer, which is a common and serious problem for solution processing of multi-layer PLEDs and photovoltaic cells. When combined with a low-bandgap polymer (PCDTBT) as the electron donor material, the PCE of the devices was improved to 6.79%. Due to drastic improvement in efficiency and easy utilization, this method opens new opportunities for PSCs from various material systems to improve towards 10% efficiency.
4:30 PM - **H2.5
Interface Science and Engineering of Organic Solar Cells.
Bernard Kippelen 1 , Yinhua Zhou 1 , William Potscavage 1 , Hyeunseok Cheun 1 , Canek Fuentes-Hernandez 1 , Jens Meyer 2 , Antoine Kahn 2
1 , Georgia Tech, Atlanta, Georgia, United States, 2 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractIn this talk, we will discuss the important role played by interface science and engineering in the operation of organic photovoltaic devices. We will first show how the modification of the electronic properties of the transparent electrode can be used to change the geometry of the solar cell and build solar cells that do not require the use of low-work function electrodes. Such inverted device architectures have superior stability compared to conventional organic solar cells. Then, we will discuss devices that do not use indium-tin-oxide or vacuum-deposited metal electrodes. In such solar cells both electrodes are comprised of conducting polymers that are modified to become either hole or electron collecting electrodes. Finally, we will describe examples of organic solar cell architectures in which all layers are comprised of polymers that can be processed from solution. We believe that these strategies pave the way to very low-cost photovoltaic technologies with light-weight and flexible form factors.
5:00 PM - H2.6
Transparent Cathode Using Organic-Inorganic Hybrid Multilayers for Organic Solar Cells.
Gwan Ho Jung 1 2 , Kihyon Hong 1 , Wan Jae Dong 1 2 , Jong-Lam Lee 1 2
1 Materials Science and Engineering, POSTECH, Pohang, Gyungbuk, Korea (the Republic of), 2 Advanced Materials Science, POSTECH, Pohang, Gyungbuk, Korea (the Republic of)
Show Abstract Thin-film solar cells based on organic and inorganic have various advantages of high efficiency, low-cost, and flexible solar cells. However, such cells using a glass substrate have the inherent disadvantages of high cost, low flexibility, and careful handle. Replacing the glass substrate with flexible opaque materials such as steel sheets or plastic films makes possible the fabrication of flexible, thin, and low-cost solar cell through roll-to-roll mass production. For the thin-film solar cells with these substrates, the direction of incoming light has to be from top to the bottom. Thus, formation of a high transparency top cathode which has high efficiency of electron collection is one of the most significant factors for high efficiency thin-film solar cells. We demonstrate a novel way of enhancing the transmittance of top cathode in OPV by using dielectric/metal/dielectric (DMD) structure. The high refractive index MoO3 (n=2.0) layer, an outer dielectric material (D2), could enhance the optical transmittance of DMD multilayer. Different optical effects between BCP, Ag layer, and outer MoO3 layer were identified in experimental measurement and theoretical calculation. From these studies, we optimized the suitable multilayer structure for transparent top cathode in OPVs. In D1MD2 cathode, the transport of electrons in the bathocuproine (BCP), employed as an inner dielectric material (D1), could be fulfilled through the gap states in the vicinity of the lowest unoccupied molecular orbital (LUMO) level of electron acceptor. Employing this BCP/Ag/MoO3 as a transparent top cathode, a high optical transmittance (~ 77%) in visible region (400 nm ~ 700 nm) and low sheet resistance (< 8.6 ohm/sq) were achieved. Organic photovoltaics (OPVs) of poly(3-hexylthiophene): [6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) with BCP/Ag/MoO3 multilayer as a transparent top cathode improved superior device performance from 0.7 % of power conversion efficiency (PCE) with Ag (10 nm) cathode to 2.3 % of PCE. Thus, we can expect this thermally evaporable BCP/Ag/MoO3 (10/10/25 nm) with high transmittance and low sheet resistance multilayer can be applicable to general transparent cathode organic opto-electronic devices.
5:15 PM - H2.7
Silver Nanowire-Polymer Composite Electrodes for Efficient Polymer Solar Cells.
Zhibin Yu 1 , Lu Li 1 , Qingwu Zhang 1 , Weili Hu 1 , Qibing Pei 1
1 , UCLA, Los Angeles, California, United States
Show AbstractSilver nanowire-polymer composite electrodes were prepared containing a highly conductive silver nanowire network in the polymer matrix. The electrodes show high conductivity, high transparency, and low surface roughness. We have developed an improved technique for the preparation of composite electrodes using a stack of silver nanowires. Polymer solar cells fabricated on the composite electrodes exhibit power conversion efficiencies comparable to control devices on indium tin oxide on glass substrate. The solar cells are highly flexible and can be bent to a maximum 8% tensile strain.
5:30 PM - H2.8
Controlling the Electronic Interface Properties in Polymer-Fullerene Bulk-Heterojunction Solar Cells.
Tobias Stubhan 1 , Ivan Litzov 1 , Hai Qiao Wang 1 , Johannes Krantz 1 , Tayebeh Ameri 1 , Luigi Pinna 1 2 , Hyunchul Oh 1 , Florian Machui 1 , Christoph Brabec 1 2
1 I-MEET, FAU Erlangen, Erlangen Germany, 2 , ZAE Bayern, Erlangen Germany
Show AbstractDuring the past decade, fast progress has happened in the field of organic photovoltaics. Polymer solar cells now offer a promising approach for a low-cost and flexible photovoltaic technology. Significant advances have led to certified efficiencies of 9.2 % and expectations are that the magic 10 % hurdle will be overcome soon. Before widespread commercialization, large area production and stability issues have to be solved. For the reliable large area production with high yield and low shunts, thick, stable, robust and printable buffer layers are a prerequisite. A promising and heavily investigated material class that can provide these properties for n- as well as p-type contacts are metal oxides. We have synthesized a series of metal oxides and characterized them with respect to their potential as interface layers in thin film electronic devices. On the n-type side we concentrated our studies on the well known ZnO and TiOx material systems and investigated their performance in solar cells as a function of conductivity (respectively doping). The effect of extrinsic doping was specifically studied for ZnO. Al- doped ZnO nanoparticles processed via high and low temperature routes, with variable sizes and doping degrees, were synthesized to tailor the opto-electronic properties of the electron injection layer. Interestingly, doped ZnO turned out to be superior to intrinsic ZnO in the thick film limit, i.e. for injection layers with a thickness in the range of 100 nm and above. Next, we developed and studied solution processed p-type metal oxides, with MoO3 nanoparticle layers as a promising and stable alternative to acidic and hygroscopic PEDOT:PSS. Employing these developments, we are able to demonstrate solar cells with both n- and p-type buffer layers made from stable, solution processed metal oxides.
5:45 PM - H2.9
Solution Processed Metal Oxide Hole Transport Layers for Organic Photovoltaic Devices.
Yun-Ju Lee 1 , Kamil Mielczarek 1 , Jian Wang 1 , Juan Yi 1 , Anvar Zakhidov 1 , Julia Hsu 1
1 , University of Texas at Dallas, Richardson, Texas, United States
Show AbstractTo maximize organic photovoltaic (OPV) device performance, various thin films have been deposited between active layer and the hole collecting electrode to block electrons, to increase electrode work function, and to optimize hole transport. However, the frequently used poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been implicated in decreased device lifetime due to acidity of the PEDOT:PSS suspension and wetting issues on top of organic films. More recently, metal oxides such as MoO3 and NiO have demonstrated improved performance and stability compared to PEDOT:PSS, but they are typically deposited in vacuum. Here, we present a simple solution based method for the synthesis and deposition of transition metal oxide thin films, and examine their performance as hole transport layers (HTLs) compared to PEDOT:PSS in model OPV devices. Metallorganic precursors are converted to metal oxide nanoparticles using solvothermal synthesis in a microwave reactor. The impact of precursor chemistry and synthesis conditions on nanoparticle composition, size, crystallinity, and stability are characterized with XRD, XPS, SEM, and other techniques. Modification of electrode work function by nanoparticle films is quantified with Kelvin probe measurements. Spin coated HTLs using e.g. MoO3 nanoparticles improve the efficiency of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester devices, and the effect of deposition conditions and post processing on device performance and lifetime are discussed.
H3: Poster Session I
Session Chairs
Tuesday AM, November 29, 2011
Exhibition Hall C (Hynes)
9:00 PM - H3.1
Characterization of Water-Soluble Thiophene Polymers for Photovoltaic Active Materials.
Michael Kurniawan 1 , Kong Fai Tai 1 , Tze Chien Sum 1 , Cheng Hon Alfred Huan 1
1 Physics and Applied Physics, Nanyang Technological University, Singapore Singapore
Show AbstractOrganic Photovoltaics hold great promise for the ease of production, flexibility, and particularly, reduction of fabrication costs. Devices comprising of thiophene polymers and fullerene derivatives are most popular and still intensely studied. Poly(3-hexylthiophene-2,5-diyl) (P3HT) is a famous choice among organic polymers and in conjunction with (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) can yield device efficiency of 5%. Driven by the increasing demand of environmentally friendly alternative energy source, few researches have been carried out to replace the usage of organic solvent in the fabrication steps with water.For a molecule to be dissolvable in water, a particular side chain is required. In our work, we compared the performance of devices made of these side-chain-modified thiophene polymers with the conventional P3HT:PCBM. Using XPS, we investigated the chemical state, chemical composition of modified-polymers in film. We also studied the properties of these modified thiophene polymers, and found that some properties of the conventional P3HT were altered as its side chain was modified. The tendency to crystallize was not observed in the modified thiophene polymers as shown from the XRD results. There was also a difference in the absorption spectra of P3HT and the modified thiophene polymers; the absence of distinct peaks in the 500 nm-660 nm regions in the modified thiophene polymers. Since the well resolved absorption peaks have been shown to relate to the crystal structure of P3HT in others’ works, this finding reveals a failure of the modified thiophene polymers to crystallize. Supplemented by the Femtosecond Transient Absorption study, we compared the charge transfer dynamics within the heterojunction consisting this modified thiophene and fullerene derivative with the conventional P3HT:PCBM.
9:00 PM - H3.10
Photovoltaic Performance of CdS/P3HT Bilayer Solar Cells: Effect of Thermal Annealing and Carbon Paint for Metal Contact.
Hugo Cortina 1 , Claudia Martinez-Alonso 1 , Karunakaran Nair 1 , Hailin Hu 1
1 , Centro de Investigacion en Energia, UNAM, Temixco, Morelos, Mexico
Show AbstractPhotovoltaic (PV) performance of a hybrid solar cell is a function of active layers’ properties such as optical absorption spectra of the electron donor and acceptor, charge carrier mobility of each one and their balance, etc. It also depends on the electrical behavior of metal contacts. In this work we report PV performance of CdS/poly3-hexylthiophene (P3HT) bilayer hybrid solar cells, prepared by chemical bath deposition of CdS thin films on transparent conductive glass substrates (ITO) and subsequence deposition of P3HT solution on top of CdS. A P3HT film was formed after solvent evaporation and a metal contact (Au) was thermally evaporated on top of P3HT. It is observed that a slow polymer film drying process and a subsequent thermal annealing at moderate temperature were very important processes to lead a higher crystallinity and larger optical absorption coefficient in P3HT films that benefit the PV performance of the cells. AFM images of P3HT films indicate that the surface roughness was reduced by the annealing process, as expected from the macromolecular rearrangement in polymeric films and at the same times the crystallinity of the same films increases with annealing temperature suggested by the corresponding XRD patterns. On the other hand, analysis of in-dark I-V curves of the cells indicates that carbon paint (CP) collocated between P3HT film and metal contact reduced one order of magnitude of cells’ series resistance and improved the cells’ PV performance as well. It is demonstrated that the introduction of CP reduced the potential difference between two metals of the cells (ITO and Au) and improved the ohmic contact between P3HT and the top metal (Au), as a consequence increases Voc and Jsc of the corresponding CdS/P3HT solar cells.
9:00 PM - H3.11
Photoinduced Effects on the Electronic Structure of the Organic/Organic and Organic/Metal Interfaces.
Senku Tanaka 1 , Koji Ogawa 2 , Ken Fukuzawa 1 , Masao Kamada 2 , Ichiro Hiromitsu 1
1 Interdisciplinary Faculty of Science and Engineering, Shimane University, Matsue, Shimane, Japan, 2 Synchrotron Light Application Center, Saga University, Saga, Saga, Japan
Show AbstractThe electronic structure at the organic/organic and organic/metal interfaces plays an important role for the photovoltaic properties of the organic solar cell (OSC). For example, it has been reported that the energy difference between the highest occupied molecular orbital of the donor layer and the lowest unoccupied molecular orbital of the acceptor layer is an important factor on the origin of the open-circuit voltage of OSC [1]. The electronic structure at the organic/metal interface also affects the efficiency of the carrier extraction from the organic layer to the external circuit [2]. Therefore the understanding and control of the electronic structure of OSC is important for the improvement of the energy conversion efficiency. In this presentation, we discuss the effect of light illumination on the electronic structure of the organic/organic and organic/metal interfaces. The photoelectron spectroscopy under the light illumination was performed to investigate the electronic structure of the organic thin films under carrier generation.As a typical donor/acceptor interface of OSC, Zn-phthalocyanine (ZnPc)/C60 interface was studied. The photoelectron spectra with and without a white light illumination were observed with incrementally deposited ZnPc and C60 on an indium-tin oxide (ITO) substrate in situ. A solar simulator was used as the white light source for the illumination. The simulated airmass 1.5 solar illumination was exposed to the sample in a vacuum chamber through a viewport. The photoelectron spectroscopy was performed by using the synchrotron radiation at the UVSOR (Okazaki, Japan) and the Saga-LS (Tosu, Japan). All the photoelectron measurements were carried out at room temperature.The photoelectron spectrum of the ZnPc layer on the ITO substrate showed no significant effect of the light illumination. In contrast, it was observed at the ZnPc/C60 interface that the photoelectron spectrum shifted toward higher kinetic energies under the light illumination. The photoinduced shift showed the dependences on both the light intensity and the thickness of the C60 layer. The shift of the photoelectron spectrum was related to the photovoltaic effect at the ZnPc/C60 interface caused by the spatial dissociation of the electron-hole pair. In the present case, the C60 layer was charged by the photogenerated electrons. The similar shifts of the photoelectron spectra under the light illumination were observed at the ZnPc/Ag interface (a Schottky interface). These preliminary results indicate that the photoelectron spectroscopy under the light illumination is a valuable method to investigate the electronic structure of the organic solar cell system under the working conditions.[1] B. P. Rand et al., Phys. Rev. B., 75 (2007) 115327. [2] S. Tanaka et al., Appl. Phys. Lett., 97 (2010) 253306.
9:00 PM - H3.12
Solid-DSSC Based on Self Assembled Nano-p-n PANI-TiO2 Heterojunctions with 1% Photovoltaic Efficiency and Energy Storage Capability.
Michael Ibrahim 1 2 , Maria Bassil 1 , Umit Demirci 2 , Georges El Haj Moussa 1 , Mario El Tahchi 1 , Philippe Miele 2
1 , LPA-GBMI, Department of Physics, Lebanese University - Faculty of Sciences, Jdeidet Lebanon, 2 , Université Lyon 1, CNRS, UMR 5615, Laboratoire de Multimatériaux et Interfaces, Villeurbanne France
Show AbstractMany advances in dye-sensitized solar cells (DSSCs) fabrication have been recorded during the last 20 years to overcome several problems. For example many sensitizers have been used to improve the overall conversion efficiency of DSSC and 11.1% efficiency has been reached [1]. The leakage problem of liquid electrolytes is solved by using gel electrolytes. Despite the development in the lab-based solar cells, large-scale production of DSSCs is still under study and hybrid systems are essential for energy storage. To overcome these latter obstacles, a new photovoltaic paint consisting of polyaniline (PANI) and titanium dioxide (TiO2) is introduced which gathers the easiness of fabrication, very low cost, stability and capability of energy storage.To form a core-shell structure between PANI and TiO2 particles (Degussa P25, 21 nm), the chemical oxidative polymerization of aniline-HCl (A-HCl) by ammonium peroxydisulfate (APS) is performed inside a TiO2 aqueous solution. By fixing the TiO2 content of the precursor solution and the A-HCl to APS molar ratio at 1:1.25 as defined by the IUPAC [2], PANI-TiO2 photovoltaic powder is synthesized. The as-formed powder is then sandwiched between two indium tin oxide (ITO) coated PET sheets and showed a photovoltaic response under sunlight. In the single-layer photovoltaic device, PANI contribution is divided into two parts: PANI is considered as a sensitizer at the photoanode and as an electrolyte deeper inside the composite layer. We show that this one-layer device acts as a single p-n junction due to charge separation between PANI and TiO2 particles where electrons from PANI are injected into TiO2 thus creating oppositely charged regions. An advanced architecture is developed to enhance the device efficiency to 1% and give it the ability to store the generated energy. First TiO2 electrodes were prepared by sandwiching titania paste over aluminum (back-contact electrode) and ITO coated PET (front electrode). PANI-TiO2 powder is mixed with ethanol to form a solution then a 2 cm x 2 cm piece of cotton fabric is placed in the mixture for several minutes and dried at room temperature. Finally the three layers are assembled together. Electrical, morphological and spectroscopic characterizations were performed in order to explain the physical phenomena governing the behavior of the device. The stability of the device is determined by measuring the time-dependence of the photocurrent under short-circuit conditions and the photovoltage under open-circuit conditions. It is shown for the first time that a photovoltaic device made by simple painting technique is capable of converting light to electricity with the ability of storing the generated power.[1] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide and L. Han, Jpn J Appl Phys 45, L638 (2006).[2] J. Stejskal and R. G. Gilbert, Pure Appl Chem 74, 857 (2002).
9:00 PM - H3.13
Fabrication and Photovoltaic Characteristics of Organic Solar Cells Using Ag Nanoparticles Network Counter Electrode.
Kimiaki Muraguchi 1 , Seimei Shiratori 1 , Kazuma Yamane 2 , Kenichi Nakata 2
1 , Keio University, Yokohama-shi, Kanagawa-ken, Japan, 2 , Toda Kogyo Corporation, Otake-shi, Hiroshima-ken, Japan
Show AbstractOrganic Solar Cells (OSCs) and Inverted Organic Solar Cells (IOSCs), in which the active materials are polymers or small organic molecules as donor and acceptor materials, have been gaining strong attention due to its potential of low fabrication cost, high mechanical flexibility of materials [1]. These photovoltaic devices may be fabricated through solution printable processing, in the future, which is most efficient in terms of the fabrication cost and large-area fabrication. However, the counter electrode (e.g. Ag or Al) of all these conventional devices needs to metal vapor deposition using dry process. In this paper, we investigate the alternative method for a counter electrode without vacuum condition. IOSCs based on the blend film of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) was used as an active layer, and ZnO nanoparticles film using sol-gel method as an electron transport layer. Ag counter electroce was fabricated by self-assembled Ag nanoparticles network technique [2]. In addition, IOSCs using Ag vapor deposition was made for the comparison. Fabricated devices were evaluated using a solar simulator with its mismatch calculated relative to AM1.5 illumination (100 mW/cm2). The light-to-electricity conversion efficiency of IOSCs using self-assembled Ag network counter electrode was reached 0.74 %. [1] Brabec, C. J., Sariciftci, N. S., Hummelen, J. C., Adv. Funct. Mater., 11, 15-26 (2001), [2] United States Pattent,2010/7736693, Garbar, et al.
9:00 PM - H3.15
Combinatorial Screening of Polymer/Fullerene Blends for Solar Cells by Inkjet Printing.
Anke Teichler 1 , Jolke Perelaer 1 , Ulrich Schubert 1
1 Laboratory of Organic and Macromolecular Chemistry (IOMC) and Jena Center for Soft Matter (JCSM), Friedrich-Schiller-University Jena, Jena Germany
Show AbstractIn the last years inkjet printing of organic semiconductors has obtained an increased interest, because of the possible applications in organic light emitting diodes (OLED) and organic photovoltaics (OPV).[1] The use of inkjet printing enables a fast and simple experimental workflow from film preparation to the study of structure-property-relationships with a very high material efficiency.[2,3]In this contribution, a combinatorial approach to screen polymer:fullerene blends by inkjet printing thin film libraries for photovoltaic devices is presented. The application of inkjet printing enabled a fast and simple experimental workflow from film preparation to the elucidation of structure-property-relationships with a very high material efficiency. Inkjet printing requires less material for the preparation of thin film libraries in comparison to other dispensing techniques, like spin-coating. The study here focused on two polymers as donating materials and two fullerene derivatives as acceptor materials for bulk heterojunction solar cells. Parameters that were screened include blend ratios, concentrations, solvent ratios, and film thicknesses. Morphological and optical properties of the inkjet printed films were investigated and compared with spin-coated films.[1]M. Singh, H. M. Haverinen, P. Dhagat, G. E. Jabbour, Adv. Mater. 2010, 22, 673.[2]A. Teichler, R. Eckardt, S. Hoeppener, C. Friebe, J. Perelaer, A. Senes, M. Morana, C. J. Brabec, U. S. Schubert, Adv. Energy Mater. 2011, 1, 105.[3]A. Teichler, R. Eckardt, C. Friebe, J. Perelaer, U. S. Schubert, Thin Solid Films 2011, 519, 3695.
9:00 PM - H3.16
Hierarchical Nanomorphologies Promote Exciton Dissociation in Polymer:Fullerene Bulk Heterojunction Solar Cells.
Wei Chen 1 , Tao Xu 2 , Feng He 2 , Wei Wang 2 , Cheng Wang 3 , Joseph Strzalka 4 , Yun Liu 5 6 , Luping Yu 2 , Seth Darling 1
1 Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois, United States, 2 Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois, United States, 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois, United States, 5 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 6 Department of Chemical Engineering, University of Delaware, Newark, Delaware, United States
Show AbstractSolar cells based on the polymer:fullerene bulk heterojunction (BHJ) represent one of the most promising technologies for next-generation solar energy conversion due to their low-cost and scalability. In the last fifteen years, research efforts have led to organic photovoltaic (OPV) devices with power conversion efficiencies (PCEs) up to ~8%, but these values are still insufficient for the devices to become widely marketable. To further improve solar cell performance, it is required to have a thorough understanding of the complex structure-property relationships in the OPV devices. In this work, we used PTB7 semiconducting copolymer, which contains thieno[3,4-b]thiophene and benzodithiophene alternating units, as an archetype of high-performance polymer:fullerene BHJ solar cells, given that PTB7 has set a historic record of PCE (7.4%) in BHJ solar cells when combined with fullerenes. Taking advantage of a simple yet effective route—altering solvents—to tune both crystalline structure and phase-separated morphology, and thereby, to improve performance, we investigated performance-related structures in PTB7:fullerene solar cells at different length scales using multiple x-ray and neutron scattering techniques. Contrary to the commonly accepted picture of ideal BHJ morphology, we demonstrated that the OPV active layer of PTB7:fullerene OPV devices involves hierarchical nanomorphologies ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains, themselves hundreds of nanometers in size. These hierarchical nanomorphologies with optimum crystallinity and intermixing of PTB7 with fullerenes are coupled to significantly enhanced exciton dissociation, which consequently contribute to photocurrent, leading to the superior performance of PTB7:fullerene BHJ solar cells. As analogous multiple-length-scale nanostructures have also been observed in the related PTB:fullerene copolymer blends, we believe these hierarchical nanomorphologies represent a structural generality in the PTB copolymer series when blended with fullerenes in the OPV devices. New insights of performance-related structures afforded by the current study should aid in the rational design of even higher performance polymeric solar cells.
9:00 PM - H3.17
Improvement of Dye Sensitized Solar Cells with Titanium Nanofiber Photoanode.
Lijun Yang 1 , Wallace Woon-Fong Leung 1
1 Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong China
Show AbstractIn order to improve the photons harvesting efficiency of Dye Sensitized Solar Cell (DSSC), semiconductor (mainly TiO2) with large surface area, such as nanoparticles, are widely studied. However, nanoparticles DSSC rely on trap-limited diffusion for electron transport that limits the efficiency of the device. Here, we report the application of TiO2 nanofibers photoanode for high efficiency DSSC by a fast and cost-effective technique, i.e.electrospinning. Nanofiber is one-dimensional nanostructure materials which can provide a direct pathway for electron transport. Our fabricated TiO2 nanofibers were mainly anatase phase with large surface area, and their diameter ranged from 50 to 120 nm. In this study, we investigated the effects of the nanofiber diameter and layer thickness of the photoanode on the performance of DSSC. The results showed that the smaller diameter nanofiber had higher dye loading capacity thus resulted in higher efficiency. Increasing the thickness of the photoanode can increase the efficiency at relatively small layer thickness of the photoanode, however, further increase in thickness of the photoanode results in a drop in efficiency due to electron recombination with the electrolyte. An optimum power conversion efficiency of 7% was obtained in DSSC with TiO2 nanofibers diameter of 60 nm and with photoanode thickness of approximately 9 μm.
9:00 PM - H3.18
Morphology Control of Octacyanophthalocyanine for Nano-Rod Array of n-Type Organic Semiconductor.
Hiroyuki Saeki 1 , Mihoko Nishimoto 1 , Yasuko Koshiba 1 , Masahiro Misaki 1 , Satoshi Horie 1 , Kenji Ishida 1 , Yasukiyo Ueda 1
1 Graduate School of Engineering, Kobe University, Kobe Japan
Show AbstractSemiconducting nano-rod array has attracted much attention as a building block for organic photovoltaic cells to enhance the charge collection in the photoactive layer. In previous study, we prepared directly thin films composed of grass-like crystals of octacyanophthalocyanine (OcPc) on metal and/or alkali halide substrates by chemical vapor deposition (CVD) of 1,2,4,5-tetracyanobenzene (TCNB). In this study, we tried to fabricate the nano-rod array of OcPc with the optimization of the reaction temperature. Preparation procedure of OcPc film was as follows. TCNB and air-cleaved KCl single crystal were put into Pyrex glass tube. After evacuating and flushing with Ar gas, the tube was sealed at 10Pa and then was heated at various temperatures in the hot air oven. OcPc films were formed on KCl substrate at the temperature range between 230 and 330°C. The SEM images of each film indicated the formation of OcPc nano-rod structure. The morphology of OcPc nano-rod heavily depended on the reaction temperature. When the reaction tube was preheated at 270°C for 6h and was heated at 300°C for 18h continuously, uniform nano-rod of OcPc with 150nm in length and 40-50nm in diameter was fabricated. The field effect transistor (FET) utilizing the OcPc films operated in the depletion mode; n-type semiconducting. OcPc nano-rod has high potential as n-type active layer in organic solar cell.
9:00 PM - H3.19
The Effect of Silicon Bridge Substitution on the Packing of Polymers for Organic Solar Cells.
Anne Guilbert 1 2 , Tiziano Agostinelli 2 5 , Jarvist Frost 1 2 , Ellis Pires 4 , Weimin Zhang 1 3 , Donal Bradley 1 2 , Iain McCulloch 1 3 , Emyr Macdonald 4 , Jenny Nelson 1 2
1 Centre for Plastic Electronics , Imperial College London, London United Kingdom, 2 Physics, Imperial College London, London United Kingdom, 5 , Plastic Logic , Cambridge United Kingdom, 4 School of Physics and Astronomy, Cardiff University, Cardiff United Kingdom, 3 Chemistry, Imperial College London, London United Kingdom
Show AbstractNowadays, the best performing designs of organic solar cell are based on blend films of a conjugated polymer and a fullerene derivative. The performance of such devices is strongly related to the blend microstructure. Therefore, in the selection of new semiconductor materials for improved light harvesting or higher photovoltage, it is essential to understand the effect of chemical structure on microstructure and to find ways to control it. Recently, the chemical substitution of the carbon bridging atom between the backbone and the side chains with a silicon atom in poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3 ,4-b']dithiophene-2,6-diyl]] (PCPDTBT) has led to a twofold increase in the power conversion efficiency up to 5.5% [1][2]. The same approach has been successfully employed with other polymers such as polyfluoren bisthienylbenzothiadiazole (PFDTBT) [3]. It has been observed that replacing the carbon bridging atom with a silicon atom increases the polymer tendency to aggregate [4], resulting in an increased crystallinity [5] and hole mobility [2]. In this work, we focus on understanding the effect of silicon bridge substitution on the packing of polymers such as PCPDTBT and PFDTBT by means of grazing incidence X-ray diffraction (GIXRD) and molecular modelling. GIXRD gives evidence that the silicon bridge enables closer chain contact in the π-π stacking direction. We then used Molecular Dynamics to investigate the effect of the volume of the silicon bridging atom on the side chain conformation [4]. This modelling indicates points out that the “stickiness” of silicon bridged polymer chains originates from an increased flexibility of the side chains and from an increased polarizability of the polymer chain. [1] J. Hou et al., Journal of the American Chemical Society, 130:16144-16145, 2008[2] M. C. Scharber et al., Advanced Materials, 22:367-370, 2010[3] E. Wang et al., Applied Physics Letters, 92:033307, 2008[4] H.-Y. Chen et al., Advanced Functional Materials, 22:371-375, 2010[5] M. Morana et al., Advanced Functional Materials , 20:1180-1188, 2010
9:00 PM - H3.2
Efficient Inverted Organic Bulk-Heterojunction Solar Cells Based on Electrochemically Deposited ZnO Thin Films.
Xiao-Feng Wang 1 , Haruhiko Ojima 1 , Ziruo Hong 1 , Tatsuhiro Takahashi 1 , Junji Kido 1
1 Faculty of Science and Engineering, Yamagata University, Yonezawa, Yamagata, Japan
Show AbstractRecently, organic solar cells (OSC) have attracted much attention than before, due to the progress in development of novel p- and n-type materials that substantially improved the power conversion efficiency. An inverted type OSC usually employs a wide band gap semiconductor material as the electron transport layer in additional of the n-type fullerene derivatives. ZnO has the following advantages compared to other inorganic semiconductor materials : 1. The ZnO morphology is much easier to control. 2. The ZnO layer can be crystallized at lower temperatures, which is useful in flexible-type OSCs based upon plastic substrates. 3. ZnO has relatively high electron mobility. In present study, we fabricated ZnO cathode at low temperature with an electrochemical deposition method. The morphology of ZnO thin films was controlled by additional of structural directing agent (SDA). The ZnO nanostructures were compact without using the SDA, but with nanopores in the wire structure by using the SDA. The thickness of the ZnO thin films has been controlled by adjusting the electrochemical deposition time. It has been found that with 10 min of electron deposition of the ZnO at 70 °C, the inverted solar cell gave rise to the highest power conversion efficiency. The highest photovoltaic performance for the P3HT/PCBM blending active layers was reported as: Jsc = 8.5 mA/cm, Voc = 0.6 V, FF = 0.52, which corresponds to a PCE = 2.7%, under the AM1.5 light irradiation (100 mW/cm2). Further improvement of the solar cells with low band-gap polymers and other fullerene derivatives has been also attempted.
9:00 PM - H3.22
Deep Level Transient Spectroscopy (DLTS) Study of P3HT:PCBM Organic Solar Cells.
Johan Lauwaert 1 , Samira Khelifi 2 , Koen Decock 2 , Marc Burgelman 2 , E. Voroshazi 3 , T. Aernouts 3 , D. Spoltore 4 , F. Piersimoni 4 , Freddy Callens 1 , Henk Vrielinck 1
1 Department of solid state sciences, University of Gent, Ghent Belgium, 2 Department of Electronics and Information Systems (ELIS), University of Gent, Ghent Belgium, 3 Organic Photovoltaics, Polymer & Molecular Electronics, IMEC, Leuven Belgium, 4 Institute of Materials Research, University of Hasselt, Hasselt Belgium
Show AbstractThe electronic structure of an organic photovoltaic bulk heterojunction cell strongly deviates from the typical textbook examples of a single sided junction used to explain electrical characterisation of defects in semiconductors. Therefore it is not so straightforward to assign the capacitance of this device or the charge in it to the presence of a depleted layer within this structure. However, conventional electronic spectroscopic techniques could give useful information to understand the electronic behaviour of the device. Therefore, in this work capacitance and charge DLTS have been performed on P3HT:PCBM solar cells.At 1MHz only negligible variation in the capacitance as a function of temperature and bias has been observed. As a result no spectrum could be recorded using a standard DLTS setup, registering the capacitance at this high frequency. To avoid this parasitic effect low frequency capacitance DLTS (40 kHz) has been performed, showing an anomalous signal with negative amplitude and an activation energy of 160meV, and a complementary positive signal could be observed altering the biases. Charge DLTS clearly revealed that both signals transients, conventional and with altered bias have the same time constants. A recent study has shown that such behaviour cannot be explained by the thermodynamic properties of capture and emission of carriers by a defect in bulk semiconductor. The validity of alternative explanations, including interface states, non-ideal ohmic contacts and effects of carrier hopping on charge mobility, will discussed.
9:00 PM - H3.23
All Ink-Jet Printed, ITO-Free Organic Photovoltaic Cells.
Nadia Grossiord 1 , Yulia Galagan 1 , Jorgen Sweelssen 2 , Harrie Gorter 1 , Mao Ren 1 , Sjoerd Veenstra 3 1 , Jan Kroon 3 1 , Paul Blom 1 , Ronn Andriessen 1
1 , Holst Centre/TNO, Eindhoven Netherlands, 2 , TNO Science and Industry, Eindhoven Netherlands, 3 Thin Film Photovoltaics, Energy Research Centre of the Netherlands (ECN), Petten Netherlands
Show AbstractAn interesting new generation of PV cells involves the use of semi-conductive polymers. The latter are very promising materials as they are soluble in many organic solvents, allowing deposition by solution printing or coating, rendering the processing of light Organic PV (OPV) cells economically competitive. In order to achieve this goal, identification of suitable printing and coating techniques for large scale OPV production is considered as a milestone. Ink jet printing is very appealing as it constitutes a non-contact, additive and mask-less approach for deposition of thin films from solutions. Moreover, it is very versatile as it allows easy change from batch to batch and a large freedom of patterning. Other attractive features of this printing technique include reduced material wastage, low-cost and scalability to large area, mass production manufacturing, a.o. We are developing high volume, roll-to-roll compatible printing and coating techniques in order to produce entirely solution-processed, ITO-free OPV modules. We prepared organic solar cells of which all layers have been ink jet printed (IJP), without using any chlorinated solvents. These all-IJP solar cells display comparable electrical performances to the ones of the corresponding devices prepared by spin coating, typically of the order of 3% for cells based on a P3HT:PCBM blend. The latter number is also comparable to the performance of cells previously reported using ITO, chlorinated solvents and containing only one (1) or two (2) IJP layers, respectively. Furthermore, since our method to formulate inks and to print the different layers of the OPV devices is general, it was also possible to extend the range of inks to other active layers as well as supporting layers, enabling the realization of IJP cells with both inverted and direct architectures. References: (1) Hoth et al. Adv. Mater. 19 (2007) 3973 & Nano Lett. 8 (2008) 2806 / Aernouts et al. Appl. Phys. Lett. 92 (2008) 033306(2) Eom et al. Org. Electron. 11 (2010) 1516
9:00 PM - H3.24
Vertical Orientation of Copper Phthalocyanine in Organic Solar Cells Using a Small Molecular Weight Organic Templating Layers.
Kyoung Soo Yook 1 , Ohyoung Kim 1 , Jun Yeob Lee 1 , Brian Lassiter 2 , Stephen Forrest 2
1 Polymer Science and Engineering, Dankook University, Yongin-si, Gyeonggi-do, Korea (the Republic of), 2 Departments of Materials Science and Engineering, Electrical Engineering and Computer Science, and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractA hole-transporting material, hexaazatriphenylene-hexacarbonitrile (HAT-CN), was grown on the indium tin oxide (ITO) as the templating layer and the device performances of copper phthalocyanine (CuPc):C60 small molecule organic photovoltaic (OPV) cells were investigated. Vertically oriented CuPc donor molecules were observed on the ITO surface by using the HAT-CN templating layer, which could reduce the series resistance of the OPVs. As a result, the fill factor and short circuit current of OPVs with HAT-CN templating layer were improved by 20% compared with those of OPVs lacking the HAT-CN templating layer.
9:00 PM - H3.25
Efficiency Enhancement of Inverted Organic Solar Cells by Pattering 3-Dimensional ITO Electrode.
Cheng Jin An 1 , Jong Kil Choi 1 , Hwan Jin Jeon 1 , Ming Liang Jin 1 , Jae Hyun Lee 1 , Hee-Tae Jung 1
1 , Korea Advanced Institute of Science and Technology, DeaJeon Korea (the Republic of)
Show AbstractWe demonstrated inverted organic solar cells in which nano-patterned ITO electrodes was extended deep within active layer, providing efficient electron transport pathway. The height and diameter of hole cylinder ITO electrodes fabricated by Second Sputtering Lithography are 100nm and 400nm respectively. The expected increase of surface area is about 160%. The titanium oxide was uniformly deposited on the nano-patterned ITO electrodes by atomic layer deposition (ALD) characterized by excellent conformality on 3-dimensional structure and accurate thickness control with nano-level. The seperated electrons in active layer can be easily transport to ITO electrode through TiO2 thin film because the conduction band of TiO2 is similar to that of PCBM. Moreover, the valence band of TiO2 is efficient to inhibit back electron or hole transfer with its high-energy barrier. The inverted organic photovoltaic cells with high aspect ratio ITO electrode, which are based on the regioregular poly(3-hexythiophene) and C61 butyric acid methyl ester bulk heterojunction, showed higher power conversion efficiency than on flat ITO substrate. The dramatically improved efficency using 3-dimensional ITO elctrode is interpreted with the enhanced-charge separation and collection by both increasing the interface area between TiO2 and active layer and shortening the traveling distance for electrons.
9:00 PM - H3.26
Effect of High Work Function Indium Tin Oxide (ITO) at Organic Semiconductor / ITO Interface.
Irfan Irfan 1 , Sachiko Graber 2 , Franky So 3 , Yongli Gao 1 4
1 Physics and Astronomy, University of Rochester, Rochester, New York, United States, 2 Devision of Science, Grinnell College, Grinnell, Iowa, United States, 3 Material Science and Engineering, University of Florida, Gainesville, Florida, United States, 4 Institute for Super Microstructure and Ultrafast Process, The Central South University, Changsha, Hunan, China
Show AbstractIndium tin oxide (ITO) is predominantly used as a transparent electrode in photovoltaic cells and organic light emitting diodes. High surface workfunction (WF) of ITO is a crucial parameter for enhanced device performance. The ITO WF is usually around 4.5 eV without any surface treatment. With surface treatment ITO WF, as high as 5.4 eV has been reported. We designed a surface treatment method with which we achieved substantially high ITO surface work function of over 6.1 eV. We investigated effect of plasma treatment with X-ray photoemission spectroscopy (XPS), ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). We observed changes in valence electronic structure and core levels, apart from surface cleaning. We also investigated interface formation of chloro-aluminum pthalocyanine (AlPc-Cl) and C60 on the high WF ITO. It was found that the high WF ITO substrate strongly lifts electronic energy levels of organic semiconductors. In the proximity of interface the highest occupied energy level of AlPc-Cl was observed to be almost pinned to the Fermi level.
9:00 PM - H3.27
Orientation Dependent Ionization Potential of CuPc and Energy Level Alignment at C60/CuPc Interface.
Chenggong Wang 1 , Irfan Irfan 1 , Alexander Turinske 2 , Yongli Gao 1 3
1 Department of Physics and Astronomy, University of Rochester, Rochester, New York, United States, 2 Department of Physics and Astronomy, University of Wisconsin Oshkosh, Oshkosh, Wisconsin, United States, 3 Institute for Super Microstructure and Ultrafast Process, Central South University, Changsha, Hunan, China
Show AbstractMolecular orientations of copper phthalocyanine (CuPc) on different substrates have been investigated with Ultra-violet photoemission spectroscopy (UPS). Orientations of CuPc molecules are expected to be flat laying in the absence of any significant interaction with highly ordered graphite as a substrate. In the case of Si (with native oxide) as a substrate, orientations are expected to be standing up. Substantial difference in ionization potential of CuPc was observed with different molecular orientation. The ionization potential of C60 was not affected by the orientation of CuPc due to spherical symmetry of C60 molecules. We observed band bending in C60 on the standing-up CuPc while on the lying-down CuPc, C60 energy levels were observed to be flat
9:00 PM - H3.28
High Efficiency Small Molecular Solar Cells Based on Blending Two Squaraine Donors.
Xin Xiao 1 , Guodan Wei 2 , Siyi Wang 3 , Mark Thompson 3 , Stephen Forrest 1 2 4
1 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Chemistry, University of Southern California, Los Angeles, California, United States, 4 Physics, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract Squaraines (SQs) are a class of promising small molecule donors for organic photovoltaic devices (OPV) due to their highly absorption constant in the visible and near-IR spectral regions [1,2]. However, it has been a challenge to increase the short circuit current (Jsc) by broadening the absorption spectrum, especially absorbing photons within the wavelength gap between 500 nm and 600 nm found in SQ/C60 donor-acceptor junctions, without also decreasing open circuit voltage (Voc) and fill factor (FF). By modifying the parent molecular structure of 2,4-bis[4-(N,N-dissobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ), we obtained a symmetric SQ (1-NPSQ: 2,4-bis[4-(N-Phenyl-1-naphthylamino)-2,6-dihydroxyphenyl] squaraine) and an asymmetric analog (DPASQ: 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] asymmetric squaraine). 1-NPSQ has an absorption band between 550 nm to 800 nm, whereas DPASQ absorption lies between 450 nm to 650 nm, which covers the gap between that of 1-NPSQ and C60. Here, we demonstrate that by blending 1-NPSQ and DPASQ, followed by vacuum deposition of C60, the power conversion efficiency η can be enhanced by 17% compared with neat 1-NPSQ/C60 OPV due to the increase of Jsc and Voc. This increase is without a reduction in FF=0.70. The 7% increase in Voc is due to the deeper highest occupied molecular orbital energy of DPASQ. The optimally blended device achieves a power conversion efficiency of 5.5% at one sun (AM 1.5G) compared with 4.7% for the control 1-NPSQ/C60 OPV. References [1] G. D. Wei et al, "Efficient, Ordered Bulk Heterojunction Nanocrystalline Solar Cells by Annealing of Ultrathin Squaraine Thin Films," Nano Letters, 10, 2010, pp.3555-3559.[2] G. D. Wei et al, "Solution-Processed Squaraine Bulk Heterojunction Photovoltaic Cells," ACS Nano, 4, 2010, pp.1927-1934.
9:00 PM - H3.29
Synthesis and Characterization of Sulfur Derivatives of Isoindigo and Diketopyrrolopyrrole for Application in Small Molecule Bulk Heterojunction Solar Cells.
Francois Grenier 1 , Mario Leclerc 1
1 Chimie, Université Laval, Québec, Quebec, Canada
Show AbstractOrganic semi-conductors are receiving broad attention because they present many interesting features. Since they are usually soluble in environmentally friendly solvents, they can be easily processed using cheap techniques such as roll-to-roll printing. Moreover, solar cells based on organic semi-conductors can be light, thin and flexible and can provide better off-angle and low-light performance than their inorganic counterparts.Diketopyrrolopyrrole (DPP) is a unit that has received a lot of attention lately. Some DPP-based copolymers are ambipolar which mean that they can transport either holes or electrons. Some DPP-based copolymers have low bandgap and deep HOMO and LUMO energy levels which is extremely interesting for the development of highly efficient polymer-based bulk heterojunction (BHJ) solar cells. Up to now, 5.5% is the best power conversion efficiency (PCE) reported in literature [1].Isoindigo represents an interesting analog to DPP. It is an electron-withdrawing unit and has low-lying HOMO and LUMO energy levels. While this unit is relatively new, some isoindigo polymers have shown promising performances in BHJ solar cells with PCE up to 3%[2].Both Isoindigo and DPP have small bandgap, which is extremely important for small molecules as the conjugation length is much shorter than for polymers. This essential characteristic, along with the easy synthesis of these units, makes them suitable for small molecules bulk heterojunction (SMBHJ) solar cells. Small molecules present several advantages over polymers. They are easier to purify, easier to characterize, may offer better batch-to-batch reproducibility and are monodisperse. Power conversion efficiency of 2.2% have been reported for SMBHJ solar cells using isoindigo while PCE of 4.4% have been reported using DDP [3,4]. In order to tune the bandgap of DPP and isoindigo derivatives and to enhance the power conversion efficiency of SMBHJ solar cells, we reported the synthesis and characterization of new sulfur-based DPP and isoindigo derivatives.[1] J. C. Bijleveld, V. S. Gevaerts, D. Di Nuzzo, M. Turbiez, S. G. J. Mathijssen, D. M. de Leeuw, M. M. Wienk, R. A. J. Janssen, Adv. Energy Mater., 2010, 22, E242-E246.[2] E. Wang, Z. Ma, Z. Zhang, P. Henriksson, O. Inganäs, F. Zhang, M. R. Andersson, Chem. Commun., 2011, 47, 4908-4910.[3] G. Zhang, Y. Fu, Z. Xie, Q. Zhang, Macromolecules, 2011, 44, 1414-1420.[4] B. Walker, A. B. Tamayo, X. D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tantiwiwat, T.-Q. Nguyen, Adv. Funct. Mater., 2009, 19, 3063-3069.
9:00 PM - H3.30
Polymer Crystallization of Partially Miscible Polythiophene/Fullerene Mixtures Controls Morphology.
Derek Kozub 1 , Kiarash Vakhshouri 1 , Enrique Gomez 1
1 Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractMixtures of polythiophene and fullerene are intensely studied for organic photovoltaic applications. Control of nanoscale morphology of these materials is critical for device performance, but characterization and understanding of this morphology and how it arises is lacking. We use energy-filtered transmission electron microscopy (EFTEM) to obtain high-contrast images of P3HT nanocrystals in organic semiconductor mixtures. Grazing-incidence small angle X-ray scattering correlates well with the length scales obtained from EFTEM images; we combine the two techniques to follow the morphology evolution under different material processing histories. EFTEM also measures local P3HT concentration in PCBM-rich regions, proving that these components are partially miscible. We determine the P3HT-PCBM χ parameter and Flory-Huggins phase diagram, which predicts miscibility for P3HT volume fractions above 0.42. This miscibility suppresses fullerene crystallization. The nanoscale morphology in these materials, critical for solar cell performance, is driven by P3HT crystallization from a partially miscible blend.
9:00 PM - H3.31
Increase in Organic Solar Efficiency by the Enhanced Electric Field induced by Dipole Moments in the Cathode Interface.
Jae Yong Lee 1 , Hui Joon Park 2 , L. Jay Guo 1 2
1 Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, Michigan, United States, 2 Macromolecular Science and Engineering, The University of Michigan, Ann Arbor, Michigan, United States
Show AbstractImproving charge transport in bulk hetero-junction (BHJ) organic solar cells can benefit device performance by mitigating charge recombination, and can facilitate the collection of electrons and holes at their respective electrodes as well. One means of enhancing charge transport processes in BHJ devices is generating a higher internal electric field within the active layer.The higher electric field in the organic semiconductor layers in a BHJ device can be generated by a larger work function difference between the cathode (Al) and anode (ITO). In this study, we exploit dipole moments in the cathode interface to effectively increase the work function difference. Among possible dipole moment-inducing materials, tris(8-hydroxyquinoline) aluminum (Alq3) is used to create the desired dipole moments in BHJ devices that have (P3HT:PCBM) blend as the active layer. Our experimental results show that the BHJ organic solar cells with thin Alq3 layer of 0.7 nm added to the cathode side in the Al/LiF/Alq3 configuration give higher power conversion efficiency (PCE) than the conventional BHJ organic solar cells with only cathode Al/LiF layers. The average PCE is increased by 12.3% from 3.66% to 4.11% with standard deviation of less than 0.11%. The Jsc and Voc are also increased in the optimum range of Alq3 layer thickness, which is sub 1.0 nm.The increase of Jsc represents enhanced electron charge transport in an active layer. This is possible due to the broadened density of states in a thin Alq3 layer, which facilitates charge transport. In a control experiment, Voc increase is observed in devices without electron acceptors, which verifies the increased work function difference created by dipole moments. At the same time, we also observe increase in Jsc, further proving that an Alq3 layer facilitates charge transport in the active layer, while minimizing undesirable charge recombination. Accordingly, we suggest a promising solution to improve the performance of BHJ organic solar cells simply by inserting dipole moment-inducing materials in the cathodes. The increase in the effective work function difference produces enhanced internal electric field, leading to improved solar cell performance. This benefit of the enhanced internal field is dominant over the slight increase in series resistance due to the added Alq3 layer.
9:00 PM - H3.32
Dye-Sensitized Back Contact Solar Cells.
Dongchuan Fu 1 , Patrick Lay 1 , Udo Bach 1
1 , Monash University, Melbourne, Victoria, Australia
Show AbstractDye-sensitized solar cells (DSCs) are promising candidates for the future solar energy exploitation, due to their low costs and promising solar energy conversion efficiencies of up to 10.4 %[1]. Typical DSCs use two pieces of transparent conductive oxide (TCO) coated glass substrates, one being the working electrode to support the TiO2 nanocrystalline films and the other being the counter electrode for triiodide reduction. The TCO electrodes in these devices contribute a major fraction of the fabrication costs, while providing limited optical transparency and electrical conduction. In novel designs such as back-contact configurations[2-3], optical losses are avoided by colocating both electrodes on one common side of the TiO2 film. TCO electrodes can be avoided in these devices, facilitating the use of cheap and highly conductive materials such as metals to make DSC electrode. Here we present a novel back-contact DSC design.[4-6] The device comprises a pair of two interdigitated finger electrodes with one of them being electrochemically coated with Pt to form a charge selective contact. Photogenerated charges are selectively collected by the two back contact electrodes. Fully constructed devices show promising solar energy conversion efficiency under one sun (AM1.5, 100 mW/cm2) illumination. Recent works on the other novel back-contact design comprising coplanar electrodes will also be presented. References1. Green, M., Emery, K., Hishikawa, Y. & Warta, W. Solar Cell Efficiency Tables (Version 34), Progress in photovoltaics 17, 320-326 (2009).2. Fuke N., Atsushi F., Ryohichi K., Ashraful I., Yasuo C., Masatoshi Y., Ryohsuke Y. & Han L., New approach to low-cost dye-sensitized solar cells with back contact electrodes, Chemistry of materials 20, 4974-4979 (2008).3. Kashiwa Y., Yoshida Y. & Hayasea S.. All-metal-electrode-type dye sensitized solar cells (transparent conductive oxide-less dye sensitized solar cell) consisting of thick and porous Ti electrode with straight pores Applied physics letters 92, (2008).4. Fu, D., Zhang, X., Barber, R. & Bach, U. Dye-Sensitized Back-Contact Solar Cells. Advanced materials 22, 4270 (2010).5. Highlighted in Technology Focus, 'Dye-sensitized Cell Redesign', Nature Photonics, Vol4, Sept. 2010, p 597 6. Research Highlights, 'Solar Cells: Things are getting clearer', Nature Publishing Group, doi:10.1038/asiamat.2010.141.
