Venkat Bommisetty South Dakota State University
Niyazi Serdar Sariciftci Johannes Kepler University of Linz
K. S. Narayan Jawaharlal Nehru Centre for Advanced Scientific Research
Garry Rumbles National Renewable Energy Laboratory
GG1: Small Molecule OPV
Tuesday PM, April 06, 2010
Room 3002 (Moscone West)
9:30 AM - **GG1.1
Nanoscale Bulk Heterojunction Solar Cells by Solution and Vapor Phase Processing.
Stephen Forrest 1 , Mark Thompson 2 Show Abstract
1 EECS & Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Chemistry, USC, LA, California, United States
In this talk we will look at several promising approaches to creating nanoscale morphology in small molecular weight thin films. Moving from the conventional vacuum deposited CuPc/C60 based system, we will examine a number of different routes to creating nanostructures based on new materials such as squaraines, carbon nanotubes and subphtalocyanine. Combinations of solution and vapor phase growth (by both oblique incidence vacuum thermal evaporation and organic vapor phase deposition) will be discussed. Routes to demonstrating very high efficiency single junction and tandem architectures with these materials and growth combinations are considered.
10:00 AM - **GG1.2
Enhancing Open-circuit Voltage in Organic Solar Cells by Interface Engineering.
Hideyuki Murata 1 , Yoshiki Kinoshita 1 , Toshinori Matsushima 1 Show Abstract
1 School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan
We investigated the role of interface engineering on enhancing the open-circuit voltage (Voc) of organic solar cells based on small molecules. The interfaces modulation at (i) donor layer/acceptor layer and (ii) anode/donor layer clearly affect to the Voc. Regarding the donor/acceptor interface, we confirm that the upper limit of Voc was determined by the difference between the highest occupied molecular orbital (HOMO) level of donor material and the lowest unoccupied molecular (LUMO) level of acceptor material. However, the Voc expected by the HOMO-LUMO gap of donor-acceptor interface can be realized when the built-in-potential determined by the difference of the work functions of anode and cathode exceeds the HOMO-LUMO gap at donor/acceptor interface.We fabricated the devices with the structure of ITO/pentacene (50 nm)/phtalocyanines (x nm)/C60 (40 nm)/BCP (10 nm)/Ag (100 nm), where the HOMO levels of donor materials are -5.00 eV (pentacene) and -5.10 eV (Cu-phthalocyanine and Zn-phthalocyanine). By inserting thin layer of phthalocyanines at the interface between pentacene and C60, we observe enhanced Voc in accordance with the increased energy difference between the LUMO level of the C60 (-4.50 eV) and the HOMO level of the donor material. When 2 nm-thick CuPc was replaced with ZnPc, short-circuit current (Jsc) increase in addition to the enhancement of Voc. Higher Jsc was ascribed to the higher exciton dissociation efficiency at ZnPc/C60 interface compared with that of CuPc/C60 interface since the lifetime of singlet excited state of ZnPc (3.3 ns) is much longer than that of CuPc (6 ps).Recently, we demonstrate that the work function of ITO/MoO3 anode increase from 4.92 eV to 5.92 eV by increasing the thickness of MoO3. Using this technique, we can tune a built-in potential of organic solar cells from 0.62 eV to 1.62 eV since Ag cathode has a work function of -4.30 eV. By using 5,10,15,20-tetraphenylporphyrine (HOMO = -5.50 eV) as donor material and C60 as an acceptor material, we can assume that the maximum value of Voc is about 1 V. Although the Voc of the device without MoO3 was limited to 0.57 V, the Voc significantly increased to 0.97 V with increasing MoO3 thickness. The observed Voc of 0.97 V is consistent with the theoretical value of that estimated from HOMO-LUMO gap. Importantly, the enhancement in the Voc was achieved without affecting the Jsc and the fill-factor. Thus, the power conversion efficiency of the device increased linearly from 1.24% to 1.88%.
10:30 AM - **GG1.3
Optimizing Nanostructures for Organic Photovoltaics.