9:00 PM - H3.33
A Photovoltaic Device Using a Periodic Mesoporous Organosilica Thin Film as a P-Type Layer.
Masamichi Ikai 1 2 , Yoshifumi Maegawa 1 2 , Yasutomo Goto 1 2 , Norihiro Mizoshita 1 2 , Takao Tani 1 2 , Shinji Inagaki 1 2
1 , Toyota Central R&D Labs., Inc., Nagakute, Aichi Japan, 2 CREST, Japan Sci & Technol Agcy (JST), Kawaguchi Japan
Show AbstractPeriodic Mesoporous Organosilicas (PMOs) are a new class of organic-inorganic hybrid materials in which organic groups are densely and covalently embedded within a silica framework. They have great potential as a p-type layer of efficient organic thin film photovoltaic devices because the ordered mesoporous structure can form ideal interface of p-n heterojunction by filling a n-type material (e.g. PCBM) in the mesopores. We fabricated a photovoltaic device with the structure of Al/LiF/PCBM/p-type layer/PEDOT:PSS/ITO using a tetraphenylpyrene (TPPy)-bridged PMO film with an absorption edge of 450 nm as a p-type layer. The external quantum efficiency of the device using a TPPy-bridged PMO film was increased more than 2-fold compared with that of the reference device using a nonporous TPPy-silica hybrid film (from 8% to 20% at 390nm). This can be due to suppressing exciton annihilation and/or deactivation by the formation of the ordered p-n heterojunction structure on the order of a few tens of nanometers. In addition, we designed and synthesized a novel low-bandgap dithienyl-benzothiadiazole (DTBT)-based organosilane precursor with an absorption edge of 650 nm, aiming for effective harvesting of sunlight. We will also present the photovoltaic characteristics of the device using a DTBT-bridged organosilica film.
9:00 PM - H3.34
Bulk Heterojunction Solar Cells Containing 6,6-Dicyanofulvenes as Fullerene Substitutes.
Trisha Andrew 1 , Vladimir Bulovic 1
1 EECS, MIT, Cambridge, Massachusetts, United States
Show AbstractP3HT/PC61BM bulk heterojunction solar cells containing varying amounts of different 6,6-dicyanofulvenes (DCFs) were fabricated and characterized. Up to 4.5 % power conversion efficiencies were obtained in photovoltaic cells containing ternary mixtures of P3HT, PC61BM, and a 6,6-dicyanofulvene (DCF) in the active layer, compared to 2.9% for reference P3HT–PC61BM solar cells. It was found that 1,4-dimethyl-2,3-diphenyl-6,6-dicyanofulvene could replace up to 50 weight percent of PC61BM in 1:1 P3HT–PC61BM solar cells without sacrificing device performance. The substituents at the 1,4-positions of the 6,6-dicyanofulvene core were found to influence the efficacy of the DCF additives, and the presence of bulky phenyl moieties proved detrimental to additive performance. Moreover, the use of alkyl dihalide additives and extensive optimization were not necessary to achieve substantially improved device performance (relative to P3HT–PC61BM devices) with the DCF additive. Although DCFs do not yield practically functional photovoltaic devices when used in binary mixtures with P3HT, our preliminary studies suggest that DCFs are promising additives that increase the short circuit current and fill factor of polymer-based OPVs.
9:00 PM - H3.35
Evaluation of Novel Conjugated Polymer for Organic Photovoltaic Cell by Microwave Technique and Device Characterization.
Masashi Tsuji 1 , Akinori Saeki 1 2 , Atsushi Asano 1 , Yoshiko Koizumi 1 , Shu Seki 1
1 , Department Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan, 2 , PRESOT, Japan Science and Technology Agency (JST), Japan, Kawagichi, Saitama, Japan
Show AbstractAn organic photovoltaic cell (OPVc) is simple and low-cost to be produced. It is known that an OPVc with the bulk heterojunction (BHJ) architecture has a high power conversion efficiency (PCE). It consists of an interpenetrating network of electron donor and electron acceptor components sandwiched by electrodes. With the combination of poly(3-hexylthiophene) (P3HT) as an electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor, the PCE of 4~5% has been achieved. In recent years, it was reported that OPVcs based on the low-bandgap polymer using the push-pull approach of conjugated electrons realized the value of PCE over 7%(ref); however it is still lower than that of inorganic counterparts.Here we report the synthesis and characterization of a donor-acceptor type copolymer with a benzobisthiazole structure. In addition to the low band-gap enabling the harvest of more sunlight, benzobisthiazole unit is expected to have a high degree of planarity due to N-S interactions. This strategy would improve the PCE of an OPV as a result of a high charge carrier mobility. To optimize the mixing ratio of the blend film, we utilized a speedy screening method: flash-photolysis time-resolved microwave conductivity (FP-TRMC). This technique allows a facile and stable evaluation of photoconductivity without using electrodes, reflecting the nanometer-scale charge carrier mobility and photocurrent generation efficiency. Ref)Yongye Liang, Zheng Xu, Jiangbin Xia, Szu-Ting Tsai, Yue Wu, Gang Li, Claire Ray, and Luping Yu, Adv. Mater. 2010, 22, E135–E138
9:00 PM - H3.37
Organic Solar Cells Active Layer Morphology Management Using Additives.
Badrou Aich 1 2 , Serge Beaupre 1 , Jianping Lu 2 , Ye Tao 2 , Mario Leclerc 1
1 chemistry, Laval university, Quebec, Quebec, Canada, 2 Institute of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada
Show AbstractAfter intense research efforts on the bulk heterojunction (BJH) solar cells using poly(3-hexylthiophene) (P3HT) as the donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor, the power conversion efficiency (PCE) of organic solar cells is still limited to around 5%. It has become increasingly apparent that the efficiency of solar cells based on the couple P3HT/PCBM is approaching its limit [1]. Two strategies are widely used in the literature to obtain more efficient solar cells.First strategy is to develop new polymers with low energy bandgaps [2-4]. These polymers have demonstrated PCE up to 7%.The second strategy is to manage the morphology and nano-scale domain structure of the photovoltaic active layer in solar cell devises. Therefore, the best device efficiencies are expected when the spatial dimensions of the domains correspond to twice of the exciton diffusion length (~20-40 nm). Two approaches can be used to obtain an optimized BHJ morphology. A classical approach already used in making efficient P3HT based solar cells is thermal annealing,[5] and it works well for crystalline materials. However, this approach is not applicable to all solar cell materials, especially when the materials are amorphous. The second approach introduced by Heeger’s group is to use processing additives.[6,7] By incorporating a few volume percent of specific processing additives into the host solvent, it is possible to control the phase separation between the donor and acceptor in BHJ systems and to significantly improve the device performance. Thus by controlling the solvent quality using these additives in solar cell fabrication, it is possible to control the domain structure in the active layer and enhance the solar cells performance.In this work, we developed a device fabrication process using those additives for new polymers. The efficiency of the BHJ solar cells was improved from 5.5% to 6.8%. More importantly, a fill factor as high as 71% was obtained, indicating an optimized morphology in the active layer. [1]. B.C. Thompson, J.M. Frechet, J. Angew. Chem., Int. Ed 47, 58-77 (2008). [2]. S.H Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J.S. Moon, D. Moses, M. Leclerc, K. Lee, A.J. Heeger, Nature Photonics 3, 297-302 (2009).[3]. T.-Y. Chu, J. Liu, S. Beaupré, Y. Zang, J.-R. Pouliot, S. Wakim, J. Zhou, M. Leclerc, Z. Li, J. Ding, Y. Tao, J. Am. Chem. Soc., 133, 4250 (2011).[4]. J. Hou, H-Y. Chen, S. Zhang, R. I. Chen, Y. Yang, Y. Wu, and G. Li, J. Am. Chem. Soc., 131, 15586 (2009).[5]. S. Cho, J. K. Lee, J. S. Moon, J. Yuen, K. Lee, A. J. Heeger., Organic Electronics. 9, 1107 (2008).[6]. J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim, K. Lee, G. C. Bazan, and A. J. Heeger, J. Am. Chem. Soc. 130, 3619 (2008).[7]. C. V. Hoven, X-D Dang, R. C. Coffin, J. Peet, T-Q. Nguyen, and G. C. Bazan., Adv. Mater. 22, E63 (2010).
9:00 PM - H3.39
A New Structural Motif in Polycarbazole Based Conjugated Copolymers.
Xinhui Lu 1 , Htay Hlaing 1 2 , David Germack 1 , Jeff Peet 3 , Won Ho Jo 4 , Benjamin Ocko 1
1 Condensed Matter Physics and Materials Science, Brookhaven National Lab, Upton, New York, United States, 2 Physics, Stony Brook University, Stony Brook, New York, United States, 3 , Konarka Technologies, Lowell, Massachusetts, United States, 4 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractWe report a new structural motif in thin films of poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), a thermally stable “donor-acceptor” semiconducting copolymer, a discovery based on grazing incidence wide-angle x-ray scattering measurements. PCDTBT, composed of alternating carbazole, benzothiadiazole and thiophene segments where two alkyl chains are attached to the nitrogen atom of the carbazole segment, has achieved 7.2% power conversion efficiency in solar cell devices [1]. The room temperature x-ray scattering pattern, prior to annealing, exhibits broad and weak scattering rings, consistent with the previously reported amorphous structure [2]. However, annealing to 200C results in the appearance of a 2D lattice of sharp Bragg peaks - not observed in previous measurements [2] - which shows the existence of a 2D crystalline structure. On the basis of the x-ray measurements and mass density calculations, we propose a structural model where the polymer backbones pair along the 19 Å thick lamellar stacking direction with the liquid-like, alkyl chains in the interstitial region. Along the backbone direction these pairs undulate with a repeat distance twice the ~21 Å monomer segment length. The liquid-like, alkly chains is supported by FTIR measurements. This paired layering model, analogous to a phospholipid bilayer, is only possible in polymers where the alkyl tails originate from the same side of the backbone, such as in PCDTBT. Although this high temperature phase has inferior device performance, the same structural motif which gives rise to the pairing is very likely present in the highly efficient amorphous phase that forms in pre-annealed films. This new structural motif provides guidance for the molecular design of high performance conjugated polymers. Finally, a new metric for assessing the relative volumes for alkyl and conjugated regions will be proposed. [1] Yanming Sun, Christopher J. Takacs, Sarah R. Cowan, Jung Hwa Seo, Xiong Gong, Anshuman Roy, and Alan J. Heeger*, Adv. Mater. 2011, 23, 2226–2230. [2] Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Photon 2009, 3, 297–302.
9:00 PM - H3.4
In Situ Light Scattering to Monitor Structure Development during the Spin-Casting of P3HT Solutions.
Wen Yin 1 , Cameron Lee 1 , John Gilmer 3 , Mark Dadmun 1 2
1 Department of Chemistry, Univ of Tennessee-EPS, Knoxville, Tennessee, United States, 3 Chemistry, King College, Bristol, Tennessee, United States, 2 Chemical Science Division, Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Show AbstractOrganic photovoltaics (OPV) have attracted considerable interest as lightweight, inexpensive, and easily processable replacements of inorganic photovoltaics. Spin-casting is vastly employed to prepare conjugated polymer and conjugated polymer/fullerene composite thin films for electric devices. However, few studies have been performed to understand the development of structure during the fabrication of these thin films. For this purpose, a time resolved light scattering and light reflectivity instrument has been constructed to track the thickness and morphology change in thin films during the spin-coating process. In our studies, 1, 2 and 3 wt% P3HT solution in 1,2-dichlorobenzene have been spin-cast at different spin rates and monitored in-situ. The results suggest the formation of ordered structures with dimensions of 5-10μm for the 2 and 3 wt% P3HT solutions with spin rates of 500 and 1000 rpm. The ordered structure is retained in the finished film for only the 3 wt% P3HT solution spun at 1000 rpm, while this structure disappears when the films dry for the remaining samples. This structure is missing during the entire spin-casting process for 1 wt% P3HT solution at all spin rates, and at 2000 rpm for 2 and 3 wt% P3HT solutions. Further analysis is underway to understand these results and correlate them to opto-electronic function.
9:00 PM - H3.40
Synthesis and Characterization of New Poly(thieno[3,4-d]thiazole) Derivatives for Photovoltaic Applications.
Nicolas Allard 1 , Serge Beaupre 1 , Badrou Reda Aiech 1 2 , Ahmed Najari 1 , Ye Tao 2 , Mario Leclerc 1
1 , Université Laval, Québec, Quebec, Canada, 2 , Institute of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada
Show AbstractOver the past few years, great progress has been made in the field of photovoltaic devices using alternating conjugated polymers. It has been proven that the utilization of push-pull structures is an efficient way to tune both HOMO and LUMO energy levels and the bandgap to increase the power conversion efficiency (PCE) of polymer-based bulk heterojunction solar cells. Lately, power conversion efficiencies exceeding 7% have been reported. According to the literature, a lot of work has been done to synthesize new ‘’push’’ units such as 2,7-carbazole, benzo[1,2-b:4,5-b’]dithiophene and dithieno[3,2-b:2’,3’-d]silole. Nowadays, these push units led to the most efficient polymer-based BHJ solar cells. On the other hand, to fill the needs of new ‘’pull’’ units, several electron-deficient compound have been synthesized. One can think of 2,1,3-benzothiadiazole (BT), diketopyrrolo[3,4-c]pyrrole-1,4-dione (DPP), thieno[3,4-c]pyrrole-4,6-dione (TPD), thieno[3,4-b]thiophene (TT) and benzo[d][1,2,3]triazole (TAZ). These ‘’pull’’ moieties has also proven to be efficient and have led to high power conversion efficiency in BHJ devices.We report the synthesis of new push-pull conjugated polymers for bulk heterojunction solar cells based on benzo[1,2-b:4,5-b’]dithiophene (push) and thieno[3,4-d]thiazole (TTz) (pull) derivatives. Stille polycondensation between distannyl-BDT derivatives with 4,6-dibromo-2-octylthieno[3,4-d]thiazole led to two alternating copolymers, namely PBDTTTz-1 and PBDTTTz-2, which have a bandgap of 1.8 eV and 1.7 eV, respectively. Both copolymers are stable in air with HOMO energy level of -5.3 eV for PBDTTTz-1 and -5.4 eV for PBDTTTz-2. Both copolymers have been tested in bulk heterojunction solar cells and preliminary results (without any optimisation process) show promising power conversion efficiency of 1.4% for PBDTTTz-1 and 1.7% for PBDTTTz-2.
9:00 PM - H3.41
Bulk Heterojunction Solar Cells Based on a New Low-Band-Gap Polymer: Morphology and Performance.
Yanguang Zhang 1 , Zhao Li 2 , Jianping Lu 1 , Jianfu Ding 2 , Ye Tao 1
1 , NRC Institute for Microstructural Sciences, Ottawa, Ontario, Canada, 2 , NRC Institute for Chemical Process and Environmental Technology, Ottawa, Ontario, Canada
Show AbstractPolymer-fullerene bulk heterojunction (BHJ) solar cells have gained much attention in the past few years due to their potential as a low cost photovoltaic technology. The state-of-the-art power conversion efficiency (PCE) has been improved to 7% regime by the recent development of thieno[3,4-b]thiophene based low-band-gap polymers. Here we reports a new low-band-gap polymer (PTTBDT-C8) with alternating thieno[3,4-b]thiophene and benzo[1,2-b:4,5-b’]dithiophene units for applications in BHJ solar cells. By optimizing the nano-scale morphology with DIO (1,8-diiodooctane) additive, the PCE of BHJ solar cells based on this new polymer has been improved dramatically from 1.4% to 4.8%. The correlation between photovoltaic performance and film morphology has been established for this new polymer/fullerene system.
9:00 PM - H3.42
Photo-Induced Charge Recombination Kinetics in Low Bandgap PCPDTBT Polymer:CdSe Quantum Dot Bulk Heterojunction Solar Cells.
Josep Albero 1 , Emilio Palomares 1
1 , Chemical research Institute of Catalonia, Tarragona Spain
Show AbstractThe interfacial charge transfer recombination processes under working conditions that limit the device performance in polymer:CdSe quatum dot bulk heterojunction hybrid solar cells have been measured. The recombination lifetimes for electrons and holes in the device show an exponential dependence in a similar way to that observed for other molecular based solar cells such as bulk heterojunction organic solar cells (OSC) and dye sensitized solar cells (DSSC). The implications of this unprecedented observation on the design of novel devices are discussed as well as the relationship between the charge accumulation in these devices under operation and open-circuit voltage.
9:00 PM - H3.43
Influence of OVPD Parameters on the Performance of Organic Solar Cells Utilizing Pentacene/PTCDI Absorption Layers.
Sebastian Axmann 1 , Michael Brast 1 , Michael Heuken 2 , Holger Kalisch 1 , Andrei Vescan 1
1 Device Technology, RWTH Aachen University, Aachen Germany, 2 , AIXTRON SE, Herzogenrath Germany
Show AbstractAs global energy demand is steadily growing, renewable energy generation by solar cells is becoming increasingly important. The use of mono- and polycrystalline silicon solar cells, which nowadays dominate the market, is limited by wafer size, rigidness of substrates and the requirement of large energy amounts for manufacturing. Organic solar cells (OSC) have the potential to overcome these limitations; especially the organic vapour phase deposition (OVPD) technology offers the possibility of reproducible, large-scale production at low temperatures also on flexible substrates.We report on planar heterojunction OSC utilizing an active layer of Pentacene/N, N’-ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI) fabricated by OVPD. The influence of growth conditions chamber pressure, deposition rate and substrate temperature was studied using atomic force microscopy (AFM) on single layers as well as electrical characterisation with and without illumination of fully processed solar cells.AFM images indicate that crystallization of Pentacene layers can be widely influenced by all three factors to grow crystals up to 1µm width. The PTCDI layer atop of these uniformly covers the crystallites and smoothens the organic stack. Open circuit voltage was found to be 0.4 V and short circuit current densities beyond 1 mA/cm^2 were measured under illumination. Fill factors were determined to range up to 41%.Overall results show that an optimal set of parameters using the materials mentioned above can be found to increase the solar cell performance in terms of short-circuit current and fill factor. Further improvements are possible by introducing additional layers and introduction of a bulk heterojunction structure.
9:00 PM - H3.44
Control of Zinc Oxide Surface Properties with Chemical Modifiers.
Gang Chen 1 , Darick Baker 1 , Tommas Brenner 1 , Heather Hunt 1 , Cecile Ladam 3 , Dana Olson 5 , Christian Weigand 2 , Andrea Yocom 4 , David Ginley 5 , Thomas Furtak 1 , Reuben Collins 1
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 3 Materials and Chemistry, SINTEF, Trondheim Norway, 5 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Electronics and Telecommunication, NTNU, Trondheim Norway, 4 Physics, University of Oregon, Eugene, Oregon, United States
Show AbstractZinc Oxide (ZnO) is a versatile large bandgap material with a wide range of chemical and electronic applications. ZnO can also be easily and inexpensively deposited in thin film form or fabricated into different types of nanostructures. The interface between ZnO and some other material is of importance in nearly all of its applications. However, the surface of ZnO is very reactive and may also need to be tailored to the requirements of a specific application. This is particularly true in hybrid organic photovoltaic devices, in which charge transfer at the interface between a conjugated polymer and ZnO is the essential issue. These problems can be addressed through chemical treatments and adsorbed molecular layers. Both alterations can change the alignment of electron energy levels across the interface, the overall surface energy, and the molecular order of the polymer at the interface. We have studied the results of the preparation and functionalization of the surface of ZnO sol-gel films, planar single crystals, and nanowire arrays using contact angle tests, Kelvin probe measurements, polarization modulated and transmission infrared spectroscopy, atomic force microscopy, and x-ray photoemission spectroscopy. Additionally, we have investigated the influence of our surface alterations through the performance of hybrid P3HT/ZnO solar cells. We have found that the chemical, optical, and thermal preparation of the ZnO surface changes the surface properties in ways that have not been previously reported. We functionalized the ZnO surface with organic molecules using the siloxane, thiol and carboxyl group attachment chemistries and have characterized how the surface and interface properties are affected by both the attachment chemistry and the functional part of the molecule. Our conclusions demonstrate how surface preparation and functionalization can be used to control the ZnO surface and interface properties for a variety of applications.
9:00 PM - H3.46
Engineering the Interface between P3HT and ZnO Nanorod Arrays by Intermediate Molecular Layers for Photovoltaic Applications.
Bert Conings 1 , Linny Baeten 2 , Hans-Gerd Boyen 1 , Donato Spoltore 1 , Anitha Ethirajan 1 , Patrick Wagner 1 3 , Marlies Van Bael 2 3 , Jean Manca 1 3
1 Institute for Materials Research - Materials Physics, Hasselt University, Diepenbeek Belgium, 2 Institute for Materials Research - Inorganic and Physical Chemistry, Hasselt University, Diepenbeek Belgium, 3 IMOMEC, Imec vzw, Diepenbeek Belgium
Show AbstractInorganic nanorod arrays are capturing the attention of the scientific community as the implementation of these one-dimensional structures in hybrid solar cells is stipulated to provide the ideal compromise between charge transport and light harvesting. ZnO based devices are generally performing less than TiO2 based ones, mostly due to poor interfacial properties between metal oxide and light absorber. Recently, we have shown that for solid-state solar cells based on a P3HT-infiltrated ZnO nanorod array, the morphology of the P3HT near its interface with the nanorods is inconvenient for vertical charge transport.[1,2] In the current work, several interfacial molecular modifiers (such as pyridine derivatives) are introduced for the functionalization of ZnO nanorods, in order to improve its interfacial properties with P3HT and thus enhance the performance of the resulting solar cells. XPS reveals the interaction between the mediating molecules and their ZnO support, while impedance spectroscopy and transient photovoltage measurements are used to characterize the effect of the interface modifier regarding transport properties. [1] L. Baeten, B. Conings., H.-G. Boyen, Jan D’Haen, An Hardy, Marc D’Olieslaeger, Jean V. Manca, M.K. Van Bael, “Towards efficient hybrid solar cells based on fully polymer infiltrated ZnO nanorods”, Adv. Mater. 2011, DOI: 10.1002/adma.201100414.[2] B. Conings, L. Baeten, H.-G. Boyen, D. Spoltore, L. Grieten, P. Wagner, M.K. Van Bael, J.V. Manca, “Influence of interface morphology onto the photovoltaic properties of nanopatterned ZnO/poly(3-hexylthiophene) hybrid solar cells – an impedance spectroscopy study”, submitted.
9:00 PM - H3.47
Carbon Nanotubes and Solar Cells.
Gerhard Lackner 1 , Ingolf Endler 2 , Frank Meissner 2 , Viktor Bezugly 3 , Richard Boucher 3 , Jan Meiss 4 , Doru Lupascu 1
1 Institute for Materials Science, University of Duisburg-Essen, Essen Germany, 2 , Fraunhofer Institute for Ceramic Technologies and Systems, Dresden Germany, 3 Institute for Materials Science, Dresden University of Technology, Dresden Germany, 4 Institute for Applied Photophysics, Dresden University of Technology, Dresden Germany
Show AbstractCarbon nanotubes (CNT) have attracted much attention due to their fabulous electrical, mechanical, and chemical properties. Especially single-walled carbon nanotubes (SW-CNT) based on their semi conductivity properties offer great opportunities in the field of new electrical devices, e.g. field effect transistors (CNT-FET). CNT are also used in organic photovoltaics (OPV) as acceptor material, to enhance charge carrier transport within the organic layers, or as transparent electrodes. The applications of CNT as acceptor material as well as for charge carrier transport enhancement are the main focus in this work. We studied the photovoltaic device performances of different material combinations of CNT, region regular Poly(3-Hexylthiophen-2,5-diyl) (rr-P3HT), Phenyl-C61-butyric acid methyl ester (PCBM) and copper phthalocyanine (CuPc). Furthermore, different device architectures were investigated and compared with each other.
9:00 PM - H3.48
Indolo[3,2-b]indole-Based Copolymers with Alternating Donor and Acceptor Moieties for Organic Photovoltaics.
Zbyslaw Owczarczyk 1 , Wade Braunecker 1 , Andres Garcia 1 , Ross Larsen 1 , Nikos Kopidakis 1 , David Ginley 1 , Dana Olson 1
1 NCPV, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractOn the basis of a novel computational approach employing time-dependent density functional theory and a simple model for monomer-monomer coupling, a series of indolo[3,2-b]indole-based D-π-A copolymers have been designed and synthesized for organic photovoltaics. The size of alkyl substituents in the donor unit ( D) has a paramount effect on structural distortion and aggregation among polymer’s chains, which consequently influences intramolecular charge transfer (CT) between the donor (D) and acceptor ( A) components. Our study reveals a good correlation between the theoretical calculations performed on the selected materials and the experimental HOMO, LUMO, absorption spectra and band gap energies of the corresponding copolymers. Energy levels and band gaps of the conjugated copolymers can be tailored by the electron-withdrawing nature of the acceptor ( A) unit in order to meet requirements of the fullerene acceptor and to provide better overlap with the solar spectrum in the region of 1.5 - 2.0 eV. In addition to photophysical, electrochemical, and electrical properties of these materials, OPV device data and synthesis of the copolymers will be presented.
9:00 PM - H3.49
Effect of Thermal Annealing on P3HT Nanowires: PCBM Blend Thin Films for Organic Photovoltaic Devices.
Jong Soo Kim 1 3 , Safa Shoai 2 3 , Steve Spencer 4 , Fernando Castro 4 , Patrick Nicholson 4 , Wing Chung Tsoi 1 3 , Myungsun Sim 5 , Craig Murphy 4 , Kilwon Cho 5 , James Durrant 2 3 , Ji-Seon Kim 1 3
1 Physics, Imperial College London, London United Kingdom, 3 , Center for Plastic Electronics, Imperial College London, London United Kingdom, 2 Chemistry, Imperial College London, London United Kingdom, 4 , National Physical Laboratory (NPL), Teddington, Middlesex United Kingdom, 5 Chemical engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractRecent studies in organic photovoltaic (OPV) devices have been focused on poly(3-hexylthiophene) (P3HT) nanowires (NW) structured by self-assembly of P3HT molecules with advantages of their enhanced crystallinity and high carrier mobility. Here, we investigate the effects of post thermal annealing in P3HT NW: PCBM blends with a particular focus on thin film morphology and associated OPV device performance. In terms of morphology, first, the changes in chain conformation and packing structure of P3HT NW in blend films are measured as a function of the annealing temperature by Raman spectroscopy and X-ray diffraction techniques. The lattice spacing of P3HT NW is changed and the crystallinity in the blend films is enhanced gradually and saturated through the annealing process. Second, the vertical phase segregation between P3HT NW and PCBM upon annealing is monitored by x-ray photoelectron spectroscopy. Even in the same annealing process, we observe the PCBM rich region at the air/film interface in the NW based film, which is very different from a normal blend of P3HT: PCBM. Third, changes in optical and electrical properties of P3HT NW: PCBM blend films are observed by transient absorption spectroscopy (TAS) and photocurrent atomic force microscopy (PC-AFM). We note that Jsc changes are consistent with the initial signal amplitude of TAS and observe that variation in transient absorption signal amplitude is assigned to an increase in the yield of long-lived dissociated charges. In PC-AFM, we show an apparent increase in density of NW upon annealing the P3HT NW: PCBM blend films. Based on the morphological changes we observed, we explain the different device performance of P3HT NW: PCBM blends upon annealing including the changes in Voc, Jsc and PCE with the comparison to the normal P3HT: PCBM blend devices. Final overall PCE in both of films is similar with around 3%, but individual parameters have very different behaviors in particular Voc and Jsc. Our work clearly identifies the effects of thermal annealing on thin film morphology of P3HT NW and PCBM blend systems and correlates these results with OPV device performance. This work was supported by the EPSRC SUPERGEN Excitonic Solar Cell Consortium (EP/G031088/1).
9:00 PM - H3.5
Naphthodithiophene-Diketopyrrolopyrrole Small Molecule Donors for Efficient Solution-Processed Solar Cells.
Stephen Loser 1 2 , Carson Bruns 1 , Hiroyuki Miyauchi 1 , Rocio Ortiz 1 , Antonio Facchetti 1 3 , Samuel Stupp 1 4 5 , Tobin Marks 1 2
1 Chemistry, Northwestern University, Evanston, Illinois, United States, 2 , Argonne-Northwestern Solar Energy Research (ANSER) Center, Evanston, Illinois, United States, 3 , Polyera Corporation, Skokie, Illinois, United States, 4 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 5 Department of Medicine and the Institute for BioNanotechnology in Medicine, Northwestern University, Evanston, Illinois, United States
Show AbstractA new class of small molecule donors based on the 5,10-bis(2-ethylhexyloxy)naphtho[2,3-b:6,7-b′]dithiophene (NDT) moiety has been designed and synthesized for organic photovoltaic (OPV) applications. The extended π-conjugation system of NDT is expected to afford strong intermolecular orbital overlap and electron-donating properties when utilized in a “push–pull” architecture. When the electron-rich NDT is doubly terminated with thiophene-capped diketopyrrolopyrrole (TDPP) units the resulting NDT(TDPP)2 exhibits broad, high oscillator strength visible absorption absorption, crystalline order, and a high hole mobility. Standard OPV cells using NDT(TDPP)2 and PCBM in a 1.5:1.0 ratio yield power conversion efficiencies (PCE) >4.0% - a record for a PCBM-based small-molecule OPV.
9:00 PM - H3.50
Polymer:Fullerene Solar Cells Based on Doped Layers with Improved Performances.
Enrico Da Como 1 , Felix Deschler 1 , Antonietta De Sio 2 , Ali Tunc 2 , Elizabeth von Hauff 2
1 , LMU Munich, Munich Germany, 2 , Carl-von-Ossietzky University, Oldenburg Germany
Show AbstractThe power conversion efficiency of organic solar cells is mainly limited by recombination phenomena and the amount of light absorbed in the active layer. Unfortunately, these two aspects are entangled, as thicker active layers, designed to have a complete absorption of the incident photons, increase the transit time of carriers towards the electrodes. The transient time is limited by the low mobility of carriers in these materials and can lead to a largely increased recombination and thus poor photovoltaic efficiency.In this communication, we propose a new approach to tune the electrical characteristics and increase the device power conversion efficiency of bulk heterojunction solar cells based on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and the fullerene derivative PCBM. The approach is based on molecular doping to induce p-doping of PCPDTBT by co-solution of the small molecule 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) during blend processing. Field effect measurements indicate that doping at low concentrations leads to an increase in both the hole conductivity and mobility in PCPDTBT films and PCPDTBT:PCBM blends. It is shown that for optimal doping concentrations the mobility can be increased by a factor of five. This is expected to decrease the probability for recombination of the photogenerated charge carriers. Solar cells prepared with doped blends demonstrate higher photocurrents, and increased efficiencies, by up to 30%, compared to non-doped blends. Optical spectroscopy experiments based on photoinduced absorption and external quantum efficiency measurements show that the p-doping improves the photocurrent generated by light absorption in the polymer, clearly indicating the beneficial effect of F4TCNQ on recombination. In addition, we present the possibility to prepare solution processed active layers based on the same materials with thicknesses above 150 nm and spatially localized doped regions close to the electrodes. The results show a novel all solution based approach to tune the electrical characteristics of polymer:fullerene solar cells and improve their power conversion efficiency.
9:00 PM - H3.51
Effective Light Trapping and Solar Concentration Using Volume Holography.
Mei-Li Hsieh 1 2 3 , Shawn-Yu Lin 1 , Ken-Yu Hsu 2 , Shiuan-Huei Lin 4
1 The Future Chips Constellation and The Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Photonics & Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu Taiwan, 3 Institute of Electro-Optical Science and Technology, National Taiwan Normal University, Taipei Taiwan, 4 Department of Electro-Physics, National Chiao-Tung University, Hsinchu Taiwan
Show AbstractThe main challenge of today’s solar cell concerns cost-reduction, efficiency improvement as well as its easy-adoption to a building’s non-planar structure. Current approaches include the use of dye-sensitized PV cells and low cost concentrators in combination of high efficiency cells. Here, we propose a simple technique to realize a compact and nearly all-angle solar energy concentrator using a volume holographic element [1]. Our calculation predicts up to a fivefold concentration of energy per unit area of photovoltaic material. Hence, our holographic approach can offer an exceedingly high solar-concentration, a universal solution easily integrated to any solar cells. The design and analysis of the holographic element will be described and demonstrated. During the recording process, a cylindrical converging-beam and edge-lit reference-beam are incident to record a volume hologram in a piecewise fashion in the medium. The recording medium (PQ doped PMMA photopolymer with 10mm x 10 mm x 2mm [2]) is shifted and the recording is repeated until all the pieces of strings are recorded. After recording, the cylindrical lens is removed and the sunlight (as a plane wave) from different angles all can be guided to the edge of the recording slab via the corresponding grating components. It is shown that even the “reading” beam is incident from different angles, the “diffracted” beam is always trapped along the sample edge. It is further shown that the collection angle can be controlled by varying the Numerical-Aperture of the cylindrical lens or by doubling or tripling the recording exposures. References [1] R. Kostuk et al., High and Low Concentration for Solar Electric Applications III, Proc. of SPIE, Vol. 7043, p70430I, 2008.[2] K.Y. Hsu et al., Opt. Eng. 42(5), p1390, 2003.
9:00 PM - H3.52
Observation and Characterization of Changing Recombination Mechanisms in Bulk Heterojunction Organic Photovoltaic Materials.
Christopher Lombardo 1 , Eric Dianelson 1 , Ananth Dodabalapur 1
1 Microelectronics Research Center, The University of Texas at Austin, Austin, Texas, United States
Show AbstractThe understanding of recombination mechanisms in bulk heterojunction (BHJ) organic photovoltaic cells is of vital importance for understanding how to best harvest photogenerated charges from OPV cells as well as to target ways for increasing the power conversion efficiency of these devices. In order to characterize the recombination mechanisms within BHJ organic photovoltaic cells, photocurrent measurements on lateral solar cell structures have been utilized. Recombination rates within these devices have been examined as a function of light intensity (1 mW/cm2 to 1000 mW/cm2), light spectrum (AM1.5 and 532 nm), electric field, and carrier transit length (100 nm to 20 μm) in order to determine the mechanisms of recombination. By fitting experimental photocurrent data as a function of light intensity, we can determine the functional form of the recombination rate as it is related to carrier concentration (U α nν). This functional form provides insight on the recombination processes occurring within these devices. For this study, P3HT:C61-PCBM and P3HT:C71-PCBM have been employed due to their wide use among researchers as well as their potential for commercialization. In these material systems, there is clear evidence of a shift in recombination mechanism. At low electric field strengths the recombination mechanism is unimolecular. As the electric field strength increases, there is a shift from unimolecular recombination to higher order recombination. This shift in recombination mechanism is a result of modulating the relative carrier populations of electrons and holes and has been explored as a function of device length, electric field, and light spectrum.
9:00 PM - H3.53
Driving Force Effects on Polymer Blends for Improving Performances of Bulk-Heterojunction Polymer Solar Cells.
Wallace C.H. Choy 1 , Feng-xian Xie 1 , Xiaolong Li 2 , Zhong Li 2
1 Electrical and Electronic Engineering, the University of Hong Kong, Hong Kong Hong Kong, 2 Shanghai Synchrotron Radiation Facility, Chinese Academy of Sciences, Shanghai China
Show AbstractBulk-heterojunction polymer solar cells (PSCs) have attracted considerable attention for its remarkable advantages[1, 2] such as simple fabrication procedure, physical flexibility and low material cost. The solution based film-growth dynamics of polymer blends become one of the crucial processes to improve the efficiency from bulk-heterojunction structures[3, 4]. However, the nanoscale network structure in this blend system may change for different solution parameters and spin-coating conditions[3, 4].In the work, we will study driving force effects on the order and phase of crystallization of the blended polymer in solution based self-assembly, which is a key factor of self-assembly of soft materials[5] but has been neglected in studying PSC morphology. We investigate driving force effects by flipping the substrate up-side-down after spin-coating the wet polymer blend. We introduce Brown’s capillarity theory to explain the polymer film formation and study the effects of downward driving force. Our results of absorption spectrum, atomic force microscopy (AFM) and 2D grazing incidence X-ray diffraction (GIXRD) measured using synchrotron radiation show that better chain packing and local order of polymer is formed by the up-side-down method as compared to that by the conventional method. Therefore, the hole transport, carrier balance, short-circuit circuit (Jsc), and fill factor (FF) and power conversion efficiency (PCE) improve. For un-annealed devices, PCE improves from 1.68% (conventional device) to 2.64% (up-side-down). For annealed PSCs, PCE of up-side-down devices is also better than that of the conventional ones. When the thickness of active layer increases to 300nm, the PCE of the up-side-down device increases to 3.58% when that of conventional device drops to 2.55%.[1]G. Dennler, et al., "Polymer-Fullerene Bulk-Heterojunction Solar Cells," Advanced Materials, vol. 21, pp. 1323-1338, 2009.[2]B. C. Thompson and J. M. J. Fréchet, "Polymer–Fullerene Composite Solar Cells," Angewandte Chemie International Edition, vol. 47, pp. 58-77, 2008.[3]G. Li, et al., "High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends," Nat Mater, vol. 4, pp. 864-868, 2005.[4]Y. Yao, et al., "Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells," Advanced Functional Materials, vol. 18, pp. 1783-1789, 2008.[5]C. J. Morris, et al., "Self-assembly for microscale and nanoscale packaging: steps toward self-packaging," Advanced Packaging, IEEE Transactions on, vol. 28, pp. 600-611, 2005.
9:00 PM - H3.54
Effects of Metallic Back Nanostrips on the Angular Response of Organic Solar Cells.
Wei E.I. Sha 1 , Wallace C.H. Choy 1 , Weng Cho Chew 1
1 Electrical and Electronic Engineering, the University of Hong Kong, Hong Kong Hong Kong
Show AbstractGenerally, unless a solar panel is mounted on an expensive tracking system, most of the time, light is incident on the array obliquely. Any improper design of the angular response [1–4] will significantly degrade the energy-conversion efficiency of solar cells (SCs) due to the poor photon absorption or harvesting. For the thin film photovoltaic cells, the thickness of active layer must be smaller than the minority carrier or exciton diffusion length to avoid bulk recombination. Typically, the light absorption of SCs strongly depends on the incident angle [5], which is governed by Lambert’s cosine law [6]. Hence, comprehensive and rigorous study of the angular response is particularly important for the optimized design of the thin-film SCs. Although some works have been done for the angular response of silicon SCs [1] and thin film organic SCs [3], it is highly desirable to study the angular response of periodic plasmonic SCs, which can boost the device performance. Moreover, the generalized equation of Lambert’s cosine law for any periodic nanostructured SCs is important for understanding their angular response.In this work, we systematically study the angular response of the thin-film organic SC with the periodic metal back nanostrips. Recently, silver gratings as the front electrode have been investigated [7, 8]. Here, we employ the periodic Au strips at the bottom of the SC and therefore the blockage effect in short wavelengths can be significantly reduced. Besides, the generalized equation of Lambert’s cosine law for arbitrary periodic structure is formulated. We find that the periodic strip structure can achieve wide-angle absorption enhancement compared with the (planar) non-strip structure for both the s- and p-polarized light. The guided complex waves (GCWs) supported by the periodic strip structure contribute to the enhancement for the s-polarized light. The surface plasmon resonances (SPRs) excited by the subwavelength Au nanostrips contribute to the enhancement for the p-polarized light.1. J. L. Balenzategui and F. Chenlo, Sol. Energy Mater. Sol. Cells 86, 53 (2005).2. S. B. Rim, S. Zhao, S. R. Scully, M. D. McGehee, and P. Peumans, Appl. Phys. Lett. 91, 243501 (2007).3. D. Cheyns, B. P. Rand, B. Verreet, J. Genoe, J. Poortmans, and P. Heremans, Appl. Phys. Lett. 92, 243310 (2008).4. L. Dominici, L. Vesce, D. Colonna, F. Michelotti, T. M. Brown, A. Reale, and A. Di Carlo, Appl. Phys. Lett. 96, 103302 (2010).5. H. A. Atwater and A. Polman, Nat. Mater. 9, 205 (2010).6. H. P. Garg, Treatise on Solar Energy: Fundamentals of Solar Energy. (Wiley, New York, 1982).7. R. A. Pala, J. White, E. Barnard, J. Liu, and M. L. Brongersma, Adv. Mater. 21, 3504 (2009).8. C. J. Min, J. Li, G. Veronis, J. Y. Lee, S. H. Fan, and P. Peumans, Appl. Phys. Lett. 96, 133302 (2010).
9:00 PM - H3.55
Metal Xanthates as Precursors for Efficient Solution Processed Copper Indium Sulfide - Polymer Nanocomposite Solar Cells.
Thomas Rath 1 2 , Achim Fischereder 1 2 , Michael Edler 1 2 , Stefan Moscher 1 2 , Roman Trattnig 1 3 , Gernot Mauthner 1 3 , Wernfried Haas 1 4 , Sebastian Dunst 2 5 , Ferdinand Hofer 4 , Emil J. W. List 3 6 , Gregor Trimmel 1 2
1 CDL for Nanocomposite Solar Cells, Graz University of Technology and NanoTecCenter Weiz Forschungsgesellschaft mbH, Graz Austria, 2 Institute for Chemistry and Technology of Materials, Graz University of Technology, Graz Austria, 3 , NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz Austria, 4 Institute for Electron Microscopy and Fine Structure Research, Graz University of Technology, Graz Austria, 5 , Polymer Competence Center Leoben GmbH, Leoben Austria, 6 Institute of Solid State Physics, Graz University of Technology, Graz Austria
Show AbstractInorganic-organic nanocomposite solar cells have recently gained increasing attention in the scientific community. In these solar cells inorganic semiconductor nanoparticles are combined with conjugated polymers. The absorption of suitable inorganic semiconductor nanoparticles together with the absorption of a conjugated polymer can cover a wide range of the solar spectrum. Also, different sizes and shapes of nanoparticles can be produced, which serves as a tool for designing active layers with improved charge transport. However, similar fabrication methods and device geometries maintain the advantages of polymer solar cells, like low-cost roll-to-roll (R2R) production on light weight, flexible and thin substrates. Numerous publications in recent years report on the preparation of nanocomposite solar cells by blending a polymer with nanoparticles stabilized with a capping agent. The main drawback of such approaches is that these agents remain at least partly in the active layers where they act as an impurity and thus limit the efficient charge separation and transport in the final device.To solve this problem we have focused on the development of in-situ preparation methods of inorganic metal sulfides via novel metal xanthate precursors directly in the matrix of a conjugated polymer. Copper indium sulfide (CIS) was selected as the inorganic semiconductor due its favorable properties as solar absorber material. The main advantage of our approach is that the organic xanthate counterions are decomposed to gaseous products during the heat induced nanoparticle formation. Thus, a high purity of the resulting nanocomposite layers can be obtained. Furthermore, the aforementioned xanthates are soluble in apolar organic solvents, which allows solution processing together with common conjugated polymers used in organic photovoltaics. The mild conversion temperatures (below 200°C) are compatible with the thermal stability of most conjugated polymers.In this contribution the formation process as well as the morphology of the nanocomposite layer was analyzed by time resolved X-ray scattering techniques and complementary spectroscopic and electron microscopic methods, which show CIS nanoparticles with sizes between 3 and 5 nm which form a dense network in the polymer matrix. Solar cells were characterized by IV-curves and IPCE spectra.By using PSiF-DBT - poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl(9,9-dioctyl-9H-9-silafluorene-2,7-diyl)-2,5-thiophenediyl] as polymer and optimizing the CIS:polymer ratio as well as the annealing temperature nanocomposite solar cells with efficiencies of up to 3 % were obtained. Investigations concerning the stability of the nanocomposite solar cells showed that the efficiency remained constant for over 2500 hours of continuous illumination.This work was funded by the Austrian Ministry of Economy, Family and Youth and Isovoltaic AG.
9:00 PM - H3.56
Controlled Molecular Orientation in Organic Solar Cells.
Kyohei Nakano 1 , Hiroaki Iino 1 2 , Takayuki Usui 1 2 , Jun-ichi Hanna 1 2
1 Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa Japan, 2 , JST-CREST, Yokohama, Kanagawa Japan
Show AbstractRecent progress in organic solar cells has been led by adopting so-called bulk hetero junction (BHJ) structure. It increases donor-acceptor junction area, while the charge carrier transport channels for photo-generated electrons and holes are maintained. However, the resulting solar cells inherit poor carrier transport properties from amorphous tissues of a donor-acceptor mixture. For further cell performance, we have to improve charge carrier transport properties in the BHJ structure. We can point out liquid crystalline materials are attractive for organic solar cells because of their high carrier mobility and self-organization ability. It is expected the charge carrier transport channel can be formed using self-organization ability and molecular orientation controllability of liquid crystalline material.In this study, we have investigated a new type of solar cells with calamitic (rod-like) liquid crystals as a p-type organic semiconductor material, which make it possible to control molecular orientation in the BHJ in self-organized manner. We fabricated a BHJ type of solar cells with a liquid crystalline pyrrolopyrrole derivative and C61PCBM.We found that the power conversion efficiency of the cells was improved from 0.052% as fabricated to 0.78% after thermal annealing at 60°C for 30 min. Form texture observation by polarized optical microscope and X-ray diffraction study, we elucidated that this significant improvement of the efficiency was due to the change of molecular orientation of the pyrrolopyrrole from perpendicular to parallel orientation as we expected. We concluded that the charge carrier transport was much enhanced in the parallel orientation of the pyrrolopyrrole, in which hole can be transported effectively in the molecular layers induced by thermal annealing. We discuss the device performance and its limiting factors, in addition to the availability of rod-like liquid crystals in organic solar cells.
9:00 PM - H3.57
Enhancing Organic Photovoltaic Performances of ZnPc:C60 Bulk Heterojunction by Structural Modification.
Yosei Shibata 1 2 , Tetsuya Taima 2 3 , Ying Zhou 2 , Tetsuhiko Miyadera 2 , Noboru Ohashi 2 , Toshihiro Yamanari 2 , Yuji Yoshida 1 2 , Michio Kondo 1 2
1 Interdisciplinary Graduate school of science and engineering, Tokyo Tech., Yokohama, Kanagawa, Japan, 2 Research Center for Photovoltaic Technologies, AIST, Tsukuba, Ibaraki, Japan, 3 Japan Science and Technology Agency, PRESTO, Kawaguchi, Saitama, Japan
Show AbstractBulk heterojunction (BHJ) structure has been drastically improved power conversion efficiency (PCE) of organic photovoltaic cells. On small-molecule-based solar cell, BHJ structure was formed by co-evaporation method. As well known, the morphology of vacuum-evaporated thin films strongly depends on growth conditions {i.e. substrate temperature (Tsub) and deposition rate (Vdepo)}. Recently, S.Pfuetzner et.al investigated the influence of Tsub for Zinc-phthalocyanine (ZnPc):fullerene (C60) BHJ cell performance [1]. However, the detailed relationships between the performance and BHJ structure have been not yet investigated by the control of both of Tsub and Vdepo. In this study, we focused on crystalline control of BHJ thin film. We investigated the effects of the growth conditions on ZnPc:C60=1:1 BHJ thin films. The cell structure was ITO/ZnPc:C60=1:1/C60/Bathocuproine(BCP)/Aluminum. The deposition pressure was 10-5 Pa. Total thickness of active layer was about 90 nm. The following layers on ZnPc:C60 layer were deposited at room temperature. Surface morphology and crystallinity of ZnPc:C60 film were observed by atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. The cell performance and crystal morphology were highly dependent on Tsub but also Vdepo. We compared the morphology of ZnPc:C60 BHJ film at the highest-performance cell with BHJ film at the lowest-performance cell. In case of the lowest-performance cell (PCE=1.39 %), surface morphology was very rough which was caused the aggregation of amorphous C60 along out-of-plane direction. We found that formation of a good BHJ network was hampered by discrete island of C60. In contrast, in case of the highest-performance cell (PCE=2.56 %), surface morphology was quite smooth. The average grain size of C60 was estimated about 50 nm at ZnPc:C60 on the highest-performance cell. We concluded that growth of nano-crystal domain is required for higher performance BHJ cell.Reference:[1] S.Pfuetzner et.al Appl.Phys.Lett.94,253303(2009)
9:00 PM - H3.58
Effect of Anisotropic Gold Nanoparticles on Morphology and Optical Properties of P3HT/PCBM Blends.
Laura Fabris 1 2 , Robert Wadams 1
1 MSE, Rutgers University, Piscataway, New Jersey, United States, 2 , IAMDN, Piscataway, New Jersey, United States
Show AbstractThe development of low-cost energy sources is one of the issues that society is currently facing. Photovoltaic devices such as solar cells have been capable of partially addressing this problem but their production costs still remain high. In the last decade, a significant decrease in costs has been achieved by the introduction of organic solar cells. The development of organic bulk heterojunction solar cell constructs has partially narrowed the gap between the efficiencies achievable in organic photovoltaic systems and the inorganic counterparts [1]. Despite this improvement however, the optimized efficiencies of the former are still significantly lower than those obtained with the latter [2].The incorporation of plasmonic nanostructures in photovoltaic thin film devices has the potential to solve the issue of efficient light trapping [3]. Surface plasmons can be thought of as the coherent oscillation of conduction electrons at the interface between a metal and a dielectric. In the presence of metallic nanostructures (e.g. noble metal nanoparticles, NPs), non-propagating electron oscillations (also called localized surface plasmons) are generated when the system is exposed to an electromagnetic field. As a consequence of the curvature of the metallic surface typical of NPs, these SPRs can be excited by direct sunlight illumination, as opposed to what happens for propagating plasmons. In order to improve the light harvesting efficiency it is necessary to design nanostructures that have high cross sections and a broad absorption spectrum insensitive to the angle of incidence. Herein, I will present our recent results on the integration of plasmonic gold nanorods (NRs) and nanobones (NBs) in P3HT/PCBM blends to be used as active layers of bulk heterojunction solar cells, and will compare their influence on the morphology and the optical properties of the blends with that exerted by spherical gold NPs and with the properties of the system in the absence of plasmonic nanostructures. We believe that the effect of NRs and NBs is evident not only in terms of their influence in improving light trapping (through scattering) and enhancing light absorption (via plasmonic near field effects), but also in terms of increasing ordering and crystallinity of P3HT owing to their anisotropic, elongated morphology. 1. G. Dennler, M. C. Scharber, C. J. Brabec. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 21, 1323 (2009).2. Source: National Renewable Energy Laboratory, (2010).3. V. E. Ferry, J. N. Munday, H. A. Atwater. Design Considerations for Plasmonic Photovoltaics. Adv. Mater. 22, 4794 (2010).
9:00 PM - H3.60
Metal Electroplating of Three Dimensional (3D) Electrode in Electrolyte-Less Dye-Sensitized Solar Cells (ELDSC).
Duen Yang Ong 1 2 , Meng Keong Lim 2 , Ziyu Jin 1 , Chee Lip Gan 1 , Kam Chew Leong 2 , Chee Cheong Wong 1 3
1 , School of Materials Science and Engineering, Singapore Singapore, 2 , GLOBALFOUNDRIES Singapore Pte Ltd., Singapore Singapore, 3 , Singapore-MIT Alliance, Nanyang Technological University, Singapore Singapore
Show AbstractThe first Electrolyte-less dye sensitized solar cell (ELDSC) is proposed with the architecture of FTO-TiO2-dye-metal. In the ELDSC design, the most significant contact is the TiO2-dye-metal interface, whereby the metal electrode acts as the dye replenishment layer as well as the external electrode. In previous work, ELDSC has an inferior Fill Factor (FF) value due to the insufficient coverage of metal electrode resulting from a top-down metal deposition method. In this work, a three dimensional (3D) metal network throughout the mesoporous TiO2 is achieved through bottom-up metal electroplating. In this paper, Field emission scanning electron microscopy (FESEM) and Transmission electron microscopy (TEM) techniques were carried out to study the planar TiO2 and mesoporous TiO2 network. On the planar TiO2 plated specimen, gold (Au) islands formation were observed initially. These Au islands became worm-like structure as they coalesce. As the plating duration is increased, a continuous layer of Au is formed on the planar TiO2. On the other hand, electroplating carried out on a FTO-planar TiO2-mesoporous TiO2 substrate results in a 3D Au network within the mesoporous TiO2. In the mesoporous TiO2, Au cords were observed as the connections among Au spheres. This study demonstrates that a continuous metal layer can be plated on a columnar insulating TiO2 structure. In addition, 3D metal network can be formed in the mesoporous TiO2.
9:00 PM - H3.62
Indium-Free, Acid-Resistant Anatase Nb-Doped TiO2 Electrodes Activated by Rapid Thermal Annealing for Cost Effective Organic Photovoltaics.
Han-Ki Kim 1 , Seong Jun Kang 1
1 Dept. of Advanced Materials for Information and Electronics, Kyung Hee University, Yongin, Gyeonggi, Korea (the Republic of)
Show AbstractIndium-free and acid-resistant anatase Nb-doped TiO2 (NTO) electrodes show promise as economical substitutes for high-cost Sn-doped In2O3 (ITO) films used in organic photovoltaics. By rapid thermal annealing under an ambient vacuum, an insulating amorphous NTO film of low transparency was changed dramatically into a transparent and conductive anatase NTO electrode. Metallic conductivity of the annealed NTO electrode could be attributed to formation of the anatase phase and activation of the Nb dopant. Based on synchrotron x-ray scattering and high resolution transmission electron microscopy, the electrical properties of the NTO electrode could be correlated with the microstructure of the NTO film. The acid-stability of NTO film also supports its use as a substitute for ITO electrode. Unlike Ga:ZnO and Al:ZnO films, which were easily etched by acidic PEDOT:PSS solution, the NTO film was stable against this reagent. Importantly, the annealing temperature influenced the performance of the organic solar cell fabricated with the NTO electrode. This indicates that activation of Nb dopants and formation of the anatase phase play an important role in the extraction of carrier from the organic layer to the anode electrode.
9:00 PM - H3.63
Nano-Sized Ag-Inserted Amorphous ZnSnO3 Multilayer Electrodes for Cost-Efficient Inverted Organic Solar Cells.
Han-Ki Kim 1
1 Dept. of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin, Gyeonggi, Korea (the Republic of)
Show AbstractA sheet resistance- and optical transmittance-tunable amorphous ZnSnO3 (ZTO) multilayer electrode created through the insertion of a nano-scale Ag layer is demonstrated as an indium-free transparent conducting electrode for cost-efficient inverted organic solar cells (IOSCs). Due to the antireflection effect, the ZTO/Ag/ZTO/glass exhibited a high transmittance of 86.29% in the absorption wavelength region of the organic active layer and a low resistivity, even though the ZTO/Ag/ZTO electrode was prepared at room temperature. The metallic conductivity of the electrode indicates that its electrical conductivity is dominated by the nano-scale Ag metal layer. In addition, optimization and control of the thickness of the nano-scale Ag layer is important in obtaining highly transparent ZTO/Ag/ZTO electrodes, because antireflection is strongly influenced by Ag thickness. Moreover, IOSCs fabricated on optimized ZTO/Ag/ZTO electrodes with Ag thicknesses of 12 nm showed power conversion efficiencies (2.55%) comparable to that of an IOSC prepared on a crystalline ITO electrode (2.45%), due to the low sheet resistance and high optical transmittance in the range of 400-600 nm. The performances of ZTO/Ag/ZTO multilayer electrodes indicate that ZTO/Ag/ZTO multilayers are promising as indium-free, transparent electrode substitutes for conventional ITO electrodes in cost-efficient IOSCs.
9:00 PM - H3.64
Omnipresent Carrier Harvesting for Polymer-Based Solar Cells Using Embedded Nano-Electrodes.
Min-Hsiang Hsu 1 , Chia-Hua Chang 1 , Jen-Hsien Huang 2 , Chih-Wei Chu 2 , Peichen Yu 1
1 Photonics, National Chiao Tung University, Hsinchu Taiwan, 2 Research Center of Applied Science, Academia Sinica, Taipei Taiwan
Show Abstract The introduction of bulk heterojunctions (BHJs) has been a milestone in the progress of organic solar cells which successfully soars the power conversion efficiency to exceed 7%. Nevertheless, owing to the random nature of BHJs, carrier-transport issues remain a technological barrier that limits the power conversion efficiency (PCE) of polymer-fullerene-based solar cells from achieving the expected value of 10%. Main-streamed research efforts have been focused on controlling the blend ratio, annealing conditions, and so on to achieve good carrier collection. Besides, recent advances in nanofabrication technologies have permitted the formation of bi-continuous and interdigitated networks, but no clear evidence has been demonstrated as to the improvement of carrier conduction. Here, we introduce an alternative cell structure that employs distinguishing indium-tin-oxide (ITO) nanorods serving as embedded anodic electrodes for polymer solar cells. Enabled by a controllable electrochemical deposition (ECD) of a hole conducting layer (HCL) wrapping around individual nano-electrodes (NEs), we exhibit evidence of balanced electron and hole transport in polymer-fullerene based solar cells. The 3-D NEs protruded into the photoactive material provide shorter routes for hole conduction than the planar electrodes (PEs). As a result, the time it took for holes to arrive at the anode is reduced, prompting an increase in effective hole mobility with respect to the PEs. Besides, such needle-like electrodes also show broadband improvement in the internal quantum efficiency (IQE) analysis, implying the excellent carrier conduction inside the active material. Moreover, according to the measurement in the short circuit current (Jsc) at various angles of incidence (AOIs) and theoretically optical model, the calculated result reveals that devices with buried NEs even show the slow degradation in IQE at oblique angles. Therefore such distinctive structures exhibit great potentials to the practical applications for OSCs owing to its omnipresent carrier harvesting.
9:00 PM - H3.65
Solid-State Dye-Sensitized Solar Cells Based on Novel Conducting Polymers by Photoelectrochemical Polymerization.
Inyoung Song 1 , Sung Hae Park 1 , Taiho Park 1
1 Chemical engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractSince conducting polymers were invented in 1970s, researchers with background of organic electronics have tried to utilize it for various electronics such as organic field effect transistor (OFET), organic photovoltaic cell (OPV) and solid-state dye-sensitized solar cell (DSSC). It could be useful because the electrical properties of continuous sp2 carbon hybridized system and easy processibilities which is own properties of polymer. Solid-state dye-sensitized solar cells, however, it had not been allowed to make good efficiency. Even though it can prevent the leakage and corrosion of electrodes, leading to a decrease in the lifetime and the efficiency of the system, poor pore-filling was the bottle neck. In 2008, Yanagida reported in-situ photoelectrochemical polymerization (PEP) technique with high efficiency. They showed 2.9 % of power conversion efficiency by using bis-3,4-ethylenedioxythiophene (bis-EDOT). It was only successful case since the PEP technique was invented in 1998. For example, polyaniline and polypyrrole were applied but these polymer-based solar cells showed poor performance such as 0.1 % and 0.8 %. Recently, 6 % of power conversion efficiency of PEP-based system was achieved by Ramakrishna, but it was also bis-EDOT-based system. Here, we suggested novel conducting polymers which were EDOT-based monomers having aromatic rings between EDOTs. From this concept, we can provide functionalities such as hydrophobicity/hydrophilicity, capturing salts, and tunning the energy level to the resulting polymers. The designed monomers we synthesized were expected to longer conjugation at oxidized state and push-pull effect on their backbones which can enhance the power conversion efficiency of the PEP-based solar cells.
9:00 PM - H3.66
Efficiency Improvement of Organic Bulk Heterojunction Solar Cells by Functionalized Polythiophene Additives.
Jose Lobez 1 , Trisha Andrew 1 , Vladimir Bulovic 1 , Timothy Swager 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractThe efficiency of organic bulk heterojunction solar cells has been increased in the previous years, but their performance is still limited compared to silicon based solar cells. This limitation originates mainly from lower short circuit currents presented by devices based on organic materials. Here we present a strategy to improve the performance of the benchmark system P3HT/PCBM, by using functionalized polythiophene derivatives. Very small amounts of these additives (0.25% in weight) result in large increases of the power conversion efficiency, mainly due to improvements of the short circuit current.
9:00 PM - H3.67
Thermal Annealing Effect on P3HT:CdSe Nanoparticle Bulkheterojunction Solar Cells.
Donggu Lee 1 , Jaehoon Lim 2 , JunYoung Kim 1 , Myeongjin Park 1 , Youngjun Ko 1 , Kookheon Char 2 , Seonghoon Lee 3 , Changhee Lee 1 , Jeonghun Kwak 4
1 School of Electrical Engineering and Computer Science, Inter-university Semiconductor Research Center, Seoul National University, Seoul Korea (the Republic of), 2 School of Chemical and Biological Engineering, Seoul National University, Seoul Korea (the Republic of), 3 Department of Chemistry, Seoul National University, Seoul Korea (the Republic of), 4 Department of Electronics Engineering, Dong-A University, Busan Korea (the Republic of)
Show AbstractSolution-processed bulk hetrojunction(BHJ) solar cells, consisting of polymer-polymer, polymer-fullerene derivatives, or polymer-nanocrystal blends, have been actively studied for developing low-cost, thin-film solar cells. Inorganic semiconductor nanocrystals have many advantages for BHJ solar cell applications as electron acceptors since their bandgap can be easily controlled through the modification of their size and shape and they have high intrinsic charge carrier mobilities. The surfaces of nanocrystals are passivated with surfactant to prevent aggregation of nanocrystals, but the typical surfactants are electrically insulating organic ligands. Therefore, carrier transport between nanocrystals are limited by such surfactants. In this study, we treated CdSe nanocrystals, prepared with the colloidal synthesis, with hexanoic acid and hexylamine to reduce nanoparticle-to-nanoparticle distance and fabricated polymer-inorganic hybrid solar cells with poly(3-hexylthiophene) (P3HT) as a donor and CdSe nanocrystal as an acceptor. The P3HT:CdSe hybrid solar cells show impoved performance after post thermal-annealing at 150 centigrade. The performance and morphology changes in the active layers induced by thermal annealing were investigated.
9:00 PM - H3.68
Photo-Curable Antireflective Coating with Antifouling Property Based on Fluoropolymers.
Suhan Kim 1 , Jong-Wook Ha 1 , Soo-Bok Lee 1 , In Jun Park 1
1 , Korea Research Institute of Chemical Technology, Daejeon Korea (the Republic of)
Show AbstractRecently, antireflective coatings (ARCs) have much interest for their many rising applications such as solar cells and displays. In a solar cell application, cell efficiency can be improved by increasing transmission of sun light into the cell devices by using ARCs. ARCs can also increase display imaging quality by removing ghost images formed by reflection of light on to the display surface. In this study, a fluoropolymer was selected for the ARC material because it has relatively low refractive index compared with other polymers and its low surface energy also gives antifouling property. Polyperfluoroether (PFPE) oligomers below 5 repeating units were synthesized and fractionated depending on chain lengths. End groups of PFPE oligomers were changed to methacrylates for the polymerization. Prepared PFPE oligomers were copolymerized with photo-curable moieties to fabricate solid polymer coatings. Chemical structures of oligomers and polymers were characterized by H-NMR and F-NMR. Synthesized polymers were spin-casted on a glass and solidified by exposure of UV light with the wavelength of 340 nm. A film thickness was controlled to about 100 nm for the maximum transmittance at the visible light region. Transmittance and reflectance of ARC coated glass were measured by UV/Vis spectroscopy. The glass with ARC on both sides showed maximum transmittance of 96.7 % at the region of UV/Vis region. To calculate surface energy of ARC, contact angles of water, n-hexadecane and diiodomethane on the ARC coated glasses were measured. Calculated surface energy of ARC using Owens-Wendt-Rabel-Kaelble method was 13.2 dyn/cm.
9:00 PM - H3.7
Highly Efficient Solar Cells Based on Conjugated Polymers.
Ken Yoshimura 1 2 , Takehito Kato 1 , Akiko Mitani 1 , Yasunori Uetani 1 , Kunihito Miyake 1 , Kei Matsumoto 2 , Shuichi Hayase 2 , Toshiyuki Itoh 2
1 Tsukuba Research Laboratory, Sumitomo Chemical Co. Ltd., Ibaraki, Tsukuba, Japan, 2 Graduate School of Engineering, Tottori University, Tottori, Tottori, Japan
Show AbstractAn organic photovoltaic device (OPV) with high efficiency has attracted recent years for producing a future photovoltaic system. A bulk heterojunction (BHJ) device has been proposed to obtain such efficiency. The corresponding active layer is prepared by casting a mixed solution of p-type and n-type semiconductors. Controlling the morphology of the active layer is generally difficult, however, and has long been viewed as one of the most important challenge to OPV production. Herein we present our recent progress in the development of highly efficient OPV devices based on control of the morphology of the active layer. We employed the p/n interfacial length using transmission electron microscopy (TEM) analysis to determine the parameter of phase separation and found that the device efficiency depended on the interfacial length and was controlled by the casting solvent. The maximum efficiency was recorded by means of optimization of the interfacial length. In this way, generated excitons easily reached the p/n interface and caused significant enhancement of quantum efficiency. Another important method considered to increase conversion efficiency was to create long-wavelength absorption materials, which could absorb wider wavelength range than that of conventional polymers, such as poly (3-hexylthiophene). We attempted to design such a polymer using alternate bonding between electron-donor segments and electron-accepter segments with the help of computational chemistry, and we were successful in developing a novel low band gap polymer, which absorbed the light up to 900 nm. Enhancing Voc is another important factor to realize better efficiency of OPV. The Voc level has been significantly associated with a gap between HOMO of p-type and LUMO of n-type materials. Therefore, we expected that combination of the p-type materials, which have lower HOMO level, and/or the n-type materials that possess higher LUMO level might afford higher Voc. We have developed cells using p-type materials with lower HOMO levels and novel fullerene derivatives, which have higher LUMO level compare to conventional C60 PCBM; the combination did indeed higher Voc. After evaluation of an appropriate solvent to control the morphology of active layer, we thus succeeded in preparing an excellent OPV device that showed 8.1 % efficiency as a single cell.This research was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through its program of "High Performance PV Generation Systems for the Future".
9:00 PM - H3.9
Annealing Effects of Three-Layered p-i-n Organic Photovoltaic Devices Using Organofullerene/Solvent Co-Crystals.
Hideyuki Tanaka 1 , Yutaka Matsuo 1 , Eiichi Nakamura 1
1 Department of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
Show AbstractSolution processable organic photovoltaic devices (OPV) using organofullerene acceptors are attracting a tremendous interest. In this work, we synthesized 1,4-bis(phenylsilylmethyl)(o-anisylsilylmethyl)[60]fullerenes (SIMEF-Ph, o-An) for use as electron-accepting materials in OPV. The SIMEF-Ph, o-An co-crystallizes with various aromatic solvents such as toluene, benzene, pyridine, and chlorobenzene. And we found that the heating of the SIMEF-Ph, o-An/toluene co-crystal at 110-120 °C results in the loss of the solvent and creates a mesophase, and that the electron mobility and the thermal profile of the device performance can be correlated to this thermotropic property of the solvent co-crystals. The power conversion efficiency (PCE) of our p-i-n OPV [1] was 4.2% at 65 °C, and significantly increased to 4.7% at 120 °C where the phase transition to the mesophase was expected. This PCE value decreased to 3.9% at 180 °C due to the decrease of the fill-factor (FF) from 0.59 to 0.51. This temperature range corresponds to the temperature at which the compound undergoes phase transition from the mesophase to the crystalline state. The study thus suggested that a mesophase created by partial loss of solvent molecules cause improvement of the OPV performance as compared with the solvated or non-solvated crystalline phase. It suggests in turn that a judicious choice of a semiconductor molecular and an additive can create a new mesophase, which may perform better than pure material. [1] Matsuo, Y.; Sato, Y.; Niinomi,
Symposium Organizers
Gang Li University of California, Los Angeles
Thuc-Quyen Nguyen University of California-Santa Barbara
Dana C. Olson National Renewable Energy Laboratory
Moritz Riede Technische Universitaet Dresden
H6/T6: Joint Session: Solar Cells
Session Chairs
Wednesday AM, November 30, 2011
Grand Ballroom (Sheraton)
9:00 AM - **H6.1/T6.1
Designing Organic Semiconducting Polymers for Transistors and Solar Cells.
Iain McCulloch 1 , Martin Heeney 1
1 Department of Chemistry, Imperial College, London United Kingdom
Show AbstractThe evolution of organic electronics is now poised to enter the commercial phase, with the recent market introduction of the first prototypes based on organic transistors fabricated from solution. Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable these devices. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their optical and electrical performance requires further improvement in order to fulfil their potential. Organic solar cell efficiencies are currently increasing rapidly based on organic bulk heterojunction devices fabricated from solution. Central to these device efficiency improvements are the development of new photoactive semiconducting donor and acceptor materials, designed at the molecular level to optimise both absorption of the long wavelength region of the solar spectrum and generation of high cell voltages. This presentation will examine some of the key design strategies to control the molecular orbital energy levels and microstructure of donor polymer semiconductors and illustrate with examples and their characterisation. Specifically, the systematic reduction of the bandgap in a series of bridged ladder type indacenodithiophene and DPP copolymers, in combination with the progressive lowering of the HOMO energy level will be shown. Analogues of these polymers also exhibit high charge carrier mobilities, and we will present transistor data.
9:30 AM - **H6.2/T6.2
Ultrasonic Spray Coating of Smooth Films for Large Area Polymer Solar Cells and Modules.
Barry Rand 1 , Claudio Girotto 1 , David Cheyns 1 , Luuk van Willigenburg 1 , Robert Gehlhaar 1
1 , imec, Leuven Belgium
Show AbstractThe promise of solution processed organic solar cells lies in their low-cost high-throughput manufacturability. However, this low cost aspect can only be fully realized if all of the layers are deposited via in-line compatible methods. Spray coating is a high-rate deposition technique characterized by the ability to deposit thin films over large areas by the superposition of femtoliter-scale droplets. Here, with an ultrasonic spray coater, we have conducted an in-depth study in order to optimize the ink properties (viscosity, surface tension, boiling point) for hole transport/injection materials, active layers, and metallic top contacts. In particular, we investigate poly(3-hexyl thiophene) (P3HT):(6,6)-phenyl C61-butyric acid methyl ester (PCBM) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solutions, and by selecting an appropriate two-solvent system in order to control both the wetting and the drying behavior of the liquid layer, we are able to prevent pin holes, coffee ring effects, and thickness variation in the multilayer solid film. Quite significantly, the characterization of the film quality, deduced from atomic force microscopy, contact profilometry and absorption measurements, confirms that we can deposit these materials in thin, smooth and uniform layers with photovoltaic characteristics (average efficiency of 3.5%) comparable to spin coated devices (average efficiency of 3.6%). Furthermore, we have spray coated and sintered Ag nanoparticle (NP) based solutions as a patterned metal top-contact, replacing vacuum evaporation. Ultimately, for practical purposes, we demonstrate large area modules produced with the spray coating technique. For P3HT:PCBM-based solar cells, we achieve an efficiency of 2.5% for a module with area of greater than 15 square cm, a performance similar to spin-coated modules.
10:00 AM - **H6.3/T6.3
Morphology-Optimized Bulk Heterojunction and Bilayer Polymer Solar Cells Realized by Scalable Fabrication Processes.
Hui Joon Park 2 , L. Jay Guo 1 2
2 Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 1 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractBulk heterojunction (BHJ) solar cells are promising candidates to realize high performance polymer photovoltaics (PV) cells, owing to the interpenetrating nanoscale networks of electron-donor and electron-acceptor with domain sizes on the order of exciton diffusion length and large interfacial areas between the domains. Optimizing blend morphology to provide effective charge separation and transport pathway is one of the most crucial issues to achieve high efficiency BHJ polymer solar cells.Thermal and solvent annealing treatments after spin-casting the P3HT/PCBM blend film are widely used approaches to control the blend morphology and crystallinity of organic semiconductors. The nano domain formation is a result of the phase separation between the two components. However, phase separation also causes non-uniform distribution of the components in the vertical direction, with P3HT phase dominant near the cathode and PCBM phase dominant near the anode. Such phase separation is opposite to the desired configuration for high efficiency solar cell. In addition, both annealing processes require relatively long processing time (e.g. tens of minutes for thermal annealing and a few hours for solvent annealing); and spin-casting deposition can only be applied to small and rigid substrate. Both presents serious limitation to practical large area and mass production of polymer solar cells. To address these issues, we introduce a new fabrication process (termed ESSENCIAL) utilizing a gas-permeable cover layer for solvent evaporation that protect the otherwise free surface, and simultaneously induce sheer flow of the blend solution by an applied pressure. The process leads to optimized morphology with more uniform distribution and crystallinity of the components favorable for charge generation and transportation that cannot be achieved by conventional thermal and solvent annealing methods. Structures fabricated by different methods were compared by measurements of quantum efficiency, absorbance spectra, X-ray photoelectron spectroscopy, and hole- and electron-mobilities. Furthermore, the effects of domain features of the components on efficient exciton dissociation were studied using atomic force microscopy and photoluminescence. Our results revealed that the new ESSENCIAL method not only induces much uniformly distributed interpenetrating continuous pathways having smaller nanodomains with high crystallinity, but is also applicable to high-speed dynamic process which is demonstrated in a roll-to-roll process.Based on the success of the ESSENCIAL process, we also developed a new route to fabricate bilayer polymer PVs that can generate efficient photoactive layers. The process significantly facilitates interdiffusion between the donor and acceptor layers, inducing optimized morphology favorable for charge generation and transportation. This new process suggests an alternative and practical way toward high efficiency polymer PV cells.
10:30 AM - **H6.4/T6.4
Scalable OPV Inks for Photovoltaic Device Manufacturing to Enable Low-Light Indoor Energy Harvesting Applications.
Darin Laird 1
1 , Plextronics, Pittsburgh, Pennsylvania, United States
Show AbstractOrganic Photovoltaics (OPV) have promise to be a viable source of low-cost renewable energy for on and off-grid deployment, provided challenges related to device efficiency and lifetime can be overcome. While the technology is still developing, however, it lends itself very well to more immediate niche applications such as indoor Energy Harvesting (EH). Basic EH applications (watches, calculators,etc.) have been served for over 30 years with amorphous silicon PV, however, OPV can be competitive with a-Si. Here we focus on utilizing OPV to harvest indoor light, particularly fluorescent emission, and provide an energy solution for new low-power electronic devices.Energy Harvesting is forecast to be a $4B market by 2019, with approximately 60% of that market being owned by photovoltaics. Some applications are in consumer products such as laptops, mobile phones, and so on. Other early applications include wireless sensors (temperature, pressure, chemical, etc.) and bi-stable displays used in Point of Purchase (PoP) advertising and electronic shelf labels.Coupled with an appropriate power management system such as thin-film secondary batteries, energy harvesting via photovoltaics can enable high penetration into the emerging EH markets. This paper will present organic photovoltaic technology with emphasis on the EH market, early applications and near and long term prospects for application of OPV technology, in particular the scaling of established ink systems tailored for EH and the initial OPV-based applications for this emerging market.
H7/T7: Joint Session: Large OPV
Session Chairs
Wednesday PM, November 30, 2011
Grand Ballroom (Sheraton)
11:15 AM - **H7.1/T7.1
From Powder to Power: The Development of OPV Panels.
Yue Wu 1 , Gang Li 1 , George Wu 1 , Vishal Shrotriya 1 , Zheng Xu 1 , Wei Wang 1
1 , Solarmer Energy Inc., El Monte, California, United States
Show AbstractOver the past a couple of years, great progress has been made in pushing OPV efficiency over 8%. As the first company who broke the 8% psychological Barrier, this presentation will review Solarmer’s recent progress in OPV materials development, ink formulations, module development, roll to roll production, and device stability. Particularly the challenges and opportunities of OPV production, from powder to power, will be discussed, as well as the roadmap of OPV technology in the near future.
11:45 AM - H7.2/H7.2
Self-Aligning, R2R Compatible Patterning Method for Slot Die Coating for OPV.
Juliane Gabel 1 , Ike Vries 1 , Charlotte Kjellander 1 , Ronn Andriessen 1 , Nadia Grossiord
1 Large Area Printing, Holst Centre / TNO, Eindhoven Netherlands
Show AbstractOrganic photovoltaic (OPV) is a fast developing research field aiming at light-weight, flexible and low-cost photovoltaic (PV) panels. High speed R2R processing of active material containing solutions on flexible substrates meets these goals. Slot die coating is a suitable technology for applying thin and homogeneous layers complying with the demands for device performance. It also meets the needs of a cost-efficient production. Solution-based deposition of thin films for OPV by slot die coating has already been demonstrated [1, 2, 3]. Unfortunately, this technology is limited regarding the patterning of the deposited materials for functional devices. However, for OPV and other printed electronics patterning is needed for two reasons: (i) The contact between anode and cathode as well as the coverage of electrode contacts by non-electrode layers has to be avoided (ii) Additionally, the functional layers of OPV devices are vulnerable to contact with oxygen and water. Therefore the devices must be encapsulated with barrier material with an outstandingly low water vapour transmission ratio. Complete encapsulation of an OPV module is essential in order to meet the desired lifetime requirements [4]. To avoid side leakages the functional layers need to be patterned.Currently mainly two methods for the patterning of slot-die coated layers are used: (i) the in-situ patterning using stripe-coating and intermittent coating and (ii) the post-patterning by means of removal of material where it is not needed/wanted after the coating step by e.g. using laser ablation or re-dissolving. Both methods are restricted, amongst others in speed, accuracy and/or resolutionTo overcome these problems we modify the surface energy of the substrate and make use of self-organized patterning of the deposited materials as it has already been demonstrated for thicker layers and water based inks [5]. We enhanced the already demonstrated technology [5] to make it compatible with organic coatings and the high homogeneity demands, for OPV as well as for other applications like OLED lighting panels.We will discuss the self-organization of slot die coated multilayered OPV considering (i) the chemistry and processing of the surface energy patterns, (ii) the physical processes of dewetting and the flow to the desired patterned areas, and (iii) the implementation of the fundamental understandings to optimize the layout and processability of the devices. We will demonstrate R2R compatible technologies for the deposition of such patterns by CVD and plasma printing under atmospheric conditions. Parallels will be drawn with patterning techniques of OPV materials, as recently been reported.1 Y. Galagan et al., Chemical engineering and processing 50 (2011) 454-4612 F. C. Krebs, Nanoscale 2 (2010) 873 - 8863. L. Blankenburg et al., Solar Energy Materials & Solar Cells 93 (2009) 476 - 4834.F. C. Krebs, Org Electr. 10 (2009) 7615 C. L. Bower et al., AIChE J, 53 (2007) 1644–1657
12:00 PM - H7.3/T7.3
From Studying Nanomorphology to the Beginning of Large Scale Production: Small Molecule OPV on Its Way from Lab to Fab.
Karsten Walzer 1
1 , Heliatek GmbH, Dresden Germany
Show AbstractDuring the last few years, Heliatek’s organic solar cells [1] have seen a fast progress both in efficiency and lifetime. Efficiencies well above 8% have been demonstrated and certified on technically relevant area. Lifetime of several thousand hours have been shown, such that the strict durability tests of inorganic thin film photovoltaics come into reach also for OPV. OPV modules made on lab scale equipment prove the scalability of OPV to practically relevant dimensions. All these progresses make OPV ready for transfer to first production lines. We will report on latest developments on oligomer-based, vacuum deposited organic solar cells developed at Heliatek. Starting with chemical synthesis, material testing and standard test cells, we will report about Heliatek’s OPV cell efficiency and lifetime progress. A crucial part of it is the existence of a long-term stable, appropriate nanomorphology, both for the photoactive absorber layer, and for the overall layer structure. This nanomorphology is studied by different methods, among them variable angle scanning ellipsometry (VASE) and transmission electron microscopy (TEM).[2] We demonstrate that the preferred orientation of the molecular electronic dipole moment perpendicular to the direction of incident light increases the efficiency of the solar cell – a phenomenon that is known from polymer OPV, but hardly studied for oligomer OPV. The transfer from rigid glass to flexible plastic substrates and the progress on upscaling will be dealt with in the last part of the talk. Here, Heliatek follows a proprietary route of vacuum based roll-to-roll production, which is under set-up and further development.[1] in part developed in collaboration with TU Dresden, Institute of Applied Photophysics, Dresden, Germany[2] in collaboration with Institute of Polymer Science, Dresden, Germany
12:15 PM - **H7.4/T7.4
All-Solution Processed Polymer: Fullerene Solar Cells.
Yulia Galagan 1 , Nadia Grossiord 1 , Sjoerd Veenstra 2 , Wiljan Verhees 2 , Lenneke Slooff 2 , Jan Kroon 2 , Ronn Andriessen 1 , Paul Blom 1
1 , Holst Centre/TNO, Eindhoven Netherlands, 2 , ECN Solar Energy, Petten Netherlands
Show AbstractConventional polymer solar cell consists of a glass substrate, a sputter coated and patterned ITO layer, a PEDOT:PSS layer, an absorber layer (typically a polymer:fullerene blend) and a thermally evaporated metal electrode (often: LiF/Al). For Roll-to-Roll processed OPV devices, glass needs to be replaced by a flexible substrate with sufficient barrier properties and both the ITO and LiF/Al electrodes should be substituted by printable electrodes. To replace ITO, a device concept is developed based on highly conductive and solution processed PEDOT:PSS in combination with printed silver grids. However, due to the relative high work function of PEDOT, this composite electrode is especially suitable to contact the valence band of the polymer or donor material of the absorber layer. In other words it can only be used as anode to collect holes from the photovoltaic device. Preparing a solution processed composite cathode is difficult. Here we make use of a recombination layer formed by the combination of ZnO and highly conductive PEDOT. The ZnO contacts the conduction band of the acceptor material to collect electrons from the absorber layer. Since ZnO is a wide bandgap material, it also acts as an exciton and hole blocking layer. Electrons collected by the ZnO layer easily recombine with holes from the highly doped, high conductive PEDOT layer. In addition, the high conductive PEDOT:PSS layer allows lateral charge transport of holes from the metal grid to the recombination site. Thus the combination of metal grid, high conductive PEDOT and ZnO effectively acts as semi-transparent, negative contact to the photovoltaic device. Using this cathode a polymer solar cell structure is manufactured in which all layers, including electrodes, are solution processed. This device is ITO free. All process steps are vacuum free and roll-to-roll compatible. Power conversion efficiencies reach up to 2.5 % using P3HT:[C60]PCBM as photoactive layer, which is 80% of the performance of the benchmark device.
H8: New Architectures
Session Chairs
Wednesday PM, November 30, 2011
Grand Ballroom (Sheraton)
2:30 PM - **H8.1
High Performance OPV Devices in Tandem Structure.
Yang Yang 1 2 , Letian Dou 1 2 , Jingbi You 1 , Jun Yang 1
1 Materials Science and Engineering, UCLA, Los Angeles, California, United States, 2 California Nanosystem Institute, UCLA, Los Angeles, California, United States
Show AbstractA general solution to improve the PCE of OPV devices is to increase the absorption spectrum by using a smaller bandgap polymer in the photoactive layer. However, this results in a lower open-circuit voltage (Voc) of the cell. A Tandem structure, a stacked structure of two or more bulk heterojunction (BHJ) cells with complementary absorption spectra, have thus spurred much interest due to their advantage of harvesting a broader range of the solar spectrum and to reduce the quantum-loss of the open circuit voltage. It consists of sub-cells connected in series through a nano inter-connection layer (ICL). This interlayer acts as a charge recombination layer. An ideal tandem photovoltaic cell must allow the summation of open circuit voltage (Voc) from the individual cells without any loss of photocurrent or fill factor. In this presentation, we will present our latest results on single layer and tandem solar cell results.
3:00 PM - H8.2
Optical Cavity Design Considerations in High-Performance Inverted Organic Photovoltaics.
Erik Garnett 1 , Yi Cui 1 , Michael McGehee 1 , Mark Brongersma 1
1 Materials Science, Stanford University, San Francisco, California, United States
Show AbstractMany of the recently published high-performance organic photovoltaics (OPV) use polymer/fullerene active layer blends that show maximum efficiency at thicknesses below 100 nm, where absorption is relatively poor. Due to the deeply subwavelength nature of these films, optical cavity effects can dramatically change the absorption. Here we present simulations and preliminary experimental data suggesting that “optimized” standard OPVs with an experimentally demonstrated efficiency of 5.5% and active layer thickness of 63 nm can be improved by as much as 30% without altering the active layer by optimizing the optical design to improve absorption. This can be done using an inverted geometry with optical spacers, more reflective metal contacts and a sparse metal nanowire mesh or low-index transparent conductor. These dramatic improvements cannot be achieved in OPVs with thicker active layers, such as the most well-studied P3HT/PCBM system, because of weaker electric field confinement and lower sensitivity to the position of the electric field maximum. This talk will focus on the differences between standard and inverted devices and provide several case studies that illuminate the importance of optical cavity design in high-performance inverted OPVs.
3:15 PM - H8.3
Inverted Parallel Tandem Organic Photovoltaic Cells with Transparent Single Wall Nanotube Common Cathode.
Kamil Mielczarek 1 , Alexander Cook 1 , Prakash Sista 2 , Yun-Ju Lee 3 , Antti Kaskela 4 , Albert Nasibulin 4 , Esko Kauppinen 4 , Mihaela Stefan 2 , Julia Hsu 3 , Anvar Zakhidov 1
1 Alan G. MacDiarmid NanoTech Institute, University Of Texas At Dallas, Richardson, Texas, United States, 2 Department of Chemistry, University Of Texas At Dallas, Richardson, Texas, United States, 3 Department of Materials Science, University Of Texas At Dallas, Richardson, Texas, United States, 4 Department of Applied Physics, Aalto University, Espoo Finland
Show AbstractWe demonstrate an organic photovoltaic (OPV) cell connect in a parallel electrical configuration utilizing polymers with complementary absorption spectra and transparent single wall carbon nanotubes (SWNT) as an interlayer common electrode. Tandem cells are of importance because they can append to the limited spectral coverage of available organic semiconducting materials, furthermore tandems configured in a parallel circuit are of interest because there is no need to achieve current balancing as is the case with serial configurations. In contrast to our previous work [1] in which a common anode made up of transparent multiwall carbon nanotubes was used to couple polymeric and molecular photovoltaic cells in a parallel tandem, the present works demonstrates an inversion of the device stack so that the device uses a common CNT cathode. The inversion is preformed using both charge selective metal oxide layers (ZnO and MoO3) and n-type doped organic materials. Each cell was characterized independently and the short circuit current of the tandem device is shown to be larger than that of each sub cell. An overall increase in efficiency is observed and attributed to enhanced spectral coverage due to active layers with complementary absorption spectra and the effective use of transparent SWNTs. The authors are thankful for the support of grants AFRL/Rice via CONTACT consortium, Welch Foundation grant AT-1617 and DOE STTR gran DE-SC0003664.[1] S. Tanaka, K. Mielczarek, R. Ovalle-Robles, B. Wang, D. Hsu, and A. A. Zakhidov,Appl. Phys. Lett., 94, 113506, 2009
3:30 PM - H8.4
Fabrication of Organic Polymer Solar Cells by a Novel Solution-Based Vapor-like Mist Deposition Method.
Takumi Ikenoue 1 2 , Shizuo Fujita 2
1 Department of Electronic Science and Engineering, Kyoto University, Kyoto Japan, 2 Photonics and Electronics Science and Engineering Center, Kyoto University, Kyoto Japan
Show Abstract Organic photovoltaic devices have attracted increasing interest due to its rapid efficiency-improvement, ease of processing, and cost-effective large-area processability. The power conversion efficiency (PCE) of polymer solar cells has continuously increased up to ~ 7%. Many of polymer solar cells have generally been fabricated by spin-coating, but material efficiency of this method is low and it is not suitable for large area fabrication. In addition, indium-tin oxide (ITO), which is used as a transparent electrode, is fabricated by vacuum processes, therefore it cannot take full advantages of the solution based processes. We report, in this presentation, a novel vapor-like-deposition technique based on a solution process, namely ultrasonic spray assisted mist deposition technique. This technique offers many advantages, for example, (i) it is possible to grow high quality thin organic films, (ii) this technique is suitable for large area substrates and roll-to-roll processes, (iii) source materials can be formed into films without many wastes, and (iv) owing to a vapor-deposition technique this deposition method allows hard-mask patterning without photolithography, which can simplify device fabrication processes, and easy formation of multilayer structures. Additionally, transparent conductive oxides such as ITO and zinc oxide can be fabricated by this technique without any vacuum systems. In the mist deposition technique, the source solution is ultrasonically atomized and the aerosol particles formed are supplied onto a substrate using nitrogen carrier gas to form thin films. We used a methanol solution of indium acetylacetonate and tin acetate, an aqueous solution of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), and a chlorobenzene solution of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) as a starting source of a transparent electrode, a hole transport layer, and a bulk-hetero junction layer, respectively. By optimizing the deposition conditions, thicknesses of each layer have been well controlled and flat surface films were obtained. An ITO layer has high transmittance and low sheet resistance less than 10 Ω/square. Recent experimental results in our laboratory showed that the short circuit current density and PCE of organic polymer solar cells fabricated by mist deposition increased by 10% compared to those fabricated by spin-coating. The detailed performance of the organic polymer solar cells will be demonstrated at the presentation.
3:45 PM - H8.5
Vapor-Deposited Multijunction Organic Photovoltaics with a Broad Spectral Response.
Jill Rowehl 1 , Richard Lunt 2 , Tim Osedach 2 , Patrick Brown 3 , Miles Barr 4 , Vladimir Bulovic 2
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 3 Physics, MIT, Cambridge, Massachusetts, United States, 4 Chemical Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractWe demonstrate a multijunction organic photovoltaic fabricated entirely by vapor-deposition with a broad spectral response. The devices comprise a near-IR-absorbing subcell recently reported in a transparent solar cell architecture [R.R. Lunt, V. Bulovic, Appl. Phys. Lett. 98, 113305 (2011)] and an efficient, complementary green-absorbing subcell. The multijunction PVs show >5.5% power-conversion efficiency with high Voc of 1.65V and FF of 0.68. An efficient charge recombination zone consisting of layers of BCP/Ag nanoclusters/MoOx connects the subcells with negligible voltage loss. Optimization of the subcell design, accomplished using the transfer-matrix optical field modeling of completed structures, and accurate subcell characterization of the external quantum efficiency will be discussed.
4:30 PM - **H8.6
Solution-Processible Crystalline Organic Semiconductors for Photovoltaic Application.
Shinji Aramaki 1
1 , Mitsubishi Chemical Group Science & Technology Reseach Center, Inc., Yokohama, Kanagawa, Japan
Show AbstractThe performance of Organic Photovoltaics (OPV) devices has improved at a significant rate these days and it has been proved to be a good candidate for a future photovoltaics technology by showing power conversion efficiency higher than 7% up to 10%.Mitsubishi Chemical has been developing organic photovoltaic devices internally and with many collaborators using transformation semiconductors, which is a technology based on tetrabenzoporphyrin and its precursor. We succeeded in demonstrating OPV devices showing higher efficiency than 7% with this transformation semiconductor technology and it is now an important class of organic electronics in addition to solution processed polymers and vacuum deposited small molecules. It is unique because highly crystalline (highly insoluble) organic pigment materials are deposited by coating and gives unusual film morphology suitable for OPV devices. It requires various kinds of science and technology, including material synthesis, processes of coating, conversion, and crystallization, device structure optimization, chemical and physical characterization of the material and the device.In this talk, we review and discuss applications of the transformation semiconductors to OPV.
5:00 PM - H8.7
Simple, Highly Efficient Vacuum-Processed Single-Cell and Tandem-Cell Solar Cells Based on Merocyanine Dyes.
Vera Steinmann 1 , Nils Kronenberg 1 , Martin Lenze 1 , Klaus Meerholz 1 , Hannah Buerckstuemmer 2 , Frank Wuerthner 2
1 Department für Chemie, Universität Köln, Köln Germany, 2 Institut für Organische Chemie and Röntgen Research Center for Complex Material Systems, Universität Würzburg, Würzburg Germany
Show AbstractIn the last years, high demand for low-cost, renewable energy has spurred strong developments in the research of organic photovoltaics. So far polymer solar cells have the lead with efficiencies up to 8%. However, the research on small-molecule solar cells has been catching up lately, reporting efficiencies beyond 5%. In contrast to most polymers, small molecules show remarkably high absorption coefficients and can be easily synthesized at large scale and high purity.We study merocyanine dyes as electron donor compounds in small molecule bulk heterojunction solar cells. Throughout the last years, we have tested a vast variety of merocyanine dyes by solution processing, reaching efficiencies up to 4.0%.[1] Most simple-structured merocyanines have the major advantage of being soluble as well as thermally stable and thus sublimable, which leads to more flexibility in the device processing. Recently, we presented a direct comparison of highly efficient solution- and vacuum-processed bulk heterojunction solar cells based on merocyanines with a simple device stack consisting of only three organic layers. In this study, the most efficient devices exhibited an efficiency of 4.9%.[2]Following this, we succeeded in optimizing the vacuum-processed merocyanine solar cells while maintaining the same simple layer stack. Here, we reported efficiencies up to 6.1%, which is – to the best of our knowledge – the highest value for single vacuum-processed small molecule bulk heterojunction solar cells to date.[3]Due to their remarkably high and easily tunable absorption, merocyanine dyes are also promising candidates for tandem structures. Very recently, we successfully implemented merocyanine dyes in tandem-cell devices. High efficiencies up to 4.7% were achieved by simply connecting two identical single-cells in series. These devices also displayed remarkably high open-circuit voltages beyond 2V. In tandem devices with two different electron donor compounds we achieved efficiencies beyond 4%, which is a clear improvement over the best single-cell device efficiency of only 3.1%.[4]
5:15 PM - H8.8
Achieving Higher than 8% Power Conversion Efficiency by Modified Electron Extraction Layer in Polymer Bulk Heterojunction Solar Cells.
Song Chen 1 , Cephas Small 1 , Chad Amb 2 , Tzunghan Lai 1 , Kenneth Graham 2 , John Reynolds 2 , Franky So 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Chemistry, University of Florida, Gainesville, Florida, United States
Show AbstractThe pursuit of polymer solar cells with high power conversion efficiency (PCE) has been greatly promoted since donor-acceptor (D-A) polymer with low bandgap and deep HOMO showed its advantage of maximizing short circuit current (Jsc) and open circuit voltage (Voc). The application of solvent additive helps to improve the interpenetrating nano-scale morphology and gives solar cell with enhanced short-circuit current and fill factors. Most recently published high efficiency polymer solar cells have a maximum PCE up to 7.4%. Here, we report the first polymer solar cells with PCE over 8%.We recently reported solar cells1 with a power conversion efficiency of 7.3% using a new D-A polymer based on stille polycondensation of a distannyl-dithienogermole (DTG) derivative with 1,3-dibromo-N-octyl-thienopyrrolodione (TPD). To reduce the photo-current loss, we modified the electron extraction layer used in the devices. As a result, devices with a modified electron extraction layer give a 12% enhancement in Jsc. Along with an enhancement in FF and a Voc as high as 0.87 V, the PCE of the final device was enhanced up to 8.5%. In order to understand the physical origin of the enhancement, we studied the defects presence in the electron extraction material by photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). The results confirm the reduction of defects states in the electron extraction material. Transient-photocurrent measurements also show that the cells with an unmodified electron extraction layer gives shorter photo-carrier lifetime compared with the modified electron extraction layer, indicating that the modified electron extraction layer does indeed increases the carrier lifetime. Finally, the enhanced device performance was tracked over a period of two weeks. All the three parameters: Jsc, Voc and FF maintain original value and show that the post-treatment of defects in the electron extraction material results in stable devices.1 C.M. Amb, S. Chen, K.R. Graham, J. Subbiah, C. E. Small, F. So. And J.R. Reynolds, JACS, 2011.
5:30 PM - H8.9
Transparent Metal Oxide pn Heterostructures with High Current Rectification.
K. Xerxes Steirer 1 2 , Kai-Lin Ou 1 2 , Erin Ratcliff 1 2 , Neal Armstrong 1 2
1 Chemistry and Biochemistry, University of Arizona, Tucson, Arizona, United States, 2 Center for Interface Science: Solar Electric Materials, University of Arizona, Tucson, Arizona, United States
Show AbstractNew materials and insights into metal oxide based middle electrodes (ME) are needed so that tandem-organic solar cells (TOPV) may perform to the high efficiency (ca. 10%) and stability (ca. 20 years) required for commercialization. Charge collection in organic solar cells (OPV) often require selectivity of the metal oxide contact with respect to the charge type and is known to increase OPV performance by blocking opposite charges and thereby reducing surface recombination. In this study, we utilize wide band-gap semiconducting metal oxides that have demonstrated enhanced carrier selectivity for hole and electron collecting contacts in standard OPVs to fabricate and test the chemical and electronic properties of oxide-oxide pn heterostructures intended for use as MEs in high performance TOPVs.We present measured interfacial, optical and JV responses for metal oxide (MOx) pn heterostructures in order to understand and maximize electron-hole recombination in the ME, which may enable high device rectification and power generation in TOPVs. MOx thin films are deposited using different vapor phase techniques for each layer. NiOx is deposited by reactive electron beam evaporation, TiO2 by chemical vapor deposition, and ZnO by sputtering. Reactive electron beam deposition of NiOx is tested over a large area of 170 cm2 for uniformity and reproducibility with the goal of very large area depositions for high throughput applications. Conformal thin films for each MOx are verified with field emission scanning electron microscopy. Ultraviolet and x-ray photoelectron spectroscopies reveal the interfacial chemistry and shifts in the valence band edge offsets as the device is built up from the bottom transparent conducting oxide. Plasma surface treatments are employed to tune the surface chemistry and the work function of NiOx in order to optimize the band edge energy offsets. By systematically varying layer thicknesses for NiOx/ZnO pn heterostructures, current-voltage response curves (JV) are shown to depend strongly on the NiOx and less so on the ZnO layer. Photodoping of ZnO in the devices is performed under UV exposure while simultaneously measuring JVs. We find that for thin NiOx layers (< 10 nm) both reverse and forward currents largely increase while for thicker NiOx layers (> 20 nm) the reverse current is slightly reduced while under UV light exposure. We also correlate device JVs with the shunt resistance, which is reduced by increasing the thickness of the p-type NiOx layers while forward currents are further increased by NiOx surface treatments and optimal processing conditions. Device performance has already reached 10^4 rectification ratio in our NiOx/ZnO heterostructure devices with tin doped indium oxide and Ag contacts. Optical transparency of the NiOx/ZnO heterostructure is better than 85% at 550 nm, which is in the peak region for solar irradiation and will allow for maximal light to reach the subcell in TOPVs.
5:45 PM - H8.10
Fabrication of Organic Single-Crystal Photovoltaic Cells and Analysis of Exciton Diffusion Length.
Tetsuhiko Miyadera 1 , Noboru Ohashi 1 , Tetsuya Taima 1 2 , Toshihiro Yamanari 1 , Yuji Yoshida 1
1 Research center for photovoltaic tecnologies, AIST, Tsukuba, Ibaraki, Japan, 2 , JST-PRESTO, Kawaguchi, Saitama, Japan
Show AbstractExciton diffusion length (LD) in organic semiconductors dominates the efficiency of organic photovoltaic (OPV) cells. The typical value of the LD in organic thin films is reported to be about several tens of nanometers. The short length of the LD limits the thickness of the active layer of OPV cells, which limits the efficiency. In order to overcome such an issue, bulk-heterojunction type OPV cells have been produced. Few reports have been, however, reported to extend the LD itself. Organic single crystals are expected to show long LD because of its uniform molecular stacking and trap free feature. In this study, fabrication technique of the single-crystal OPV cell was developed and LD was analyzed by means of incident photon-to-current efficiency (IPCE) simulation and photoluminescence (PL) measurement. The p-type organic single crystals (rubrene, tetracene) were fabricated by means of train sublimation method. The single crystal was mounted on the glass substrate with indium tin oxide electrode and poly (3,4-ethylenedioxythiophene) : poly (styrenesulfonate). Fullerene (C60) layer and Al electrode were deposited on the single crystal to form the OPV cell. Although the single crystal layer is relatively thick (~μm), sufficient PV characteristics were observed under light illumination of AM1.5G, 1 Sun. The current density was 20 times higher than that of the OPV cells with polycrystalline rubrene film (thickness ~μm). The LD dependent IPCE spectra were simulated and the best fit with the experiment was obtained when the LD in the tetracene single crystal is about 170 nm. The LD value is larger than those of poly-crystal. The large LD is also observed in PL quench analysis. Based on the large LD, power conversion efficiency was simulated for various p-type organic materials. The efficiency was calculated to be improved with the increase of LD.
H9: Poster Session II
Session Chairs
Thursday AM, December 01, 2011
Exhibition Hall C (Hynes)
9:00 PM - H9.1
Unconventional Morphology Control of PEDOT:PSS Composite Films via Polar Solvent Vapor Annealing and Its Application to Transparent Electrodes in Optoelectronic Devices.
Jun-Seok Yeo 1 2 4 , Jin-Mun Yun 1 2 , Byung-Kwan Yu 1 4 , Seok-In Na 2 , Dong-Yu Kim 1 3 4
1 School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Wanju Korea (the Republic of), 4 Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractThe poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) composite is one of the promising transparent conducting material due to superior properties such as solution processability, excellent electrical property, and high transparency in the visible range. Therefore, the PEDOT:PSS composite films are suitable for next generation transparent electrodes in optoelectronic devices alternative to indium tin oxide (ITO). Unfortunately, the conductivity of the pristine PEODT:PSS is too low to be a transparent electrode. Therefore, the high-dielectric solvents such as dimethylsulfoxide, ethylene glycol and N,N-dimethylformamide are generally used to enhance the conductivity of PEDOT:PSS. However, the usual method which is addition of these polar solvents to PEDOT:PSS solution would form the non-equilibrium morphology due to fast evaporation of polar solvent from PEDOT:PSS films. Also, addition of polar solvent to aqueous solution alters the wetting property and degrades film quality in large area. In this study, we applied the solvent vapor annealing technique to the fabrication of high conductive transparent electrode without film damage and obtained superior electrical property of PEDOT:PSS films. Polymer light-emitting diodes and organic solar cells based such PEDOT:PSS films showed an improved performance compared to the devices which are based on conventional PEDOT:PSS films.