Ximin He 2 , Feng Gao 1 , Richard Friend 1 , Wilhelm Huck 2 , Neil Greenham 1 Show Abstract
2 Melville Laboratory, University of Cambridge, Cambridge United Kingdom, 1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom
The ideal structure for an organic donor-acceptor photovoltaic device is thought to be an array of columns of one material surrounded by a complementary matrix of the other material, with the column diameter comparable with the exciton diffusion length. Ideally, the donor and acceptor columns are each connected to the appropriate electrode through a thin continuous layer to prevent carriers from reaching the wrong contact. We report how this type of structure can be fabricated using a double imprinting technique where one polymer is first imprinted with a silicon stamp, and then itself imprinted into a second polymer layer. Solvent vapour is used to assist the nanoimprinting processes. Feature sizes as low as 25 nm can be achieved. Devices using poly(3-hexylthiophene) and the fluorene copolymer F8TBT give power conversion efficiencies of 1.9%, considerably higher than bilayer or blend devices made from the same materials, indicating that efficient exciton dissociation and charge collection is achieved. Modeling of the effect of nanostructure on device performance will also be reported.
11:00 AM - GG1:Small Mol
GG2: OPV Imaging
Tuesday PM, April 06, 2010
Room 3002 (Moscone West)
11:30 AM - **GG2.1
The Role of Heterogeneous Charge Transport in Polymer Solar Cells.
David Ginger 1 Show Abstract
1 , University of Washington, Seattle, Washington, United States
We correlate processing and nanoscale film morphology with the performance of several model polymer solar cells using electrical scanning probe microscopy. We image directly how the hole transport, electron transport, and photocurrent collection networks evolve with annealing in the archetypal polythiophene (P3HT)/fullerene (PCBM) blend system using both conductive and photoconductive atomic force microscopy (cAFM and pcAFM). We show that local heterogeneity in carrier transport properties can limit performance, and discuss the implications of local heterogeneity for polymer solar cell modeling. We also demonstrate how scanning probe methods can be used to study both vertical composition gradients and carrier trapping with high spatial resolution. These measurements show that unfavorable vertical gradients can exists in many blends, and that the formation of carrier traps due to photochemical degradation can be spatially resolved using time-resolved electrostatic force microscopy (trEFM).
12:00 PM - GG2.2
Nanoscale Resolution of the Exciton Transport in Organic Solar Cells by Tip-enhanced Tunneling Luminescence Microscopy.
Manuel Romero 1 , Anthony Morfa 1 , Thomas Reilly 1 , Jao van de Lagemaat 1 , Mowafak Al-Jassim 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
In organic solar cells, the efficiency of the exciton transport and dissociation across donor-acceptor (D/A) interfaces is primarily controlled by the nanoscale distribution of the donor and acceptor phases. The observation of photoluminescence quenching is often used as confirmation for efficient exciton dissociation, but provides no information on the nanoscopic nature of the exciton transport. Here we demonstrate nanometer resolution of the exciton transport in films consisting of the conjugated polymer poly(3-hexylthiophene) (P3HT, electron donor) blended with the C60 derivative 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM, electron acceptor) by a tunneling luminescence spectroscopy based on scanning probe microscopy. The excitonic luminescence is significantly enhanced when the conjugated polymer is coupled to the plasmon excitation at the metallic tip (tip-enhanced luminescence). This effect allows one to dramatically improve the detection efficiency of the excitonic luminescence and, consequently, resolve individual domains of the conjugated polymer in which the exciton will recombine before dissociation at the D/A interface.We have investigated the influence of the annealing temperature on the nanoscale exciton transport of P3HT:PCBM films using this tip-enhanced luminescence effect. Under thermal annealing conditions promoting the segregation of the donor and acceptor phases, a clear increase of the luminescence is seen from polymer-rich regions, consistent with domains of dimensions much larger that the exciton diffusion length. Direct measurements of the exciton diffusion length are also accessible in this setup.
12:15 PM - GG2.3
Effect of Solvent on the Nanoscale Phase Separation and Surface Potential Distribution in P3HT/PCBM Blends.