9:00 PM - H9.11
Doping Effect on Chloroindium Phthalocyanine (ClInPc)/C60 Solar Cells.
Weining Wang 1 , Neal Armstrong 2
1 Physics, Seton Hall Unviersity, South Orange, New Jersey, United States, 2 Chemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractFor inorganic semiconductor solar cells, controlled doping is important because it can be used to achieve high build-in voltage (thus high open-circuit voltage VOC) and ohmic contact at the metal-semiconductor interface. It has been shown that, for organic semiconductors, doping also causes a Fermi level shift and a change in the electronic structures at the interface as it does for inorganic semiconductors; but little work has been done to study how the properties of organic semiconductor solar cells are affected by doping and what the correlation between them is. This work will focus on our recent characterization of chloroindium phthalocyanine (ClInPc)/C60 heterojunction solar cells, where tetrafluoro-teracyano-quinodimethane (F4-TCNQ) is used to dope ClInPc layer. Two difference doping scenario are studied: doping in the bulk of ClInPc layer and doping at the ITO/ClInPc interface only. Ultraviolet photoemission spectroscopy (UPS) is used to investigate the ITO/ClInPc and ClInPc/C60 interfaces. Different frontier orbital energy offsets are observed for those heterojunctions at different doping levels. We will correlate these doping effects on the frontier orbital energy offsets with determination of the OPV properties of these heterojunctions, especially VOC and JSC.
9:00 PM - H9.12
Efficient All-Polymer Bulk Heterojunction Solar Cells with High Fill Factors Based on Blends of Poly-3-Hexylthiophene: Poly(perylene diimmide-alt-terthiophene).
Erika Kozma 1 , Dariusz Kotowski 1 , Marinella Catellani 1 , Silvia Luzzati 1
1 ISMAC, CNR, Milan Italy
Show AbstractAll polymer bulk heterojunction solar cells, where a donor polymer is blended to an acceptor polymer, have raised the attention due to the easier tailoring of the electronic properties in conjugated polymers respect to fullerene derivatives. Several combinations of donor and acceptor polymers have been tested as active materials in bulk heterojunction solar cells, but the devices efficiencies, ranging between 1.3 to 2.2%, are lower respect to polymer:fullerene systems. One of the main issues limiting the efficiency of all-polymer bulk heterojunctions is the poor charge separation and transport, as testified by the relatively low fill factors (FF) reaching values up to 0.4-0.5. With the aim to increase the all-polymer device performances and their FF, n-type polymers with good accepting properties and good electron mobilities able to compete with fullerenes have to be designed. In this regard, a new class of acceptor polymers based on the alternation of perylene diimmide and donor moieties recently emerged as quite promising polymeric n-type semiconductor. Recently, we have prepared an acceptor perylene copolymer based on alternating perylene diimide and terthiophene units (PEK3). This n-type polymer shows a good compatibility with poly-3-hexylthiophene (P3HT) as donor polymer and their blends have the suitable morphology to favour both exciton dissociation and electron and hole transport. In this contribution we report on the realization of all-polymer solar cells made upon blending PEK3 polymer as acceptor with P3HT as donor component. The optimised devices exhibit power conversion efficiencies of 1.6 %, with Voc:0.61, Jsc: 4.16 mA/ cm2, FF: 0.63 under AM1.5 G solar simulation (100 mW/cm2). To our knowledge, this is the first time that efficient all-polymer bulk heterojunction are reported with FFs comparable to optimised polymer/fullerene devices.
9:00 PM - H9.14
Benzotriazole Bearing Donor-Acceptor Type Polymers in Organic Photovoltaics.
Derya Baran 1 , Christoph Brabec 1
1 , Institute of Materials for Electronics and Energy Technology, Erlangen Germany
Show AbstractOne key feature of organic semiconductors including conjugated conducting polymers is the possibility to tune their electrical properties via altering the conjugated backbone. Nowadays, the research move towards the synthesis and characterization of structures with specific and optimized properties for a certain field of application. D-A systems, structures with alternating Donor and Acceptor moieties in the conjugated backbone, fall into this category. BHJ solar cell performances have been gradually improved; power conversion efficiencies (PCE) of 7-9 % have been reported. The majority of the high performance polymers are based on so called push-pull co-polymers, whereas the interplay between the donor and acceptor unit allows to fine tune the opto-electronic properties with unprecedented precision. In this contribution we discuss a new acceptor unit for the design of low bandgap polymers for photovoltaic applications: Benzotriazole (BTz). BTz containing polymers have recently emerged in organic electronic applications, mainly electrochromics and photovoltaics. The photovoltaic performance of BTz containing polymers is already as high as 7 %, certainly matching the performance of the Benzothiadiazole (BTd) pendent co-polymers. In this study, the performance of benzotriazole incorporated polymers will be analyzed and reviewed from a general perspective in terms of their potential use in bulkheterojunction solar cells. The differences of BTz vs BTd containing push-pull polymers are investigated for a series of donor units (Th, PCz, PF, PCPDT etc.)
9:00 PM - H9.15
Study the Effect of the Charge Transport Layers in the Electrical Characteristics of the Organic Photovoltaics.
Ronak Rahimi 1 , Alex Roberts 1 , Vishal Narang 2 , Vamsi Kumbham 1 , D. Korakakis 1
1 Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia, United States, 2 Department of Physics, West Virginia University , Morgantown, West Virginia, United States
Show AbstractSignificant progress in fabrication and optimization of organic photovoltiacs (OPVs) has been made during the last decade. The main reason for popularity of OPVs is due to their low production cost, large area devices and compatibility with flexible substrates [1-3]. Various approaches including optimizing morphology of the active layers [1, 2], introducing new materials as the donor and acceptor [3, 4], new device structures such as tandem structure [5, 6] have been adapted to improve the efficiency of the organic photovoltaics. However, electrical characteristics of the OPVs do not only depend on the active layer materials or device structure. They can also be defined by the interface properties between active layers and the charge transport layers or the metal contacts. Within this paper, the effect of the thickness variation of the charge transport layers in the electrical properties of the bilayer heterojunction OPVs has been studied. Several devices with CuPc/PTCDI-C8 as the donor/acceptor layers have been fabricated with different thicknesses of charge transport layers. MoO3 and Alq3 have been used respectively as the hole transport layer (HTL) and the electron transport layer (ETL). It has been shown that the S-shape effect in the current–voltage curve is attributed to the accumulation of the charge carriers at the interface between the active layer and the charge transport layer [5, 7]. -----------------------------------1.D. Wynands, M. Levichkova, M. Riede, M. Pfeiffer, P. Baeuerle, R. Rentenberger, P. Denner, K. Leo, Journal of Applied Physics, v 107, n 1 (2010).2.Yang Fan, M. Shtein, S.R. Forrest, Journal of Applied Physics, v 98, n 1 (2005).3.R. Schueppel, K. Schmidt, C. Uhrich, K. Schulze, D. Wynands, J.L. Bredas, E. Brier, E. Reinold, H.-B. Bu, P. Baeuerle, B. Maennig, M. Pfeiffer, K. Leo, Physical Review B (Condensed Matter and Materials Physics), v 77, n 8 (2008).4.R. Mondal, H.A. Becerril, E. Verploegen, Kim Dongwook, J.E. Norton, Ko Sangwon, N. Miyaki, Lee Sangjun, M.F. Toney, J.-L. Bredas, M.D. McGehee, Zhenan Bao, Journal of Materials Chemistry, v 20, n 28, (2010).5.Srinivas Sista, Mi-Hyae Park, Ziruo Hong, Yue Wu, Jianhui Hou, Wei Lek Kwan, Gang Li, Yang Yang, Advanced Materials, v 22, n 3 (2010).6.R. Timmreck, S. Olthof, K. Leo, M.K. Riede, Journal of Applied Physics, v 108, n 3, (2010).7. J.C. Wang, X.C. Ren, S.Q. Shi, C.W. Leung, Paddy K.L. Chan, Organic Electronics: physics, materials, applications, v 12, n 6 (2011).
9:00 PM - H9.16
Insight into the Electronic Properties of Organic Active Layers in OPV Devices: Establishing Correlation between Electronic and Molecular Structures, and Corresponding Device Performance.
Olga Griffith 1 , John Anthony 2 , Matthew Bruzek 2 , Jeremy Gantz 1 , Neal Armstrong 1
1 Chemistry and Biochemistry, University of Arizona, Tucson, Arizona, United States, 2 Chemistry, University of Kentucky, Lexington, Kentucky, United States
Show AbstractEstablishing a correlation between molecular electronic properties and molecular structure of organic semiconductors at electrode interfaces enables understanding of phenomena which limit the efficiencies of emerging solar cell technologies. We have recently focused on interlayers using single monolayers, or multilayers of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) which lie between donor layers and the oxide (ITO) or metal (Au) contact. Donor materials (D) of interest are pentacene and 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]-pentacene (or TP-5 pentacene); C60 is used as an acceptor material (A). UV and X-ray photoemission spectroscopies have been used to characterize ionization potentials, polarization energies, and local vacuum level shifts of donor and acceptor materials. The results show that the ionization energies and the work function of pentacene films do not depend on a substrate (Au or ITO), meaning that pentacene-pentacene interactions in the solid are stronger than pentacene-substrate interactions. That in turn leads to very similar polarization effects in pentacene films on ITO and Au, meaning that morphology of pentacene films on these substrates with or without PTCDA interlayer stays the same. Whereas in case of TP-5 pentacene, polarization effects change with thickness on ITO (from single monolayer to the bulk), but stay the same on Au. The presence of the PTCDA interlayer affects the TP-5 pentacene-substrate interactions, which are observed in the corresponding energy offset values. Additional XRR, AFM, and OPV studies show correlation between the observed molecular surface structures, energetics, and the device performance. The insight into the electronic properties of organic active layers provides an important pathway to understanding the ohmic nature and selectivity of contacts, and the electrical characteristics of corresponding devices.
9:00 PM - H9.17
Improved Efficiency of ITO/TiO2-nc/CdS/P3HT:PCBM/Ag Inverted Solar Cells by Depositing CdS on TiO2 Nanocrystalline Film.
Chong Chen 1 , Mukesh Kumar 1 , Venkat Bommisetty 1
1 , SDSU, Brookings, South Dakota, United States
Show AbstractInverted polymer solar cells (PSCs) have attracted considerable interest due to the their advantages over conventional bulk heterojunction PSCs, such as stability and cost-effective solution processing. Inverted PSCs utilize air-stable high-work-function metal (Ag) electrode deposited on the active layer to collect holes, while metal oxides such as TiO2 and ZnO act as the electron-selective contact at the ITO interface. Recent studies showed that ITO/TiO2-nc/P3HT:PCBM/Ag inverted solar cells have a low efficiency of 0.13% due to large recombination at the interface between P3HT:PCBM film and the ITO electrode. This report illustrates the incorporation of CdS nanoparticles on nanocrystalline-TiO2 using chemical bath (CBD) method to enhance the absorption in UV-vis region and to increase the interfacial area between electron donor and electron acceptor. Moreover, CdS, a n-type semiconductor can serve as an electron-selective layer to reduce the recombination between photo-generated electrons and holes. Results show that the performance parameters, such as the short circuit current (Jsc), fill factor (FF), and the open circuit voltage (Voc) of the cells with CdS are increased significantly, compared to those without CdS. The efficiency of ITO/TiO2-nc/CdS/P3HT:PCBM/Ag inverted solar cells was 1.6% compared to 0.15% efficiency in the devices without CdS layer. This study reveals the importance of CdS in enhancing the efficiency of ITO/TiO2-nc/P3HT:PCBM/Ag cells.
9:00 PM - H9.18
Processing Conditions of Active Layer Determines Whether the Dominant Recombination Mechanism in Polymer Solar Cells is Bimolecular or Interfacial.
Kanwar Nalwa 1 , Hari Kodali 2 , Baskar Ganapathysubramanian 2 , Sumit Chaudhary 1
1 Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa, United States, 2 Department of Mechanical Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractIn order to understand the effect of processing conditions such as spin coating speed and drying rate on the carrier recombination mechanism; three types of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells (A, B, and C) were synthesized by spin-coating P3HT:PCBM blend at 400, 600 and 1000 rpm for 30, 60, and 60 seconds, obtaining solvent drying time of ~ 40, 7 and 1 minutes respectively. Experiments suggested that higher drying time of device A lead to reduction of sub-bandgap traps by an order of magnitude as compared to fast drying time of device C. The slow growth assists the formation of self-organized ordered structure in the P3HT/PCBM blend system diminishing morphological defects in P3HT chains. We also demonstrate that the open circuit voltage and its light intensity dependence are strongly affected by interfacial recombination of carriers at sub-gap defect states. By coupling our experimental results with simulations, we show that at one sun condition, device A has bimolecular recombination as the major loss mechanism which exceeds the trap-assisted interfacial recombination loss by an order of magnitude. However, for the case of device C with high trap density, the trap-assisted recombination dominates over the intrinsic bimolecular recombination by a factor of 10. It is also observed that the recombination rate is 3-4 times higher in device C as compared with device A at short circuit condition, owing to higher trap-assisted recombination. The enhanced recombination losses in device C lower the fill factor and short circuit current.
9:00 PM - H9.19
Donor-Acceptor Additives for Enhancing Dye Sensitized Solar Cell Performance.
Akshay Kokil 1 , Matthew Chudomel 2 , Paul Hominck 2 , Paul Lahti 2 , Jayant Kumar 1
1 Center for Advanced Materials, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Department of Chemistry, University of Massachusetts Amherst, Amherst, Massachusetts, United States
Show AbstractDye sensitized solar cells (DSSCs) are of notable interest as an alternative energy source, due to their impressive performance with reported power conversion efficiencies (PCEs) of up to 12%. However, limited improvement in DSSC efficiencies has been obtained since a seminal report. A variety of N-heterocycle electrolyte additives such as 4-tert-butylpyridine were reported to improve open circuit voltage (Voc). However, short circuit current density (Jsc) of these devices was reportedly unaffected upon incorporation of the additives in the electrolyte. To broaden DSSC absorption bandwidth, sensitizing dyes which relay energy through Förster resonance energy transfer (FRET) were recently utilized and an impressive improvement in the DSSC performance reported. In an alternate approach, here we present a strategy for significant improvement in DSSC performance, by using novel donor-acceptor (DA) molecules as additives in the electrolyte containing iodide/triiodide redox couple. A series of DA additives was tested with concentrations more than an order of magnitude lower than typically used for commonly used N-heterocycle additives. The DA molecules were selected so that their highest occupied molecular orbital energies (Ehomo) were situated between the sensitizing dye Z907 Ehomo and the equilibrium redox potential of the electrolyte. Significant improvements in the DSSC performance were observed and will be discussed. A multiplicity of effects can be attributed for the obtained results and will be discussed.
9:00 PM - H9.2
Enhanced Photovoltaic Properties of Organic Solar Cells with Insert of Highly Crystalline Titanium Oxide Layer.
Kenchi Sasaki 1 2 , Toshihiro Yamanari 1 , Shingo Takano 3 , Yoshida Yuji 1 , Yasukiyo Ueda 2
1 , National Institute of Advanced Industrial Science and Technology, Tukuba-shi, Ibaraki-ken Japan, 2 , Kobe University, Kobe-shi, Hyogo-ken Japan, 3 , Sumitomo Osaka Cement Company, Limited, Funabashi-shi, Chiba-ken Japan
Show AbstractOrganic solar cells are much attractive for the realization of a low cost, lightweight, flexible, large-area, and ecological energy source. Especially polymer solar cells based on bulk-heterojunction (BHJ) structure are an expected worldwide research target because of their ease of fabrication and high performance. Recently, the high efficiency of the cell has been reported when thin TiOx inter layer was introduced between Al electrode and active layer. The TiOx inter layer is prepared by sol-gel process at low temperature of 150 °C. The inter layer served as effective optical spacer, which induces spatial redistribution of interfering light intensity inside the cell (optical interference effect). It gives effective light absorption, and increasing photo-current density of the cell. However, the sol-gel process is poor-reproducibility, because the process has to control crystallization of titanium oxide under ambient condition. In some cases, the TiOx inter layer shows high internal resistance, which causes negatively affects for solar cell performance. In this study, we tried to fabricate high crystalline titanium oxide inter layer (TiO2 inter layer). The layer was formed from a dispersion liquid of pre-crystallized titanium oxide nano-particle (SUMITOMO OSAKA CEMENT Co.,Ltd.). Because the methods no need to control crystallization in the air, high reproducibility can be expected. The dense TiO2 layer was composed of the spherical particles with (the size of) 10 nm in diameter. The value of TiO2 series resistance is 1/3 of the TiOx. The TiO2 layer is more effective optical spacer than TiOx layer. High efficiency of 3.91 % was achieved in the cell with TiO2 layer, and the efficiency is 27 % higher than that without inter layer.
9:00 PM - H9.20
Small Organic Molecular Dyes Modification onto ZnO Nanorods Surface for Polymer–Inorganic Hybrid Solar Cells.
Pipat Ruankham 1 , Lea Macaraig 1 , Takashi Sagawa 1 , Hiroyuki Nakazumi 2 , Susumu Yoshikawa 1
1 Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan, 2 Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka, Japan
Show AbstractThe power conversion efficiency (PCE) of P3HT/ZnO hybrid solar cell is commonly low due to ineffective potential barrier at the depletion region and a narrow absorption range of the photo-active materials. In our work, dye molecules, serving as interface modifiers and photosensitizers, were adsorbed onto the surface of ZnO nanorod arrays in order to improve the depletion region at the P3HT/ZnO interface and widen the absorption range of the devices. The effects of dye-modification on the device mechanism are investigated in order to find appropriate properties of dyes for inverted ZnO nanords/P3HT devices, by the use of the four dyes (N719, NKX2677, D205, and squaraine derivative). The dye molecules attach to the ZnO surface by their carboxylate group, confirmed by FT-IR analysis. It is found that the use of D205 dye, whose dipole moment are directing away from the ZnO surface, suppresses the reverse saturation dark current density (J0) by strengthening the potential barrier of depletion region and increasing the internal electric field of the ZnO nanorods. These lead to 3.2 times enhancement of PCE with remarkable 1.5 times enhancement on open-circuit voltage (Voc) as compared with that of non-modified one. Particularly, the light harvesting ability of the device improves due to the non-overlap absorption spectra of squaraine dye to that of the ZnO and P3HT, resulting 3.8 times increment of the short-circuit current density (Jsc) (as compared non-modified one) with the highest PCE among the four types of dyes.
9:00 PM - H9.21
Morphological and Electrical Effects of Solvent Annealing Squaraine/C60 Photovoltaic Devices.
Jeramy Zimmerman 1 , Christopher Renshaw 2 , Xin Xiao 1 , Siyi Wang 3 , Vyacheslav Diev 3 , Mark Thompson 3 , Stephen Forrest 1 2 4
1 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 2 Physics, University of Michigan, Ann Arbor, Michigan, United States, 3 Chemistry, University of Southern California, Los Angeles, California, United States, 4 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOrganic photovoltaic efficiency is governed by the polaron-pair dynamics at the donor-acceptor interface; however, the effects of defects at this interface are not fully understood.1 We show that the interface defect state population, and thus the device efficiency, is strongly affected by solvent annealing 2,4-bis [4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ)-C60 junctions. Similar to previous work, the devices are deposited using a combination of solution processing and vacuum thermal evaporation.2,3 Exposure to dichloromethane vapor for greater than 10 min changes the morphology of DPSQ films from a relatively featureless film with a RMS roughness of 1.7 nm, to one with crystallites as large as 800 nm and a RMS roughness of 12 nm. Ultraviolet and X-ray photoelectron spectroscopy and temperature dependent current-voltage characteristics suggest that defects are being driven towards the free surface during the solvent anneal. These defects become interface traps between the DPSQ and the C60, increasing polaron-pair recombination rates that reduce the open-circuit voltage (VOC), short circuit current (JSC), and fill factor (FF) by approximately 10%. Additionally, the external quantum efficiency (EQE) due to absorption in C60 is reduced, but the exciton harvesting efficiency from the DPSQ is increased, suggesting an increased polaron-pair recombination rate and improved exciton diffusion in the DPSQ. Optimized annealing conditions reduce the degradation of the DPSQ/C60 interface, and the EQE can be increased by over 60% while maintaining a nearly constant VOC and FF, leading to power conversion efficiencies greater than 6.1% at 100 mW/cm2 simulated AM1.5G illumination, with FF=0.73, VOC=0.96 V, and JSC=8.7 mA/cm2. 1. N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, et al., Phys. Rev. B 82, 155305 (2010).2. G. D. Wei, R. R. Lunt, K. Sun, et al., Nano Lett. 10, 3555 (2010).3. B. E. Lassiter, G. Wei, S. Wang, et al., Appl. Phys. Lett. 98, 243307 (2011).
9:00 PM - H9.22
Morphology Control of Dibenzotetraphenylperiflanthenethin (DBP) Layers for High Performance Organic Photovoltaic Cell.
Tetsuya Taima 1 2 , Ying Zhou 1 , Yosei Shibata 1 3 , Tetsuhiko Miyadera 1 , Toshihiro Yamanari 1 , Yuji Yoshida 1
1 RCPVT, AIST, Tsukuba, Ibaraki, Japan, 2 , JST-PRESTO, Kawaguchi, Saitama, Japan, 3 , Tokyo Tech., Yokohama, Kanagawa, Japan
Show AbstractSmall-molecular semiconductor, dibenzotetra-phenylperiflanthenethin (DBP) is one of the candidate materials for high performance organic photovoltaic (OPV) cell. It was reported that DBP/C60 p-n heterojunction OPV cells evaporated without substrate temperature control show high short-circuit current density (Jsc) due to the high absorption coefficient of DBP material [1]. We also reported a new device structure “alternating multilayer structure” for this excellent DBP material [2]. We succeed in improving photovoltaic (PV) properties by this structure. We found that the control of film morphology and crystallinity is the key to improve PV performance. Unfortunately, the control of DBP film morphology and crystallinity had not been investigated yet. Here, we study the DBP film morphology and crystallinity by changing of the evaporation speed and substrate temperature and discuss the effects on PV properties. The device structure was ITO/20nm DBP/80nm C60/0.1nm LiF/Al. The DBP layers were evaporated at approximately 10-6 Pa using Knudsen cell. The substrate temperatures were adjusted from room temperature to 150 oC. To avoid the structural variations in C60 films, after deposition of DBP films, the samples were transferred to another vacuum system by using a glove box. C60 films and negative electrodes were evaporated on 4 different substrates at one time. The surface morphology was studied by AFM and the crystallinity of DBP films was discussed by MCP-RHEED.From room temperature to 90 oC, Morphologies are changed from smooth to well-directional grain structures and the crystallinities are changed from amorphous to poly-crystal. The power conversion efficiencies (PCEs) and Jscs are dramatically improved, while substrate temperatures are increased. This result indicates that the excellent crystallized film leads to higher PV performance possibly due to its higher carrier transport properties. Finally, the OPV cell using DBP film grown at 90 oC shows almost 60% higher PCE than that of the cell grown at room temperature. This work was supported by the Precursory Research for Embryonic Science and Technology (PRESTO) program from the Japan Science and Technology Agency (JST). [1] D. Fujishima, et al.,Sol. Energy Mater. Sol. Cells., 93, 2009, pp. 1029-1032. [2]T. Taima, et al., Record of 35th PVSC, 2010 pp.001621-001623.
9:00 PM - H9.23
Novel Low Band-Gap Conjugated Polymers Based on Diketopyrrolopyrrole for High Performance Organic Electronics.
Jae Woong Jung 1 , Feng Liu 2 , Thomas Russell 2 , Won Ho Jo 1
1 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts, United States
Show AbstractOver the last decade, organic material based electronics have attracted considerable interest due to the promise of low cost, light weight, and flexibility. Among the various organic materials, conjugated polymers are very important for use in polymer solar cell (PSC) and organic field-effect transistor (OFET) devices. Their solution processability, mechanical, and unique electrochemical properties allow access to a new generation of flexible device and cheap energy source. Current state of the art polymer-based electronics have reached OFET with mobility of 1 cm2/Vs and PSC with power conversion efficiency (PCE) of 7 %. Recently diketopyrrolopyrrole (DPP) has been emerged as an attractive building block for the conjugated polymer for the active material of both OFET and PSC, because of its extended conjugation and high molar absorptivity. Also, electron deficient nature of DPP unit enables construction of the donor-acceptor type low band-gap conjugated polymer. So far, various low band-gap conjugated copolymers composed of thiophene-capped DPP and thiophene derivatives were reported. When these copolymers are used in the PSC device, high hole mobility, high current density, and consequently, high PCE were reported. Despite of its promising result, the DPP-based copolymers usually exhibited relatively high-lying highest occupied molecular orbital (HOMO) level, and so, the PSC device showed low open circuit voltage (VOC). The copolymer composed of DPP and thiophene (PDPP3T) showed HOMO level of -5.17 eV, and the PSC device exhibited Voc of 0.65 V. Whene thieno[3,2-b]thiophene was coupled with DPP, the copolymer (PDBT-co-TT) exhibited -5.25 eV of HOMO level. The benzene or furan based DPP copolymer also showed ca. -5.2 eV HOMO level. Therefore a novel DPP-based copolymer having low-lying HOMO level is strongly demanded.Herein, we report novel DPP containing conjugated copolymers based on benzo[1,2-b:4,5-b']dithiophene (BDT) (PBDTDPP) and naphtho[1,2-b:5,6-b']dithiophene (NDT) (PNDTDPP). The novel copolymers exhibited low band-gap around 1.4-1.5 eV, and low-lying HOMO level of around 5.4-5.5 eV. These copolymers exhibited high molar absorptivity and crystallinity due to extended conjugation and highly planar nature of BDT and NDT. These copolymers also exhibited high hole mobility, and promising PCE when it is mixed with [6,6]phenyl-C71-butyric acid methyl ester, and therfore these copolymers are promising active materials for high performance organic electronics. Furthermore since BDT and NDT is electron-deficient electron donating moiety, the effect of electron deficiency of electron donating moiety in donor-acceptor type low band-gap copolymer on electrochemical characteristics of polymer can be evaluated. In this presentation synthesis, molecular characteristics, optical, and electrochemical properties of new copolymers will be addressed, and morphology and photovoltaic characteristics of the new copolymers will also be discussed.
9:00 PM - H9.24
Selective SiO2 Deposition on Dye Sensitized Solar Cells.
Xinwei Wang 1 , Ho-Jin Son 2 , Chaiya Prasittichai 2 , Roy Gordon 1 , Joseph Hupp 2
1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Chemistry and Argonne-Northwestern Solar Energy Research Center (ANSER), Northwestern University, Evanston, Illinois, United States
Show AbstractFor nanocrystalline TiO2 based dye sensitized solar cells (DSSCs), electron recombination at the TiO2/dye/electrolyte interface is one mechanism that limits the DSSC performance. Since the TiO2 surface is not fully covered by the dye molecules, the open area where TiO2 is in direct contact with the electrolyte provides a pathway for electrons to recombine. We were able to reduce this recombination rate by selectively coating an ultra-thin layer of SiO2 on these open areas of the TiO2. The SiO2 coating was found to be uniform in thickness and conformal on the TiO2 particles, with no SiO2 coating taking place on or under the dye molecules. We will present current-voltage curves for DSSCs as a function of the thickness of the silica layer, showing that the DSSC efficiency can be increased by 50%. We will also characterize the DSSCs by electrochemical impedance spectroscopy and transient-photovoltage techniques in order to quantify the kinetics of interfacial electron transfer.
9:00 PM - H9.25
Influence of the Chemical Structure of Methanofullerenes on the Performance of Polymer Solar Cells.
Hung-Wei Liu 2 1 , Tze-Wei Chou 1 , Leeyih Wang 1 2
2 Institute of Polymer Science and Engineering, National Taiwan University, Taipei Taiwan, 1 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan
Show AbstractA series of novel soluble fullerene derivatives with different alkylbenzyl substitutions, including phenyl (P-C60), p-methylphenyl (MP-C60), p-ethylphenyl (EP-C60), p-isopropylphenyl (iPP-C60) and p-tert-butylphenyl (tBP-C60), were synthesized via Bingel-Hirsch reaction and employed as acceptor to fabricate polymer solar cells. The compatibility between these C60 derivatives and poly(3-hexylthiophene) (P3HT) increased with the increase of bulkiness of alkyl substitutions. Both MP-C60 and EP-C60 tend to crystallize whereas iPP-C60 and tBP-C60 can homogeneously distribute inside polymer matrix upon the spin-drying of their blend solutions with P3HT. UV-vis, PL, TEM and XRD were used to characterize the blending films after annealing. Besides, the mobility of electrons and holes were measured to analyze the photoelectric properties of the films. The results clearly indicate that both interfacial properties of two phases and mobility of electrons and holes play an important role in the performance of devices. The device based-on tBP-C60 exhibited the highest current density of 10.03 mA/cm2 and the best energy conversion efficiency of 3.59%.
9:00 PM - H9.26
High Efficiency Small-Molecule Based Photovoltaic Cells.
Ziruo Hong 1 , Guo Chen 1 , Hisahiro Sasabe 1 , Daisuke Yokoyama 1 , Xiaofeng Wang 1 , Yang Yang 2 , Junji Kido 1
1 the Research Center of Organic Electronics, Yamagata Univ, Yonezawa, Yamagata, Japan, 2 Materials Science and Engineering, UCLA, Los Angeles, California, United States
Show AbstractOrganic photovoltaic has been considered as a promising candidate for alternative solar power sources in the near future. Recently low molecular weight materials have attracted much attention as electron donating materials in organic solar cells, since power conversion efficiencies >4% have been demonstrated from single junction photovoltaic cells, and >8% from multiple junction devices. Various device architectures, such as planar and bulk heterojunctions, can be realized using wet or dry coating processes. Further efforts in molecular and devices design are desired to improve efficiency.In this work we investigate photovoltaic cells based on a series of squaraine compounds and fullerene heterojunctions. From planar heterojunction devices, optical properties of squaraine thin films are studied via optical simulation. In comparison with experimental data, we derived exciton diffusion length of ~50 angstrom in neat films, indicating insufficient exciton collection in layer-by-layer structures. Therefore, we fabricated photovoltaic cells of various device architectures. Among them, the bulk heterojunction demonstrated >5% power conversion efficiency from squaraine/fullerene bulk heterojunction photovoltaic cells via device optimization. We discussed device performance in term of materials purity, optical properties, and balanced carrier mobilities. The low fill factors were shown to be the major limiting factor for efficiency. Our results suggested a large room to improve efficiency through combination of multiple channels, including control of intermolecular interaction of donor materials via molecular design, and manipulation of film morphology via external treatments.
9:00 PM - H9.27
Performance Enhancement by Multijunction Organic Solar Cells.
Jingchuan Wang 1 , Dennis Leung 2 , Paddy Chan 3
1 Mechanical Engineering , The Hong Kong Polytechnic University, Hong Kong Hong Kong, 2 Applied Physics, The Hong Kong Polytechnic University, Hong Kong Hong Kong, 3 Mechanical Engineering, The University of Hong Kong, Hong Kong Hong Kong
Show AbstractTandem structure in organic photovoltaics (OPVs) can be expected to provide a wider absorption spectra of solar radiation and enlarge the output potential of the charge carriers with the current density unchanged. The main drawback of tandem device is the photogenerated currents of each individual cell need to be matched with other cells to prevent charge accumulation. As a result, the high potential was obtained at the cost of low current density in most of tandem organic solar cells. To enhance the efficiency of OPV, here we adopted the organic multijunction structure which, in contrast to tandem structure, was expected to achieve higher current density while maintained the open circuit voltage. We exhibit two heterojunctions for exciton dissociation. The performance of the multijucntion device (TTPA/SubPc/C60) shows a near 45 % increases in the power conversion efficiency while comparing with a single junction device. Our finding allows better understanding of the energy alignment and the charge transport characteristic throughout the device. Also, the detailed improvement mechanism caused by the TTPA layer would also be discussed.
9:00 PM - H9.28
Semiconducting Polymer-ZnO Hybrid Bulk Heterojunction Photovoltaic Solar Cells with Semiconducting Surfactants.
Jason Amsden 1 2 , Insun Park 2 , Younhee Lim 2 , Donggu Lee 1 , Michael Meister 3 , Jun-Mo Park 2 , Ki-Young Yoon 2 , Jun Young Kim 1 , Raja Ashraf 4 , Iain McCulloch 4 , TaeLim Choi 2 , Laquai Frederic 3 , Changhee Lee 1 , Klaus Muellen 3 , Do Yoon 2
1 Electrical Engineering and Computer Science, Inter University Semiconductor Research Center, Seoul National University, Seoul Korea (the Republic of), 2 Department of Chemistry, Seoul National University, Seoul Korea (the Republic of), 3 , Max Planck Institute for Polymer Research, Mainz Germany, 4 Department of Chemistry, Imperial College London South Kensington Campus, London United Kingdom
Show AbstractHybrid photovoltaic devices comprised of semiconducting nanocrystal acceptors and organic polymer donors are an intriguing alternative to traditional organic photovoltaic devices based on polymers and C61-butyric acid methyl ester (PCBM). Semiconducting nanocrystals such as CdSe, TiOx, and ZnO have several potential advantages including relatively high electron mobility, tunable band gap, and various morphologies. In particular, ZnO is an attractive possibility due to its environmentally friendly solution processing and non toxicity. However, to date, hybrid photovoltaic device performance lags far behind that of devices using PCBM as the electron acceptor. The reason for the poor performance is thought to be a result of poor device morphology. In this talk I will discuss recent progress in the development of semiconducting surfactants for use with ZnO/MDMO-PPV hybrid photovoltaic devices. We compared device morphology and performance using two different surfactants including an insulating surfactant oleic acid and a newly developed semiconducting surfactant, 2-(2-ethylhexyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinoline-6,7-dicarboxylic acid (BQ) with devices fabricated without surfactant. Analyzing hybrid films using TEM and AFM, we found that use of the surfactants significantly improved the morphology of the films as compared to a control film without surfactant. However, only when using the semiconducting surfactant, BQ did we find significantly increased device performance.[1] In addition, I will discuss recent progress using ZnO with the BQ surfactant and low band-gap polymers.1.Park, I., et al., Enhanced photovoltaic performance of ZnO nanoparticle/poly(phenylene vinylene) hybrid photovoltaic cells by semiconducting surfactant. Organic Electronics, 2011. 12(3): p. 424-428.
9:00 PM - H9.30
Charge Photogeneration by PC70BM Excitons Dissociation in Polymer/Fullerene Solar Cells.
Stoichko Dimitrov 1 , Christian Nielsen 1 , Ying Soon 1 , Pabitra Shakya Tuladhar 1 , Junping Du 1 , Iain McCulloch 1 , James Durrant 1
1 Department of Chemistry, Imperial College London, London United Kingdom
Show AbstractIn this work, we focus upon the role of PC70BM excitons in the charge photogeneration in a low bandgap polymer/fullerene bulk heterojunction (BHJ) solar cells. Transient absorption spectroscopy (TAS) was used to monitor the formation and decay dynamics of charges in thin films of a benzotrithiophene-DPP copolymer/PC70BM. Charge generation yields were estimated as a function of excitation energies. A comparison of the TAS results with the internal quantum efficiency of the corresponding device reveals a strong dependence of the charge photogeneration efficiency on the energy of the absorbed photons; namely, light absorption by PC70BM and not by the polymer contributes to the device photocurrent. We conclude that in the studied system charge photogeneration happens by dissociation of PC70BM excitons - hole transfer from the fullerene HOMO to the polymer HOMO. This result emphasizes on the importance of the acceptor species in BHJ photovoltaics and suggests why devices with PC70BM show better performance than devices with PC60BM. Remarkably, despite charge photogeneration being largely limited to the dissociation of PC70BM excitons, organic solar cells fabricated using this photoactive blend layer yielded device photocurrents of 6.0mA/cm2, maximal EQE’s of 26% and a device efficiency of 2.9%.
9:00 PM - H9.31
Nanoimprint Lithography for Nanostructured Organic Solar Cells.
Holger Hesse 1 , Claudia Palumbiny 1 , Ricky Dunbar 1 , Thomas Pfadler 1 , Lukas Schmidt-Mende 1
1 Physics, University of Munich, Munich Germany
Show AbstractThe morphology of the donor-acceptor material in organic solar cells plays a crucial factor for device efficiency. Unfortunately the standard film processing methods, such as spin-coating or ink-jet printing do not allow a control of this nano-architecture. Nanoimprint lithography however enables this control down to nanometer size. We successfully prepared organic nanowires arrays used in solar cells with wires of <50nm diameter and different length. We present these nanostructured organic solar cells and demonstrate the influence of the imprint on the device physics. Furthermore it is possible to also structure the electrode inducing some light scattering and plasmonic effects.
9:00 PM - H9.33
Analyzing Open-Voltage of Double-Layer Organic Solar Cells Using Optical Electric-Field-Induced Second-Harmonic Generation.
Dai Taguchi 1 , Xiangyu Chen 1 , Takaaki Manaka 1 , Mitsumasa Iwamoto 1
1 Physical Electronics, Tokyo Institute of Technology, Tokyo Japan
Show AbstractOrganic solar cells (OSCs) are promising candidates for the next generation device, where low-cost and printable methods are available in the device fabrication. It is well known that donor-acceptor (D-A) interface plays a key role for efficient electron-hole dissociation that results in photovoltaic effect. Double-layer OSCs are helpful for studying the details of carrier dynamics at the interface. Under photoillumination, excitons created in donor and acceptor layers, and they diffuse to the D-A interface where exciton dissociation provokes. Resulting holes transport through the donor layer to anode, while the counter-part electrons transport through the acceptor layer to cathode. Accordingly a current flows continuously from the anode to the cathode through the external circuit. During these processes, excess charges accumulate at the interface due to the Maxwell-Wagner effect [1]. Probing these excess charges is a challenging task to understand the generation mechanism of open voltage. We have been developing an optical electric-field-induced second–harmonic generation (EFISHG) method that can directly probe carrier dynamics in organic devices, and visualized charge propagation in the channel of organic field-effect transistors [2], and charge accumulation/decay processes in organic light-emitting diodes [3]. The results were very helpful for analyzing carriers in terms of device performance [4, 5]. This motivated us to use the EFISHG measurement for analyzing mechanism of photovoltaic effect in organic solar cells [6, 7]. We prepared the double-layer (pentacene/fullerene) OSCs, and directly probed charge accumulation/decay processes at the pentacene/fullerene interface by using the EFISHG measurement. Results evidently showed that the accumulated excess charge was Qs=-1.7×10-9 C/cm2 under illumination at an intensity of 0.05 mW/cm2 (wavelength 630 nm), and it saturated with Qs=-3.8×10-9 C/cm2 at 0.5 mW/cm2. Accordingly, open-circuit voltage increased from Voc=0.18 V to the saturated value of 0.26 V. We analyzed this charge accumulation process using an equivalent circuit model, and showed that electrostatic energy stored in OSCs made a significant contribution of generated open-circuit voltage.[1] R. Tamura, E. Lim, T. Manaka, M. Iwamoto, J. Appl. Phys., 100, 114515 (2006).[2] T. Manaka, F. Liu, M. Weis, M. Iwamoto, J. Phys. Chem. C, 113, 10279 (2009).[3] D. Taguchi, S. Inoue, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, J. Phys. Chem. Lett., 1, 803 (2010).[4] M. Weis, J. Lin, D. Taguchi, T. Manaka, M. Iwamoto, Jpn. J. Appl. Phys., 49, 071603 (2010).[5] D. Taguchi, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, J. Phys. Chem. C, 114, 15136 (2010).[6] D. Taguchi, T. Shino, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, Appl. Phys. Express, 4, 021602 (2011).[7] D. Taguchi, T. Shino, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto, Appl. Phys. Lett., 98, 133507 (2011).
9:00 PM - H9.34
Nanostructure Control of P3HT:PCBM Bulk Hetero-Junction Polymer Solar Cells.
Jaewook Seok 1 , Sen Li 2 , Eliot Gann 2 , Xinhui Lu 3 , Htay Hlaing 3 , Benjamin Ocko 3 , C.Maurice Balik 1 , Harald Ade 2 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Physics, North Carolina State University, Raleigh, North Carolina, United States, 3 National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractOrganic photovoltaics (OPV) are an actively researched solar technology due to the simple fabrication requirements, relatively low cost and potential applications on flexible substrates. Bulk hetero-junction (BHJ) polymer/fullerene solar cells are a particularly popular system. Highly regioregular Poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are a promising model system for electron donor and acceptor materials, respectively. For the better P3HT:PCBM BHJ solar cells, not only smaller domain size which is approaching to the exciton diffusion length (~10nm) but also the orientation of P3HT crystalline should be optimized. Smaller domain results in the effective charge separation and minimizes the charge recombination at the interface between P3HT and PCBM. Additionally, face-on crystalline orientation of P3HT in which the π- π stacking direction is parallel to the electric field enhances the charge carrier mobility.In this paper, we are presenting a new strategy to have increased face-on P3HT crystalline orientation and smaller domain size in P3HT/PCBM BHJ solar cells. The effects of in-situ polymerization of 2,5-dibromothiophene (DBT) vapor on the morphology and P3HT orientation in P3HT:PCBM BHJ devices improved up to 30% of PCE relative to that with thermal annealing alone.
9:00 PM - H9.35
Ultrasonication-Assisted Carrier Transport Enhancement in Poly(3-hexylthiophene) Thin Film Transistors.
Kui Zhao 1
1 , King abdullah university of science and technology, Jeddah Saudi Arabia
Show AbstractThe packing structure and ordering of poly(3-hexlthiophene) (P3HT) during crystallization from solution-processes is a determining factor for carrier transport in organic thin-film transistor (OTFTs). The chain entanglement caused by the semi-rigid backbone and long side-chains during the aggregation process can inhibit crystallization and long range order, which in turn hinders carrier transport. Here, we report using ultrasonication of the P3HT solution as a means of de-entangling the side-chains, resulting in significant device performance improvements as compared to the untreated solution. We correlate these improvements with microstructural changes initiated in the solution and observed in the solid state via transmission electron microscopy (TEM), atomic force microscopy (AFM), and grazing incidence wide angle X-ray diffraction (GIWAXS).The marginal solvent toluene was chosen, as it is known to favor P3HT aggregation and is a classic choice for solution-processed P3HT OTFTs. Upon ultrasonication of the P3HT (87 kDa) solution, significant steady-state aggregation is observed in the solution, resulting in a more ordered solute, which precipitated from the solution, as detected by quartz crystal microbalance measurements. TEM investigation of the resulting thin film indicated improved crystallinity and larger fiber density with decreased π-π stacking distance, while GIWAXS showed increased lamellar spacing and much better mosaicity after treatment than in the case of untreated solutions. OTFTs fabricated from ultrasonicated P3HT solutions showed two orders of magnitude improvement in field-effect mobility. We attained another order of magnitude enhancement with the combination of surface treatment of the dielectric and reached a mean field-effect mobility of 0.05 cm2V-1s-1 at 4 min ultrasonication time. After 4 min treatment time the carrier mobility declined and reached to untreated levels after 8 min ultrasonication, suggesting an optimal process window.
9:00 PM - H9.37
Electrical In Situ Characterization of Organic Semiconductor Blends during Film Growth.
Max Beu 1 , Andre Dragaesser 1 , Christopher Keil 1 , Derck Schlettwein 1
1 Institute of Applied Physics, Justus-Liebig-University Giessen, Giessen Germany
Show AbstractEvaporated bulk heterojunctions consisting of phthalocyanines and buckminsterfullerene C60 blends have proven to be a relevant material for organic photovoltaic cells. Film growth at elevated temperature proved useful for device performance [1,2]. In the present study, the influence of the substrate temperature during film growth as well as annealing of films subsequent to preparation was studied by in-situ measurements of conduction and photoconduction of molecular blends of phthalocyaninatocopper - (II) (PcCu) and C60. Analysis during film growth allowed a real-time analysis of electrical properties [3] as opposed to cell characterization subsequent to film deposition. The results were discussed in the context of subsequent structural and morphological changes observed in scanning electron microscopy (SEM) and UV/vis transmission measurements. Deposition of bulk heterojunctions at elevated substrate temperature featured improved percolation pathways leading to increased conductivity, photoconductivity and exciton dissociation yield.[1]K. Fostiropoulos and W. Schindler, phys. stat. sol. (B) 246, 2840-2843 (2009).[2]S. Pfuetzner, C. Mickel, J. Jankowski, M. Hein, J. Meiss, C. Schuenemann, C. Elschner, A.A. Levin, B. Rellinghaus, and K. Leo, Org. Electron. 12, 435-441 (2011).[3]C. Keil and D. Schlettwein, Org. Electron. 12, 1376-1382 (2011).
9:00 PM - H9.38
Investigation of the Effects of PEDOT:PSS with a Change of the Solvents for Organic Solar Cells.
Jeong Suk Yang 1 , Sang Hoon Oh 1 , Hyun Jae Kim 1
1 Electrical and Electronic Engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractWe reported the effects of the conductivity of poly (ethylene-3, 4-dioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) buffer layer treated by various solvents, such as glycerol, dimethylformamide, acetic acid, ethanol, and methanol. When some solvents were added to PEDOT:PSS solution, the resistivity of PEDOT:PSS film was changed with the solvent. Through the addition of glycerol, the resistivity was highly decreased from 3.6 to 0.005 Ωcm. Specifically, the resistivity of PEDOT:PSS with glycerol was 700 times lower than that of pristine PEDOT:PSS. In this respect, the electrical properties of organic solar cell based (OSC) on poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were investigated with PEDOT:PSS mixed various solvents. Conventional OSC using pristine PEDOT:PSS has the short circuit current density (J_SC) of 5.43 mA/cm^2, the open circuit voltage (V_OC) of 0.58 V, the fill factor (FF) of 56.08% and the power conversion efficiency (PCE) of 1.79%. The performance of photovoltaic devices was improved with the increased conductivity of modulated PEDOT:PSS layer. Especially, the effect of glycerol on PEDOT:PSS films showed significant increasing the PCE of 5.31% and the J_SC of 16.76 mA/cm^2 without changing the V_OC and the FF compared with conventional OSC. This showed that holes can transfer to an anode layer smoothly without the recombination and trapping of holes via the modulated buffer layer. In this study, we confirmed the electrical characteristic of PEDOT:PSS buffer layer treated various solvents in OSC. In the future, we expect to show the results of surface morphology of modulated PEDOT:PSS and mechanism of the solvent reaction.
9:00 PM - H9.39
Estimation of HOMO-LUMO Energy Gap at Donor-Acceptor Interface by the Internal Photoemission Spectroscopy Technique.
Eiji Itoh 1 , Kenta Okuhara 1
1 Department of Electrical and Electronic Engineering, Shinshu Univ., Nagano Japan
Show AbstractOrganic solar cells(OSCs) have attracted much attention due to their potential application of low cost, flexibility, and light weight film base solar cells. The open-circuit voltage (Voc) in organic solar cells depends either on the difference in the energies of HOMO of the donor and LUMO of the acceptor, the work functions difference of electrodes sandwiching the organic layer, and on temperature. Therefore, the estimation of the electronics structure at donor/ acceptor interface is essentially important. In this study, we have investigated the correlation between the open-circuit voltage of organic solar cells and the energy difference between donor-acceptor interface by internal photoemission spectroscopy (IPE) techniques. We used two types of device structure (i) multilayered(ML) solar cells by vacuum evaporation technique and (ii) inverted type bulk heterojunction(BHJ) polymer solar cells with the mixture of conducting polymer and fullerene derivative(PCBM) on thin titanium oxide (titania) film. ML cells consists of ITO/15nm-thick NiO/donor/50nm-thick fullerene (C60)/ 10nm-thick bathocuproine(BCP)/ Al structures, and they were prepaered by the conventional thermal evaporation technique. Inverted bulk- heterojunction (IBHJ) solar cells consists of ITO/200nm-thick titanium oxide/ ca. 150nm-thick active layer/ 30nm-thick MoO3/ Ag structures. The titania layer was prepared on ITO substrate by sol-gel technique. After spin-coating the polymer:PCBM solutions, the films were heat-treated at 423K for 5min in nitrogen atmosphere. Finally, molybdenum oxide and Ag electrode were evaporated. The active electrode area was 4mm2, and all measurements were carried out in a vacuum chamber equipped with a cryostat at the various temperatures between 40 and 350K. The photovoltaic properties were measured under the white light illumination(100mWcm-2) from ITO side. The HOMO-LUMO energy gap between the donor and acceptor materials were measured by the threshold energy of the photocurrent (approximately the quantum efficiency of about 0.00001%). From IPE spectra, the threshold energy becomes 1.05, 1.2, 1.35eV for P3HT/PCBM, MEH-PPV/PCBM, and F8T2/PCBM interface in IBHJ cells, while the open circuit voltage becomes 0.57, 0.81, and 0.98V at 40K and they decreases to 0.57, 0.74, and 0.92V at 300K for P3HT, MEH-PPV, and F8T2/PCBM IBHJ cells, respectively. That is the Voc decreases by 0.3~0.5V from the threshold energy of IPE curves. Similar change was again observed for ML cells. And the energy difference between the threshold energy (probably the HOMO-LUMO energy gap) and the open circuit voltage decreases from upon cooling the samples. The strong correlation between the threshold energy and the open-circuit voltage indicates that IPE measurement is useful tool for estimating the interfacial electronic structures at the donor-acceptor interface. The details will be discussed in the conference.
9:00 PM - H9.4
Inverted Organic Solar Cells with Polyfluorene Derivative as Cathode Interfacial Layer.
Rira Kang 1 3 , Seung-Hwan Oh 4 , Tae-Soo Kim 1 3 , Dong-Yu Kim 1 2 3
1 School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 4 Radiation Research Division for Industry & Environment, Korea Atomic Energy Research Institute (KAERI), Jeongeup-si, Jeollabuk-do , Korea (the Republic of), 2 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractEfficient inverted organic solar cells(OSCs), based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), with a polyfluorene derivative (WPF-oxy-F) as cathode interfacial layer are investigated. Ultraviolet Photoelectron Spectroscopy (UPS) studies demonstrated that the WPF-oxy-F reduces cathode work function due to the interfacial dipole. In addition, the efficiency of the solar cell increases as the post-annealing temperature is varied from room temperature to 170°C. Measurements of the water contact angle were performed on the surface of cathode/ WPF-oxy-F layer annealed under various temperature. The dark current-voltage curves of inverted OSCs with the WPF-oxy-F layer showed that the leakage current decreases as temperature increases. As a result, post-annealed inverted OSCs with WPF-oxy-F raised open-circuit voltage, fill factor and power conversion efficiency were improved as temperature increases.
9:00 PM - H9.40
Fabrication of the Inverted Bulk Heterojunction Organic Solar Cell on Ultra-Thin Titania Nanosheet.
Eiji Itoh 1 , Yasutake Maruyama 1 , Katsutoshi Fukuda 1
1 Department of Electrical and Electronic Engineering, Shinshu Univ., Nagano Japan
Show AbstractOrganic solar cells (OSCs) with conjugated and fullerene based bulk hetero junction (BHJ) composites are considered promising candidate for the solar cells because of their light-weight, low-cost, and simple fabrication for large area processing. Recent advances in polymer and fullerene based materials improved the power conversion efficiency of BHJ OSCs up to 5% in conventional device structures consisting of PEDOT-PSS as the anodic buffer layer, P3HT: PCBM mixture as the active layer, and amorphous TiOX as the cathodic buffer layer. Since the more effective crystalline TiO2 (anatase phase), requires high temperature heat-treatment at ~800K, the inverted structure consisted of ITO/ TiO2/ active layer/ MoOX/ Ag or Au structure is preferable for fabricating the BHJ cell with titanium oxide. However, the high temperature process for fabricating crystalline TiO2 is a severe disadvantage for the application of organic solar cell, and the low temperature (below 470K) process is required. Recently, Sasaki et al. developed a set of transition metal oxides with 2 dimensional structure (called “nanosheet”) including titania. The nanosheet has several important features: (i) ultimate 2-dimensionality with a thickness of ~1nm and a lateral size of micrometers, (ii) high crystallinity and well-defined composition, and (iii) enhanced physical properties due to the quantum size effect. Moreover, the exfoliated titania nanosheet can be deposited by low temperature process.In this study, we have investigated the deposition condition of titania nanosheet onto ITO coated glass substrate and silica substrate. We have then investigated the electrical or photovoltaic properties of the inverted BHJ cell in ITO/ titania nanosheet/ P3HT: PCBM active layer/ MoOX/ Ag multilayerd structure. The devices were fabricated on 150nm-thick ITO coated glass substrates. We deposited titania nanosheet by layer-by-layer deposition technique. Deposition cycles were repeated for a desired number of times to obtain a multilayered nanosheet film at room temperature. After heat-treatment of the substrate, P3HT/PCBM mixture film was spincoated onto titania nanosheet film from chlorobenzene solution followed by short-time heat-treatment, and the evaporation of MoOx (20nm) and Ag (>30nm) layers. The effective area was 4mm2. Electrical characterization were measured in argon atmosphere at ambient temperature with a source meter under 1 sun global AM 1.5 simulated solar illumination. The JSC, VOC, FF and η in the device without titania nanosheet are 8.04mA/cm2, 0.31V, 0.39 and 0.97% respectively. On the other hand, those values in the device with titania nanosheet becomes 8.0mA/cm2, 0.54V, 0,39, 1.70% (1 cycle), and 8mA/cm2, 0.56V, 0.42 and 1.80% (2 cycle), respectively. That is, only a few nanometer thick titania nanosheet improves the photovoltaic properties. The detail will be discussed in the conference.
9:00 PM - H9.41
Solution Processable Small Molecules for Organic Solar Cells.
Scott Watkins 1 , Fiona Scholes 1 , Richard Evans 1 , Akhil Gupta 1 , Mei Gao 1 , Kevin Winzenberg 1 , Peter Kemppinen 1 , Noel Clark 1 , Doojin Vak 1 , Noel Duffy 2 , Chris Fell 3 , Gerry Wilson 1
1 Materials Science and Engineering, CSIRO, Melbourne, Victoria, Australia, 2 Energy Technology, CSIRO, Melbourne, Victoria, Australia, 3 Energy Technology, CSIRO, Newcastle, New South Wales, Australia
Show AbstractIn this paper we will describe several of our approaches to improving the performance of solution processed organic photovoltaics. In particular we will describe the application of two classes of solution processable small molecules: dibenzochrysenes and donor-acceptor oligothiophenes. For the former we will describe deposition induced changes to the material work function that correlate with changes to the device open circuit voltage. For the latter, we will highlight the enhancement of light absorption that can be achieved through the use of changes to the acceptor group. For both classes of small molecules we will discuss the deposition of these in roll-to-roll printed devices. Finally, we will present details of our custom-built, outdoor testing station for photovoltaic modules. This rig is capable of testing up to 16 modules facing in all directions. The modular approach enables us to rapidly compare different materials under real-world conditions – a key challenge to the commercialisation of organic photovoltaic devices.
9:00 PM - H9.42
Single-Layer Graphene Cathodes for Organic Photovoltaics.
Marshall Cox 1
1 , Columbia University, New York, New York, United States
Show AbstractWe have integrated single-layer graphene as a cathode into organic photovoltaic devices. The properties of these devices indicate that graphene offers two potential advantages over conventional photovoltaic electrode materials; work function matching via contact doping, and increased power conversion efficiency due to transparency. These findings indicate that flexible, light-weight all carbon solar cells can be constructed using graphene as the cathode material.
9:00 PM - H9.43
Controlling Side-Chain Density of Electron Donating Polymers for Improving Their Packing Structure and Photovoltaic Performance.
Chul-Hee Cho 1 , Hyunbum Kang 1 , Tae Eui Kang 1 , Han-Hee Cho 1 , Ki-Hyun Kim 1 , Bumjoon Kim 1
1 Department of Chemical and Biomolecular Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractThe ability to tune the LUMO/HOMO levels of electroactive polymers is critical in controlling their optical and electrochemical properties that are important for organic electronic applications. The HOMO and LUMO offsets between the polymer donor and the electron acceptor strongly affect charge separation and the open circuit voltage (Voc) of a solar cell. Here, we present a series of novel conjugated polymers, poly[3-(4-n-octyl)phenylthiophene] (POPT), poly[3-(4-n-octyl)phenyl-5,2',5',2''-terthiophene] (POPTT) and poly[3-(4-n-octyl)phenyl-5,2',5',2'',5'',2'''-quaterthiophene] (POPQT) that include different numbers of thiophene groups added to their conjugated backbone and thus retain different side chain density. Our electro-optical measurements showed that the addition of thiophene units to the polymer backbones reduced their LUMO/HOMO levels, resulting in higher Voc values in the photovoltaic device. Bulk-heterojunction solar cells fabricated from these polymers, consisting of POPT:PCBM, POPTT:PCBM and POPQT:PCBM, showed increasing Voc values of 0.58, 0.63 and 0.75 V, respectively, resulting in the highest power-conversion efficiency of over 3.4% for the POPQT:PCBM device. In addition, the grazing incidence-angle wide angle X-ray scattering (GIWAXS) study revealed that the decrease in the side-chain density of the conjugated polymers allowed for side-chain interdigitation and thus promoted a three-dimensional packing structure in POPQT, which may explain the dramatic increase in the polymer hole mobility. Our simple approach of tuning the side chain density in the conjugated polymers provides a model system for investigating the effects on the LUMO and HOMO levels of polymers and suggests a design rule for higher Voc and better device performance.
9:00 PM - H9.44
Bi2S3 Nanocrystals as a Novel Electron Acceptor for NIR-Sensitized, Non-Toxic Polymer-Nanocrystal Solar Cells.
Luis Martinez 1 , Maria Bernechea 1 , Pelayo Garcia de Arquer 1 , Gerasimos Konstantatos 1
1 , ICFO - The Institute of Photonic Sciences, Castelldefels, Barcelona, Spain
Show AbstractPolymer solar cells have seen tremendous progress in the recent decade with efficiencies in excess of 6%. This progress has been the result of the optimization of the nanomorphology of the bulk heterojunctions formed between polymers and PCBM and of the design of high efficiency polymers with their bandgap approaching that for optimal solar harnessing. However, little progress has been made on the electron accepting materials of polymer solar cells. PCBM has been to date the main player due to its favorable electronic properties. PCBM however has low absorption coefficient, thus photocurrent generation takes place mainly in the polymer phase. Great benefits are to be expected if both phases (electron donor and acceptor) can contribute in light absorption. Polymer - quantum dot solar cells have been introduced based on PbS [1, 2] and CdSe [3] QD. PbS QD, despite being a p-type material, offer the extension of polymer sensitivity to the infrared whereas CdSe QDs allow for more efficient hybrid solar cells, yet their absorption is limited in the visible. Given the toxic contents of these materials, non-toxic alternatives have been introduced based on Si quantum dots[4] and high bandgap oxides[5,6] which do not offer a second light absorbing phase. We will introduce Bi2S3 nanocrystals as a novel n-type solution processed semiconductor and demonstrate its successful incorporation with PbS QD in the first solution processed p-n heterojunction between p-type PbS and n-type Bi2S3 nanocrystals[7]. We will then demonstrate the first polymer-nanocrystal hybrid solar cell structure based on non-toxic inorganic semiconductors and solar harnessing extended to the NIR due to the favorable bandgap of Bi2S3 of 1.3 eV[8]. Our structure consists of a bilayer heterojunction of P3HT and Bi2S3. The solar cells demonstrate power conversion efficiency in excess of 0.4%, among the highest reported in polymer bilayer structures, with EQE reaching as high as 20%. We will present optical modeling of our structures using FDTD simulations which suggest that the Bi2S3-P3HT interface provides a very efficient exciton dissociation junction. We will further report the underlying physical mechanisms of our devices using intensity dependent EQE, carrier lifetime and Voc and we will conclude by proposing novel configurations towards high-efficiency, NIR-sensitive, non-toxic polymer-nanocrystal solar cells.[1]S.A. McDonald et al. Nat. Mater. 2005, 4, 138-42.[2]K.M. Noone et al., Nano Lett. 2010, 10, 2635–2639.[3]W.U. Huynh et al., Science 2002, 295, 2425-7.[4]C.-Y. Liu et al., Adv. Funct. Mater. 2010, 20, 2157-2164.[5]W.J.E. Beek et al., J. Mater. Chem. 2005, 15, 2985.[6]S. Lu, et al. J. Mater. Sci.: Mater. Electron. 2009, 21, 682-686.[7]A.K. Rath et al. Adv. Mater.2011, accepted, doi 10.1002/adma. 201101399[8]L. Martinez et al. Adv. Mater. 2011, submitted
9:00 PM - H9.6
Polythiophenes with Increased Crystallinity for Efficient Bilayer Organic Solar Cells.
Nico Seidler 1 , Giovanni Lazzerini 1 , Giovanni Li Destri 2 , Giovanni Marletta 2 , Franco Cacialli 1
1 Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, London United Kingdom, 2 Laboratory for Molecular Surfaces and Nanotechnology (LAMSUN) Department of Chemical Sciences, University of Catania Viale A. Doria, Catania Italy
Show AbstractWe used di-tert-butyl peroxide (DTBP) to initiate the formation of nanofibres of poly(3-hexylthiophene) (P3HT) directly in solution, using solvents such as chlorobenzene or dichlorobenzene.The absorption spectra of films spin-cast from the P3HT+DTBP solution show a pronounced vibronic structure with a strong enhancement of the low-energy transition compared to the pure P3HT film, which indicates a high degree of crystallinity. These results are also supported by atomic force microscopy (AFM) measurements, which reveal that the film consists of P3HT nanofibres of a length exceeding 2 µm.Furthermore, the spin-cast films show increasing insolubility with increasing DTBP concentration, reaching retention factors of more than 80%, and field-effect transistor measurements show a more than sixfold increase in hole mobility.Insolubility as well as the enhanced absorption of red light makes the film well-suited as donor material for the use in bilayer heterojunction solar cells because it allows the subsequent deposition of an acceptor layer, spin-cast from the same solvent as the P3HT. Using phenyl-C61-butyric acid methyl ester (PCBM) as acceptor material, we fabricated unoptimised devices with power conversion efficiencies exceeding 1% . However, optimisation of layer thickness and thermal treatment is expected to further increase the performance of the bilayer devices.
9:00 PM - H9.7
Comparison of Planar and Bulk Heterojunction Evaporated Tandem Cells.
David Cheyns 1 , Bregt Verreet 1 2 , Barry Rand 1 , Paul Heremans 1 2
1 SSET / OPV, imec vzw, Heverlee Belgium, 2 esat, KULeuven, Leuven Belgium
Show AbstractWhile the transition from planar heterojunction (PHJ) to bulk heterojunctions (BHJ) was very successful for polymer - fullerene based solar cells, it is not trivial to achieve the same performance enhancement with devices based on evaporated small molecules. One reason is the outstanding initial efficiency of evaporated PHJ compared to the ones of polymers, due to a typically larger absorption coefficient, mobility and exciton diffusion. Another more important reason is the bad charge collection characteristic of most small molecule BHJ produced via coevaporation, leading to low fill-factors for thicker layers. This is due to a bad phase separation of donor and acceptor, and low charge mobility in the blended layers.In this work, we demonstrate the optimization of coevaporated BHJ devices for two different donor materials. Chloroaluminum phthalocyanine (ClAlPc) in combination with C60 showed a large efficiency enhancement, from 3.2% to 4.3% in going from a PHJ to a BHJ. This was achieved by using a) a thin wetting layer of pure ClAlPc and b) evaporation of the blend at elevated temperatures. The combination of these two factors induces a crystalline growth of ClAlPc, which leads to a low series resistance in the device. In the case of boron subphthalocyanine (SubPc), a high proportion of C60 is needed to obtain the required charge collection efficiency, and evaporation at elevated substrate temperatures does not improve performance. In fact, open-circuit voltage (VOC) is shown to decrease as a function of substrate temperature. The optimized structure leads to an efficiency increase of 20% (from 3.4% for PHJ to 4.1% for BHJ).As the absorption spectra of SubPc and ClAlPc are nicely complementary, the combination is a good candidate for incorporation in a tandem configuration. Here, we use the optimized bulk heterojunction subcells to create a highly efficient series connected tandem cell. The recombination zone connecting the two subcells is kept as thin as possible, in order to utilize the first interference peak of the incident light for both subcells. In this configuration, efficiencies above 5.5% are achieved, with VOC values approaching 1.8 V. Compared to similar tandem devices based on two PHJ subcells, the short-circuit current density increases by more than 50%, with an increase in power conversion efficiency from 4.1% to 5.5%. Moreover, this efficiency is relatively stable over a wide range of light intensities, from 1 mW/cm2 to 530 mW/cm2, a beneficial characteristic for both outdoor and indoor real-world applications.
9:00 PM - H9.8
Small Molecular Organic Photovoltaic Cells Having Exciton Blocking Layer at Anode/Donor Interface.
Masaya Hirade 1 , Chihaya Adachi 1 2
1 Center for Future Chemistry, Kyushu Univ., Fukuoka Japan, 2 I2CNER, Kyushu Univ., Fukuoka Japan
Show AbstractRecently, the power conversion efficiency (ηPCE) of organic photovoltaic (OPV) cells has been increased dramatically. However, their ηPCE value are still lower than those of inorganic ones. To enhance the device performance, an increase of the exciton diffusion efficiency (ηED) to donor/acceptor interfaces is crucial. To prevent the exciton quenching at metal electrodes, many devices introduced exciton blocking layer (EBL) at acceptor/cathode interfaces. On the other hand, a poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) layer is used for an anodic buffer layer, providing smooth anode surface and higher stability. Although PEDOT:PSS is well known as an exciton quencher, insertion of an exicton blocking layer into PEDOT:PSS/donor interface have not discussed. Thus, in this study, we introduced an exciton blocking layer into anode/donor interfaces and achieved enhancement of ηPCE. Tris-[4-(5-phenylthiophen-2-yl)phenyl]amine (TPTPA) and tetraphenyldibenzoperiflanthene (DBP) were used as an anodic EBL and a donor layer, respectively. Since they possess almost the same highly occupied molecular orbital (HOMO) level of ~5.4 eV, there is no net energy barrier between a TPTPA and DBP layers. Using these materials, we fabricated the devices of ITO/PEDOT:PSS/TPTPA (X nm)/DBP (20-X nm)/C60 (50 nm)/bathocuproine (BCP) (10 nm)/Al (100 nm). We changed X from 0 nm to 15 nm, and evaluated the thickness dependence of a TPTPA layer. Although the reference device (X=0 nm) showed short circuit current density (JSC) of -5.82 mA/cm2 and ηPCE of 3.88 %, the device performance was enhanced with increasing X. In the case of X=10 nm, the highest JSC of -7.15 mA/cm2 and ηPCE of 5.14 % were obtained. From the action spectra, the enhancement of incident photon to current efficiency (IPCE) was remarkable between 500 nm and 600 nm, corresponding to the DBP absorption region. From the AFM images of several TPTPA layers deposited on a PEDOT:PSS layer, the 3 nm-thick TPTPA layer showed a nearly continuous layer. To estimate the exciton blocking effect of the 3 nm-thick TPTPA layer, we compared the PL intensity of the 20 nm-thick DBP layer deposited on a PEDOT:PSS layer with and without a 3 nm-thick TPTPA EBL between a PEDOT:PSS layer and DBP layer. In the case of a DBP film deposited on a PEDOT:PSS layer, the PL intensity was decreased compared with a DBP film without a PEDOT:PSS layer. On the other hand, a DBP film with a TPTPA EBL between a PEDOT:PSS interface showed almost the same PL intensity as the DBP sole film. This result indicates that a PEDOT:PSS layer acts as an exciton quencher and a 3 nm-thick TPTPA EBL completely suppressed the exciton quenching at the PEDOT:PSS interface. From these results, we can conclude the combination of the exciton blocking and the exciton confinement into thin DBP layer by the TPTPA layer enhanced the device performance.
9:00 PM - H9.9
Conjugated Donor-Acceptor Diblock Copolymers for All Polymer Solar Cells.
Sven Huettner 1 , Kerr Johnson 1 , Rhiannon Mulherin 1 , Carol Huang 1 , Michael Sommer 2 , Peter Kohn 1 , Joanna Slota 2 , Dorota Niedzialek 3 , David Beljonne 3 , Wilhelm Huck 2 , Neil Greenham 1 , Richard Friend 1
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Melville Laboratory, University of Cambridge, Cambridge United Kingdom, 3 Chemistry of Novel Materials, University of Mons, Place du Parc 20 Belgium
Show AbstractPurely polymer based solar cells rely on an interpenetrating morphology, but also on an efficient charge separation with reduced geminate recombination. Focus of our research are systems based on P3HT and F8TBT type polymers, which form the currently most efficient all polymer organic solar cells. We use donor-block-acceptor polymers in order to modify the morphology as well as the interface of polymer blends. Our investigations include morphological studies, device characterisation as well as time resolved transient absorption spectroscopy to track the photophysical processes from a sub-picosecond to millisecond timescale.In order to study the conjugated connection between the donor and the acceptor block we use model systems which consist of P3HT and one unit of F8TBT. We find that there are significant differences regarding its photophysical properties depending on whether the connection is through the fluorene unit or the thiophene unit. In the latter case, a new charge transfer state forms settling on the connection between donor and acceptor. Molecular modelling and time resolved transient absorption spectroscopy is used to identify and characterize these states. It seems that this state is formed through an ultra-fast intramolecular energy transfer process within each polymer chain.Regarding photovoltaic devices, our research is extended to donor-block-accepetor block copolymers that allow the control of the heterojunction morphology. The block copolymers based on P3HT and polyfluorenes have the ability to create thermodynamical stable morphologies forming a standing lamellar nanostructure with highly ordered domains in the range of ∼15 nm - towards the optimum structure for polymer solar cells. The block copopolymers can be used either as a material on its own or as a compatibilizer in a blend of polymers. This is a very elegant way to directly take influence on the phase separation and create thermodynamical stable morphologies, independent of elaborated post annealing protocols.
Symposium Organizers
Gang Li University of California, Los Angeles
Thuc-Quyen Nguyen University of California-Santa Barbara
Dana C. Olson National Renewable Energy Laboratory
Moritz Riede Technische Universitaet Dresden
H13: Poster Session III
Session Chairs
Thursday PM, December 01, 2011
Exhibition Hall C (Hynes)
H10: Device Physics
Session Chairs
Thursday PM, December 01, 2011
Grand Ballroom (Sheraton)
9:00 AM - **H10.1
Exciton Management in Organic Photovoltaics.
Mark Thompson 1 , Sean Roberts 1 , Viacheslav Diev 1 , Robert McAnally 1 , Stephen Bradforth 1 , Sarah Conron 1 , Jeramy Zimmerman 2 , Stephen Forrest 2
1 Chemistry, University of Southern California, Los Angeles, California, United States, 2 Physics and Electrical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract We have explored the use of a number of molecular materials, including metallo-porphyrins and organic dyes, for extending the active wavelengths for photovoltaic devices into the near infrared. I will discuss the use of these materials as donor materials in OPVs, illustrating how these materials can be used to enhance both efficiency and Voc. Controlling the location, lifetime and energy of the exciton is essential to achieving high OPV efficiency. We have investigated methods for tuning exciton energies and controlling their migration paths, both intramolecularly and within a thin film. In particular we have used ultrafast transient absorption spectroscopic methods to monitor the formation and evolution of both singlet and triplet exactions in OPV materials. These studies have included both neat materials and donor/acceptor materials combinations, with an eye to tracking both exciton generation and charge separation in real time. I will discuss our most recent work with porphyrinic materials for OPVs. This involves a careful materials design study that leads to both low energy absorption (into the nearIR) to efficiently harvest photons through the entire visible spectrum. Both long wavelength and broad absorption are achieved with high extinction (α > 105 cm-1). Moreover, we have measured the intra- and intermolecular exciton dynamics for both singlet and triplet excitons in these materials. The implications of these studies for light harvesting by singlet fission routes will be discussed.
9:30 AM - H10.2
Quantifying Bimolecular Recombination Losses in Organic Bulk Heterojunction Solar Cells.
L. Jan Anton Koster 1 2 , Martijn Kemerink 1 , Martijn Wienk 1 , Klara Maturova 1 , Rene Janssen 1
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands, 2 Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
Show AbstractBimolecular recombination of photogenerated charge carriers is one of the efficiency-limiting processes in organic solar cells. It is therefore important to establish just how many carriers are lost through this process. So far, quantification of bimolecular losses has proven problematic. We present a new and straightforward method to do just that. In order to interpret our results, we derive a compact analytical expression for the photocurrent generated by an organic solar cell. Surprisingly, we find that bimolecular losses can amount to tens of % under solar illumination while the short circuit current versus light intensity curve seemingly remains linear. The latter is commonly taken as an indication for negligible bimolecular recombination.Our method is based on measuring the differential current density ΔJ induced by a small modulation in light intensity (ΔI) in the presence of a background light intensity I. This steady-state differential current (SSDC) measurement is carried out by illuminating the solar cell at short-circuit with a cw laser and a mechanically modulated monochromatic light source of a much smaller intensity ΔI. A lock-in amplifier is used to pick up the current ΔJ induced by the modulated light source. By using a set of neutral density filters the intensity I of the cw light incident on the solar cell is modified. As a model system a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron donor and acceptor respectively, is used.The analytical solar cell model includes drift and diffusion of charge carriers, as well as bimolecular recombination and predicts photocurrents as a function of bias and light intensity. In combination with the experimental methodology, the model allows one to determine the fraction of photogenerated charges that is lost to bimolecular recombination to within a few percent.Application of the above to the experimental data of P3HT:PCBM cells yields that the bimolecular recombination constant is suppressed relative to the Langevin rate by a factor 3-10, depending on the annealing temperature. Moreover, bimolecular losses in this system can amount to 17% under solar illumination for devices annealed at 100 °C.
9:45 AM - H10.3
Interplay between Non-Geminate Recombination and Local Absorption Profiles in Bulk Heterojunction Organic Solar Cells.
John Tumbleston 1 2 , Yingchi Liu 2 , Edward Samulski 3 , Rene Lopez 2
1 Physics, North Carolina State University, Raleigh, North Carolina, United States, 2 Physics and Astronomy, University of North Carolina - Chapel Hill, Chapel Hill, North Carolina, United States, 3 Chemistry, University of North Carolina - Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractNon-geminate recombination in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices should be directly related to the efficiency of free carrier transport, since it is known to depend on the density of free charges. Therefore, recombination should be influenced by the distance carriers are required to travel prior to collection at the electrodes. Thus far, this has been difficult to measure except when comparing separate devices that have undergone different processing conditions. Herein, we demonstrate a clear link between recombination and free carrier transport in a single device by utilizing characteristic local light absorption profiles to probe non-geminate recombination. The local absorption profiles are controlled by changing the incident wavelength and side of illumination for semi-transparent devices, which in turn modifies the distance free carriers must travel in order to be extracted. Not only do we explicitly show that non-geminate recombination is dependent on the local absorption profile, we also observe anisotropy in recombination depending on whether holes or electrons have a longer distance to travel prior to extraction. For a variety of donor polymers, including P3HT, MDMO-PPV, and PCDTBT, each blended with PCBM, we determine whether hole or electron transport forces the onset of non-geminate recombination along with the corresponding transport distance of each carrier species.
10:00 AM - H10.4
The Role of Doping Density in Organic Photovoltaic Bulk-Heterojunctions.
Alexandre Nardes 1 , Andres Garcia 1 , Peter Graf 1 , Sean Shaheen 2 , Dana Olson 1 , Nikos Kopidakis 1
1 Basic Sciences, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Dept. of Physics and Astronomy, University of Denver, Denver, Colorado, United States
Show AbstractOrganic photovoltaics (OPVs) are being extensively investigated as a promising class of photovoltaic technology due to their potential for low cost, high throughput manufacturing. An often-used step to produce high efficiency polymer bulk-heterojunction (BHJ) devices is thermal annealing of the active-layer film and/or the complete device, which is typically assumed to improve the morphology of the active layer. In this work, we studied the effects of thermal annealing before (pre-annealing) and after (post-annealing) aluminum deposition on devices of neat P3HT and bulk-heterojunctions of P3HT blended with different fullerene-based acceptors and standard ITO/PEDOT:PSS bottom contacts. Characterization was done by current-voltage (JV), charge extraction by linearly increasing voltage (CELIV), impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS) measurements. Our results suggest that both the top and bottom electrical contacts as well as the morphology of the active layer are affected by thermal annealing. Compared to pristine and pre-annealed devices, the post-annealed devices presented enhanced open-circuit voltage (Voc) and fill factor (FF) and lower reverse saturation current (Js) that increase the PCE of devices by about 30 to 40%. Mott-Schottky analysis and CELIV measurements revealed that the dark carrier concentration of devices that were subjected to a post-annealing step is nearly one order of magnitude lower (~1015 cm-3) compared to pristine and pre-annealed devices (~1016 cm-3). Furthermore, after either a pre- or post-annealing step the charge carrier mobility remains constant, suggesting a de-doping process and improved interface formation at the electrical contacts as the mechanism for improvement upon annealing. Using numerical drift-diffusion simulations we were able to reproduce and explain the experimental results. Implications for OPV basic research and manufacturing are discussed.
10:15 AM - H10.5
Significantly Reduced Bimolecular Recombination in a Silole-Based Donor/Acceptor Polymer:Fullerene Blend.
Tracey Clarke 1 , Attila Mozer 1 , Deanna Rodovsky 2 3 , Andrew Herzing 3 , Jeff Peet 2 , Gilles Dennler 4 2 , Dean DeLongchamp 3 , Christoph Lungenschmied 2
1 Intelligent Polymer Research Institute, University of Wollongong, Wollongong, New South Wales, Australia, 2 , Konarka Technologies Inc., Lowell, Massachusetts, United States, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 4 , IMRA Europe, Sophia-Antipolis Cedex France
Show AbstractOrganic photovoltaic devices formed from blends of semi-conducting polymers and substituted fullerenes are gaining considerable interest for low cost solar energy conversion. The function of these solar cells is based on a light-induced charge separation reaction between the polymer electron donor and fullerene electron acceptor. As such, elucidating the processes of charge generation, recombination and transport in the blends is of great importance in order to improve the device efficiencies. Bimolecular recombination of photogenerated free charges, in particular, is one of the most important limiting processes in organic photovoltaic devices. In Langevin bimolecular recombination, the recombination rate is limited by the diffusion of the charges. Annealed P3HT:PCBM is the only example of strongly suppressed (non-Langevin) bimolecular recombination that has been clearly demonstrated in the literature. This behaviour has typically been attributed to the morphology of the annealed P3HT:PCBM; the well-defined phase separation and two-dimensional lamellar structure of the polymer domain help prevent recombination.In this work we focus on the charge transport and recombination mechanisms of organic photovoltaic devices fabricated from two silole-based donor-acceptor polymers, KP115 and Si-PCPDTBT, blended with PCBM. This was accomplished utilising the techniques of time-of-flight and photo-CELIV (charge extraction by linearly increasing voltage), which allow the charge carrier density, mobility, and the rate of bimolecular recombination to be ascertained. It was discovered that Si-PCPDTBT:PCBM shows clear evidence of Langevin bimolecular recombination while KP115:PCBM possesses non-Langevin behaviour, one of the few polymer:PCBM systems in the literature to do so. As a result of such rare but desirable behaviour, this KP115:PCBM blend has a long charge carrier drift length and can therefore possess substantially thicker active layers while maintaining a high fill factor. This is of considerable importance for commercial viability, particularly for printing and high-throughput applications.Furthermore, the morphologies of the Si-PCPDTBT:PCBM and KP115:PCBM blends (imaged by energy-filtered transmission electron microscopy) do not appear to be strongly correlated with their respective recombination behaviour, as previously reported for P3HT:PCBM. This suggests that non-Langevin recombination has a more complex origin than previously thought and may not be correlated with morphology in all organic photovoltaic blend systems. Elucidating this point would be significant progress in furthering the understanding of bimolecular recombination and its limitations on organic photovoltaics, which in turn would be a key step towards large-scale deployment of this promising technology.
11:00 AM - **H10.6
Distorted S-Shaped I-V Curves of Organic Solar Cells in Experiment and Simulation: Rules and Methods for Distinction of the Reasons and Ways of Avoidance.
Wolfgang Tress 1 , Steef Corvers 1 , Karl Leo 1 , Moritz Riede 1
1 Institut fuer Angewandte Photophysik, Technische Universitaet Dresden, Dresden Germany
Show AbstractOrganic solar cells sometimes suffer from highly distorted S-shaped current-voltage characteristics (I-V curve) with very low fill factor. Although various reasons are discussed, a common picture is missing. However, understanding this phenomenon is essential for a systematic improvement of organic solar cell performance.In experiment and simulation we study a variety of small-molecule planar and bulk heterojunction solar cells with different donor-acceptor combinations and hole transport layers. The experimental I-V curves are reproduced and explained by the drift-diffusion simulations, which visualize the influence of several mechanisms separately. The major effects are energetically misaligned contact layers forming injection and extraction barriers or low-conductive charge transport layers. However, also the active material itself can be responsible for an S-shape caused by highly imbalanced mobilities.The microscopic reasons for S-shapes revealed by simulations are a pile-up of photogenerated charges or a diffusion current, which is reverse to the drift current, due to energetically misaligned contacts. Thus, apart from describing S-shapes, these investigations give a deeper insight into the working principle and driving forces required for a well-working organic solar cell.To further confirm the field and charge carrier distribution within the device we perform transient measurements, where the time response of the photo current upon an illumination signal is monitored in the microsecond regime. By this simple measurement we are able to experimentally differentiate the effects causing the S-shape. In the case of barriers overshoots in the current signal show a pile-up of charges, whereas low charge carrier mobilities are seen in very high response times. Thus, these measurements confirm the predictions of the (steady-state) drift-diffusion simulations and, furthermore, are themselves reproduced by transient simulations.Our findings are very general and hence beneficial for other types of small molecule and polymer solar cells as well. The proposed transient photo current measurement technique can be applied as simple means of detecting the reason for an S-shape. As the S-shape is an undesired effect, we propose and show ways of decreasing its strength, e.g., by applying doping of the hole transport layer.
11:30 AM - H10.7
Built-in Potential and Validity of Mott-Schottky Analysis in Organic Bulk Heterojunction Solar Cells.
Markus Mingebach 1 , Carsten Deibel 1 , Vladimir Dyakonov 1 2
1 Experimental Physics VI, Julius-Maximilians University of Würzburg, Würzburg, Bavaria, Germany, 2 , Bavarian Centre for Applied Energy Research (ZAE Bayern), Würzburg, Bavaria, Germany
Show AbstractThe built-in potential is an important key parameter of organic bulk heterojunction (BHJ) solar cells since it determines the internal electric field profile in the device but also gives an estimate for the open circuit voltage [1] and hence the solar cell efficiency. Therefore the correct determination of the built-in potential is essential to further understand the potential of a given material combination used as the active layer of organic solar cells. This is a nontrivial task, since zero field inside the BHJ device is not easy to achieve. There are two cases to be distinguished: the flat band conditions inside the bulk of the solar cell but with diffusion induced band bending at the interfaces, the so called case of quasi flat bands (QFB), and the case of tilted energy bands (nonzero field) inside the bulk but equipotential electrodes [2]. To separate these contributions to the built-in potential, we applied pulsed photocurrent method to poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester BHJ solar cells and compared the results with the Mott-Schottky analysis of capacitance-voltage characteristics, which is a well-established tool to determine the built-in potential in inorganic devices. By applying the Mott-Schottky analysis to our solar cells we found considerably lower built-in voltages than expected from the difference of the electrode work functions. Moreover, a dependence of the built-in voltage, determined this way, on the active layer thickness of the devices was observed. We conclude that Mott-Schottky analysis seems not to be generally appropriate for the determination of the built-in potential in organic BHJ. On the other hand, the pulsed photocurrent technique allowed us to identify the voltage corresponding to the quasi flat band situation. Furthermore, we were able to deduce the influence of band bending and to determine the built-in potential from the temperature dependent pulsed photocurrent measurements. [1] D. Rauh, A. Wagenpfahl, C. Deibel and V. Dyakonov, Appl. Phys. Lett. 98, 133301 (2011)[2] M. Limpinsel, A. Wagenpfahl, M. Mingebach, C. Deibel and V. Dyakonov, Phys. Rev. B 81, 085203 (2010)
11:45 AM - H10.8
Incorporation of Novel Bis-C60 Derivatives in High Efficiency Polymer Photovoltaic Cells with an Insight into the Limit of Open-Circuit Voltage.
Eszter Voroshazi 1 2 , Karolien Vasseur 1 3 , Tom Aernouts 1 , Paul Heremans 1 2 , Xiang Xue 4 , Angela Herring 4 , Tom Lada 4 , Andreas Athans 4 , Henning Richter 4 , Barry Rand 1
1 PV/OPV, imec, Leuven Belgium, 2 ESAT, Katholieke Universiteit Leuven, Leuven Belgium, 3 MTM, Katholieke Universiteit Leuven, Leuven Belgium, 4 , NanoC Inc., Westwood, Massachusetts, United States
Show AbstractRecent and continuous efficiency increase of solution processed organic solar cells has been driven by the development of novel donor-type polymers, enabled by an enhanced understanding of structure-property relationships in these materials. Meanwhile, development of novel acceptor materials is starting to receive attention, and promises similar improvements to device performance. Currently, fullerene derivatives remain the most promising acceptors, and engineering the sidechains attached to the C60 or C70 cage presents a considerable flexibility to alter their properties. Bis-fullerene adducts recently emerged as a promising class of materials, but only few studies addressed the implication of their altered properties beyond the shift in frontier orbital levels. Here, we report the application of novel mono-and bis-adducts in bulk heterojunction (BHJ) photovoltaic devices, and through the optimization of their photovoltaic parameters we will shed light into their critical differences compared to the commonly used phenyl-C61-butyric acid methyl ester (PCBM).Here, we investigate mono-and bis-oquino-dimethaneC60 (oQDMC60) blended with poly(3-hexylthiophene), producing devices that yield open-circuit voltage (Voc) of 640 mV and 820 mV, while preserving high short-circuit current and fill factor, resulting in efficiencies of 4.1% and 5.2%, respectively. Firstly, the increased Voc stems from an energy level shift of 70 meV and 250 meV with respect to PCBM, as estimated by both quantum chemical calculations and cyclic voltammetry measurements. Secondly, the high photocurrent results from an optimization of the BHJ morphology, since phase segregation is strongly inhibited by the high solubility of the bis-oQDMC60, as demonstrated by absorption spectroscopy, X-ray reflectivity and atomic force microscopy. An order of magnitude higher solubility, compared to PCBM, is an inherent property of bis-fullerene adducts as indicated by solubility measurements following ASTM standards on various bis-adducts. Hence, the optimization not only serves as a guide for implementation of similar fullerene derivatives, but also further hints at an insight into solubility-morphology relationship in BHJs.Finally, we also compare bis-oQDMC60 with two other bis-adducts which have been finding use recently in blends with P3HT, to gain further understanding of this emerging class of materials.To further complement the assessment of bis-oQDMC60, we present its incorporation into BHJs with various amorphous polymers, with the aim to establish the maximal Voc enhancement without incurring a loss in photocurrent. Specifically, we determine the necessary energy level offset between the polymer and bis-oQDMC60 to ensure enhanced device performance compared to blends with PCBM.
12:00 PM - H10.9
Characteristics of Organic Bulk Heterojunction Solar Cells Investigated by Impedance Spectroscopy.
Juan Bisquert 1 , Germa Garcia-Belmonte 1
1 , Universitat jaume I, Castello Spain
Show AbstractAnalysis of the recombination and series resistances extracted from impedance spectroscopy of polythiophene:fullerene organic solar cells allows to determine general and important characteristics of the steady-state performance (current-potential curve). We determine the recombination flux dependence on the internal voltage, associated with the splitting of electron-hole Fermi levels, in conditions of dark or illumination. We formulate expressions to construct the current-voltage characteristics from a few measurements. Further, the determination of capacitance as function of voltage provides information about carrier density, energetics levels in the solar cell, and limitations to photovoltage. The analysis of the impedance results as function of voltage therefore provides a range of crucial information both to establish essential physical mechanisms and for technical analysis of the cells.
12:15 PM - H10.10
Capacitance Spectroscopy on Organic Photovoltaic Devices.
Matthew White 1 , Stefanie Schlager 1 , Stefan Kraner 1 , Niyazi Sariciftci 1
1 Physical Chemistry, Johannes Kepler University, Linz Austria
Show AbstractImpedance spectroscopy to measure capacitance and resistance in organic semiconductor devices is an important technique that will likely contribute a great deal of information. However it also has an extremely high potential to yield misleading data. This work will demonstrate some of the different uses of the technique to characterize organic semiconductor materials properties and organic photovoltaic devices in reverse and forward bias. It will also highlight the extreme caution that must be taken when interpreting data as extended calculations based on subtle measurement phenomenon can give extremely large apparent signals.
H11: Interfaces and Hybrid OPV
Session Chairs
Wolfgang Tress
Wolfgang Tress
Matthew White
Thursday PM, December 01, 2011
Grand Ballroom (Sheraton)
2:30 PM - H11.1
Ferroelectric Organic Photovoltaic-for Higher Efficiency and New Functionalities.
Jinsong Huang 1 2
1 Department of Mechanical and Materials Engineering, University of Nebraska Lincoln, Lincoln, Nebraska, United States, 2 Nebraska Center for Materials and Nanoscience , University of Nebraska Lincoln, Lincoln, Nebraska, United States
Show AbstractThe ferroelectric-photovoltaic (FE-PV) device, in which homogeneous ferroelectric material is used as a light absorbing layer works with a distinctly different mechanism from p-n junction photovoltaic. The polarization electric field is the driving force for the photocurrent in FE-PV. The anomalous photovoltaic effect, in which the voltage output along the electric polarization direction can reach tens to ten-thousand volts, has been frequently observed in FE-PV devices. However its efficiency is low because of the low solar light absorption efficiency, low electric conductivity, and short lifetime of photoinduced charge carriers of the large band-gap ferroelectric oxides and polymers. In this talk, I will present a method for overriding these constraints by combining the strong polarization electric field from ferroelectric material with the strong absorption and good conduction capability of polymer semiconductors. A direct combination of these two gives poor performance. But with rationale device design, the efficiency of ferroelectric organic photovoltaic can be higher than that of both OPV and FE-PV. The FE-OPV also shows unique functions such as switchable polarity and efficiency.1 It is critical to control the morphology of the ferroelectric film otherwise it would reduce the efficiency of OPVs by its large interface resistance. Our recent progress on how to make a right thin ferroelectric thin film in order to achieve higher efficiency will also be reported. 1Y. B. Yuan, T. J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang, and J. S. Huang. Efficiency enhancement in organic solar cells with ferroelectric polymers, Nature Materials 10 (4), 296 (2011).
2:45 PM - H11.2
Low-Temperature, Solution-Processed Molybdenum Oxide Hole-Transport Layer for Organic Photovoltaics.
Scott Hammond 1 , Jens Meyer 2 , Nicodemus Widjonarko 1 , Paul Ndione 1 , Joseph Berry 1 , Alexander Miedaner 1 , Antoine Kahn 2 , David Ginley 1 , Dana Olson 1
1 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractWe have utilized a commercially available metal-organic precursor to develop a low-temperature, solution processed molybdenum oxide (MoOx) hole-transport layer (HTL) for organic photovoltaic (OPV) devices. Thermogravimetric analysis indicates complete decomposition of the metal-organic precursor by 115 °C in air. Acetonitrile solutions spin-cast in a N2 atmosphere and annealed in air afford continuous thin films of MoOx. Ultraviolet and inverse photoemission spectroscopy confirms the formation of MoOx and, along with Kelvin probe measurements, provides detailed information about the energetics of the HTL. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard HTL: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoOx devices exhibit significantly enhanced fill factors with respect to the PEDOT:PSS control devices, on the order of 68%, with slight drops in short circuit densities due to absorption of the MoOx in the blue end of the spectrum. We have also evaluated the use of MoOx with deep highest occupied molecular orbital energy polymers such as PCDTBT.
3:00 PM - H11.3
Improved Photoinduced Charge Carriers Separation in Organic-Inorganic Hybrid Photovoltaic Devices.
Yana Vaynzof 1 , Dinesh Kabra 1 , Lihong Zhao 2 , Peter K. Ho 2 , Andrew T. Wee 2 , Richard Friend 2
1 Cavendish laboratory, University of Cambridge, Cambridge United Kingdom, 2 Department of Physics, , National University of Singapore, Singapore Singapore
Show AbstractWe demonstrate enhanced performance of a hybrid photovoltaic device, where poly[3-hexylthiophene] (P3HT) is used as active material and a solution-processed thin flat film of ZnO modified by a self-assembled monolayer (SAM) of phenyl-C61-butyric acid (PCBA) is used as electron extracting electrode. Ultraviolet photoemission spectroscopy measurements reveal an increase in the substrate work function from 3.6 to 4.1 eV upon PCBA SAM deposition due to an interfacial dipole pointing away from the ZnO. External quantum efficiency (EQE) of the SAM modified devices reached 9%, greatly improved over the 3% EQE of the unmodified devices.[1] Similarly, an increase in performance was observed for Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,7-bis(3-hexylthiophene-5-yl)-2,1,3-benzothiadiazole)-2',2''- diyl] (F8TBT) on SAM modified ZnO. Time Correlated Single Photon Counting (TCSPC) measurements revealed a decrease in the exciton life time in both P3HT and F8TBT for the PCBA modified substrates, confirming the improved charge carrier separation at the PCBA modified interface. We further improved the performance of our devices, by employing ZnO nanowires (ZnO NWs) substrates, which offer a much larger interface area between the ZnO and the polymer. Our results for P3HT indicate that the EQE of such devices is increased from 8% to 15% upon PCBA modification.Reference:[1] Vaynzof et al, Appl. Phys. Lett. 97, 033309 (2010).
3:15 PM - H11.4
Investigation on Chemical Reactivity for Improving Lifetime in Dye-Sensitized Solar Cells.
Sang-Hyun Eom 1 , Ji-Won Lee 1 , Nam-Seok Baek 1 , Duckhyun Kim 1 , Sanghoon Kim 1 , Kyung-Soo Kim 1 , Byung-Chul Shin 1 , Si-Young Cha 1
1 , Samsung, Kyunggi-Do Korea (the Republic of)
Show AbstractResearch on dye-sensitized solar cells (DSSCs) has begun in early 1990, and now several companies are looking for their applications in building-integrated photovoltaic devices (BIPVs). The recent maximum efficiencies in DSSCs are over ~11%, which are not also comparable to those of thin-film based inorganic photovoltaic devices but also exceeding those of organic based photovoltaic devices (OPVs). However, improving lifetime of DSSCs is still a major obstacle for moving forward to the mass-production. In terms of lifetime issues in DSSCs, most researchers have focused on developing robust sealing and encapsulation technology for last two decades, therefore, the decision of organic solvents for electrolytes in DSSCs is typically made upon considering the volatility of organic solvents. Here, we demonstrate that the lifetime of DSSCs is significantly improved by preventing chemical reactions between ruthenium-based organometallic dyes and several components of liquid electrolytes. It is found that chemically unstable NCS ligands of organometallic dyes are replaced with specific components of organic solvents and additives in liquid electrolytes, leading to the significant reduction of short-circuit currents. Optical and chemical analysis methods were used to provide obvious evidences of chemical reactions, and accelerated lifetime tests such as under heat (@ 85°C) and light-soaking (@ 1sun & 60°C) conditions were applied to demonstrate improved lifetime based on large-size (100cm2) DSSCs.
3:30 PM - H11.5
A New Approach to TCO-Free Dye Sensitised Solar Cells.
Grant Mathieson 1 2 , Klaudia Wagner 1 2 , David McDonald 1 3 , Gordon Wallace 1 2 4 , David Officer 1 2 4 , Gerry Sweigers 2 4
1 Cooperative Research Centre for Polymers, University of Wollongong, Wollongong, New South Wales, Australia, 2 Intelligent Polymer Research Institute, University of Wollongong, Wollongong, New South Wales, Australia, 3 Centre of Microphotonics, The Swinburne University of Technology, Hawthorn, Victoria, Australia, 4 ARC Centre of Excellence for Electromaterials Science, University of Wollongong, Wollongong, New South Wales, Australia
Show AbstractSince the discovery of the dye sensitised solar cell (DSSC) by Graetzel and O’Regan in 1991, a wide variety of DSSC structures have been developed with a view to improving the potential ease and cost of DSSC manufacture. One of the most exciting developments was that of the back contact DSSC (BCDSSC) introduced by a number of research groups in 2007/2008, in which the TCO can be eliminated using a porous titanium dioxide-coated photoanode with a cathode placed behind it. Fuke and his co-workers [1] achieved this by coating a film of titanium dioxide on glass with 15 microns of porous titanium and assembling this dyed photoanode into a normal DSSC sandwich structure. This TCO-free BCDSSC displayed a remarkable 8.4 % efficiency. Hayase et al. obtained the same efficiency using a similar approach, creating the pores in the metallic photoanode layer by using a removable ZnO whisker [2]. In contrast, Fan and co-workers could only achieve a 2% DSSC efficiency using a metal mesh-based photoanode [3]. Subsequently, a number of alternative approaches to BCDSSC have been examined including using wires and titanium dioxide mesosponges on titanium metal.The challenge for the development of any large scale solar cell is ensuring good current collection. The use of metal foils as the electrodes in the solar cell resolves this problem. Therefore, the introduction of a metal foil into the BCDSSC appeared a promising way to not only remove the TCO but also improve large scale current collection. We have now shown that a 5% BCDSSC (1 sq cm) can be achieved using a porous metal foil as the anode, and that this structure is amenable to the preparation of large DSSCs. This BCDSSC has considerable potential both for large scale production of DSSCs but also for integration into a variety of structures. [1] Fuke et al., Chem. Mater. 2008, 20, 4974; [2] Hayase et al., Appl. Phys. Lett. 2008, 92, 33308; [3] Fan et al., Appl. Phys. Lett. 2007, 90, 073501.
3:45 PM - H11.6
Efficiency Enhancement of Bulk-Heterojunction Hybrid Solar Cells towards 3.5% by Postsynthetic Surface Treatments of Semiconductor Nanocrystals.
Michael Krueger 1 2 , Dilek Celik 3 , Phenwisa Niyamakom 3 , Frank Rauscher 3 , Clemens Veith 4 , Birger Zimmermann 4 , Hans Frieder Schleiermacher 4 , Sybille Allard 5 , Ines Dumsch 5 , Ullrich Scherf 5 , Yunfei Zhou 1 2 , Michael Eck 1 2
1 Freiburg Materials Research Centre, University of Freiburg, Freiburg Germany, 2 Institute for Microsystems Technology (IMTEK), University of Freiburg, Freiburg Germany, 3 , Bayer Technology Services GmbH, Leverkusen Germany, 4 , Fraunhofer Institute for Solar Energy Systems ISE, Freiburg Germany, 5 Institut fuer Polymertechnologie, University of Wuppertal, Wuppertal Germany
Show AbstractThe direct utilization of colloidal semiconductor nanocrystals in hybrid solar cells is usually limited due to long chain insulating organic capping molecules. Postsynthetic surface treatments have to be applied in order to reduce or to replace the insulating capping for improving charge separation between the polymer and the nanocrystals as well as charge transport between individual nanocrystals. The decrease of the insulating ligand sphere around CdSe nanoparticles by various surface treatments was confirmed by TGA-MS. By applying a combination of different postsynthetic surface treatments and by the use of a low bandgap conjugated polymer, improved power conversion efficiencies up to 3.5% have been achieved for bulk heterojunction hybrid solar cells.
H12: Morphology I
Session Chairs
Thursday PM, December 01, 2011
Grand Ballroom (Sheraton)
4:30 PM - **H12.1
Architectures and Processes for Organic Solar Cell Production.
Madleine Heyder 1 , Luigi Pinna 2 , Florian Machui 2 , Johannes Krantz 2 , Tobias Stubhan 2 , Dery Baran 2 , Tayebeh Ameri 2 , Peter Kutka 2 , Ning Li 2 , Christoph Brabec 1 2
1 , Bavarian Institute for Applied Energies (ZAE Bayern), Erlangen Germany, 2 , Institute Materials for Electronics and Energy Technology, Erlangen Germany
Show AbstractThe technology of organic solar cells has matured to an extent that commercialization of first products did happen. Heavy research efforts were placed on novel organic semiconductors with potential for higher power conversion efficiency. That strategy has proven very successful, and within the last three years efficiencies have evolved from the 4 % into the 9 % regime, with the 10 % hurdle expected to be overcome by 2011 / 2012. However, with the first products pushing into the market, the research community realizes that a qualified product requires more than only a high efficiency number. Device architecture, materials related to this and concurrent production processes need to be all aligned to achieve a good combination between performance, lifetime and efficiency. This requires revisiting the organic solar cell process materials and device architecture with respect to production, reliability and yield. In this contribution we will discuss and review the passive components of the organic semiconductors relevant to guarantee a reliable production process (interface materials, electrodes, ...) and their production processes. Generic interface and electrode materials will be introduced and their performance for various organic semiconductors reviewed. This contribution will shed some light on the role and importance of the layer thickness of the interface materials, on their mulitfunctionality, and on design rules for the production processes. Specifically alternative electrode and interface materials are investigated, as their impact on the device architecture and device lifetime can not be separated from the semiconductor performance. The summary will present suggestions for organic solar cell architectures, optimized with respect to production aspects, and will compare these architectures to their inorganic thin film pendants.