Pavel Dutta 1 , Yu Xie 1 , Dorin Cengher 1 , Jing Li 1 , David Galipeau 1 , Qiquan Qiao 1 , Venkat Bommisetty 1 Show Abstract
1 Electrical Engineering, South Dakota State University, Brookings, South Dakota, United States
The charge collection efficiency in bulk-heterojunction solar cells critically depends on the nanoscale morphology and electronic quality of the donor-acceptor (DA) network. Specifically, short exciton length necessitates nanoscale phase separation between DA phases with uniform potential distribution. This phase seperaration is determined by process conditions such as solvent, temperature, annealing time and annealing method. Present report details the effect of solvent on the morphology and phase separation of P3HT/PCBM heterojunction blends from chlorobenzene and two different isomorphs of dichlorobenzene (1, 2-dichlorobenzene and 1, 3-dichlorobenzene). The nanoscale morphology of the blends, electrical conductivity and surface potential distribution at DA interfaces was studied using scanning probe microcopy based methods. Atomic force microscopy images showed a strong influence of the solvent on morphology (both nanoscale and macroscale) and phase separation. Films prepared using chlorobenzene has largest clusters (~120 nm) whereas 1,2 dichlorobenzene produced smallest clusters (~40 nm). Surface potential images were acquired using scanning Kelvin probe force microscopy (KFM) to measure the effect of morphology and phase separation on local work-function differences between the DA phases. The phase and surface potential images corresponding to 1, 2-dichlorobenzene based blend show a uniform potential distribution induicating a homogenous phase DA separation. Random chain-like structures were observed in the 1, 3-dichlorobenzene based blend whereas chlorobenzene based blen has very narrow potential variation. These results may explain the reason for improved performance of the 1, 2-dichlorobenzene based devices due to short exciton diffusion length. Results connecting the belnd morphology and potential distribution with quantum effciency and solar cell performance will be presented.
12:30 PM - **GG2.4
Visualization of Charge Transport in Polymer Solar Cells: Influence of Electrodes.
Harald Hoppe 1 Show Abstract
1 Institute of Physics, Ilmenau University of Technology, Ilmenau, Thuringia, Germany
Charge transport through polymer solar cells requires not only proper nanoscale morphology of donor and acceptor domains within the photoactive layer, but also efficient extraction or injection of charge carriers at the organic-electrodes interfaces and their transport within the electrodes. All of these processes contribute to the total series and parallel resistances limiting the performance of these photovoltaic devices. Imaging methods such as electroluminescence and lock-in thermography were applied to visualize different aspects of charge transport of complete solar cells and monolithic modules. Patterns of charge transport obtained by these methods shall be correlated with device parameters and compared to simple calculations describing the current flow and associated dissipative losses within solar cells.
GG3/HH7/II2: Joint Session: Molecular Engineering of OPV Materials
Jao van de Lagemaat
Tuesday PM, April 06, 2010
Room 3001 (Moscone West)
2:30 PM - **GG3.1/HH7.1/II2.1
Conjugated Materials for Opto-electronics.
Seth Marder 1 , Xuan Zhang 1 , Timothy Steckler 2 , Raghunath Dasari 1 , Shino Ohira 1 , Stephan Barlow 3 , San-Hui Chi 1 , Joseph Perry 3 , William Potscavage 3 , Shree Prakash Tiwari 3 , Severine Coppee 1 , Stefan Ellinger 3 , Jean-Luc Bredas 2 , Bernard Kippelen 1 , John Reynolds 3 Show Abstract
1 School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Department of Chemistry, University of Florida, Gainesville, Florida, United States, 3 School of Electrical and Computer Engineering and Center for Organic Photonics and Electronics, University of Florida, Atlanta, Georgia, United States
In this presentation, I will review some recent results on conjugated materials with long wavelength absorptions for use in photovoltaic, nonlinear optical and organic field effect transistor applications.
3:00 PM - **GG3.2/HH7.2/II2.2
Materials for Photon Harvesting in Organic Solar Cells.
Paul Burn 1 , Paul Meredith 1 Show Abstract
1 Centre for Organic Photonics & Electronics, University of Queensland, Brisbane, Queensland, Australia
Polymers, small molecules and dendrimers have been used in the two main families of ‘organic’ photovoltaic devices, namely dye sensitised and bulk heterojunction solar cells. In the field of organic semiconductors the term ‘organic’ is generally used to include all organic materials as well as organometallic complexes. In both device architectures the devices are essentially excitonic in nature, that is, an exciton is formed before charge separation. Irrespective of the organic solar cell device platform (thin film bulk heterojunction or dye sensitised) the challenges for device architecture and materials design remain the same - one must absorb as much of the solar spectrum as possible and efficiently separate and transfer the generated charge. These processes require optimisation and careful design of the absorber (donor) and acceptor electronic properties and control of their nanophase behavior. The overall aim is to achieve this optimisation in materials that can be solution processed to create large area devices. In this presentation we will discuss recent progress in applying molecular engineering to optimize the properties of materials that can be used in organic solar cells. We will present the development of our latest materials: their synthesis, optical and electronic properties, processing, and device performance. We will discuss the relationship between the structure and photon harvesting capacity of the materials.
3:30 PM - **GG3.3/HH7.3/II2.3
Crystal Engineering for Organic Devices.
John Anthony 1 , Marsha Grimminger 1 , Ying Shu 1 , Oana Jurchescu 3 , Brad Conrad 3 , David Gundlach 3 , Yee-Fun Lim 2 , George Malliaras 2 Show Abstract
1 Chemistry, University of Kentucky, Lexington, Kentucky, United States, 3 Electronics and Electrical Engineering Laboratory, NIST, Gaithersburg, Maryland, United States, 2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
The primary and secondary crystal packing motifs of small organic semiconductors plays a critical role in their device performance. A the smallest scale, interactions between any neighboring molecules are directly related to the strength of electronic coupling between these molecules, and thus directly impact charge transport. However, the potential for excellent transport of charge carriers between two molecules is not a sufficient criterion for the development of high performance devices. The charge transport must be able to occur in a direction appropriate to the device being studied (in-plane, for devices such as transistors, and through-plane, for devices such as photovoltaics), which requires engineering materials to have either planes of pi-stacks, or columnar / slipped columnar arrangements. Finally, the overall morphology of the material can be controlled by secondary crystal packing arrangements. For example, materials with clear shear planes tend to exhibit the two-dimensional growth beneficial to transistors, while those where substituents interdigitate lead to three-dimensional crystal growth suited to photovoltaic applications. These requirements for crystal growth and film formation of course still must be paired with molecular functionalization to tune stability, optical properties and dominant carrier type.This talk will survey our work in designing chromophores for organic electronics, focusing in particular on the crystalline motifs that appear to yield the best performance for each device class. For transistor applications, we will present single-crystal studies that allow us to begin untangling the difficult relationship between crystal packing and field-effect mobility. We emphasize that not all pi-stacking relationships yield significant electronic coupling, and also show that small changes in intermolecular interactions can lead to dramatic changes in transport properties. Photovoltaic applications require a very different crystal packing. In a wide variety of electron-deficient pentacenes (and concomitantly, across a broad range of open-circuit voltages), certain crystal packing motifs repeatedly lead to the highest photocurrent generation. Discussion of this class of acceptors will focus on the features of the crystal packing that likely lead to improved bulk heterojunction performance.
4:00 PM - GG3/HH7/II2
4:30 PM - **GG3.4/HH7.4/II2.4
New Conjugated Polymers for Plastic Solar Cells.
Mario Leclerc 1 Show Abstract
1 Chemistry, Université Laval, Quebec City, Quebec, Canada
Photovoltaic cells (PCs) based on conjugated polymeric materials have received much attention due to their numerous advantages such as production on flexible and large-area substrates by solution processing which dramatically reduces the manufacturing costs. In such PCs, the absorption of photons leads to the formation of excitons that will dissociate into free charge carriers at an interface between two components with different electron affinities and ionization potentials. As the exciton diffusion length for most organic materials is below 20 nm, only excitons generated in a small region within ≤ 20 nm from the interface contribute to the photocurrent. To overcome this problem, bulk heterojunction (BHJ) structures have been widely used in polymer-based solar cells. In BHJ structures, the two components of the active layer form an interpenetrating network which creates a large donor/acceptor interface. For efficient exciton dissociation, a charge separation interface should be at the vicinity of exciton generation site and each component should form continuous pathway to the respective electrode. Therefore, appropriate control of the nanoscale morphology in the blend is necessary for efficient exciton dissociation and charge transport.Along these lines, the bulk heterojunction (BHJ) solar cells based on a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) derivatives have been widely investigated. Power conversion efficiencies (PCE) as high as 4.8-5.2% have been reported. To increase the efficiencies of single layer BHJ solar cell up to 10%, new polymeric materials should be developed. This lecture will focus on photovoltaic devices based on new poly(2,7-carbazole) and poly(2,7-germafluorene) derivatives; two promising classes of conjugated polymers based on bridged phenylenes. New polythiophene derivatives will also be discussed. It is our aim to establish some structure-property relationships and to identify the remaining challenges to develop conjugated polymers that could contribute to a new generation of efficient solar cells.