5:00 PM - H12.2
Device Characteristics of Bulk-Heterojunction Polymer Solar Cells Are Independent of Interfacial Segregation of Active Layers.
He Wang 1 2 , Enrique Gomez 1 , Jong Bok Kim 1 , Zelei Guan 2 , Cherno Jaye 3 , Daniel Fischer 3 , Antoine Kahn 2 , Yueh-Lin (Lynn) Loo 1
1 Department of Chemical and Biological Engineering , Princeton University, Princeton, New Jersey, United States, 2 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThe electron donor, poly(3-hexylthiophene), P3HT, and electron acceptor, [6,6]-phenyl-C61-butyric acid methyl ester, PCBM, within bulk-heterojunction polymer solar cells are chemically incompatible, and thus tend to phase segregate. Given that these active layers are only ca. 150nm thick, surface and substrate energy differences can also induce phase separation of the active layers in the vertical direction. Such preferential segregation characteristics is said to affect solar cell performance. We carried out experiments to examine how the device characteristics of bulk-heterojunction polymer solar cells are influenced by interfacial segregation of active layers. We used near-edge X-ray absorption fine structure spectroscopy to quantify the composition at the surface, and at the buried organic-bottom electrode interface of active layers comprising P3HT and PCBM. We find the surface of the active layer to comprise 97 wt% P3HT and the buried active layer-anode interface to comprise 65 wt% P3HT. By delaminating the thin active layer and transferring it onto a device platform to form electrical contacts, we have been able to construct devices having prescribed composition profiles. Specifically, we can delaminate the as-spun film and transfer this film directly onto a new platform. This once-transferred film has the same interfacial segregation characteristics as that of the as-spun film. Both the as-spun and the once-transferred films yield comparable device characteristics, suggesting that this delamination and transfer technique enables efficient electrical contacts. Alternatively, we can flip the delaminated film before transferring it onto a new device platform. Such twice-transferred films exhibit the opposite interfacial segregation characteristics as those of the as-spun and once-transferred films. Surprisingly, polymer solar cells made with all three films show comparable device characteristics in the dark and under illumination. Our experiments thus indicate that the device characteristics are insensitive to the interfacial segregation characteristics of the bulk-heterojunction active layer.
5:15 PM - H12.3
Time-Resolved Characterization of the Nanoscale Structure and Morphology of Spin-Cast Bulk Heterojunction Solar Cells.
Kang Wei Chou 1 , Ruipeng Li 1 , Buyi Yan 1 , Erqiang Li 2 , Robert Gassaway 3 , Alan Biocca 3 , Steven Alvarez-Rivera 3 , Alexander Hexemer 3 , John Anthony 4 , Sigurdur Thoroddsen 2 , Aram Amassian 1
1 Materials Science and Engineering, KAUST, Thuwal Saudi Arabia, 2 Mechanical Engineering, KAUST, Thuwal Saudi Arabia, 3 Advanced Light Source, LBNL, Berkeley, California, United States, 4 , University of Kentucky, Lexington, Kentucky, United States
Show AbstractDespite significant improvements in organic photovoltaic (OPV) technology there still remain several issues that hinder the wide-spread use and profitable commercial production of OPVs. One major challenge is the weak control over the manufacturing processes to get tailored morphologies of the photoactive thin films via solution processing. Current state-of-the-art approaches to understand the morphological evolution and tailoring of the manufacturing processes for high efficiency organic solar cells are limited to post-process characterization and provide limited insight. The main reason for that is the incompatibility of many traditional characterization techniques with solution processes. These challenges hinder our ability to understand and subsequently control the interaction of multiple factors affecting the morphological evolution. We report the in situ monitoring of the nanoscale phase separation and crystallization process in spin-cast P3HT:PCBM blended thin films. A compact spin coater and liquid dispensing system were built and time-resolved grazing incidence x-ray scattering (small angle and wide angle) measurements were performed during the spin coating process with sub-second time resolution. The evolution of the liquid layer was monitored in situ using fast CCD camera imaging and optical reflectometry. The results show a vertical phase separation of the blend in less than 1 second with a characteristic spacing of 1.57 nm. In addition, the formation of PCBM aggregates was imaged with a spacing of 14.78 nm. These experiments also demonstrate the feasibility of using in situ time-resolved characterization techniques to monitor solution processes in conditions of relevance to device fabrication.
5:30 PM - H12.4
Novel Insights into the 3D Functional Morphology of Photovoltaic Blends by Low Energy-Loss Electron Spectroscopic Imaging.
Martin Pfannmoeller 1 , Harald Fluegge 2 , Katrin Schultheiss 2 5 , Gerd Benner 3 , Irene Wacker 4 , Wolfgang Kowalsky 2 5 , Rasmus Schroeder 1 5
1 CellNetworks, University of Heidelberg, Heidelberg Germany, 2 Institute for High-Frequency Technology, TU Braunschweig, Braunschweig Germany, 5 , InnovationLab, Heidelberg Germany, 3 , Carl Zeiss NTS, Oberkochen Germany, 4 , Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractThe nanoscale morphology of bulk heterojunctions (BHJs) of photoactive donor and acceptor materials is critical for the efficiency of corresponding solar cell devices. It is thus of greatest interest to understand the deterministic relationship between inherent structure and electric performance. Amongst various other techniques transmission electron microscopy (TEM) is employed to visualize material domains [1]. However, bright field imaging is not suitable to distinguish typical donor/acceptor materials, which usually show low contrast. Hence we use electron spectroscopic imaging (ESI) to obtain spectroscopic material contrast, i.e. electronic excitations are used to visualize chemical contrast of photovoltaic materials [2,3]. So far information from different plasmonic excitations has been exploited to discriminate domains. Here we show that by extending the spectral information included in the analysis to the optical to ultraviolet energy range we can better distinguish spectral characteristics and thus better separate and localize materials up to a resolution of 1-2 nm. We apply electron microscopic imaging using a monochromated beam, which offers the possibility to acquire spectroscopic images at an energy loss as low as 1-2 eV. To analyze local spectroscopic information we use nonlinear, multivariate statistical analysis and machine learning, so that minor spectral differences are employed for classifying spectra into groups.We processed ESI data from as-cast and annealed blends P3HT and PCBM. Through data analysis three different phases can be identified at nanometer resolution, which correspond to P3HT-rich, PCBM-rich and intermediate domains, the latter being a homogeneously mixed, interconnecting phase. Morphology of floated layers as well as ultra¬-thin cross-sections orthogonal to the layer planes was analyzed to compare lateral and vertical material phases. Segmenting the three different material phases in 3D reveals information about structural changes of the phase distribution in blends when comparing as-cast to annealed layers: It is observed that in general domain sizes grow larger by annealing and thus possibly extend the charge transport paths. Most interestingly, the expanse of the mixed phase diminishes upon annealing. The findings illustrate that energy-extended ESI imaging is a versatile tool for investigations of photovoltaic materials with distinct spectral features that are suitable for creating material contrast. We are currently incorporating low energy-loss ESI into electron tomography, which allows the direct reconstruction of the morphology of single layers in 3D, i.e. volume maps of the “three-phase” network.[1]Bavel, S. S. v.; Sourty, E.; With, G. d.; Loos, J. Nano Lett 2008, 9, (2), 507-513.[2]Drummy, L. F.; Davis, R. J.; Moore, D. L.; Durstock, M.; Vaia, R. A.; Hsu, J. W. P. Chem Mat 2010, 23, (3), 907-912.[3]Herzing, A. A.; Richter, L. J.; Anderson, I. M. J Phys Chem C 2010, 114, (41), 17501-17508.
5:45 PM - H12.5
Nanoscale Infrared Spectroscopic Analysis of Organic Photovoltaic Materials.
Michael Lo 1 , Curtis Marcott 2 , Roshan Shetty 1 , Kevin Kjoller 1
1 , Anasys Instruments, Santa Barbara, California, United States, 2 , Light Light Solutions, Athens, Georgia, United States
Show AbstractOrganic photovoltaic (PV) materials, such as poly(3-hexylthiophene) (P3HT) and P3HT doped with (6,6)-phenyl-C61-butyric acid methyl ester (PCBM), have been heavily researched in harnessing solar power as a source of alternate energy. Local material variations, i.e. phase separation, can contribute to the overall solar conversion efficiency of the solar cell. Recently, the release of a breakthrough bench-top AFM-infrared (AFM-IR) instrument enables IR microspectroscopy in the size scale of 100 nm, which is at least an order of magnitude higher in resolution than conventional IR systems. Using the AFM-IR apparatus, spatially resolved chemical analysis of P3HT and P3HT/PCBM samples is performed. Spectral changes within ca. 100 nm are observed when a series of spectra is collected across a flat area of P3HT. Phase separation of the P3HT and PCBM within a surface defect can be detected by comparing the spectra of interest with the pure component spectra. Subtle differences are realized. Surface topology is also correlated with both chemical information and relative mechanical stiffness.
H13: Poster Session III
Session Chairs
Friday AM, December 02, 2011
Exhibition Hall C (Hynes)
9:00 PM - H13.1
Effect of Side Chain Number in Indene-Based Multiadduct Fullerenes on Their Opto-Electronic Properties and Open-Circuit Voltages (Voc) in Organic Photovoltaics.
Hyunbum Kang 1 , Chul-Hee Cho 1 , Tae Eui Kang 1 , Hyeong Jun Kim 1 , Ki-Hyun Kim 1 , Han-Hee Cho 1 , Sung Cheol Yoon 2 , Bumjoon Kim 1
1 Department of Chemical and Biomolecular Engineering, KAIST, Daejeon Korea (the Republic of), 2 Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejeon Korea (the Republic of)
Show AbstractThe ability to tune the lowest unoccupied molecular orbital (LUMO)/ highest occupied molecular orbital (HOMO) levels of fullerene derivatives used as electron acceptors is critical in controlling the optical and electrochemical properties of these materials, which is important for organic electronic applications. In addition, the HOMO and LUMO offsets between the polymer donor and the electron acceptor strongly affect charge separation and the open circuit voltage (Voc) of solar cells. Here, we report a series of indene fullerene multiadducts (ICMA, ICBA and ICTA) in which different numbers of side chains are present in the fullerene molecule. Our electro-optical measurements showed that the addition of indene units to fullerene raised its LUMO/HOMO level, resulting in higher Voc values in the photovoltaic device. Bulk-heterojunction solar cells fabricated from a series of indene fullerene multiadducts consisting of P3HT:ICMA, P3HT:ICBA and P3HT:ICTA showed Voc values of 0.66, 0.84 and 0.92 V, respectively. The effects of the added side chains on the opto-electrical properties of the fullerene derivatives were carefully investigated to elucidate the molecular structure–device function relationship. Our study provides a model system for investigating the effects of added indene groups on the properties of fullerene derivatives and their behavior in BHJ solar cells.
9:00 PM - H13.10
Density Functional Theory Study of the Electronic Structure of Thiophene and Pyrrole Copolymers.
Ben Williams 1 , V. Barone 1 , Brian Pate 1
1 Physics, Central Michigan University, Mount Pleasant, Michigan, United States
Show AbstractWe present a density functional theory study of the electronic properties of thiophene and pyrrole copolymers, employing a variety of density functionals such as LSDA, GGA, hybrids, and the more recent screened exchange HSE functional. The effect of the copolymer’s monomer ratio on the electronic structure is explored and the variation of the bandgap and HOCO levels with the composition is analyzed. Our calculations show a linear relationship between the HOCO level and the ratio of monomers, a result that provides a sound theoretical basis for the pursuit of synthesizing copolymers with a statistical gradient of electron binding energies for applications in photovoltaics.
9:00 PM - H13.11
Electrical Doping as a Route towards Highly Conductive Optical Spacers in Multi-Junction Organic Solar Cells.
Alexander Mityashin 1 2 , Eszter Voroshazi 1 2 , David Cheyns 1 , Barry Rand 1 , Jan Genoe 1 , Paul Heremans 1 2
1 , imec, Leuven Belgium, 2 ESAT, Katholieke Universiteit Leuven, Leuven Belgium
Show AbstractHighly conductive, transparent optical spacers are essential building blocks of multi-junction organic solar cells with complementary absorption [1]. Using such spacers, overall device absorption can be enhanced by optimizing the light distribution in the corresponding sub-cells of a tandem structure. However, most of the transparent organic materials are intrinsically poor conductors, which limit device performance when using thick spacers. To cope with a similar limitation in transport layers of organic-light emitting diodes, electrical doping is very successfully used to enhance layer conductivity. It has been shown that at doping concentrations of several percent, material conductivity can be increased by several orders of magnitude, without any significant change of its optical properties [2]. In this work we extend the use of electrical doping towards multi-junction solar cells and demonstrate how metal doped electron transport layers can be used as electron conducting optical spacers. First, we study spacer impact on optical, electrical and lifetime properties of the prototype CuPc/C60 heterojunction by testing various spacer materials both in standard and reverse device architectures. We demonstrate that spacer thickness can be increased tenfold without affecting series resistance of the cell when low work function metals are used. Device fill factor remains above 60% with no significant decrease in 5-150 nm range of the spacer thickness, while photo-current lost due to the spacer absorption is minimized. The use of a high work function metal does not lead to electrical doping, and the electrical properties of the device are therefore not enhanced. After that, we discuss integration of the spacers in tandem structures, where the use of thick optical spacers opens the possibility to use both the first and the second maximum of the interference pattern in multi-junction stack.[1] D. Cheyns, B.P. Rand, P. Heremans, Applied Physics Letters 97(3):033301. (2010).[2] B. Maennig, M. Pfeiffer, A. Nollau, et al., Physical Review B 64(19):1-9 (2001).
9:00 PM - H13.12
Approaching the Ultimate Open Circuit Voltage in Thiophene Based Single Junction Solar Cells by Applying Diindenoperylene as Acceptor.
Ulrich Hoermann 1 , Julia Wagner 1 , Mark Gruber 1 , Andreas Optiz 1 , Wolfgang Bruetting 1
1 Institute of Physics, University of Augsburg, Augsburg Germany
Show AbstractThe efficiency of a photovoltaic cell is directly proportional to its open circuit voltage. This in turn is eventually set by the donor–acceptor energy gap, i.e. the energy of the intermolecular charge-transfer state in organic solar cells. In this contribution we study diindenoperylene (DIP) as a new molecular acceptor in combination with various molecular donors. We show that planar heterojunctions of thiophene derivatives and DIP yield extraordinarily high open circuit voltages (Voc) of approximately 1.2 V for poly(3-hexylthiophene) and almost 1.4 V for heat treated α-sexithiophene. Considering the respective transport gaps, those values are close to the maximum Voc attainable for these material systems.
9:00 PM - H13.13
Quantifying Sensitivity of Morphology Evolution to Solvent Effects during Fabrication of Organic Solar Cells.
Olga Wodo 1 , Aram Amassian 3 , Baskar Ganapathysubramanian 1 2
1 Mechanical Engineering Department, Iowa State Univeristy, Ames, Iowa, United States, 3 Materials Science and Engineering, Division of Physical Sciences and Engineering, King Abdullah University for Science and Technology (KAUST), Thuwal Saudi Arabia, 2 Electrical and Computer Engineering Department, Iowa State University, Ames, Iowa, United States
Show AbstractThe multiplicity of fabrication process variables (solvent type, solvent blends, evaporation profile, annealing, spinning rate, solvent atmosphere, blend ratio, substrate effect, additives) makes tailoring fabrication processes a challenging task. In particular, the evaporation rate is a strong function of the solvent type, and changes with film thickness by at least one order of magnitude. It is of interest to quantify the effect of solvent type and evaporation profiles on morphology evolution, since they offer the easiest means to tune morphology. Current state-of-the-art approaches to quantifying morphology evolution for tailoring fabrication process is limited to combinatorial trial-and-error based experimental investigation. This serves as a rationale for integrating computational modeling to augment experimental efforts for tuning morphology in organic solar cells. We use a phase field based thermodynamic approach to develop a predictive theory for the evolution of morphology during solvent-based fabrication of organic solar cells. We use experimentally informed, time dependent evaporation rate profiles in the model. The model is validated by (a) comparing the final 3D morphology predictions with AFM characterization, (b) in situ monitoring of the growth of thin films via time-resolved grazing incidence x-ray scattering (small angle and wide angle) measurements with sub-second time resolution. We use the validated model to investigate the effect of solvent and evaporation profile on the morphology evolution. In particular, we quantify the sensitivity of (a) characteristic length of features; (b) percolation pathways; and (c) compositional gradients to the evaporation rates and solvent types for three solvents: chloroform, chlorobenzene and xylene. This provides quantitative metrics for rationally choosing solvents and evaporation profiles for achieving desired morphological features.
9:00 PM - H13.14
Hole Mobility Effect in the Efficiency of Bilayer Heterojunction Polymer/C60 Photovoltaic Cells.
Lucimara Roman 1 3 , Andréia Macedo 1 , Cleber Marchiori 1 , Isabel Grova 3 , Marlus Koehler 1 , Leni Akcelrud 2 3
1 Physics, UFPR, Curitiba, Paraná, Brazil, 3 PIPE – Programa pós-graduação em Engenharia, UFPR, Curitiba Brazil, 2 Chemistry, UFPR, Curitba Brazil
Show AbstractWe report here bilayer heterojunction solar cells fabricated by using poly[9,9’-hexyl-fluorene-alt-bithiophene] (LaPPS43) polymer as active layer. The power conversion efficiency (η) displays a 7-fold increase upon annealing at 200 οC, reaching the value of 2.8 %. This result is comparable to the highest η reported so far for bulk heterojunction solar cells using the same polymer. Simulation, absorbance spectra and current versus voltage results indicate that the π-π stacking in solid state is enhanced after annealing with a reduction of traps and thus reflecting in higher hole mobility.
9:00 PM - H13.15
Synthetically Tuned Subphthalocyanines as New Light Harvesting Electron Acceptors.
Paul Sullivan 1 , Amelie Duraud 1 , Ian Hancox 1 , Nicola Beaumont 1 , Ross Hatton 1 , Michael Shipman 1 , Tim Jones 1
1 Department of Chemistry, University of Warwick, Coventry United Kingdom
Show AbstractImproved understanding of the factors influencing the performance of small molecule organic photovoltaic (OPV) devices has led to increased interest in the discovery of new photoactive materials. The relative abundance of suitable commercially available materials has led to a number of candidate donor materials being reported showing improvements in open-circuit voltage (VOC) and/or short-circuit current (JSC). However, the list of acceptor materials remains small with most work relying on fullerene derivatives. In the simplest terms, a photoactive heterojunction requires only an energy level offset to function, and hence one approach to new acceptor materials is the synthetic modification of existing donor materials to provide acceptor characteristics.In this work new light harvesting electron acceptors based on traditional donor materials, primarily boron subphthalocyanine chloride (SubPc), have been synthesized and integrated into planar heterojunction OPV devices. By selective substitution of the periphery of the conjugated system, energy level tuning to maximize the interface gap formed with a range of donor materials is presented. As an example, a planar SubPc/Cl6-SubPc heterojunction device exhibits significantly improved VOC compared to the SubPc/C60 device, with comparable power conversion efficiency. Under continuous illumination these devices also exhibit significantly improved stability as compared to the same device structure employing C60 as the electron acceptor, further proving their considerable potential as a replacement for C60 in OPVs.
9:00 PM - H13.17
Organic Tandem Solar Cells in Three-Terminal Structure with a Highly Transparent and Conductive ZTO/Ag/ZTO Middle Electrode.
Hans Schmidt 1 , Stephan Schmale 1 , Thomas Winkler 1 , Harald Fluegge 1 , Hans-Hermann Johannes 1 , Sami Hamwi 1 , Torsten Rabe 1 , Wolfgang Kowalsky 1
1 , Technische Universität Braunschweig - Institut für Hochfrequenztechnik, Braunschweig Germany
Show AbstractWe present an organic tandem solar cell comprising a wet processed polymer and a vacuum processed small molecule subcell. The two subcells exhibit complementary absorption profiles leading to an improved overall absorption. For the first subcell an inverted architecture is used, whereas the second subcell is prepared in a conventional stack. The middle electrode is used as a common anode for both cells. It consists of a highly transparent multilayer electrode prepared by rf sputtering of zinc tin oxide (ZTO) and thermal evaporation of silver (Ag) embedded between two ZTO layers. Recently we have shown the electrical and optical properties of the ZTO/Ag/ZTO (ZAZ) multilayer electrode and used it as bottom and top contact for semitransparent bulk hetero junction (BHJ) solar cells with an inverted device architecture [1]. Furthermore films of transition metal oxides (TMO) as tungsten oxide WO3) or molybdenum oxide (MoO3) have been evidenced to work as efficient buffer layers to prevent organic layers from damages due to the sputter deposition process of the top electrode [2, 3]. These highly transparent TMOs can be thermally evaporated on top of organic layers without introducing damages and have widely been used for organic solar cells due to their favorably electronic structure. Efficient semitransparent organic solar cells with rf sputtered indium tin oxide (ITO) top contact using a TMO buffer layer have been shown [2]. With this tandem solar architecture we study the two subcells in the stack detached from it due to the three electrodes in the device structure. We will show that the J-V characteristic of the tandem cell device is an addition of the J-V characteristics of the two subcells.
9:00 PM - H13.18
Characterizing the Charge Collection Efficiency in Efficient Organic Photovoltaic Cells Based on a Donor-Acceptor Bulk Heterojunction.
Richa Pandey 1 , Russell Holmes 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractOrganic photovoltaic cells (OPVs) based on electron donor-acceptor (D-A) bulk heterojunctions have realized high power conversion efficiencies exceeding 8%. In these systems, the high efficiency comes as a result of optimized exciton diffusion/dissociation and charge carrier collection efficiencies. While a variety of techniques exist to characterize exciton diffusion in OPVs, fewer studies have developed explicit experimental approaches to assess the charge collection efficiency. In this work, we explore the performance of bulk heterojunction OPVs based on the D-A pairing of boron subphthalocyanine chloride (SubPc) and C60. Measurements of the external quantum efficiency under reverse bias permit the calculation of the charge collection efficiency without any additional input from optical field simulations. For the SubPc:C60 system, optimum device performance is realized for films containing 80 wt.% C60. The origin of this C60-rich composition is explained in terms of a high charge collection efficiency of 77% under short-circuit conditions. A separate characterization of the charge transport in mixed D-A films using hole- and electron-only devices indicates that this optimum composition also coincides with a peak in the hole mobility of the mixture. Overall, the power conversion efficiency of mixed OPVs containing 80 wt.% C60 reaches (4.0±0.1)% under 100 mW/cm2 simulated AM1.5G solar illumination. By also considering the external quantum efficiency of this device under forward bias, a reduction in the charge collection efficiency is observed, leading to a low device fill factor that ultimately limits the overall device performance.
9:00 PM - H13.19
Semiconducting Carbon Nanotubes as Near-Infrared Absorbers for Photovoltaics.
Dominick Bindl 1 , Meng Yin Wu 2 , Frederick Prehn 1 , Michael Arnold 1
1 Materials Science and Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States, 2 Electrical Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States
Show AbstractSemiconducting single walled carbon nanotubes (s-SWCNTs) are polymer-like materials with direct and tunable NIR bandgaps, absorption coefficients > 10^5 cm-1, carrier mobilities > 10^4 cm2V-1s-1, solution processability and strong resistance to photo-oxidation. This suite of properties is reminiscent of semiconducting polymers, with significantly enhanced carrier mobility, band gaps inherently of order 1eV, and much stronger photo-oxidation resistance. Recently, we demonstrated internal quantum efficiencies (QE) in s-SWCNT planar devices approaching 100%, which indicates near perfect exciton dissociation and charge collection, and further motivates the use of SWCNTs as photovoltaic absorbers. Achieving high-efficiency, s-SWCNT-based photovoltaics is met with challenges analogous to challenges in polymer/fullerene systems; namely, addressing exciton diffusion lengths and optimizing morphologies to suppress carrier recombination. Exciton diffusion in s-SWCNTs is highly anisotropic; diffusion lengths along the backbone of a nanotube are expected to be > 200 nm, while diffusion involving inter-tube energy transfer is highly dependent on tube-tube coupling. For film morphologies whereby the majority of s-SWCNTs lie in plane, exciton diffusion to the photoactive interface is dominated by inter-tube processes.In previous s-SWCNT planar devices, photovoltaic performance was limited by out-of-plane exciton diffusion lengths of order 5nm. Here, we demonstrate the removal of solubilizing ligands from s-SWCNTs, significantly enhancing inter-tube coupling and thus, exciton diffusivity. These enhancements in exciton diffusion result in a factor of two increase in responsivity, NIR external QE >20% and monochromatic power conversion efficiencies >7% at 1052nm. We discuss the nature of the diffusivity enhancement and strategies for further improvement. Additionally, we demonstrate novel fabrication methods for creating a 3-D, hierarchical s-SWCNT/acceptor interface with partial vertical alignment of the s-SWCNTs with respect to the substrate. This morphology stands to exploit the exceptional intratube exciton diffusion length. Only with recent advances in sorting s-SWCNTs from as-produced carbon nanotube powders have these exceptional properties been unlocked to photovoltaics. S-SWCNTs have marked potential for implementation as standalone absorbers, NIR dyes in existing devices, or NIR sensitive layers in tandem cells. The findings presented represent the first steps in the optimization of s-SWCNT based systems towards the realization of a high efficiency, low cost photovoltaic technology. [1] Bindl, D. J.; Wu, M.-Y.; Prehn, F. C.; Arnold, M. S., Efficiently Harvesting Excitons from Electronic Type-Controlled Semiconducting Carbon Nanotube Films. Nano Lett. 2011, 11 (2), 455-460 [2] Bindl, D. J.; Arnold, M. S.; Semiconducting Carbon Nanotube Photovoltaic Photodetectors, International Journal of High Speed Electronics and Systems. 2011, (In Press)
9:00 PM - H13.2
Energy Level Alignment at Metal/Organic Semiconductor Interfaces with Artificially Structured Thin Oxide Layers.
Nyun Jong Lee 1 , Yu Jeong Bae 1 , Tae Hee Kim 1 , Hyunduck Cho 2 , Changhee Lee 2 , Luke Fleet 3 , Atsufumi Hirohata 3 , Eisuke Ito 4
1 Department of Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 School of Electrical Engineering and Computer Science, Seoul National University, Seoul Korea (the Republic of), 3 Department of Electronics, The University of York, York United Kingdom, 4 Flucto-Order Functions Research Team, RIKEN Advanced Science Institute, Wako, Saitama, Japan
Show AbstractSpin transport in molecular organic semiconductors (OSCs) has attracted tremendous attention for new science and future molecular based technology. [1] The injection of spin-polarized carriers could be used to control the light emitted by organic light emitting diodes (OLEDs). Therefore, the multi-functionality using photons to control spin current and vice versa can be expected with OSCs. For realistic applications, a fundamental prerequisite is basic knowledge about the mechanism of spin injection and spin transport. Especially, understanding the basic structural and electrical properties at metal/organic (M/O) interfaces is very important. In this work, we investigated the interfacial properties of a hetero structure consisting of a highly-qualified interface between OSC Cu-phthalocyanine (CuPc), transition metals (Co and Fe), and oxide layers (Co-O, Fe-O and MgO(100)). We focus on understanding the interface electronic states and energy alignment by using x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. The experimental determination of binding energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states of organic layers with respect to the Fermi level (E
F) of the metal electrode was carried out. Based on these values, the energy barriers for hole and electron injection from the metal to organic layer can be estimated. Our results show that the shifts of CuPc HOMO energy level strongly depending on the metal electrode; the bigger shift of the HOMO energy level observed when a nominal 1.8 nm thick MgO(100) layer is inserted between Fe(100) and CuPc layers. This HOMO energy level shift increases monotonically as the CuPc thickness decreases ranging from 10 to 1 nm. However, a slight shift was observed for the metal electrodes of naturally oxidized Fe(100) and poly crystal Fe films. The interface microstructure of these heterostructures was also investigated by using high-resolution TEM.This work is supported by the National Research Foundation of Korea (NRF) through the Quantum Meta-Materials Research Center and NRF grant (NRF-2010-0006749). [1] V. Dediu et al.. Nature Mater. 8, 707-716 (2009).* e-mail:
[email protected]9:00 PM - H13.20
Boron Subpthalocyanine Chloride as an Electron Acceptor for High Voltage Fullerene Free Organic Photovoltaics.
Nicola Beaumont 1 , Sang Wan Cho 2 , Paul Sullivan 1 , David Newby 2 , Ross Hatton 1 , Kevin Smith 2 , Tim Jones 1
1 Chemistry department, Warwick University, Coventry United Kingdom, 2 Physics Department, Boston University, Boston, Massachusetts, United States
Show AbstractThe lack of available electron accepting materials remains a significant limitation in the development of efficient organic photovoltaic (OPV) cells, with the vast majority of devices using fullerenes such as C60, C70 or their soluble derivatives. Although C60 is extremely efficient as an electron accepting material, successful donor / acceptor heterojunctions depend on well tuned orbital energies and due to the relatively small band gap of C60 the open circuit voltage (Voc) is largely limited in these devices. We report high efficiency fullerene-free single heterojunction (SHJ) OPV devices consisting of two typical “donor” organic semiconductor materials, Tetracene (Tc) and Boron subphthalocyanine chloride (SubPc). Optimisation of Tc thin films in Tc / C60 and Tc / SubPc OPV devices was completed and soft x – ray photoelectron spectroscopy (PES) was used to study the donor / acceptor interfaces of both systems. Devices containing SubPc as a replacement for C60 were tested in an attempt to increase the Voc and short circuit current density (Jsc) and by using a 35 nm layer of SubPc the Voc was found to increase significantly from 0.76 V to 1.24 V. This voltage is amongst the highest Voc values achieved to date in single heterojunction OPVs and an overall improvement of ~ 60 % in power conversion efficiency from 1.8 % to 2.9 % was also achieved. Although organic semiconductors show a preference for having donor or acceptor character, we have shown that when employed in devices as long as the offsets at the heterojunction are sufficient to dissociate charges efficiently, a typical donor material can be utilised as an acceptor. This leads the way for interesting donor – acceptor material combinations allowing for better overlap of the solar spectrum and improved device performance.
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Quality Control of Roll-to-Roll Processed Polymer Solar Modules by Complementary Imaging Methods.
Roland Roesch 1 , Frederik Krebs 2 , David Tanenbaum 2 3 , Harald Hoppe 1
1 Institute of Physics, Ilmenau University of Technology, Ilmenau Germany, 2 Risø National Laboratory for Sustainable Energy, Technical University of Denmark , Roskilde Denmark, 3 Department of Physics and Astronomy, Pomona College, Claremont, California, United States
Show AbstractWe applied complementary imaging methods to investigate processing failures of roll-to-roll solution processed polymer solar modules based on polymer:fullerene bulk heterojunctions. For investigation of processing deficiencies in solar modules dark lock-in thermography (DLIT), electroluminescence (ELI) and photoluminescence imaging (PLI) has been employed and was complemented by optical imaging. The investigation can be divided in a fast DLIT –overview of the module, and a successive more detailed analysis of the suspicious region by all imaging methods. The combination of all high resolution images allowed us to allocate the origin of processing errors to a specific deposition process, i.e. the insufficient coverage of an electrode interlayer, which demonstrates these complementary imaging techniques to be a powerful tool for offline characterization of processing failures and thus quality control of the production process.
9:00 PM - H13.23
Flexible Polymer Solar Cells and Roll-to-Roll Processed Polymer Solar Modules – Investigations on Degradation by Complementary Imaging Methods.
Roland Roesch 1 , Frederik Krebs 2 , Mikkel Jorgensen 2 , David Tanenbaum 2 3 , Burhan Muhsin 1 , Marco Seeland 1 , Maik Baerenklau 1 , Harald Hoppe 1
1 Institute of Physics, Ilmenau University of Technology, Ilmenau Germany, 2 Risø National Laboratory for Sustainable Energy, Technical University of Denmark , Roskilde Denmark, 3 Department of Physics and Astronomy, Pomona College, Claremont, California, United States
Show AbstractRoutes and patterns of degradation occurring within flexible solution processed polymer solar cells and modules were investigated by complementary imaging methods. In detail, we have employed dark lock-in thermography (DLIT), light beam induced current (LBIC), electroluminescence and photoluminescence imaging (ELI and PLI), all complemented by optical imaging (OI). The combination of these methods allows us to allocate degradation issues laterally on the device. Furthermore these techniques increase the understanding of the origin of the degradation, due to the fact that these methods are sensitive to different layers within the device that are subject to different failure mechanisms. Specific emission patterns are discussed in the light of luminescence modeling.References:[1]M. Seeland, R. Rosch and H. Hoppe, Luminescence imaging of polymer solar cells: Visualization of progressing degradation, J. Appl. Phys. 109, p. 064513 (2011).
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Uncovering Some More Realities of Solvent-Additives in Organic Solar Cells.
Rakesh Mahadevapuram 1 , Yuqing Chen 2 , John Carr 2 , Kanwar Nalwa 2 , Sumit Chaudhary 1 2
1 Materials Science and Engineering, Iowa State University, ames, Iowa, United States, 2 Electrical Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractHigh boiling point solvent additives have been largely employed to improve the performance of polymer-fullerene based solar cells as an alternative to thermal annealing. Reports till date have ascribed the improved photovoltaic performance to better nanomorphology/phase separation. However, the influence of solvent additives on several device characteristics is still unknown, and several questions remain open. Some of them are: (1) How does solvent additives affect defect densities in polymer domains? (2) Are recombination rates/mechanisms affected? (3) What are the relative effects on horizontal and vertical phase separations? (3) Do the additives always have to be high-boiling-point solvents, or can low-boiling point solvents also help in some way? In this report, we attempt to answer these questions. Some of our key findings/observations are: (1) A low-boiling-point solvent additive led to a fill-factor of 68%, which is highest ever reported for unannealed polythiophene: fullerene solar cells with aluminum cathode. (2) There was a one-to-one correlation between hole mobilities of polymer and the fill factors. (3) Higher defect densities do not necessarily mean worse photovoltaic performance (4) Most popular solvent additives alkanedithiols are the toughest to reproduce because they are non-solvents for one of the species in the bulk-heterojunction blend, thus slight deviations in solution preparation and film deposition conditions can sometimes lead to very poor devices due to undesirable aggregation.
9:00 PM - H13.25
Organic Dye Design Tools for Efficient Photo-Current Generation in Dye Sensitized Solar Cells; Exciton Binding Energy and Electron Acceptor.
Bong-Gi Kim 1 , Chang-Gua Zhen 2 , EunJeong Jeong 2 , John Kieffer 2 , Jinsang Kim 1 2
1 Macromolecular Sci and Eng, University of Michigan, Ann Arbor, Michigan, United States, 2 Material Sci and Eng, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe generated photo-current in a dye-sensitized solar cell, a type of excitonic solar cell, is inextricably related with the charge dissociation energy which is quantitatively proportional to the exciton binding energy of the dye. In order verify this, the chemical structure dependent exciton binding energy of several pure organic dyes was explored using density functional theory, and its effect on light-to-electric energy conversion efficiency was investigated with dye sensitized solar cell devices. The input photon to current conversion efficiency measurement of applied dyes revealed that exciton binding energy and quantum yield are inversely correlated, implying that dyes having lower exciton binding energies more efficiently produce electric current from absorbed light. When a strong electron-accepting moiety was inserted in the middle of the dye framework, light-to-electric energy conversion behavior was unexpectedly deteriorated likely due to electron localization near the electron deficient group, which was verified by electronic structure calculations. Obtained results provide insights on organic dye design toward efficient photo-sensitizers for dye sensitized solar cell application. Finally, we validate this design principle by devising a new dye (EB-01) exhibiting over 9% power conversion efficiency.
9:00 PM - H13.26
Energy Level Modulation and Prediction in Conjugated Polymers for Organic Photovoltaic Application.
Bong-Gi Kim 1 , Yutaka Ie 2 , Chang-Gua Zhen 3 , Elizabeth Coir 3 , David Bilby 3 , John Kieffer 3 , Jinsang Kim 1 3
1 Macromolecular Sci and Eng, University of Michigan, Ann Arbor, Michigan, United States, 2 , Osaka University, Osaka Japan, 3 Material Sci and Eng, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractConjugated polymers (CPs) based organic photovoltaic cells (OPVCs) have drawn great attention as a cost-effective alternative to silicon-based solar cells. Since one of the fundamental requirements for photovoltaic energy conversion is adjusting the CP’s absorption range to match the high photon flux region of the solar spectrum, the optical band-gap of CPs in OPV devices is crucially important for efficient solar energy harvesting. In addition, lowering the highest occupied molecular orbital (HOMO) of CPs helps increase the open circuit voltage (Voc), yet the lowest unoccupied molecular orbital (LUMO) of the CP still must be higher than the acceptor’s LUMO for efficient charge dissociation. To investigate a way to precisely modulate the HOMO and LUMO of CPs, CPs having alternating donor/acceptor monomer units were synthesized. We fixed a donor monomer and changed acceptor monomers and investigated the correlation between the HOMO/LUMO of the monomer units and the resulting CPs. The stronger the electron withdrawing group in the acceptor molecule of CP was, the lower the HOMO and LUMO of the resulting CP were. However, the trend in the change of HOMO and LUMO level of the CPs was somewhat different. The LUMO level dropped more rapidly than the HOMO level, which ultimately resulted in a narrowed band-gap. Interestingly, the difference between the HOMO of CP and the average value of the HOMO of the donor and the acceptor was constant regardless of the difference in the monomers’ HOMO values. The same holds true for the LUMO as well. Considering that hybridization of each monomer’s molecular orbitals results in the CP’s energy levels within its effective conjugated length, we are likely to apply this method to anticipate a CP’s energy levels based on the monomers energy levels when the CP has very similar conformational geometry (and thus effective conjugation length).
9:00 PM - H13.27
Tailoring Hybrid Photovoltaic Morphologies: The Role of KCl in ZnO Growth and Device Performance.
Jonathan Downing 1 , Mary Ryan 1 , Natalie Stingelin 1 , Martyn McLachlan 1
1 Department of Materials Science and Engineering , Imperial College London, London United Kingdom
Show AbstractDevice heterostructures with nanostructured interfaces between inorganic and organic materials, so- called ‘hybrid cells’, have the potential to achieve improved efficiency whilst minimizing both material and processing costs. Desirable properties are obtained from each constituent. The metal oxide, in this case ZnO, has high electron mobilities and can be prepared with numerous morphologies. For organic materials, and in particular poly(3-hexylthiophene) (P3HT), excellent hole transport properties are combined with strong light-absorption. Large area deposition is possible through solution processing of both materials. Tailoring of ZnO nanorods morphologies is of interest for increased performance, which is thought possible by increasing the surface area of the inorganic structure and the ease with which it can be filled. This can be accomplished by respectively increasing the nanorods density and their alignment to the substrate. Law et al. [1] and Zhou et al. [2] previously reported on the addition of the surfactant polyethylienamine (PEI) to hinder nanorods growth in the lateral direction. We report on the use of ionic species, specifically KCl, to control growth in the dominant <002> direction. Rod dimensions have been quantified and a correlation between rod length, diameter and density with KCl concentration is observed. The adsorption of KCl to the polar (002) face affects rod morphology primarily by controlling nucleation, surface termination and via these factors can control growth. In particular use of ionic species to promote homogeneous nucleation on the surface, resulting in increased uniformity of nanorods length is investigated. It is seen that surface termination control is also possible because of the ability of ionic species to stabilise the polar (002) face. XRD data are presented and give a qualitative measurement of the alignment of these nanostructured films. Device performance of hybrid cells constructed with these films will be presented in order to show the effect of tailoring rod morphologies in hybrid photovoltaics. [1] Law, M. et al. Nat. Mater. 4, 455 (2005)[2] Zhou, Y. et al. Mater. Res. Bull. 43, 2113 (2008).
9:00 PM - H13.28
Correlating the Effect of Molecular Weight and Additives on Device Morphology and Performance of OPVs Based on Low Band Gap Polymer-Fullerene Blends.
Christopher McNeill 1 , Brian Collins 2 , Zhe Li 3 , Xiaoxi He 3 , Eliot Gann 2 , Cheng Wang 4 , Harald Ade 2
1 Materials Engineering, Monash University, Clayton, Victoria, Australia, 2 Physics, NC State University, Raleigh, North Carolina, United States, 3 Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractIn an effort to increase the solar absorption of organic photovoltaic devices (OPVs), new semiconducting polymers have recently been developed that exhibit a smaller electronic band gap relative to previous standard materials such as poly(3-hexylthiophene). The use of these ‘low gap’ polymers blended with the fullerene [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) have resulted in a dramatic improvement in solar power conversion efficiencies (PCE). However, the material parameters in these systems have not yet been systematically explored. In this study, we present the performance and morphology of devices based on the polymer poly[(4,4-bis(2-ethylhexyl)-cyclopenta-[2,1-b;3,4-b′]dithiophene)-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PCPDTBT) while varying the polymer’s molecular weight (Mw) and the presence of the additive 1,8-octanedithiol (ODT). We show that PCE is highly dependent on the Mw of the polymer with the highest Mw resulting in the best device performance. However, without the presence of ODT, Mw does not affect performance. These results are correlated with morphological studies conducted on identically prepared films using resonant soft X-ray scattering (RSoXS) and show that the device performance is linked to the formation of domains on the 100 nm length scale. The thermodynamic miscibility of PC71BM in PCPDTBT was also measured to be nonzero in this system, suggesting these domains may not be pure. The correlation between nanoscale and molecular morphology with device performance in this system will be discussed along with general implications for other silole-containing polythiophenes.
9:00 PM - H13.29
Synthesis and Photovoltaic Properties of Poly(dithieno[3, 2-b:2’, 3’-d]germole) Derivatives.
David Gendron 1 , Pierre-Olivier Morin 1 , Philippe Berrouard 1 , Nicolas Allard 1 , Badrou Reda Aich 1 2 , Christian Garon 1 , Ye Tao 2 , Mario Leclerc 1
1 Chemistry, Université Laval, Ste-Foy, Quebec, Canada, 2 , Institute of Microstructural Sciences, Natianol Research Council of Canada,, Ottawa, Ontario, Canada
Show AbstractSolar energy will play a bigger role in the coming years due to the abundance of the ressource. To adress the cost issue of this technology, plastic solar cells made of polymer bulk heterojunction has shown promising advance with power conversion efficiencies over 7% when blended with fullerene derivatives. Among these classes of polymers, many are composed of a dithienosilole, a benzothidiazole or a thieno[3,4-c]pyrrole-4,6-dione unit. Our group first reported germole derivatives based on fluorene unit.1 To further study the potential of germanium bridge atom, we synthesized a new analog of the dithienosilole with the atom of germanium, i.e., dithienogermole.To gain a better knowledge of this derivative, a series of three dithieno[3,2-b;2’,3’-d]germole-based copolymers have been synthesized and characterized. Comonomer units such as the benzothiadiazole and the thieno[3,4-c]pyrrole-4,6-dione have been utilized. Different polymerization methods have been investigated. The polymers were then characterized by size-exclusion chromatography, thermal analyses (TGA, DSC), and their optical and electronic properties were investigated by UV-vis absorption spectroscopy and cyclic voltammetry. These low bandgap polymers (1.3 to 1.7 eV) have also been tested for photovoltaic applications; the best result was achieved with poly[(4,4’-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-alt-1,3-(5-octylthieno[3,4-c]pyrrole-4,6-dione which shows a power conversion efficiency of 4.1 %.[1]Nicolas Allard, Réda Badrou Aïch, David Gendron, Pierre-Luc T. Boudreault, Christian Tessier, Salima Alem, Shing-Chi Tse, Ye Tao & Mario Leclerc, Macromolecules, 2010, 43, 2328.
9:00 PM - H13.3
Thiazolothiazole–Thiophene Copolymers for High-Efficiency Solar Cells; Control of Molecular Ordering and Orientation by Rational Design and Molecular Weight Optimization.
Itaru Osaka 1 2 , Masahiko Saito 1 , Hiroki Mori 1 , Tomoyuki Koganezawa 3 , Kazuo Takimiya 1
1 Applied Chemistry, Hiroshima University, Higashi-Hiroshima, Hirohima, Japan, 2 Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, Chiyoda-ku, Tokyo, Japan, 3 , Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo, Japan
Show AbstractWe examined a series of thiazolothiazole–thiophene semiconducting polymers, which have been originally developed for organic field-effect transistors by the author (Osaka), McCullough, and co-workers, as the p-type material for bulk heterojunction solar cells with PC61BM. With two thiophenes and one thiazolothiazole in the main chain and both branched and linear alkyl groups in the side chains, and thus compromised optical and physical properties, such as ionization potential, molecular ordering and orientational structures, and solubility, PTzBT-14HD showed the best power conversion efficiency, 3.2%, among the polymers examined. We then further worked on PTzBT-14HD, and found that the increase of molecular weight leads to high efficiencies of 5.7% with Mn = 33 kDa. Structural studies of the polymer thin films revealed that, in fact, the increase of molecular weight brought about high crystallinity and, interestingly, drastic changes in molecular orientation when blended with PC61BM. While in the polymer-only thin film, the polymer tended to orient in an edge-on manner, in the presence of PC61BM, the polymer showed a strong tendency to form a face-on orientation with the increase of molecular weight, which critically rationalizes the high efficiencies in the high-molecular weight polymers. These results demonstrate the importance of molecular design, by taking into the account the backbone units, the alkyl chain combination, and the molecular weight, for controlling the molecular ordering and orientation of semiconducting polymers in order to achieve high power conversion efficiencies in solar cells.
9:00 PM - H13.30
Systematic Process Development for Optimization of Manufacturable Organic Solar Cells.
Joseph Weiss 1 , Lei Zhu 1 , Babak Arfaei 1 , Peter Borgesen 1
1 System Science and Industrial Engineering, Binghamton University, Johnson City, New York, United States
Show AbstractTo steadily increase the efficiency of organic solar cells a systematic approach is needed instead of empirical trials. The vast number of factors, response variables, and factor levels makes a full factorial design of experiment unfeasible. Consequently, a systematic approach must be implemented to understand the phenomena at work and to predict the response as factors are changed. Only in this way can we move toward finding a true optimum instead of local optima. Although novel structures have shown promising efficiencies, a plan for manufacturability must always be kept in mind if these solutions are to have a meaningful impact instead of remaining just an academic curiosity. Our goal is to produce an approach that can be used to derive the best manufacturable cell in terms of efficiency, fabrication and installation cost, and reliability. In order to accomplish this, a detailed understanding of the charge generation process is needed as well as an in depth understanding of degradation and failure mechanisms of the cell. Each of the four basic steps of charge generation in organic solar cells must happen in series to produce electricity – absorption, exciton diffusion, charge transfer, and carrier collection. Analytical tools such as UV-vis, photoluminescence, lifetime measurements, mobility measurements, and current-voltage curves (I-V curve) can be used to independently assess a cell’s performance in each of these steps and optimize each step individually. We use these tools to determine the annealing conditions that optimize both the ability of excitons to reach an interface as well as the ability of separated charge carriers to exit the junction. A detailed discussion of the I-V curve explains the physical processes in the cell and shows how series and shunt resistances can be extracted and used to identify specific types of defects. Finally, the I-V curve is used to quantify the potential gains that can be achieved by removing these defects.
9:00 PM - H13.33
Influence of Interfacial Layers on the Performance of Bulk-Heterojunction Solar Cells.
Gopal Mor 1 , Enrique Gomez 1 2
1 Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania, United States, 2 Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractIn bulk-heterojunction solar cells, the efficient splitting of photo-generated excitons inside semiconducting polymers into free charge carriers, their transport along the respective hole and electron transporting materials, and finally their injection into the current collecting electrodes dictates solar cell performance. Thus, interfaces within organic solar cells can greatly affect the performance of devices. Through a combination of the shadow-Focused Ion Beam technique and energy-filtered transmission electron microscopy, we have examined the presence of wetting layers near the semiconductor-electrode interface of the active layer. The shadow-FIB technique enables characterization of the organic film without the extensive damage characteristic of FIB preparation protocols when employed on soft materials. By cross-sectioning devices, we also find that aluminum is preferentially oxidized at the organic semiconductor interface. In this poster, we will demonstrate how processing conditions alter the presence of wetting layers and aluminum oxidation, and in-turn, how these parameters affect device performance.
9:00 PM - H13.34
New Ways for the Mediation of the Charge Transfer Mechanism in P3HT:PCBM Bulk Heterojunction Solar Cells.
Pieter Robaeys 1 , Emilie Bourgeois 1 , Ken Haenen 1 2 , Kiran Kimura 3 , Kian Loh 3 , Milos Nesladek 1 2 , Jean Manca 1
1 Institute for Materials Research (IMO), Hasselt University, Diepenbeek Belgium, 2 IMOMEC, IMEC vzw, Leuven Belgium, 3 3Department of Chemistry, National University of Singapore, Singapore Malaysia
Show AbstractOrganic photovoltaic solar cells are attractive for indoor applications with current external quantum efficiency (EQE) over 7%. One of t the great challenges is the enhancement of the Open circuit voltage (Voc), controlling the EQE [1]. Graphene has attracted important attention as electrodes in photovoltaic solar cells. Several strategies including chemical coupling routes have been used to construct photovoltaic solar cell including graphene mixtures with inorganic and organic compounds. However one application not studied yet is to enhance the charge transfer mechanisms by incorporating grapheme in bulk heterojunctions solar cells.In our work we concentrated on one of the most studied systems P3HT: PCBM solar cells which reach high collection efficiencies [1]. We report on novel mechanism to enhance the Voc by using graphene as the charge transfer mediator. By using photocurrent methods, Photo thermal Deflection Spectroscopy (PDS) and Fourier Transform Photocurrent Spectroscopy (FTPS) we study the charge transfer mechanism in these organic photovoltaic structures. These techniques are known for high sensitive spectral measurements of the optical absorption. The photocurrent could be measured over 6 orders in magnitude. We could clearly establish that graphene addition is controlling the shape of the charge transfer band leading for specific concentrations to enhanced Voc. [1] Vandewal K, Tvingstedt K, Gadisa A, , Nature Materials 8, p. 904-909 (2009).
9:00 PM - H13.35
Surface Contact Modeling in Hybrid Organic/Inorganic Solar Cells and Light Emitting Devices.
Tiffany Tong 1 2 , Jing Du 1 3 , Androniki Tsakiridou 4 3 , Wali Akande 1 2 , Asuo Mawusi 5 , Wole Soboyejo 1 3 5
1 Princeton Institute for the Science and Technology of Materials, Princeton Unviersity, Princeton, New Jersey, United States, 2 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Mechanical and Aerospace Engineering, Princeton Unviersity, Princeton, New Jersey, United States, 4 Picker Engineering Program, Smith College, Northampton, Massachusetts, United States, 5 Materials Science and Engineering, African University of Science and Technology, Abuja, Federal Capital Territory, Nigeria
Show AbstractThis work presents the results of a combined experimental and theoretical study of surface contacts between layers that are relevant to hybrid organic/inorganic solar cells and light emitting devices. Cantilever beam theory is used to model material deformation over particles at the interface to study the effects of dust particles introduced into the devices through processing techniques such as soft contact lamination. Atomic force microscopy (AFM) measurements are incorporated to measure the effects of adhesion between the layers on surface contact profiles. Finally, the impact of TiO2 nanoparticles in the organic polymer active layers on mechanical and electrical device properties is measured through a combination of nanoindentation and surface resistivity measurements and void distribution modeling. The implications are then discussed for the design of robust organic and hybrid organic/inorganic electronic structures.