5:00 PM - **GG3.5/HH7.5/II2.5
Polymer Semiconductor Nanostructures for Electronic and Solar Energy Applications.
Samson Jenekhe 1 2 Show Abstract
1 Department of Chemical Engineering, University of Washington, Seattle, Washington, United States, 2 Department of Chemistry, University of Washington, Seattle, Washington, United States
Our laboratory is exploring a molecular and nanoscale engineering approach to readily processable and robust, high charge carrier mobility polymer semiconductors needed for developing the next generation high-performance light-emitting devices for displays and solid-state lighting, field-effect transistors, logic circuits, and low-cost solar cells. In this talk I will describe our recent investigation of the self-assembly, nanoscale morphology, charge transport, and electronic and optical properties of various classes of polymer semiconductor nanostructures. Among such examples of nanostructured conjugated polymers are polymer semiconductor nanowires and assemblies of block copolymers with widths of 5-20 nm and aspect ratios of up to 1000, which are readily assembled from solution and have been found to be promising building blocks for nanoelectronics and solar energy applications. The results show that polymer semiconductor nanostructures can have better properties and enable high performance devices than simple two-dimensional films.
5:30 PM - **GG3.6/HH7.6/II2.6
Development of New Semiconducting Polymers for Highly Efficient Polymer Solar Cells.
Yongye Liang 1 , Luping Yu 1 Show Abstract
1 Chemistry, The University of Chicago, Chicago, Illinois, United States
A series of new polymers, synthesized via the Stille polycondensation between the ester substituted 2,5-dibromothieno[3,4-b]thiophene and dialkoxyl thieobenzothiophene distannane monomers, has exceptional power conversion efficiency. These polymers are designed to possess low band gap to most effectively harvest solar energy. The quinoidal structures give these polymers high rigidity and relatively high charge carrier’s mobility. A detailed structural variation led us to discover several polymers that exhibit power conversion efficiency of 7.5 (after spectral correction) with a open circuit voltage of 0.74 V, short circuit current of 14.01 mA/cm2, and an fill factor of 68.64. These are the best results obtained so far for single-layer polymer solar cells.
GG4: Poster Session
Tuesday PM, April 06, 2010
Exhibition Hall (Moscone West)
6:00 PM - GG4.1
WO3 Bilayer Electrodes for Photoelectrochemical Cells.
Kwang-Soon Ahn 1 , Yu-Ri Kim 1 , Yun-Jung Kim 1 , Sang-Yeup Ahn 1 , Jung-Mi Kim 1 , Jung-Won Park 1 , Sung-Jong Yoo 2 , Yung-Eun Sung 2 Show Abstract
1 , Yeungnam University, Gyeongsan, Gyeongbuk, Korea (the Republic of), 2 , Seoul National University, Seoul Korea (the Republic of)
WO3 have been extensively studied in many other technological areas such as electrochromism, photocatalysis, and gas sensors, due to their nontoxic, stable, and native n-type semiconductor properties. The WO3 is also one of few inexpensive semiconductors resistant against photocorrosion in an acidic aqueous solution and its energy bandgap is approximately 2.7 eV for the crystalline WO3. These properties suggest the use of WO3 as a promising alternative to TiO2 in photoelectrochemical (PEC) cells. In this presentation, WO3 bilayer electrodes composed of the WO3 top and bottom layers were designed for photoelectochemical (PEC) cells. The bottom layers were sputter-deposited at high temperature (500 °C), leading to large grains and suitable electrical pathways for the carrier collection. The top layer deposited at low temperature (300 °C) consisted of small grains, leading to the large electrochemical reaction sites. Due to the combination of these favorable effects, the bilayer electrodes exhibited significantly enhanced PEC performances, as compared to the WO3 monolayer electrodes deposited at 300 ° and 500 °C, respectively. We expect that it should provide good insight when developing the multi-layered electrodes for the PEC applications including photocatalysts, electrochromism, and batteries.