9:00 PM - H13.36
Enhanced Chlorophyll A Purification and Dye Sensitized Solar Cell Performance.
Komal Magsi 1 , Charles Fortmann 1
1 , Stony Brook University, Commack, New York, United States
Show AbstractDye-sensitized solar cells (DSSC) may provide an economical alternative to the present p–n junction photovoltaic devices. In contrast to these present commercial photovoltaic systems, where the semiconductor assumes the task of light absorption, charge carrier transport, and carrier collection, the functions are completely separate in DSSC’s. In DSSC’s light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge separation takes place at the interface through photo-induced electron injection from the dye into the conduction band of the solid. Various light sensitizers have been studied throughout the world; the use of chlorophyll is the focus of this work. While chlorophyll DSSC’s have previously been reported here, the relationship between purity of chlorophyll and photovoltaic performance is probed. The impure form of chlorophyll produce photovoltaic efficiencies of .2%, under simulated AM 1.5 sunlight. Highly pure forms of chlorophyll a produce much greater efficiencies (closer to that of .5%) with noticeable and significant shifts in spectral response. The mechanisms used to separate and purify chlorophyll a from spinach leaves will be described.
9:00 PM - H13.37
Organic Multi-Chromophore Systems for Photovoltaics.
Meng Guo 1 , Theodore Goodson 1
1 , University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOrganic photovoltaics have been actively investigated due to the rising demand for efficient and sustainable energy generation and storage. Recent developments in this field have enabled a realization of high efficiency of 6-8% in OPVs by reducing the optical bandgap of the semiconducting polymers and optimizing the morphology of donor-acceptors. However, there is still a big gap from the theoretical value and some of these organic optical limiting systems have only limited applications in near IR and IR range. In this study, we took the advantage of long range interactions in certain organic multi-chromophore systems to enhance the power conversion efficiency and broaden the absorption of solar spectrum. We also developed a series of ultrafast spectroscopic techniques to investigate new physical parameters related to energy and electron transfer in these materials, addressing a major challenge faced by commercially available organic light harvesting systems: the limitation in the achieved power conversion efficiency and the broad-band absorption of the solar spectrum. This deep physical understanding is crucial in producing energy efficient organic photovoltaic devices.
9:00 PM - H13.38
Development of Microwave Methods for Processing Electrodes in Dye Sensitized Solar Cells.
Robert Cotta 1 , Jeffrey Allen 1 , Eden Couillard 1 , Travis Cournoyer 1 , Cliff Timpson 1 , Clifford Murphy 1
1 Chemistry and Physics, Roger Williams University, Bristol, Rhode Island, United States
Show AbstractMicrowave synthetic chemistry methods have been shown to be effective at accelerating reactions and reducing their environmental impact, but are not generally known to be appropriate for conductive surfaces. We believe we have developed a solution to safe application of microwave chemistry to conducting surfaces (U.S. Patent No. 61/407,071, pending) and have applied these methods to the silanation of ITO and FTO transmissive electrode surfaces and to reactions that covalently couple model ruthenium complex dyes to these electrode surfaces. Electrode surfaces are first hydroxylated under basic pH conditions, and then silanated to produce surfaces with amine, ethynyl, or pyridine functionality for use in coupling reactions. The ruthenium complexes synthesized for this investigation include 2,2’,2”-terpyridyl-2,2’-bipyridylchlororuthenium (II), trans-chloronitrosyltetrapyridylruthenium (II), and cis-bis-4,4’-dicarboxylixacid-2,2’-bipyridyldithiocyanatoruthenium (II), the latter being the well-characterized Graetzel photoharvesting dye included as a benchmark compound. Functionalized surfaces are characterized by contact angle, cyclic voltammetry, and UV-Vis absorption measurements. Functionalized electrodes produced both by conventional heating and microwave methods were incorporated into dye sensitized solar cells (DSSCs) using commercially available kits from Solaronix SA to compare photocurrents for materials processed by either heating method.
9:00 PM - H13.39
Efficiency Enhancement of Inverted Low-Bandgap Polymer Solar Cells.
Kirill Zilberberg 1 , Sara Trost 1 , Ines Dumsch 2 , Sibylle Allard 2 , Ullrich Scherf 2 , Andreas Behrendt 3 , Dirk Luetzenkirchen-Hecht 3 , Ronald Frahm 3 , Thomas Riedl 1
1 Electrical Engineering, University of Wuppertal, Wuppertal Germany, 2 Macromolecular Chemistry, University of Wuppertal, Wuppertal Germany, 3 Physics of Condensed Matter, University of Wuppertal, Wuppertal Germany
Show AbstractConventional organic solar cells (OSCs) are based on a layer sequence with the anode adjacent to the transparent substrate. There may, however, be serious reasons to flip the cell layout to an inverted structure, where the bottom electrode is the cathode. In bulk-heterojunctions where the concentration of the acceptor species is increased towards the substrate, electron extraction via the bottom electrode will lead to higher power conversion efficiencies (PCEs), thus favoring an inverted cell.[1] Importantly, the inverted cell architecture may be desirable from a manufacturing point of view and in terms of improved device lifetime. By reversing the polarity of charge collection, air-stable high work-function (WF) electrodes can be used on top of the device instead of an air-sensitive cathode. Furthermore it has been shown that inverted cells are also a key to next generation semi-transparent OSCs.[2] Low band-gap donor polymers not only pave the way towards highly efficient OSCs but also allow for semi-transparent cells with favorable color-perception.[3] For P3HT:PC60BM, OSCs in both conventional and inverted architecture similar PCEs have been verified. Comparative studies for low band-gap polymer:fullerene devices in conventional and inverted geometry are still very limited. Here we study OSCs based on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) as donor and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) as acceptor. In conventional cells without any processing additives, we obtain a high Voc of 0.67 V and a PCE of 3.5% (under AM1.5, 100 mW/cm2) in agreement with previous work.[4] As an anode interlayer for improved hole extraction we used high WF sol-gel processed V2O5 instead of PEDOT:PSS.[5] Strikingly, in inverted cells using metal-oxide interlayers for both electron and hole extraction, a higher PCE of 4.3% was obtained. This is the highest PCE for inverted cells reported for the PCPDTBT:PC70BM system. Voc = 0.67 V and FF = 45% are almost the same in conventional and inverted structures. A higher short circuit current (Isc = 14.4 mA/cm2) in the inverted cells as opposed to Isc= 12.2 mA/cm2 in conventional cells is the main reason for the improved PCE. The results will be discussed in the context of depth resolved concentration analysis using XPS and optical simulation of the device stack. [1] L. M. Chen, Z. R. Hong, G. Li, and Y. Yang, Adv Mater 21, 1434 (2009).[2] H. Schmidt, H. Flugge, T. Winkler, T. Bulow, T. Riedl, and W. Kowalsky, Appl Phys Lett 94, 243302 (2009).[3] T. Ameri, G. Dennler, C. Waldauf, H. Azimi, A. Seemann, K. Forberich, J. Hauch, M. Scharber, K. Hingerl, and C. J. Brabec, Adv Funct Mater 20, 1592 (2010).[4] D. Muhlbacher, M. Scharber, M. Morana, Z. G. Zhu, D. Waller, R. Gaudiana, and C. Brabec, Adv Mater 18, 2884 (2006).[5] K. Zilberberg, S. Trost, H. Schmidt, and T. Riedl, Advanced Energy Materials 1, 377 (2011).
9:00 PM - H13.4
Plasmonic Electrodes for Organic Photovoltaics.
Deirdre O'Carroll 1 2 , Divya Vijapurapu 1 , Gary Cheung 1 , Kenan Gebizlioglu 1
1 Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Chemistry and IAMDN, Rutgers University, Piscataway, New Jersey, United States
Show AbstractArrays of plasmonic nanostructures incorporated into organic conjugated polymer thin films can render metallic electrodes both electrically and optically active and promote a macroscopic optical antenna effect in organic semiconductor devices. Recently, noble metal nanoparticles which support surface plasmon resonances have been incorporated into bulk-heterojunction solar cells, predominantly as passive optical elements dispersed randomly in the active layer. Here, we investigate the use of active plasmonic nanoparticle array electrodes on the optical characteristics of polythiophene semiconductor thin films and photovoltaic devices both experimentally and theoretically. The advantages of this approach are: (1) Plasmonic nanoparticles exhibit strongly localized surface plasmon resonances, and, when designed to resonate in the frequency range of the semiconductor absorber, can enhance the light absorption rate. (2) Nanostructured metal electrodes can act as backscatterers to increase the effective light absorption depth. (3) Additionally, the electrode interfacial area may be increased by up to 50 % compared to planar electrodes. The plasmonic electrodes are fabricated by evaporation of silver through nanoporous alumina masks, and subsequent removal of the alumina [1]. The evolution of reflected light spectra from the devices, as a function of nanoparticle height is studied experimentally and using electromagnetic finite-difference-time-domain simulations. Absorption in the active layer is enhanced by factors of 5-10 in the 570 to 750 nm wavelength range at the red of the polythiophene absorption band using nanoparticles with height:diameter aspect ratios of greater than 1. Additionally, the enhanced absorption leads to improved red response in the photocurrent spectra of fabricated test devices.[1] D. M. O’Carroll, A. X. Collopy, V. E. Ferry, H. A. Atwater. Surface Plasmon Assisted Absorption in Conjugated Polymer Thin Films and Devices. 25th EU PVSEC / WCPEC-5 Proc. 834-837 (2010).
9:00 PM - H13.40
Fabrication and Characterization of OPV Devices from Copolymers Based on Thieno[3,4-c]pyrrole-4,6-dione Derivatives.
Ahmed Najari 1 , Philippe Berrouard 1 , Serge Beaupre 1 , Yingping Zou 1 , Jean-Remi Pouliot 1 , Mario Leclerc 1
1 Chemistry, Université Laval, Québec, Quebec, Canada
Show AbstractPolymer bulk heterojunction (BHJ) solar cells offer a compelling option for tomorrow’s photovoltaics due to interesting features such as low cost, lightweight and flexibility. Although such solar cells have witnessed significant advances during the past few years, the efficiency, stability, processability, and fabrication cost still need to be improved for large scale production and commercialization. Both materials and device designs play important roles in realizing such applications.We will present several combinations of light-absorbing conjugated copolymers as the electron donor and fullerene derivatives as the electron acceptor that have been developed to fabricate bulk heterojunction (BHJ) solar cells. In our research for new electron withdrawing units, new thiophene derivatives (thieno[3,4-c]pyrrole-4,6-dione based (TPD) copolymers seem particularly promising. It is important to note that further improvements are possible from different device configurations such as annealing, solvents, additives, electrodes, acceptors, etc. More importantly, this work shows that the TPD unit and related derivatives can be efficiently utilized to tailor the optical and electronic properties of conjugated polymers and may therefore become quite simple and useful comonomers for the future design of electroactive and photoactive polymers for photovoltaic devices and for many other applications in plastic electronics.
9:00 PM - H13.41
Developing Oxide:Organic Hybrid Photovoltaics.
Joseph Franklin 1 3 , Jonathan Downing 1 3 , Robert Hewlett 1 3 , Thomas Anthopoulos 2 3 , Natalie Stingelin 1 3 , Mary Ryan 1 , Martyn McLachlan 1 3
1 Materials, Imperial College London, London United Kingdom, 3 Centre for Plastic Electronics, Imperial College London, London United Kingdom, 2 Physics, Imperial College London, London United Kingdom
Show AbstractThe development of hybrid (oxide:organic) photovoltaic devices (h-PV) is an attractive prospect, owing to the suitability of such devices for high-throughput, low-cost solution processing. Despite their promise the development of such devices has been slow - the typical efficiencies (< 0.5%) are significantly lower than even modest organic photovoltaics (OPVs). Our efforts to overcome current performance limitations have been focused on two complementary approaches, i) preparation of highly crystalline planar heterojunctions and ii) preparation of controlled interdigitated architectures.i) Using pulsed laser deposition (PLD) we have been able to deposit highly crystalline ZnO on rigid and flexible substrates at temperatures as low as 50°C. The low temperature processing has allowed the preparation of normal and inverse device architectures based on ZnO and a range of polymers (P3HT, P3HS) and small molecules (pentacene). Additionally the planar devices have allowed charge transfer at the organic:oxide interface to be studied effectively for the different compositions, allowing correlation between device performance, composition and configuration to be made unambiguously.ii) Using a modified hydrothermal growth method we have successfully formed a range of ZnO nanorods in which the aspect ratio has been controllably varied. We report the preparation of highly interpenetrating networks using these rods and P3HT infiltrated using both conventional solution and novel solid state processing techniques. The role of rod length/spacing in addition to infiltration method on measured device performance will be discussed in addition to limitations of the processing encountered during our studies.For both families of structures we describe the changes in measured device performance observed as structure and composition are varied. We pay particular attention to the bulk and interfacial properties of the oxide:organic components. The measurements are supported by detailed structural (SEM, AFM, XRD) and electronic (mobility, resistivity) characterization of the individual layers and (where appropriate) the complete device structures
9:00 PM - H13.42
Revealing the Pros-and-Cons of Different Annealing Treatments in Polymer Solar Cells.
Yuqing Chen 1 , Rakesh Mahadevapuram 2 , Kanwar Nalwa 1 , Sumit Chaudhary 1 2
1 Electrical and Computer Engineering, Iowa State University, Ames, Iowa, United States, 2 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractTo improve performance of polymer bulk-heterojunction solar cells, solvent annealing and thermal annealing (both pre and post-production) are one of the primary ways to optimize nanoscale morphology of the active-layer, polymer crystallinity etc.. However, these promising and rival treatments have not been compared comprehensively, let alone rather paucity of information about their individual pros and cons on a single device architecture. In this experiment, solvent annealing, pre-production thermal annealing and post-production thermal annealing were investigated, and their effects were compared. Photovoltaic current-voltage characterization was combined with capacitance/impedance measurements, as well as other structural and optoelectronic characterization to reveal precise correlations and anomalies among various device parameters. Our key observations are: (1) devices have higher open circuit voltage under post-production thermal annealing than any other types of annealing. (2) There’s no best annealing type for short circuit current and fill factor, which gave different results among several sets of devices. (3) Higher defect density does not necessarily mean worse photovoltaic performance. Above conclusions are based on our statistical study on several sets of devices spread out across a time scale of several months, and several devices within each set.
9:00 PM - H13.43
Nanostructured Electron Blocking Layer in Organic Photovoltaic.
Htay Hlaing 1 2 , Xinhui Lu 1 , Danvers Johnston 3 , Chang-Yong Nam 3 , Charles Black 3 , Benjamin Ocko 1
1 Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, New York, United States, 2 Physics, Stony Brook University, Stony Brook, New York, United States, 3 Center for Functional Nanomaterial, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractOne of the promising routes to increase the power conversion efficiency in organic photovoltaic (OPV) devices is to employ high surface area nanostructured electrodes [1]. However, nanostructuring transparent electrodes (gratings, pillars, etc.) on indium tin oxide (ITO) requires complex fabrication procedures [1,2] and under these conditions it is difficult to maintain highly conductive transparent films. Rather than structuring the ITO, we have nanostructured the electron-blocking layer (EBL), deposited directly on the ITO, through the use of nano-imprint lithography. OPV devices were prepared by (1) deposition of an EBL layer on ITO, (2) nano-imprinting the EBL, (3) deposition of polymer/fullerene blends, and (4) deposition of an aluminum layer. The imprinting of the EBL (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) was carried out using a silicon master template (grating profile) with periodicities ranging from 80 nm to 160 nm. The excellent fidelity of the pattern transfer from the master template to EBL was confirmed by grazing incidence small-angle x-ray scattering measurements. We present the results of grazing incidence wide-angle x-ray scattering investigations on nano-imprinted EBL layers, filled with blended materials, in order to understand the molecular packing of the polymer in this geometry. Specifically, we show how the nano-imprinted EBL layer affects the as-deposited polymer orientation and the crystallization of PCBM and contrast these results with the same materials prepared in the planar geometry. Results will be presented for P3HT and PCDTBT, conjugated polymers with very different structural properties, and these will be correlated with OPV device measurements.[1] David R., et al., “Indium tin oxide nanopillar electrodes in polymer/fullerene solar cells”, Nanotechnology 22(8), Feb 2011[2] Yasuyuki Akita, Yuki Sugimoto, Masahiro Mita, Hideo Oi, Osami Sakata and Mamoru Yoshimoto, "Fabrication of nanostructured ITO thin films on nanoimprinted glasses by pulsed laser deposition", Proc. SPIE 7201, 72011A (2009)
9:00 PM - H13.44
Using Self-Assembly of Plymer Blends to Engineer Heterodyne Junction Photoelectric Devices.
Joseph Ortiz 1 , Xu Di 1 , Dilip Gersappe 1 , Miriam Rafailovich 1
1 Department of Materials Science & Engineering, Stony Brook University, Stony Brook, New York, United States
Show AbstractEngineering heterodyne junction solar cells requires precise positioning of the photoactive polymers and the PCBM conductors such that maximum current reaches the electrodes with minimal resistive scattering. One possible method for accomplishing this may be to use polymer phase segregation in combination with the nanoparticles’ natural segregation to the interfaces. In this manner, large-scale devices can be formed using self-assembly methods, rather than fixed methods. We have used Molecular Dynamics simulation to predict the morphology of polymer blends and determine which combination of factors would yield the optimal cylindrical pattern which would contact the electrodes, while producing the largest number of interfaces. Secondly, we were also able to determine the conditions that would cause the particles to segregate and template along the interfaces, which would provide direct conductivity to the electrodes. Using thin film and bulk structures and by manipulating particle size, the attraction between the particle and the polymer component, and the amount of filler within the material, we can explore the formation of cheaper, more effective and efficient networks.
9:00 PM - H13.45
Fused Donor-Acceptor-Donor Oligomeric Dyes for DSSC’s.
Coralie Richard 1 , Dinesh Patel 1 , Arroyave Frank 1 , Leandro Estrada 1 , Kirk Schanze 1 , John Reynolds 1
1 Department of Chemistry, University of Florida, Gainesville, Florida, United States
Show AbstractOrganic dyes for dye sensitized solar cells (DSSC’s) are usually based on the same architecture: a push-pull or donor - π spacer - acceptor structure. The photophysics of these systems has been extensively studied and much information on the electron injection from the dye into the titanium dioxide conduction band has been gleaned. To better understand the electron injection process, we present here an innovative dye architecture. We have synthesized a family of Donor- Acceptor- Donor (D-A-D) dyes that are centered on a fused donor-acceptor core (A) sandwiched between two donor moieties (D). The donor moieties are hexyl thiophene. For the cores, we present two dyes centered on phenanthrene derivatives and two dyes composed of benzodithiophene isomers. The position of the donors is also varied to provide different conjugation pathways. These pathways are shown by visualizing the HOMO and LUMO surfaces. We also observe the delocalization of the electron density along the donors for the HOMO and of the electron density along the acceptor for the LUMO. Spectroscopic studies demonstrate that the addition of donor molecules leads to bathochromic shifts of the absorption maximum with extinction coefficients in excess of 40 000 M-1 cm-1. Electrochemical data shows that adding the donor moieties reduces the energy gap by raising the HOMO level. In addition, the LUMO levels of the dyes under study are above the titanium dioxide conduction band and should permit facile electron injection. A full photophysical characterization of these dyes will be presented.
9:00 PM - H13.47
Novel Cruciform Oligothiophenes as Monodisperse Electron Donors for Organic Solar Cells.
Peter Skabara 1
1 Pure and Applied Chemistry, University of Strathclyde, Glasgow United Kingdom
Show AbstractRecently, we have synthesised a series of well-defined, monodisperse oligothiophenes bearing a bridging, fused tetrathiafulvalene (TTF) unit. The materials exhibit complex redox chemistry for the longer chain analogues and they can be oxidised up to the octacation within a relatively narrow potential window. The methodology used to construct the end-capped oligothiophene chains has enabled us to explore a new range of Ge centred cruciform structures, in which the central bridging atom of the molecule provides a tetrahedral geometry. Previously, it has been suggested that cruciform structures of conjugated chains discourage aggregation and are therefore of interest as highly luminescent materials. However, in our example we observe strong intermolecular interactions between the oligothiophene Ge cruciforms. This has led to the development of organic solar cells with good efficiencies and charge transport properties from the electron donor. In this presentation we report on the synthesis, self-assembly and application of the cruciforms in organic solar cells.
9:00 PM - H13.48
Characterization of UV-Assisted Chemical Vapor Deposited TiO2 Thin Films in Inverted Bulk Heterojunction Organic Solar Cells.
Kai-Lin Ou 1 , Dlevin Tadytin 1 , Diogenes Placencia 1 , Xerxes Steirer 1 , Lingzi Sang 1 , Neal Armstrong 1 , Jeanne Pemberton 1
1 Biochemistry and chemistry , University of Arizona, Tucson, Arizona, United States
Show AbstractMetal oxide interlayers are known to adjust the energy barrier for charge transfer and serve as a blocking layer for carriers of one type between active layers and electrodes in organic photovoltaic devices(OPV). We present a novel, low temperature UV-assisted chemical vapor deposition (UV-CVD) process to create thin film TiO2 interlayers, with easily tunable thickness, as electron transport interlayers (ETL) on indium tin oxide (ITO) electrodes. We utilize these conformal interlayers in inverted bulk heterojunction (BHJ) OPV devices, and in diode-like platforms, to test the effects of these interlayers on charge selective electron-harvesting and on OPV device performance. TiO2 thin films are deposited using titanium isopropoxide (TTIP) as the precursor material. The decomposition of TTIP molecules to form the oxide is accelerated by UV exposure and by thermal effects. The resultant film qualities are tested on both rigid and flexible substrates. In this report, we deposit thin films of TiO2 from room temperature to 240 oC, and demonstrate that our UV-CVD process enables the deposition of metal oxides on flexible substrates, which may be suitable for roll-to-roll OPV fabrication processes. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) cross section studies show TiO2 films are conformal films on ITO. X-ray photoelectron spectroscopy (XPS) confirms that oxide films deposited at all temperatures is stoichiometric TiO2, but at low process temperatures we see inclusion of carbonaceous materials as well. These TiO2 thin films show good hole-blocking as studied by solution probe molecule electrochemistry, with no observed leakage current within TiO2 band gap region demonstrating that pinhole-free thin films are obtainable. The crystallinity of this TiO2 film is tuned from amorphous to anatase by increasing the substrate temperature from room temperature to 240 oC, which is still lower than the temperatures required for anatase phase TiO2 formation (>300 oC) in other processes. The individual UV-assisted and -thermal decompositions are investigated by polarization-modulation infrared reflection- absorption spectroscopy (PM-IRRAS). The characteristic vibration intensities of C-H, C-C on TTIP isopropyl groups enable us evaluate the decomposition yield from UV and thermal treatments. Inverted BHJ device performance is enhanced due to the UV-CVD TiO2 ETLs with configuration: ITO/(TiO2)/P3HT:PCBM/MoO3/Ag. Optimized device measured under AM 1.5G simulated sunlight show increased short circuit current, open circuit voltage compared to device without TiO2 film interlayers.
9:00 PM - H13.49
Plastic-Syringe Induced Silicone Contamination in Organic Photovoltaic Fabrication: A Second Look at Its Implications on Device Performance.
John Carr 1 , Kanwar Nalwa 1 , Rakesh Mahadevapuram, 1 , Yuqing Chen 1 , Sumit Chaudhary 1 2
1 Electrical and Computer Engineering, Iowa State University, Ames, Iowa, United States, 2 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractIn the global search for clean and sustainable energy sources, organic photovoltaics (OPVs) have recently gained much attention. Many facets of OPVs make them an advantageous option for the realization of green-power generation. Their solution processability tops this list, allowing for simple, low-cost fabrication techniques such as spin-casting. An inevitable part of spin-casting fabrication is the use of syringes to filter and transfer polymer solution onto a substrate for spinning. Many commercially available disposable syringes are lubricated with polydimthylsiloxane (PDMS) or other silicone based polymers, which can be easily dissolved by organic solvents and introduced into the system. A recent work by Graham et al. has shown that devices fabricated with silicone containing plastic-syringes exhibit higher power conversion efficiencies (PCEs) than those fabricated with glass syringes. With further quantification, Graham et al. showed that a deliberate 0.50 mg/mL additive of PDMS can induce a PCE improvement of ca. 72%. Here, we confirm this silicone based contamination in plastic-syringe fabricated devices and we take a second look at its implications on OPV performance. Using x-ray photoelectron spectroscopy (XPS), we find this contamination to aggregate near the surface; with ca. 8.00 ± 1.00% atomic concentration of silicone within the film’s top 10nm and < 2% throughout the bulk. In contrast to the original work, we find silicone deficient (i.e. glass-syringe fabricated) devices spun at 600rpm produce 32.50 ± 8.00% better devices when compared to silicone rich (i.e. plastic-syringe fabricated) devices of the same recipe. Detailed analysis of the ‘glass’ and ‘plastic’ films reveals the simple conclusion that glass-syringe fabrication generates a 21.50 ± 1.50% thicker film, which leads to greater absorption in these devices. We find the introduction of silicone changes the hydrophilicity of the system, increasing the wettability of plastic-syringe handled solution on PEDOT:PSS, which generates a thinner film on spinning. Upon spinning at faster speeds (800rpm and 1,000rpm), we find both glass and plastic devices produce similar film thicknesses and PCE performances. This, coupled with data from other characterization techniques (e.g. Raman spectroscopy, atomic force microscopy, dark IV, trap density of states, etc.) allows us to logically conclude that the silicone contamination affects very little aside from film thickness.
9:00 PM - H13.5
Synthesis, Characterization and Performance of Poly(dithieno[3,2-b:2’,3’-d]thiophene) Derivatives in Photovoltaic Cells.
Jean-Remi Pouliot 1 , Serge Beaupre 1 , Mario Leclerc 1
1 , Université Laval, Québec, Quebec, Canada
Show AbstractHarvesting energy directly from sunlight using photovoltaic cells is a very important way to utilize renewable energy and to address the environmental pollution. Conjugated polymers as electron donor in photovoltaic cells are being extensively studied due to their low-cost solution processing as compared to conventional inorganic semiconducting materials. The push-pull concept is a good way to achieve a significant decrease of polymer bandgap. Many examples of excellent electron-rich moieties consisting of fused heterocycles have been reported. Fused thiophene rings are well known to have planar, rigid and conjugated structures allowing good π-π staking. This feature enhances the charge transport resulting in better hole or/and electron mobility. Furthermore, this class of molecule shows better chemical stability that their acene counterpart. In our research, we investigated the copolymerization of 3,5-dialkyl-dithieno[3,2-b:2’,3’-d]thiophenes (DTT) with thieno[3,4-c]pyrrole-4,6-diones (TPD). We will show several combinations of light-absorbing conjugated copolymers as the electron donor and fullerene derivatives as the electron acceptor to fabricate bulk heterojunction (BHJ) solar cells. A careful study on each polymer led us to a better understanding of how these new polymers interact in blends with the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to give the resulting PCE.
9:00 PM - H13.50
Effect of Cathode Metal Evaporation Rate on the Deep Trapped Hole Formation in Bulk Heterojunction Organic Solar Cells.
Emre Yengel 1 , M. Saif Islam 1
1 Department of Electrical and Computer Engineering, UC Davis, Davis, California, United States
Show AbstractIn bulk heterojunction organic solar cells, open circuit voltage (Voc) is mainly dependent on the lowest unoccupied molecular orbital and the highest occupied molecular orbital of the donor/acceptor polymer pair in the active layer. However, there are other factors that contribute considerable reduction in the Voc. The active layer/cathode interface is the one of these factors. Previous studies show that e-beam evaporation of the cathode metal contact forms deep interface trap holes in the active layer which increases the open circuit voltage of the solar cells. Although these studied show the effect of these deeply trapped holes on the Voc, the mechanism behind this effect is not well understood. In this work, the effect of cathode contact annealing rate on the overall efficiency is studied. Three different devices are fabricated with varying evaporation rates of 0.1A/s, 1A/s and 5A/s. The results show that at low evaporation rates, cathode material atoms lack adequate energy to form deep trap holes. Also, above a certain value, the evaporation rate does not have a significant effect on the deep trap hole formation.
9:00 PM - H13.52
Synthesis of Conjugated Polymers by Direct Arylation.
Philippe Berrouard 1 , Chiara Ottone 1 , Mario Leclerc 1
1 Chemistry, Université Laval, Quebec, Quebec, Canada
Show AbstractThe development of efficient carbon-carbon bonds forming reactions for organic synthesis such as Suzuki and Stille cross-coupling has allowed the synthesis of new molecules by reducing the length of the synthetic route and increasing, , in some case, the reaction yields. Those reactions also had a huge impact on macromolecular chemistry, especially in the case of the synthesis of conjugated polymers. Despite their numerous advantages, both Stille and Suzuki cross coupling have some drawbacks such as the formation of toxic byproducts or in some case instability of the organometallic intermediates. Recently the development of reactions called direct arylation[1], using palladium chemistry similar as Suzuki and Stille reactions, are the subject of much attention. Those reactions can allow the formation of carbon-carbon bond between aromatics units with activated hydrogens without the use of organometallic intermediates. Actually, these reactions are mostly developed for the synthesis of small molecules. Only few works report the use of direct arylation as polymerization reaction [2][3]. Our recent work focuses on the development catalytic systems for the synthesis of conjugated polymers.To achieve efficient polymerization by direct arylation, different conditions were tested on several substrates. We optimized several parameters such as catalyst, catalyst load, ligand, solvent, concentration, and temperature. Those parameters are known to influence the molecular weight distribution in classical polymerisation methods. We synthesized and fully characterised several polymers using direct arylation and compared them with their structural homologue synthesized by Stille or Suzuki cross-coupling polycondensation.[1] G.P. McGlacken, L. M. Bateman, Chem. Soc. Rev. 2009, 38, 2447.[2] Q. Wang, R. Takita, Y. Kikuzaki, F. Ozawa, J. Am. Chem. Soc., 2010, 132, 11420.[3] W. Lu, J. Kuwabara, T. Kanbara, Macromolecules, 2011, 44, 1252.
9:00 PM - H13.53
The Role of Interactions between Polymers and Fullerene Derivatives on Transport Properties.
Rakhee Pani 1 , Benjamin Bond 1 , Yaroslava Yingling 1
1 , North Carolina State University, Raleigh, North Carolina, United States
Show AbstractOrganic photovoltaic devices, which are usually based on a blend of a conjugated polymer and a fullerene derivative, offer a promise of creating lightweight and low-cost solar cells.It is widely accepted that polymer film morphology is the key that determines photovoltaic properties and controls the efficient charge transport and device efficiency of the solar cell. Further improvements in the performance of organic solar cells require a better understanding of the mechanisms of diffusion and molecular rearrangement. In order to address the effect of chemical interactions on transport properties within these polymer nanocomposites, we applied molecular dynamics simulations to two model system of poly (3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT)with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and fullerene (C60).Our study examined the interfacial properties of PCBMand C60 on [100] and [010] surfaces of P3HT and PBTTT crystal.Simulations reveal that the decrease of the PCBM diffusion is mainly attributed to the efficient interactions between PCBMs and aromatic P3HT backbone. Also PCBM has a higher diffusion coefficient in crystalline P3HT as compared to C60, due to the decrease in binding energy of PCBM with P3HT. In the second model the PCBMs get intercalated between the arms of PBTTT, as suggested by previous experiments with increase in lattice spacing of PBTTT. Overall, a thorough understanding of the diffusion factors and quantification of the molecular mechanics energy will promote better morphological control in polymer solar cells leading to improvements in efficiency.
9:00 PM - H13.54
Phase Morphology in Poly(thiophene)-Fullerene Thin Film Devices.
Nabankur Deb 1 , David Bucknall 1 , Max Skoda 3 , Bobby Sumpter 2 , Alamgir Karim 4
1 Materials Science and Engineering, [email protected], Atlanta , Georgia, United States, 3 ISIS Facility, STFC, Didcot United Kingdom, 2 CNMS, ORNL, Oak Ridge, Tennessee, United States, 4 Polymer Engineering, University of Akron, Akron, Ohio, United States
Show AbstractOrganic or polymer based photovoltaic devices promise solar technologies that are inexpensive enough to be widely exploited and therefore provide a significant fraction of the Nation’s future energy needs. Nanoscale heterojunction systems consisting of fullerenes dispersed in conjugated polymers are promising materials candidates for achieving high performance devices. The fullerenes can be functionalized to change their electronic properties or to alter their interactions with the polymers. The morphologies in thin films is directly related to the interaction strength between the polymer and fullerene, degree of crystallinity, choice of processing conditions and substrate surface energy and roughness. These parameters provide huge degrees of versatility to the behavior of the system and yet produce significant challenges to the rational exploration of these materials because the number and types of possible variations are far too large to investigate without predictive models or well-established data and guidelines to expected behaviour.In order to understand the phase behaviour in these devices we are using multi-scale molecular modelling coupled with neutron reflection to determine the behavior of model conjugated polymer-fullerene mixtures. Neutron reflection is particularly useful for these types of thin film studies since the fullerene generally have a high scattering contrast with respect to most polymers. We are studying model bulk heterojunction (BHJ) films based on mixtures of poly(3-alkyl thiophene)s (P3BT, P3HT and P3OT) and different fullerenes (C60, PCBM and bis-PCBM). We have used neutron reflection measurements to determine film morphology normal to the film surfaces in real device configurations. The novelty of the approach over previous studies is that the BHJ layer is sandwiched between a PEDOT/PSS and Al layers. Using this model systems, we have measured the effect of typical thermal annealing processes on the film development as a function of the polythiophene-fullerene mixtures. These experimental data are compared to DFT molecular modelling studies.The results of this fundamental study are expected to lead to a more comprehensive understanding of the complex relationships between phase morphology and kinetics, and their relationship to electronic properties. This will ultimately provide the fundamental knowledge base which will lead to development of future solar device platforms with potentially very significant increases in efficiencies over current technologies.
9:00 PM - H13.55
Self-Assembly Columnar Structure in Active Layer of Bulk Heterojunction Solar Cell.
Cheng Pan 1 , Jennifer Segui 1 , Yingjie Yu 1 , Hongfei Li 1 , Bulent Akgun 2 , Sushil K.Satijia 2 , Dilip Gersappe 1 , Chang-Yong Nam 3 , Miriam Rafailovich 1
1 Materials Science & Engineering, Stony Brook University, Stony Brook, New York, United States, 2 Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractBulk Heterojunction (BHJ) polymer solar cells are an area of intense interest due to their flexibility and relatively low cost. The mixture of polythiophene derivatives (donor) and fullerenes (acceptor) is spin coated on substrate as the active layer, and are phase-separated into interconnected domains. However, due to the disordered inner structure in active layer, donor or acceptor domains isolated from electrodes and long path conduction lower the power conversion efficiency (PCE) of BHJ solar cell. Therefore, control of inner structure within active layer is highly considered to enhance the efficiency. Our research provides the method to produce ordered self-assembly columnar structure within active layer by introducing a common polymer-polystyrene (PS) into the BHJ solar cell system. After spin coating blend thin film of polystyrene, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) (at different ratio) on substrate and annealing in vacuum oven for certain time, highly ordered columns (P3HT) is uniformly dispersed in PS, while PCBM nanoparticles are well confined at the interface of P3HT and PS. The surface morphology of blend thin film is measured by atomic force microscopy (AFM) and then is etched by ion sputtering to help us investigate the vertical column structure (different polymers have different etch rates under ion sputter). Neutron reflectometry was used to demonstrate the confinement of PCBM at the interface of PS and P3HT. The different morphological structures formed via phase segregation will be correlated with the performance of the PEV cells to be fabricated at the BNL-CFN.
9:00 PM - H13.57
Solid-State Processing of Conjugated Polymer-Fullerene Blends.
David Bucknall 1 , Gabriel Bernardo 2 , Meisha Shofner 1
1 Materials Science and Engineering, [email protected], Atlanta , Georgia, United States, 2 Institute for Polymers and Composites, University of Minho, Guimarães Portugal
Show AbstractIf the future global future energy needs are to be met, renewable energies must be exploited. The possibility of cheap, but efficient organic or polymer based photovoltaic (OPV) devices are among the competing technologies, which are actively being studied. To date the most efficient OPV devices are based on conjugated polymer-fullerene bulk heterojunctions (BHJ). The choice of polymer and fullerene is a balance of electronic (donor-acceptor) properties, but due to the nature of the solubility of the species, also includes solubilising ligands. Whilst these ligands certainly induce improved solution processability, they can also affect the electronic characteristics and also morphology of the polymers and fullerenes. However, potentially interesting polymer-fullerene combinations are not being explored due to difficulties in solution processing of the mixtures. We are exploring the use of pressure on the phase behaviour of polymer-fullerene blends, following on from previous workers who showed that melt processing of polymers can be greatly influenced by application of pressure. Under moderate pressure, we have demonstrated that equivalent phase behavior in polymer-fullerene pairs observed by conventional solution processing can be induced in solid-state processing. Using a combination of experiments and molecular modeling we are exploring the effect that pressure plays in inducing phase miscibility in otherwise intractable polymer-fullerene mixtures. To date we have been exploring a number of model-fullerene mixtures, but have demonstrated that the effects of this solid-state processing method are ubiquitous. The results of this fundamental study are expected to lead to alternative methods of processing of conjugated polymers without the need for solvents.
9:00 PM - H13.6
High Performance Bilayer Polymer Photovoltaic Cells Fabricated by a New Process.
Hui Joon Park 1 , Jae Yong Lee 2 , Xiaodong Liu 2 , L. Jay Guo 1 2
1 Macromolecular Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering & Computer Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractPolymer photovoltaic (PV) cells is a promising PV technology due to the strong potential for the production of low-cost, easily processible, and flexible PV cells over a large area with acceptable efficiencies. Among various photoactive layer structures, bulk heterojunction (BHJ) structure has been exploited as one of the most successful structures giving the highest efficiencies for polymer PV cells, because interpenetrating nanoscale networks of electron-donor and electron-acceptor give domain sizes on the order of exciton diffusion length. Therefore, significant efforts have been focused on the ways to optimize and understand the BHJ structures. However, the random nature of such interpenetrating network limits the further optimization of the structures and hinders a fundamental understanding of the physical properties. In contrast, the bilayer structure of electron-donor and electron-acceptor is a simple structure that can help easy optimization of devices and better understanding about their properties. However, it has been known as inefficient architecture for OPV, because it cannot efficiently produce photo-carriers due to the limited interfacial areas between domains responsible for charge separation. Based on the success of a previously developed ESSENCIAL technique[1], in this work we introduce a novel route to fabricate bilayer polymer PVs that can generate efficient photoactive layers. The process significantly facilitates interdiffusion between the donor and acceptor layers inducing optimized morphology favorable for charge generation and transportation that cannot be achieved by the conventional BHJ structure. The PVs made of P3HT/PCBM bilayers produced by our new process could achieve about 50% higher short circuit current than that of the thermally annealed BHJ polymer PV cells. Structures fabricated by different methods were compared by the measurements of quantum efficiency, absorbance spectra, X-ray photoelectron spectroscopy, and carrier extraction by linearly increasing voltage (CELIV) method. This structure and the new process not only suggest an alternative and practical way toward high efficiency polymer PV cells, we believe it also help us understand the underlying fundamentals of the OPV operations. [1] H. J. Park, M.-G. Kang, S. H. Ahn, L. J. Guo “Facile Route to Polymer Solar Cells with Optimum Morphology Readily Applicable to Roll-to-Roll Process without Sacrificing High Device Performances.” Advanced Materials, 22, E247–E253 (2010).
9:00 PM - H13.60
Characterization of Charge Transport in Bulk Heterojunction Organic Photovoltaic Materials by Measurement of the Mobility-Lifetime Product.
Eric Danielson 1 , Chris Lombardo 1 , Zi-En Ooi 2 , Ananth Dodabalapur 1
1 Microelectronics Research Center, The University of Texas at Austin, Austin, Texas, United States, 2 , Institure of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*STAR), Singapore Singapore
Show AbstractThe mobility-lifetime product, in bulk heterojunction (BHJ) organic photovoltaic cells, is an important parameter for understanding charge transport within these materials and lateral solar cell structures have been shown to be a powerful tool for characterizing transport within these material systems. In order to measure the mobility-lifetime product in BHJ organic photovoltaic cells, Hecht’s equation has been adapted to a lateral geometry, steady-state illumination, and to the measurement of electrical current flowing in lateral solar cell devices. The result allows us to determine the carrier drift length within the BHJ material by using photocurrent measurements as a function of carrier drift length ranging from 100 nm to 20 µm. Using these measurements performed on asymmetric lateral solar cell devices, we have determined the mobility-lifetime product as a function of electric field. In addition to the mobility-lifetime product, the charge carrier generation rate from the dissociation of photogenerated excitons can also be independently determined from these measurements. These charge transport parameters have also been studied as a function of light intensity (1 mW/cm2 to 1000 mW/cm2), light spectrum (AM1.5 and 532 nm), electric field, and carrier transit length. For this study, P3HT:C61-PCBM and P3HT:C71-PCBM have been employed due to their wide use among researchers as well as their potential for commercialization.
9:00 PM - H13.61
Solution Processable ZnO Nanoparticle Coated Ag-Nanowire Films as a Transparent Electrode for Hybrid Solar Cells.
Frederik S. Morgenstern 1 , Dinesh Kabra 1 , Sylvain Massip 1 , Thomas J. Brenner 1 , Philip Lyons 2 , Jonathan Coleman 2 , Richard Friend 1
1 Cavendish laboratory, University of Cambridge, Cambridge United Kingdom, 2 Center of Research on Adaptive Nanosturctures and Nanodevices, Trinity College, Dublin Ireland
Show AbstractWe demonstrate that solution processible silver nanowire (Ag-NW) films coated directly with ZnO can be used as the transparent electrode in organic photovoltaic devices. Ag-NW films were coated with a thin layer of ZnO nanoparticles (NP) which act both as an efficient electron extraction layer and as an encapsulating agent, thus protecting the wires from oxidisation and greatly improving the mechanical stability of the Ag-NW film, as shown in figure. Scanning photocurrent microscopy showed that photocurrent generation is more efficient at the active material surrounding the wires. UV illumination as present in the solar spectrum was found to enhance the photocurrent by improving the ZnO in-layer conductivity. This is due to photo-doping of the ZnO layer and its effects are sufficient to satisfy the current spreading requirement when using Ag-NW films. These transparent conducting substrates were found to show equivalent performance for photovoltaic devices in comparison to devices made up using ITO.
9:00 PM - H13.62
Application of Mesoporous TiO2 Beads to Hybrid Solar Cells.
Bofei Xue 1 , Fuzhi Huang 1 , Dehong Chen 2 , Rachel Caruso 2 , Yibing Cheng 1
1 Depart. Materials Engineering, Monash University, Melbourne, Victoria, Australia, 2 School of Chemistry, The University of Melbourne, Melbourne, Victoria, Australia
Show Abstract Mesoporous TiO2 beads[1] have been sucessfully applied to dye-sensitized solar cells (DSC). Films made from the mesoporous TiO2 beads have unique advantages over normal films made from TiO2 nanoparticles (such as P25) - the films have dual functions: dye adsorption and light scattering.[2] More than 10 % power conversion efficiency (PCE) has been achieved with a single screen-printed film from mesoporous TiO2 beads, without an additional scattering layer.[3] This impressive result was rationalized by longer electron diffusion lengths and extended electron lifetime, achieved with the mesoporous TiO2 beads. The unique properties of mesoporous TiO2 beads make them very attractive for other solar energy conversion devices. In this paper, the mesoporous TiO2 beads are applied to hybrid solar cells (HSC). In this HSC configuration, poly-3-hexylthiophene (P3HT) is the light absorber and hole conductor, while TiO2 serves as an electron acceptor. The influence of a compact TiO2 layer underneath the mesoporous TiO2 bead film was investigated by changing its thickness and deposition methods. The mesoporous TiO2 bead film thickness was optimized. Different P3HT concentrations in different solvents were studied to achieve the the best performance. Different techniques for the anode (Au) contacts were also comparied. For comparison, TiO2 porous films were also prepared from a commercial TiO2 pastes, which is made from 18-25 nm TiO2 particles. The preliminary results demonstrate the superiority of mesoporous TiO2 bead films over normal TiO2 porous films. References:[1] D.H. Chen, L. Cao, F. Huang, P. Imperia, Y.B. Cheng, R.A. Caruso, Synthesis of Monodisperse Mesoporous Titania Beads with Controllable Diameter, High Surface Areas, and Variable Pore Diameters (14-23 nm), Journal of the American Chemical Society, 132 (2010) 4438.[2] F. Huang, D. Chen, X.L. Zhang, R.A. Caruso, Y.-B. Cheng, Dual-Function Scattering Layer of Submicrometer-Sized Mesoporous TiO2Beads for High-Efficiency Dye-Sensitized Solar Cells, Advanced Functional Materials, 20 (2010) 1301-1305.[3] F. Sauvage, D.H. Chen, P. Comte, F.Z. Huang, L.-P. Heiniger, Y.B. Cheng, R.A. Caruso, M. Gratzel, Dye-Sensitized Solar Cells Employing a Single Film of Mesoporous TiO2 beads Achieve Power Conversion Efficiencies Over 10 % ACS Nano, 4 (2010) 4420.[4] D. Chen, F. Huang, Y.-B. Cheng, R.A. Caruso, Mesoporous Anatase TiO2 Beads with High Surface Areas and Controllable Pore Sizes: A Superior Candidate for High-Performance Dye-Sensitized Solar Cells, Advanced Materials, 21 (2009) 2206-2210.
9:00 PM - H13.63
Injecting Computation into the Investigation of Morphological Evolution of the Bulk Heterojunction Layer.
Aram Amassian 1 , Olga Wodo 2 , Buyi Yan 1 , Kang Chou 1 , Ruipeng Li 1 , Kui Zhao 1 , Rachid Sougrat 1 , D. Cha 1 , Baskar Ganapathysubramanian 2 2
1 Materials Science and Engineering, Division of Physical Sciences and Engineering, King Abdullah University for Science and Technology (KAUST), Thuwal Saudi Arabia, 2 Mechanical Engineering Department, Iowa State Univeristy, Ames, Iowa, United States, 2 Electrical and Computer Engineering Department, Iowa State University, Ames, Iowa, United States
Show AbstractA major challenge towards fabricating high efficiency OPVs is the current weak control over fabrication processes to get tailored morphologies. Current state-of-the-art approaches to understanding morphology evolution and tailoring fabrication process for high efficiency organic solar cells are either limited to combinatorial experimental investigation or single scale analysis. Moreover, experimental techniques provide limited data for analysis (limited to final morphology and mostly to lateral cross sections). These challenges hinder our ability to understand and subsequently control the interaction of multiple factors affecting morphological evolution. We inject computational thinking to understand the morphology evolution during solvent-based fabrication of organic solar cells. In this context, we develop a computational model to predict morphology evolution during solution processing. This computational model is validated via in situ monitoring of the solution process. We use experimentally informed, time dependent evaporation rate profiles in the model. The final morphology is also matched using surface AFM and cross-sectional TEM measurements. The validated model is subsequently used to perform a high throughput analysis of the phase space of processing conditions: solvent type, evaporation rate profile, blend ratio on morphology evolution.
9:00 PM - H13.64
Hybrid P3HT:ZnO Solar Cells: Towards Air Stable Low Cost Photovoltaics.
Guillaume Poize 1 2 , Ivan Shupyk 2 , Robert Heinrich 1 , Joerg Ackermann 1 2
1 CINAM, CNRS UPR 3118, Marseille France, 2 , Genes'Ink, Meyreuil France
Show AbstractPolymer solar cells are actually one of the most promising approaches towards low cost printable photovoltaics. Theses solar cells use bulk heterojunction nanomaterials based on blends of donor and acceptor polymers as active layers. However organic electron acceptors have in general poor performances and stability under ambient conditions. This fact makes encapsulation layer with high diffusion barriers for oxygen and water necessary to obtain stable organic solar cells.In order to produce solar cells at low cost, simple processing in air and avoiding of expensive encapsulation is highly desirable. The replacement of the organic n-type materials by inorganic nanoparticles leading to hybrid organic-inorganic bulk heterojunction materials could be one step in this direction due to the fact that inorganic semiconductors are generally more stable in air. Indeed in some cases, hybrid solar cells with improved stability under storage in air have been reported.1 In this work, we compared organic and hybrid solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT) blended with PCBM and Zinc oxide (ZnO) nanoparticles, respectively. Both type of solar cells were processed, stored and characterized under simulated sunlight in air. Our results reveal that the hybrid solar cells retain almost 100% of the initial performance while the organic devices loose 80% of their efficiency. We will present a detailed characterization of the device performances as a function of air exposition and correlate the improved air stability of the hybrid nanomaterials to the shape of the nanoparticles. REFERENCES:[1]L. E. Greene, M. Law, B. D. Yuhas, P. Yang, Journal of Physical Chemistry C 2007, 111, 18451.
9:00 PM - H13.65
Photovoltaic Investigation of New Co-Polymers Based on Fluorene-Phenylene and Fluorene-Thiophene Units.
Natasha Yamamoto 1 , Andreia Macedo 1 , Bruno Nowacki 2 , Isabel Grova 2 , Leni Akcelrud 2 , Lucimara Roman 1
1 Physics Department, Universidade Federal do Paraná, Curitiba, Paraná, Brazil, 2 Chemistry Department, Universidade Federal do Paraná, Curitiba, Paraná, Brazil
Show AbstractIn this work a new series of electron donor copolymers was studied as active layer in organic thin-films solar cells. We have investigated five different polymers containing fluorene, thiophene and phenylene units that were synthesized through the Witting route, originating alternate copolymers with the combination of different rates of the following structures: poly(9,9-n-dihexyl-2,7-fluorenylenevinylene-alt-1,4-phenylenevinylene) and poly(9,9-n-dihexyl-2,7-fluorenylenevinylene-alt-2,5-thienylenevinylene). These co-polymers are composed of fluorene, phenylene and thiophene units, where the fluorene content is kept constant while the phenylene:thiophene ratio is varied in order to evaluate the effects on their photovoltaic properties. For every copolymer ratio, we report results of photovoltaic devices in bi-layer structure using vapor-deposited C60 as the electron acceptor. We have shown the effect of altering the thickness of the fullerene films in order to analyze the charge transport mechanism in the co-polymer/fullerene interface. Effects of annealing on the film morphology were also investigated. The best co-polymer showed a peak external quantum efficiency of 40% and AM 1.5 power conversion efficiency of 1.8 %.
9:00 PM - H13.66
Transparent Organic Photovoltaics for Widespread Window Applications.
Richard Lunt 1 2 , Vladimir Bulovic 2
1 Chemical Engineering and Material Science, Michigan State University, East Lansing, Michigan, United States, 2 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractManipulation of excitons in organic and molecular semiconductors provides opportunities for unique solar harvesting applications. For example, the presence of strongly-bound excitons leads to large optical resonances, generating structured absorption that can be utilized to produce highly transparent and efficient near-infrared photovoltaics. The obstacle of large-area solar cell deployment could be overcome, in part, with development of such a transparent photovoltaic (PV) technology where integration onto window panes enhances the functionality of already utilized transparent surfaces without requiring the acquisition of undeveloped real estate and can significantly reduce balance-of-systems and PV installation costs. Here we demonstrate a transparent planar-heterojunction organic PV device with a spectrally-corrected power conversion efficiency of 1.3±0.1% with simultaneous average transmission across the visible spectrum of > 65%. Combining this device with a transparent high-reflectivity near-infrared (NIR) mirror leads to an average transparency of >55% and a power conversion efficiency of 1.7±0.1%, which approaches the opaque cell efficiency of 2.4±0.2%. We highlight the importance of near-infrared optical interference optimization on the simultaneous enhancement in both power efficiency and transparency and will discuss theoretical and practical efficiency limits for such architectures.
9:00 PM - H13.67
Optimizing Charge Transfer Efficiency in Conjugated Polymer/Single-Walled Carbon Nanotube Hybrids via Nanotube Diameter Selection.
Josh Holt 1 , Andrew Ferguson 1 , Nikos Kopidakis 1 , Brian Larsen 1 , Fritz Prehn 2 , Martin Heeney 3 , Garry Rumbles 1 , Jeffrey Blackburn 1
1 Chemical & Materials Science Center, National Renewable Energy Lab, Golden, Colorado, United States, 2 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Department of Chemistry, Imperial College London, Madison, South Kensington, United Kingdom
Show AbstractSeveral unique properties of single-walled carbon nanotubes (SWCNTs) have motivated their investigation as potential replacements for fullerene derivatives as the acceptor phase of organic photovoltaic (OPV) devices. Although replacement of the ubiquitous fullerene acceptors by SWCNTs in OPV devices has shown limited success thus far, fundamental investigations of charge transfer between SWCNTs and conjugated polymers are limited. To properly pair conjugated polymer and nanotubes requires knowledge of energy level alignments, which thus far have not been well characterized. Since nanotube diameter affects its electronic properties, including HOMO/LUMO energy level offsets relative to those of the polymer, it is naturally expected that photo-carrier transfer efficiency depends strongly on nanotube size. Using primarily time-resolved microwave conductivity (TRMC), which is inherently sensitive to mobile charge carriers, we probe the photo-carrier generation and decay dynamics in poly(3-hexylthiophene) (P3HT) paired with a broad diameter range of SWCNTs. We witness electron transfer from the polymer to SWCNT and selective hole transfer from the SWCNT to polymer by varying the nanotube HOMO via its diameter. We further extend our investigation to an emerging semi-conducting polymer with high charge mobilities, poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT). We demonstrate that nanotube selection is indeed crucial for charge transfer and, hence, high-efficiency photovoltaics.
9:00 PM - H13.68
Rapid Surface Modification of Metal Oxide Surfaces Using Phosphonic Acids.
Anuradha Bulusu 1 , Steven Walker 1 , Samuel Graham 1
1 Mechanical Engineering, Georgia Tech, Atlanta, Georgia, United States
Show AbstractMetal oxides are critical for use in a number of applications such as transparent conductive electrodes and barrier films for photovoltaic devices. Surface modification of these oxides can affect properties such as work function, mechanical adhesion, or provide chemical functionalization to enable or prevent chemical reactions from occurring on a given surface. While the use of phosphonic acids has been shown to be effective in functionalizing metal oxides, the use of techniques such as dip coating is very slow, taking on the order of hours to functionalize surfaces. In this work, we present a new method of rapidly modifying metal oxide surfaces with strongly bonded monolayers of phosphonic acids through spray coating. The method offers the ability to form strongly bonded monolayers within a few seconds with minimal material consumption, not possible through traditional methods such as dip coating or TBAG process. Phosphonic acid sprayed at room temperature on an indium tin oxide surface showed a 60% increase in coverage compared to a sample immersed in the acid for 1 hour using XPS measurements. The effect of temperature on enhancing reaction kinetics is explored through surface coverage measurements of high temperature spray coated and dip coated surfaces. In general, surface coverage increases linearly with deposition temperature. Preliminary results show that spray coating for 30 seconds at 150C results in similar coverage as a sample dip coated at 75C for 1 hour making spray coating a viable option as a surface modification method in rapid industrial fabrication of devices which incorporate metal oxides.
9:00 PM - H13.69
Electrode Contact Engineering to Improve Charge Collection in Heterogeneous Polymer Solar Cells.
Hari Kodali 1 , Baskar Ganapathysubramanian 1
1 Mechanical Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractA major factor resulting in low efficiencies of excitonic solar cells compared to inorganic solar cells is poor charge transport and collection after generation. This is due to a) low charge mobilities and b) highly intertwined pathways for charge transport. The highest incident radiation intensity occurs close to the upper electrode (Anode) resulting in highest exciton generation close to the upper electrode. This translates to a shorter path for holes from generation to collection site, compared to electrons. The disparity in path lengths affects the electron charge collection rate even with higher electron mobilities. This limits the maximum thickness of the active layer that can be used for optimum performance. Our aim is to investigate ways to improve charge collection for thicker devices via bottom electrode patterning. We utilize an excitonic drift diffusion framework to analyze charge transport in heterogeneous BHJ morphologies with various tailored electrode patterning. We investigate the effect of introducing undulations on the surface of cathode. This corresponds to introducing nano-scale roughness to the metallic surface of the cathode which would essentially shorten the distance electrons need to travel to reach the collection site. We present results comparing the change in electron collection rate at cathode for an ideal morphology (vertical-rod microstructure) and heterogeneous morphologies with various feature sizes due to the undulations.
9:00 PM - H13.70
Influence of the Nature of Nanometric TiO2 Particles on Photovoltaic Devices.
Sophie Cassaignon 1 2 , Constance Magne 3 , Thierry Pauporte 4
1 LCMCP, UPMC, Paris France, 2 LCMCP, College de France, Paris France, 3 , Saint-Gobain Recherche, Paris France, 4 LECIME, ENSCP, Paris France
Show AbstractTitanium oxide TiO2 has found extensive use in a great variety of applications among which electrode materials for dye-sensitized solar cells. The polymorphs of TiO2, rutile, anatase and brookite exhibit specific physical properties, band gap, surface states... For many applications the size of particles is an important parameter because it determines the surface to volume ratio, which greatly influences many properties. Due to the key role of the semi-conductor, the photovoltaic cell performances of dye-sensitzed solar cells (DSSCs) are highly dependent on the phase, the shape and the size of the oxide. Most of the studies on DSSCs have focused on anatase partcles prepared by hydrothermal growth. Rutile has been described as less eficient due to a lower electric conductvity. However, recent results on rutile nanotubes and on solvothermally grown single-crystalline rutle nanowires have revived the interest for rutle-based DSSCs. Up to now, interest on the brookite phase has been almost non-existent.This presentation will present original results on pure anatase, rutle and brookite partcles for DSSCs. Nanometric particles of the three polymorphs were synthesized by precipitation of TiCl4 and/or TiCl3 in aqueous medium. The control of the precipitation conditions (acidity, nature of anions, addition of complexants, ionic strength, titanium concentration…) allows the control of crystalline structure, size and morphology of particles. After sensitization by the N719 dye, efficient cells have been produced. It will be compared in this presentation. For exemple we have see that sensitzer titraton showed that the rutle layers adsorbed less dye than anatase or brookite. Impedance spectroscopy exhibited differencies between the phases. Moreover, a best overall conversion efficiency of 5.97% without a scattering layer was found for the larger TiO2 starting nanoparticles of brookite.
9:00 PM - H13.71
Effect of p-Type Single Wall Carbon Nanotubes on Polymer Solar-Cell Performance.
Ji-heon Kim 1 , Dal-ho Kim 1 , Yeon-hui Hwang 1 , Jae-woo Shin 1 , Jea-gun Park 2
1 Advanced Semiconductor Materials and Devices Development Center, Hanyang Univ., Seoul, 11, Korea (the Republic of), 2 Department of electronics computer engineering, Hanyang Univ., Seoul, 11, Korea (the Republic of)
Show AbstractIn organic photovoltaic cells using small-molecular or polymer layer, the enhancement of the power conversion efficiency (PCE) has been facing on the huddle due to the narrow absorption spectra of small-molecular and polymer materials. Therefore, in our study, we investigated the improvement of the short circuit current (Jsc) by the single wall carbon nanotube (SWCNT) sprayed layer. The p-type SWCNT were deposited on the ITO layer at the pressure of about 1.5 mmHg with a various spray time (1, 2, 3, 5 and 10 times). The SWCNT were dissolved in IPA at a weight of 0.1wt% and ultrasonication at for more than 24 hours before spraying to form the mixed layer. The poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) were dissolved in 1,2-dichlorobenzene at a weight ratio of 2:1 and stirred at 50oC on a hot plate for more than 72 hours in a nitrogen (N2) glove box before spin casting to form the blend layer. The bar-shape ITO anode was patterned and the active area for light absorption (1.5 mm x 1.5 mm) was opened using a hard photoresist patterning. And then, the organic photovoltaic cells with a device structure of ITO / SWCNT / P3HT:PCBM / BCP / LiF / Al were fabricated with small-molecular and polymer donating materials blended layer. As a result, Jsc increases sharply with increasing SWCNT sprayed times, up to about 2 times and the variation Jsc was about 16.91% (from 11.74 mA/cm2 to 14.13 mA/cm2). But, Jsc decreased with rapidly with increasing SWCNT sprayed times and the variation Jsc was about 7.64% (from 14.13 mA/cm2 to 13.05 mA/cm2). Our result, indicates that the Jsc for organic photovoltaic cells strongly depends on the sprayed times of the SWCNT. We observed a high power conversion efficiency (PCE) of 5.4% with high short-circuit current (Jsc) of 14.13 mA/cm2, open-circuit voltage (Voc) of 0.625 V, and fill factor (FF) of 60.9% of organic photovoltaic cell.
9:00 PM - H13.72
TiO2 Nanotubes Fabricated by Atomic Layer Deposition for Solar Cells.
Mi-Hee Jung 1 , Man-Gu Kang 1
1 , Electronics and Telecommunications Research Institute (ETRI), Daejeon Korea (the Republic of)
Show AbstractTitanium (IV) dioxide (TiO2) is one of the most attractive d-block transition metal functional oxides. Many applications of TiO2 such as dye-sensitized solar cells and photocatalyst have been widely investigated. To utilize solar energy efficiently, TiO2 should be well-aligned with a high surface area and promote the charge separation as well as electron transport. Herein, the TiO2 nanotubes were successfully fabricated by a template-directed method. The electrospun PEO(Polyethylene oxide, Molecular weight, 400k)fibers were used as a soft template for coating with titanium dioxide using an atomic layer deposition (ALD) technique. The deposition was conducted onto a template at 50 °C by using titaniumisopropoxide [Ti(OCH(CH3)2)4; TTIP] as precursors of TiO2. While the as-deposited TiO2 layers onto PEO fibers were completely amorphous with atomic layer deposition, the TiO2 layers after calcination at 500 °C for 1 h were properly converted into polycrystalline nanostructured hallow TiO2 nanotube. The TiO2 nanotube with high surface area can be easily handled and reclaimed for use in future applications related to solar cell fabrications.
9:00 PM - H13.73
High Hole-Mobility Poly(3-hexylthiophene) Nanofibers Fabricated by Electrospinning for Nanostructured Solar Cells.
Surawut Chuangchote 1 , Takashi Sagawa 1 , Hiroshi Sakaguchi 1 , Susumu Yoshikawa 1
1 Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan
Show AbstractRecently, nanowires or nanofibers of conducting polymers have received much interest, especially in nanophotonic and nanoelectronic device applications, such as photovoltaic cells, thin film transistors, and light emitting diodes. The conducting polymer nanowires or nanofibers have been prepared by various methods, such as dip-pen nano-lithography, polymerization in nanoporous templates, self-assembly, and electrospinning. Among various kinds of processing techniques, electrospinning has become one of the simple processes, which utilizes electrostatic forces to produce continuous micro- or nanofibers. We reported the successful fabrications of nanofibers from hardly electrospinnable conducting polymers, i.e. poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene- vinylene) (MEH-PPV) and poly(3- hexylthiophene) (P3HT), by electrospinning of mixtures of the conducting polymers with an non-conducting, easily extractable and electrospinnable polyvinylpyrrolidone (PVP) in a mixed solvent of chlorobenzene and methanol [1,2]. Pure conducting polymer nanofibers could be obtained by selective extraction of PVP out from the composite fibers. Herewith, new strategy for fabrication of pure conducting polymers without blending with any non-conducting polymer is reported. P3HT was directly electrospun from the solution in chloroform. Electrospinnability of the P3HT solution was improved by the addition of a volatile organic salt, pyridinium formate (PF), which was easily evaporate during the electrospinning. Therefore, pure and smooth P3HT nanofibers were obtained with high crystallinity. The field-effect transistor (FET) measurement of the P3HT nanofibers exhibited a relatively high hole mobility of up to 0.01 cm2V−1s−1 rendering them suitable candidates for organic electronic applications. Applications of P3HT nanofibers in nanostructured solar cells will be reported.[1] Chuangchote, S.; Sagawa, T.; Yoshikawa, S. Jpn. J. Appl. Phys. 2008, 47, 787.[2] Chuangchote, S.; Fujita, M.; Sagawa, T.; Sakaguchi, H.; Yoshikawa, S. ACS Appl. Mater. Interfaces, 2010, 2, 2995.
9:00 PM - H13.74
DFT Investigation of Interface Alignment Energies.
Michelle Tomasik 1 , Alexie Kolpak 2 , Jeffrey Grossman 2
1 Physics, MIT, Cambridge, Massachusetts, United States, 2 Materials Science, MIT, Cambridge, Massachusetts, United States
Show AbstractWhile attractive as light-weight, flexible, and potentially cheap alternatives to conventional Si-based solar cells, organic photovoltaics are hampered by low efficiency. One potential way to increase efficiency is to design interfaces to optimize the band alignment at the organic/electrode interface such that the open-circuit voltage (V_OC) is maximized and interfacial recombination is minimized. In this study, we use density functional theory with the Delta SCF method to investigate the ground and excited state band alignment at the interface between Alq3, an organic molecule commonly used in OLEDs, and Ag nanoparticles of different size and charge. We determine the various contributions to the band alignment and discuss possible interface modifications for maximal energy extraction. Our work aims both to simulate the experimental work showing a difference between bottom electrode (clean interface) and top electrode (nanoparticle insertions into the organic layer) and to provide useful insights in improving efficiency.
9:00 PM - H13.75
Heteroheptacene Based Ssemiconducting Polymers for Bulk Heterojunction Solar Cells.
Qingdong Zheng 1 , Shanci Chen 1 , Lixin Wang 1 , Zhigang Yin 1 , Changquan Tang 1
1 , Fujian Inst. Res. Struc. Matter, CAS, Fuzhou, Fujian, China
Show AbstractSeveral semiconducting polymers using newly synthesized conjugated heteroheptacenes with the inclusion of carbazole and thiophene units are reported. The introduction of heteroatoms (sulfur, nitrogen) in the ladder-type fused-ring system leads to decreased band-gaps of these polymers. The charge carrier mobilities for the synthesized polymers were investigated. The optical absorption spectra for these polymers show that these polymers have an improved light absorbing ability with harvested photons from UV to near IR (800 nm). These heteroheptacene containing co-polymers were used as donor materials in both conventional and inverted bulk heterojunction solar cells. We also investigated the interfacial layers effect on the performance of organic bulk heterojunction solar cells.
9:00 PM - H13.76
Well-Defined Hole Mobility from Time-of-Flight Non-Dispersive Transients of Annealed and Non-Annealed Spin-Cast Films of Poly-3-Hexylthiophene (P3HT).
Chris Collison 1 3 2 , Andronique Ioannidis 2 , Harry Hu 4 , David Weiss 5
1 Chemistry, Rochester Institute of Technology, Rochester, New York, United States, 3 Microsystems Engineering, Rochester Institute of Technology, Rochester, New York, United States, 2 Nanopower Research Laboratory, Rochester Institute of Technology, Rochester, New York, United States, 4 Materials Science and Engineering, Rochester Institute of Technology, Rochester, New York, United States, 5 , University of Rochester, Rochester, New York, United States
Show AbstractP3HT is widely used as the hole transport material for experimental polymer photovoltaic devices (PV). One of the critical parameters in device efficiency is charge mobility, unambiguous measurements of which are clearly necessary in order to draw meaningful conclusions from mobility investigations based on fabrication parameters. Time-of-Flight (ToF) charge mobility measurements are the standard measurements capable of producing unambiguous results when resulting charge transients display even a small plateau. Such transients are referred to as ‘non-dispersive’, to distinguish from ‘dispersive’ transients that have no plateau.However, due to the inherent complexity in device preparation, equipment set-up for ToF measurements, and device fabrication limitations, other methods or non-optimal ToF transients often must be used to determine mobility. Accordingly, transport measurements in P3HT have been reported in the literature either using other techniques or using dispersive transients from ToF.Unfortunately, these techniques produce results that are difficult to relate directly to PV devices and/or yield poor transients that cannot define a clear mobility.In the present study, by careful selection of measurement device parameters, measurement protocol and device preparation conditions, we have obtained ‘non-dispersive’ transients, unambiguous mobility determination and some of the highest mobilities reported for p3HT in the literature, of the order of 10-3 cm2/Vs. We have subsequently made a meaningful comparison between annealed and unannealed P3HT films. Annealed P3HT has been found to have nearly an order of magnitude lower mobility than unannealed P3HT.Despite some consistency with expectations that hole transport is slower between polymer chains than within chains, this is nevertheless a surprising result when considering that annealed P3HT:PCBM devices typically have higher efficiency than unannealed devices. We present our interpretations of an additional mechanism, separate from hole mobility, limiting efficiencies in the devices.
9:00 PM - H13.77
AFM and Confocal Raman Characterization of PCBM Loaded PS-b-PEO System.
Praveen Pitliya 1 , Malik Lewis 1 , Abul Huq 2 , Jianyong Yang 3 , Wei Liu 3 , Oladapo Bakare 1 , Alamgir Karim 2 , Dharmaraj Raghavan 1
1 Chemistry, Howard University, Washington, DC, District of Columbia, United States, 2 Polymer Engineering, University of Akron, Akron, Ohio, United States, 3 , WITec Instruments Corp, Maryville, Tennessee, United States
Show AbstractFor organic photovoltaic applications, we have examined block copolymer thin films as model nanoscale templates with acceptor and donor in two different blocks so that efficient charge separation can be achieved. The acceptor material (synthesized PCBM) was incorporated at different concentration levels into the Polystyrene-b-Polyethyleneoxide (PS-b-PEO) block copolymer, where the PEO forms the cylinder phase. PCBM synthesis involved esterification of 4- Benzoyl butyric acid, conversion to its tosylhydrazone followed by diazo formation and finally a 1, 3 dipolar cycloaddition to fullerene. The synthesized (6)-1-(3-(methoxycarbonyl) propyl)-(5)-1-phenyl [5, 6]-C61 (PCBM analog) and intermediates were characterized by NMR, FT-IR and MALDI-TOFMS. AFM characterization of PCBM loaded block copolymer system showed that PCBM was well dispersed up to 30%:100% (PCBM:PS-PEO) in the block copolymer matrix. The highly dispersed sample is in close agreement to percolation threshold of PCBM at which independent donor and acceptor paths are expected to be achieved. At higher loading of PCBM, the undispersed PCBM formed aggregates, while the block copolymer microphase separated structure remained intact. Confocal Raman microscopy was used to identify PCBM rich domains in the samples. Clear identifiable PCBM signal was noticed in block copolymer system with higher PCBM loading greater than 5%. Additionally, PCBM aggregates in samples of block copolymers were occasionally noticed which validates the AFM morphological results. Studies are underway to synthesize different fullerene derivatives analogue to PCBM and evaluate the influence of chemistry of those derivatives on the morphology of loaded block co-polymer systems. The novel fullerene derivative will also enable us to achieve well tuned acceptor band gap, which will eventually yield to high efficiency organic photovoltaics (OPV’s). Sponsor :DE-FG02-10ER4779
9:00 PM - H13.78
A Novel Squaraine Dye Series Photovoltaic Performance: Nanoscale Manipulation of Aggregation with Solution Process Design.
Susan Spencer 1 3 , Harry Hu 3 , Qimeng Li 3 , Amber Monfette 3 , Cameron Gallivan 2 , Tom Debies 5 , Niki Ioannidis 3 , Jeremy Cody 2 , Kevin Belfield 4 , Chris Collison 2 3 1
1 Microsystems Engineering, Rochester Institute of Technology, Rochester, New York, United States, 3 Nanopower Research Laboratories, Rochester Institute of Technology, Rochester, New York, United States, 2 Chemistry, Rochester Institute of Technology, Rochester, New York, United States, 5 , Xerox Corporation, Rochester, New York, United States, 4 Chemistry, University of Central Florida, Orlando, Florida, United States
Show AbstractOrganic solution-processed photovoltaics offer a cost-effective, flexible, and scalable alternative to conventional photovoltaic silicon-based devices. One issue that delays their commercialization is a comparatively low efficiency. We address the efficiency problem through improvement of a near-infrared (NIR) absorbing layer that would allow the harvest of a larger portion of the solar spectrum in a tandem device than standalone P3HT:PCBM devices. We demonstrate subsequent optimization of that NIR layer’s nanoscale morphology to maximize charge separation at the donor-acceptor interface as well as maximize charge carrier mobility after separation. We present results from solution-processed spin-cast bulk heterojunction devices made from novel squaraine dyes as the near-infrared donor and PCBM as the acceptor. We demonstrate improved spectral responses with external quantum efficiencies from 22 to 24 % over the spectral range of 590 to 725 nm, and open circuit voltages (VOC) from 0.42 to 0.6 V with our best device operating at ηIQE = 2.5 %.By designing a series of novel squaraine dyes with only slight modification from molecule to molecule we examine the effect of molecular structure on the crystallinity and aggregation of the nanocrystalline squaraine moiety as measured through XRD and absorbance measurements. The changes in molecular structure will also change the donor-acceptor interface, with this change quantified by direct gains in short circuit current and overall device efficiency. Finally, the changes in molecular structure and resultant effects on the device processes will be explained in terms of Onsager-Braun theory for charge dissociation. This will allow for prescription of an optimal set of NIR absorbing materials to investigate the relationship between molecular structure and device properties.
9:00 PM - H13.79
Development of New Fullerene-Based Electron Acceptors for Efficient Organic Photovoltaic Cells.
Yutaka Matsuo 1 , Eiichi Nakamura 1
1 , The University of Tokyo, Tokyo Japan
Show AbstractBecause [60]fullerene is widely employed in the development of organic photovoltaic (OPV) devices, the control of its electronic state and morphology by addition of organic addends to the fullerene core has become a very important issue. For instance, an increasing number of addends can reduce the pi-conjugation length, raise the LUMO level, and hence raise the open circuit voltage (VOC) of the OPV device, which is beneficial for the device performance. However, the addends inevitably change the crystal packing, typically reduce the fullerene–fullerene contact, and may also reduce the carrier mobility. We report new fullerene-based electron acceptors, 1,4-bis(silylmethyl)[60]fullerenes (SIMEFs) and 56pi-dihydromethano[60]fullerene derivative that raise VOC without decreasing of short-circuit current density.
9:00 PM - H13.8
Efficient Charge Transfer Processes in Triple Junction Carbon Nanotube:Polymer:Fullerene Derivative Compounds for Hybrid Photovoltaics.
Nasrul Nismy 1 , K D G Imalka Jayawardena 1 , A A Damitha Adikaari 1 , S Ravi Silva 1
1 Advanced Technology Institute, University of Surrey, Guidlford United Kingdom
Show AbstractPhotoluminescence (PL) studies are used to investigate the charge transfer efficiency of carbon nanotube-polymer:fullerene composites, intended to be used as the photoactive layer in hybrid photovoltaic devices. Acid functionalized multiwall carbon nanotubes (O-MWCNTs) are utilized due to its better dispersion in organic solvents which leads to better nanotube distributions in the composite. A drastic reduction in the PL (53% PL quenching) is observed for O-MWCNTs incorporated films which is associated with efficient charge transfer from the conjugated polymer to the fullerene and, for more exciton dissociation at the presence of O-MWCNTs in the films. O-MWCNTs in the photo active layer support the hypothesis of increased exciton dissociation by acting as exciton dissociation centres via the formation of triple-junctions in the hybrid devices. The carbon nanotube incorporated hybrid photovoltaic devices show enhanced short circuit current densities for P3HT:O-MWCNTs:PCBM system over 5%, compared with the reference devices, while maintaining other device parameters near-optimal, under standard test conditions leading to efficiencies as high as 3.1%. The enhanced charge collection is further corroborated by incident photon conversion efficiency measurements leading to efficient hybrid photovoltaic devices.
9:00 PM - H13.80
Electrochemical Study on Thermodynamic and Kinetics of Tris(2,2’-bipyridyl) Ruthenium (II) in Nonaqueous Solutions.
Battsengel Baatar 1 , Bolormaa Gendensuren 1 , Battulga Munkhbat 1 , Naranbileg Batjargal 1
1 Chem Tech, NUM, Ulaanbaatar Mongolia
Show AbstractThis study focuses on electrochemical behavior of the ruthenium (II) tris bipyridyl complex used for organic LEDs in nonaqueous solution by a method of cyclic voltammetry and kinetics and thermodynamics parameters were calculated. After verifying a flat background of 0.1 M TBAH supporting electrolyte in ACN in range of –1.8 to 1.8 V vs. Ag/AgCl, the cyclic voltammogram of 1mM ruthenium (II) trisbipyridyl complex was recorded. The redox reaction of ruthenium (II) tris bipyridyl complex is totally quasi-reversible and it can be described as a diffusion-controlled process. As a result of the redox reaction, the kinetic parameters of the electrode process e.g. diffusion coefficient (D0), and heterogeneous rate constant (ks) were calculated using a method of cyclic voltammetry. In addition, the different thermodynamic parameters i.e. standard free energy, enthalpy change, and entropy change revealed the exothermic nature of the electrode reaction.
9:00 PM - H13.81
In situ Morphological and Electronic Studies of Solution-Processed Small Molecule Organic Photovoltaics.
Gregory Su 1 , Nancy Eisenmenger 1 , Gregory Welch 1 4 , Guillermo Bazan 1 2 4 , Edward Kramer 1 3 , Michael Chabinyc 1
1 Materials, University of California Santa Barbara, Santa Barbara, California, United States, 4 Center for Energy Efficient Materials, University of California Santa Barbara, Santa Barbara, California, United States, 2 Chemistry, University of California Santa Barbara, Santa Barbara, California, United States, 3 Chemical Engineering, University of California Santa Barbara, Santa Barbara, California, United States
Show AbstractOrganic based photovoltaic devices have gained much attention in recent years due to their potential to be high performance, low-cost, large area modules. We have studied semiconducting solution-processed small molecule donors in bulk heterojunction solar cells. Unlike their polymer counterparts, solution-processed small molecules do not suffer from the effects of polydispersity and are easily purified. Here, we examine two donor small molecules that differ only by the chemical identity of one atom: 5,5’-bis{7-(4-(5-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3’-di-2-ethylhexylsilylene-2,2’-bithiophene, (SM1) and its carbon-bridged analog, (SM2). These molecules consist of an acceptor/donor/acceptor core with donor endcapping units. Devices fabricated with blends of either SM1 or SM2 and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) show different power conversion efficiencies under similar annealing conditions, about 2.5% and 1.0%, respectively. To determine the origin of this difference, we have used a collection of in situ measurements to monitor changes that occur during the thermal annealing process. A novel in situ technique is used to track the development of current-voltage characteristics during thermal annealing. A peak in short-circuit current vs. temperature was found at about 90°C for both materials. Corresponding in situ grazing incidence wide angle X-ray scattering (GIWAXS), UV-visible, and optical microscopy studies were carried out during annealing. Both the crystallinity and kinetics of crystallization differ for SM1 and SM2. These findings highlight how chemical structure, morphology, and performance are interrelated and provide insight into the design and characterization of organic semiconducting small molecule materials for photovoltaic applications.
9:00 PM - H13.82
Investigation of Correlation Among Carrier Mobility, Morphology and Organic Photovoltaic Efficiency by Time of Flight.
Harry Hu 1
1 , Rochester Institute of Technology, Rochester, New York, United States
Show AbstractOrganic photovoltaics offer a low cost and easily implementable option for future renewable energy. However, comparing to conventional photovoltaic silicon-based devices, organic devices have comparatively low efficiency, which is considered a critical issue. Recent research suggests that the mobility of active materials is a major contributor of device performance. Since most organic photovoltaic materials have relative low mobility, falling into the 10-3 to 10-6 cm2/(V*s) range, the standard method of mobility measurement for silicon-based devices is no longer suitable for organic materials. We utilized a more accurate measure of the mobility of organic active layers. It is called transient Time of Flight Mobility Measurement (TOF). By measuring the travelling time of a charge carrier layer generated by a short laser pulse, we successfully extracted the mobility of P3HT. Furthering the research effort, a certain percentage of fullerene derivatives (PCBM) will be doped into P3HT. PCBM acts as an electron acceptor in state of the art devices. We are looking for the correlation between PCBM concentration and the holes mobility and electrons mobility of mixture materials. Also an annealing process will be applied, which is typically used to change and improve the morphology; this morphology change will also influence the mobility of devices. For the same purpose, different solvents and co-solvent are used into film processing. Because of improvement of morphology, the charge mobility is enhanced. The thickness of a TOF sample is much larger than the thickness of photovoltaic device film, hence we used UV-VIS measurement to make sure morphology changes from the annealing process for both films are identical. The same proportion of PCBM:P3HT will be used for making actual photovoltaic devices. Both power conversion efficiency (PCE) and external quantum efficiency (EQE) of these devices will be demonstrated. According to mobility change, the Jsc,the Voc and the Fill Factor which contribute to IPE are varying into different direction. The connection between mobility, PCE and EQE will be fully investigated, giving a much better understanding of the relation between mobility and device performance and providing a pathway for improved device efficiency.
9:00 PM - H13.83
Phase Morphology and Electrical Performance of Vertically Composition-Graded P3HT/PCBM Organic Photovolatics.
Min Kim 1 , Jisoo Shin 1 , Kilwon Cho 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractThe vertical phase morphology of donor and acceptor materials needs to be studied to understand vertical charge transport and consequently achieve the efficient charge collection in the organic photovoltaics (OPVs). In this study we manipulated the vertical phase morphology of photoactive layer, poly(3-hexylthiophene) (P3HT):[6,6]phenyl-C61-butyric acid methyl ester (PCBM) by using an additive, and the morphology was investigated by transmission electron microtomography (TEMT), dynamic secondary ion mass spectroscopy (DSIMS) and x-ray photoelectron spectroscopy (XPS). Based on these characterizations, we successfully analyzed the three-dimensional morphology of the processed films and proposed a film-forming mechanism for vertically segregated blend morphology. The OPV performances of the vertically segregated blend films showed strong dependency on the device architecture which determines the direction of electron and hole transport. Hole dominant diode and ambipolar bottom-gated field-effect transistor confirmed the asymmetric carrier transport at the buried interface and air surface in the vertically segregated blend films. In this sense three-dimensional morphology of blend films can be correlated with the anisotropic electrical behaviors and photovoltaic performances.
9:00 PM - H13.84
Small Molecule Organics Photovoltaics with Doped Organic Transport Layers and Doped Single Walled Carbon Nanotubes.
Alexander Cook 1 , Kamil Mielczarek 1 , Alexios Papadimitratos 1 , Anvar Zakhidov 1 , Antti Kaskela 2 , Albert Nasibulin 2 , Esko Kauppinen 2
1 , University of Texas at Dallas, Richardson, Texas, United States, 2 , Aalto University, Espoo Finland
Show AbstractDue to the short exciton diffusion length, optimal small molecular organic photovoltaic cells (OPV) are very thin; usually well below 100nm in thickness. Solar cells also require high conductivity, transparent electrodes to perform well. These two conditions would typically rule out the possibility of using carbon nanotubes as electrodes in place of either indium tin oxide or aluminum. Both of these difficulties can be addressed via doped organic transport layers. The same dopant which dopes the transport material will at the same time dope the single walled carbon nanotubes (SWCNT) resulting in a several fold increase in conductivity. The doped layers can be relatively thick (up to hundreds of nanometers) due to their high electrical conductivity which can reduce the probability of shorting the entire device with the carbon nanotubes or due to pin holes in the thin active area. Doped layers could also yield higher internal electric fields, better ohmic contact with the electrodes and allow for some optical enhancements. [1]In this presentation we will discuss our recent results with P and N doped transport layers and SWCNT. The SWCNT are graciously provided by our collaborators from Aalto University and are described and demonstrated in an OLED in [2]. The SWCNT and transport layer materials will be doped by organic p-type dopants such as 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and organic n-type dopants such as acridine orange base (AOB). The host materials for these dopants will be large band gap organic materials such as N,N’-Di(naphthalen-1-yl)-N,N’-diphenyl-benzidine (alpha-NPB) for P-type doping and C60 for N-type doping. Small molecular organic photovoltaic devices will be fabricated with these materials and active layers such as Zn-phthalocyanine (ZnPC) C60 via vacuum thermal evaporation. The devices will then be characterized. The authors are thankful for the support of grants AFRL/Rice via CONTACT consortium, Welch Foundation grant AT-1617 and DOE STTR grant DE-SC0003664.[1] Drechsel J, Maennig B, Gebeyehu D, Pfeiffer M, Leo K and Hoppe H 2004 MIP-type organic solar cells incorporating phthalocyanine/fullerene mixed layers and doped wide-gap transport layers Org. Electron. 5 175–8[2] Kaskela, A., Nasibulin, A. G., Timmermans, M. Y., Aitchison, B., Papadimitratos, A., Tian, Y., et al. (2010). Aerosol-synthesized SWCNT networks with tunable conductivity and transparency by a dry transfer technique. Nano letters, 10(11), 4349-55. doi: 10.1021/nl101680s.
9:00 PM - H13.85
Modifying Morphology by Choice of Squaraine Side-Groups for Optimizing a near-Infra-Red Organic Photovoltaic Device.
Qimeng Li 1 , Chris Collison 2 , Susan Spencer 3 , Jeremy Cody 2 , Amber Monfette 2 , Zhiyuan Hu 1 , Jason Staub 2
1 Material science and engineering, Rochester institution of technology, Rochester, New York, United States, 2 Chemistry, Rochester institute of technology, Rochester, New York, United States, 3 Microsystem, Rochester institute of technology, Rochester, New York, United States
Show AbstractOrganic Photovoltaic (OPV) solar cells give us a relative low-cost , light-weight and flexible approach for renewable energy compared to silicon-based solar cells. But the efficiency is too low for commercial application at the present stage. So, the use of multiple junction (Tandem) solar cell structure had came into our sight and provide us a strategy for increasing power conversion efficiency (PCE). An active layer made from near infra-red absorption material is very important in tandem solar cells for harvesting more of the solar spectrum. The use of squaraine dye as a near-infra-red absorption layer had demonstrated high spectral response and open circuit voltage. The overall efficiency of tandem solar cells is built up on the basis of PCE for each single layer which is depend on many factors such as absorption coefficient, exciton diffusion and mobility, and the morphology of the bulk heterojunction active layer is critical for all these factors and hence it substantially impacts the device efficiency. We have been working with different single layer device made of various types of synthesized squaraine dyes with different side groups and use them as electron donor while using PCBM as electron acceptor in the active layer. They have shown different performance and efficiency since the side groups will present different band gap energies, as well as affect how the molecules interact physically with each other and how they pack to form different aggregates. Also the use of co-solvent might be a factor affecting the morphology since it has impact on the formation of aggregates. In this work, the PCE data from solar-simulator and external quantum efficiency (spectral response) will be presented as a function of Squaraine side-groups and their resultant energy levels as represented by the changes in open-circuit voltage. Through understanding of these changes we can not only prescribe a meaningful correlation between fundamental changes in molecular structure and a measureable device property, but also provide information about optimizing device efficiency by changing the morphology.
9:00 PM - H13.86
Novel Squaraines for Improved NIR-Active Bulk Heterojunction Photovoltaics.
Chris Collison 1 2 3 , Susan Spencer 2 , Amber Monfette 3 , Jeremy Cody 1 , Kevin Belfield 5 , Cameron Gallivan 1 , Qimeng Li 4 , Harry Hu 4 , Andronique Ioannidis 3 , Victor Murcia 1
1 Chemistry, Rochester Institute of Technology, Rochester, New York, United States, 2 Microsystems Engineering, Rochetser Institute of Technology, Rochester, New York, United States, 3 Nanopower Research Laboratory, Rochetser Institute of Technology, Rochester, New York, United States, 5 Chemistry, University of Central Florida, Orlando, Florida, United States, 4 Materials Science and Engineering, Rochetser Institute of Technology, Rochester, New York, United States
Show AbstractOrganic photovoltaic (PV) solar cells are inexpensive since their manufacture relies only upon spin-coating or spray coating technology. Yet organic solar cells suffer from lower efficiencies when compared to their more expensive silicon-based counterparts. Low efficiencies ultimately mean excessive investment costs, which prevent their widespread use.We describe a variety of squaraine dyes that will be targeted specifically for optimal application in more efficient NIR tandem cells, so as to broaden the range of spectral harvesting from the sun. Squaraines form a family that is among the very few successful solution processable molecular chromophores. Our current goal is to describe how 2,4-Bis-(4-diisobutylamino-2,6-dihydroxy-phenyl)-cyclobutane-1,3-dione can be substituted in order to achieve high power conversion efficiencies for our devices. We will systematically vary the functional groups on synthesized squaraines and we will measure the impact on isolated variables; in particular the aggregation/morphology in bulk heterojunction devices with PC60BM as the electron acceptor. We will present corresponding power conversion efficiency trends, measured in our laboratory, from 0.5% to 2.5% and higher, based on improvements in processing parameters that include blend ratios, choice of annealing technique and choice of buffer layer. The results of this work will allow us to prescribe improved device efficiency through chemical tuning and design.
9:00 PM - H13.87
Characterization of Morphology in Poly 3-HexylThiophene Films Using Fluorescence Anisotropy.
Chris Collison 1 2 3 , Christopher Grieco 1 , Andronique Ioannidis 2 , Susan Spencer 2
1 Chemistry, Rochester Institute of Technology, Rochester, New York, United States, 2 Microsystems Engineering, Rochester Institute of Technology, Rochester, New York, United States, 3 Nanopower Research Laboratory, Rochester Institute of Technology, Rochester, New York, United States
Show AbstractWith the limitation on fossil fuel supply, photovoltaic devices are becoming more and more appealing. However, the efficiencies of these devices are not satisfactory. Fortunately, methods for improving device efficiency are plausible. Organic photovoltaic (OPV) devices offer low-cost synthesis and manufacture and, importantly, efficiencies can be greatly improved by improving the nanomorphology of the active layer components. One component widely used because of its correlation with higher device frequencies, is poly(3-hexylthiophene), or P3HT. Controlling the crystallization and deposition of P3HT nanofibers onto thin films is significant to the development of high-efficiency photovoltaic devices. These events are detected commonly by X-ray diffraction (XRD). However, since XRD is very costly, alternate methods of measuring the extent of crystallinity of P3HT layers would be optimal. One way to measure the organization of a fluorescent molecule, such as P3HT, is by measuring its fluorescence anisotropy (FA). FA is a value related to the common orientation of molecules in a solution or film sample. Here, we propose that FA can be used to indicate the extent of crystallinity of P3HT polymers and nanofibers in solution and on thin films in application to organic photovoltaic devices.
9:00 PM - H13.88
Further Investigation into the Paintbrush Deposition Technique for P3HT:PCBM Based Organic Solar Cells.
Aaron Thoeming 1 , John Carr 1 , Rakesh Mahadevapuram 2 , Sumit Chaudhary 1 2
1 Electrical Engineering, Iowa State University, Ames, Iowa, United States, 2 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractResearch in organic semiconductor materials and devices has increased dramatically in the last decade, particularly in the photovoltaics field. Organic based solar cells with record power conversion efficiencies of 8% to 9% have recently been reported, and future expectations for organic solar cell power conversion efficiencies are believed to approach 15%. Organic semiconductors hold interest due to their flexible mechanical properties and their ability to be solution processed. Processing techniques such as: spin-coating, drop-casting, roll-painting, and inkjet printing can be readily used to deposit dissolved solutions of polymers and fullerenes to create organic solar cells. One novel, minimally researched solution processing technique for the deposition of polymer and fullerene solutions is paint-brushing. Current research on paint-brushed solar cells has shown that solar cell efficiencies close to 5% can be obtained from active layer paint-brushing on heated indium-tin-oxide covered glass substrates. In this study, organic solar cells are fabricated from P3HT and PCBM in chlorobenzene solution via paint-brushing technique. Unlike previous studies, no heat treatment is applied to P3HT:PCBM active layers during or after device production. Paint-brushed devices are compared to spin-coated devices fabricated from the same P3HT:PCBM chlorobenzene solution in an effort to further investigate why paint-brushed solar cell devices perform better than non-annealed, spin-coated solar cell devices. Results from a full device characterization process including light current-voltage measurements, dark current-voltage measurements, external quantum efficiency measurements, trap density measurements, active layer absorption measurements, and mobility measurements for both paint-brushed and spin-coated devices will be shown and discussed. Atomic Force Microscopy is used to provide active layer thickness data and film morphology information for both paint-brushed and spin-coated devices. Practical issues concerning using the paint-brush technique for organic solar cell fabrication are also covered.
9:00 PM - H13.89
Understanding How Various Fullerene-TiO2 Binding Motifs Affect Exciton Dissociation at the Organic/Inorganic Interface in P3HT/TiO2 Solar Cells.
Bertrand Tremolet de Villers 1 , Benjamin Schwartz 1
1 Chemistry and Biochemistry, UCLA, Los Angeles, California, United States
Show AbstractSolution-processed organic and hybrid organic-inorganic excitonic solar cells are currently the focus of intense research due to their potential for inexpensive, flexible and lightweight energy production. We show that exciton dissociation is dramatically improved in P3HT/TiO2 photovoltaics by incorporating a small molecule mediator derived from C60 onto the surface of TiO2. Photoluminescence quenching measurements reveal that the surface-modified TiO2 dissociates many more excitons from P3HT compared to bare TiO2. Efficient charge transfer from the surface fullerene mediator to TiO2 is confirmed by AM1.5 solar illumination current-voltage measurements which show up to a four-fold enhancement of the short-circuit current density in devices with the surface-modified titania. In order to gain more insight into the nature of the improvement in device performance with fullerene adsorbates relative to a clean titania surface, we investigate how the chemical structure of the fullerene affects its binding affinity to the TiO2 surface. We show that by varying the fullerene-TiO2 interface, we can selectively tune the enhancement of the open-circuit voltage by changing the energetics of the interface between the polymer and titania. Electronic states at the interface due to the fullerene may also reduce charge recombination as surface-modified TiO2 devices also showed higher fill-factors.
9:00 PM - H13.9
Towards Efficient Semiconducting Carbon Nanotubes/P3HT Solar Cells.
Marco Bernardi 1 , Shenqiang Ren 1 , Silvija Gradecak 1 , Jeffrey Grossman 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractHeterojunctions of semiconducting single-walled carbon nanotubes (SWCNTs) and poly(3-hexylthiophene) (P3HT) showed ultra-fast charge separation upon illumination with visible light, and thus hold promise for organic photovoltaic devices. Previous attempts to prepare P3HT/SWCNT bulk-heterojunction solar cells yielded low AM1.5 power conversion efficiencies of less than 0.05%, likely due to the detrimental presence of metallic nanotubes.We used high-purity semiconducting SWCNTs and P3HT to fabricate solar cells with significantly higher efficiency (0.72% under AM1.5 conditions) and higher fraction of SWCNTs (up to 40% wt.) than previously reported. The fabrication process involves mixing P3HT and SWCNT prior to deposition to form nanostructures of SWCNT coated by an ordered layer of P3HT, a key process that we show to be highly beneficial to the device charge separation and transport. Electrical characterization shows a large open circuit voltage (VOC) in excess of 1V and unexpected based on the interface bands alignment, and cell characteristics strongly dependent on the SWCNT concentration. We modeled VOC using a modified diode equation that suggests that the voltage could be regulated by the dark and short-circuit currents rather than by the value of the interface gap, and that carriers recombination processes are dominant in the device. Study of the optical absorption and quantum efficiency allowed us to determine that type-II heterojunctions contributing to charge separation were formed only by SWCNTs with diameters in the range of approx. 1.3 – 1.4 nm, and showed an internal quantum efficiency of 25% . Our results contribute to the fundamental understanding of nanoscale chemical and physical processes governing the power conversion efficiency in polymer/SWCNTs devices.
9:00 PM - H13.91
A Novel Chemical Synthesis for >1 µm2 Graphene Sheets and Its Incorporation with Graphene Oxide to Enhance Organic Polymer Solar Cell Efficiency.
Rebecca Isseroff 1 , Miriam Rafailovich 1 , Paul Masih Das 2 , Nathan Akhavan 3 , Benjamin Goldman 3 , Iaonnis Kymissis 4 , Nanlo Yang 5 , Andrew Chen 6 , Sneha Chittabatini 6 , Alexandra Tse 6
1 Garcia Center, SUNY Stony Brook, Stony Brook, New York, United States, 2 , Johns Hopkins University, Baltimore, Maryland, United States, 3 , Rambam Mesivta HS, Lawrence, New York, United States, 4 Department of Engineering, Columbia University, New York, New York, United States, 5 Department of Chemistry, College of Staten Island, Staten Island, New York, United States, 6 , Lawrence High School, Lawrence, New York, United States
Show AbstractThis project developed an innovative, simple, and cost-efficient synthesis procedure, dispersing graphene oxide in an ethanol-water solvent and reducing slowly with sodium borohydride (NaBH4). It was found that when graphene oxide was reduced in a 75:25 H2O:ethanol solution with 15 mmolar NaBH4, numerous single-layered graphene sheets >1 µm2, and sometimes even >2 µm2, in size were produced. The quality of these sheets was confirmed by Raman spectroscopy, FTIR, XRD, AFM, TGA, and TEM. HRTEM displayed hexagonal arrangement of atoms with carbon nanotubes forming in multilayered areas, indicating sample purity. ED confirmed six point lattice. Graphene and graphene oxide were integrated with several variations of a conventional P3HT:PCBM/PEDOT:PSS organic polymer solar cell and tested in a Solar Simulator. Results indicate that the incorporation of GO improved the hole production of the cell, increasing the solar cell efficiency from 2.62% to 2.89%. Additionally, results of the cells tested under a halogen lamp showed that the cell utilizing GO as a hole conductor increased the voltage production by 26%, while the cell incorporating graphene as the hole conductor decreased the production by 32%.
9:00 PM - H13.92
The Role of Processing Additive in Achieving Efficient Donor-Acceptor Copolymer-Based Bulk Heterojunction Solar Cells.
Hao Xin 1 , Xugang Guo 2 , Guoqiang Ren 1 , Mark Watson 2 , Samson Jenekhe 1
1 , University of Washington, Seattle, Washington, United States, 2 , University of Kentucky, Lexington, Kentucky, United States
Show Abstract The two-phase morphology of bulk heterojunction (BHJ) polymer solar cells is critical to the performance of the devices. Recently, processing the polymer/fullerene blend film from a binary solvent mixture containing a small amount of a higher-boiling-point additive has turned out to be a very effective approach to improving the photovoltaic properties of polymer/fullerene solar cells.1, 2 Initial investigation suggested that the role of the processing additive in controlling the two-phase morphology was to increase the domain size of the polymer and/or the fullerene.2 However, more recent studies indicate that the processing additive improves performance of polymer solar cells by reducing the domain size of the phase separation and creating a more uniform BHJ morphology.3, 4 Details of how the processing additive influences the phase separation, the domain sizes of the two-phase morphology, and the photovoltaic properties of the BHJ as well as applicability of the method to other D-A conjugated copolymer films remain unknown. We have used phthalimide based donor-acceptor copolymers5 to investigate how a processing additive and film-drying rate enhance the photovoltaic performance of polymer/fullerene bulk heterojunction solar cells. An optimal interconnected two-phase morphology with 15-20 nm domains was obtained when a processing additive was used compared with 100-300 nm domains without the additive, resulting in almost 10-folder enhancement in efficiency. By a combination of photovoltaic measurements, TEM and AFM imaging, and X-ray diffraction characterization, we have found the underlying mechanism on how processing additive achieves the highly efficiency nanoscale morphology for donor-acceptor copolymer/fullerene solar cells. References1.Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C., Nat. Mater. 2007, 6, 497-500.2.Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J., J. Am. Chem. Soc. 2008, 130, 3619-3623.3.Hoven, C. V.; Dang, X.-D.; Coffin, R. C.; Peet, J.; Nguyen, T.-Q.; Bazan, G. C., Adv. Mater. 2010, 22, E63-E66.4.Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L., Adv. Mater. 2010, 22, 1-4.5.Guo, X. G.; Kim, F. S.; Jenekhe, S. A.; Watson, M. D., J. Am. Chem. Soc. 2009, 131, 7206-7207.
9:00 PM - H13.94
Low Band Gap Donor–Acceptor Copolymers of Perylene Diimide-Based Photovoltaic Materials.
Ashok Keerthi 1 , Kwan Wei Lek 2 , Suresh Valiyaveettil 1
1 Chemistry, National University of Singapore, Singapore Singapore, 2 School of Electrical & Electronic Engineering, Singapore Polytechnic, Singapore Singapore
Show AbstractDonor-acceptor conjugated systems gained significant scientific interest due to their interesting optoelectronic properties. Over the last decade, the performance of bulk-heterojunctions (BHJ) devices has improved significantly owing to advances in the design of photo- voltaic materials, as well as device optimization. To date, BHJ-type solar cells with p-type semiconducting polymers as the electron donor and fullerene derivatives as the electron acceptor have shown the highest performance. Perylene diimide (PDI) represents the class of n-type semiconductors with high molar extinction coefficient, high photo stability and high electron mobility. Research efforts have been focused on the modification of PDI structures to improve their optoelectronic and charge transport properties. The functionalization of PDI at the bay position is an easy approach to fine tune the properties of PDI. Here we report the synthesis of highly stable and processable PDI based polymers incorporated with many donor groups. The photo-physical, electrochemical and thermal properties of the synthesized polymers were studied. The charge transport and photovoltaic performance of the polymers are under progress in our laboratory.