6:00 PM - GG4.10
Efficiency Determining Mechanism of Platinum Content in Pt/Carbon Black Counter Electrode for Dye Sensitized Solar Cell.
Su-Young Lee 1 , Sang-Ho Kim 1 Show Abstract
1 , Korea University of Technology and Education, Cheonan Korea (the Republic of)
Dye sensitized solar cell (DSSC) has been attracted much attentions as a potential low-cost solar cell. Platinum has been commonly used for counter electrode materials in DSSCs for its high electric conductivity as well as chemical inertness with iodide in electrolyte. However, because the Pt is one of the most expansive precious metals, carbon black has recently been considered as a substitution material. Carbon black is inexpensive, and it has a low resistance, high eletrocatalytic activity and good I-/I3- redox properties. In this research, we investigated Pt added carbon black count electrode. The effect of Pt ratio to carbon black on the I-V, FF, Voc, Jsc and energy conversion efficiency (η) of built-in DSSC was investigated. To study how the Pt content give an effect on the each efficiency determining parameters such as FF, Voc, Jsc, the distribution of Pt in carbon black was observed using SEM, and I-/I3- redox reaction was measured using CV analysis, and the photon-electronic transformation and impedance properties were analyzed using IPCE and EIS. The energy conversion efficiency with pure Pt electrode was about 6.13%. And when 3wt% of Pt added in carbon black, the maximum energy conversion efficiency was 6.02% which is comparable value to that of pure Pt electrode. When content of Pt decreased from 5wt% to 3wt%, the energy conversion efficiency was sustained on a similar level, and further decreased below 3wt% to 1wt%, the energy conversion efficiency decreased up to 4.6%. That was caused mainly by lower Jsc and FF. It seemed to be related to impedance increase between counter electrode and electrolyte from the EIS analysis result. If the carbon black and Pt doping process will be improved smaller amount of Pt doped carbon black can replace Pt for counter electrode material in DSSCs.
6:00 PM - GG4.11
TiO2/Ni-Cr/TiO2 Multi-layers Electrode Prepared by RF Magnetron Sputtering for Efficient Dye Sensitized Solar Cells.
Min Suck Kim 1 , Jun Tak Kim 1 , Sang Ho Kim 1 Show Abstract
1 Materials Engineering, Korea University of Technology and Education, Cheonan-city, Chungnam, Korea (the Republic of)
In dye sensitized solar cells(DSSC), it is very important that electrons are injected into TCO electrode without recombination. Reduction of charge recombination at the TCO/TiO2 interface and electrolyte/TiO2 interface is indispensable to increase the energy conversion efficiency of DSSCs.In our research, Ni-Cr was deposited by RF magnetron sputtering as a BUS electrode layer between sputtered TiO2 electrode layers. Ni-Cr was investigated to enhance the electron injection from TiO2 to TCO electrode. To check an effect of BUS electrode layer in DSSCs, we compared two DSSC samples. The one was conventional sample fabricated by screen-printing method with only mesoporous TiO2 electrode, and another sample was fabricated by RF magnetron sputtering with TiO2/Ni-Cr/TiO2 multi-layers electrode. During the deposition, the substrate was heated at 400 °C, and carried out the post-annealing at 450 °C for 30min. To investigate the optical work function, grain structure, dye absorption property, conversion efficiency of cells, we used UV-vis spectrophotometer, XRD, FESEM, BET, IPCE, EIS, and solar simulator.In the result, the short circuit current density (Jsc) of cell used TiO2/Ni-Cr/TiO2 multi-layer electrode increased from 10.11mA/cm2 to 13.09mA/cm2, 29.5% and other cell parameters were enhanced, too. The improvement of cell efficiency was caused by increasing the electron transfer and charge collection between TiO2 and TCO electrode.
6:00 PM - GG4.14
Charge Carrier Transport in the Bulk and at the Surface of Nanoparticles: A Quasi-solid-state Dye-sensitized Solar Cell.
Dennis Friedrich 1 , Marinus Kunst 1 Show Abstract
1 Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany
Nanosystems offer excellent chances for the development of cheaper solar cells. The dye-sensitized solar cell (DSC) proposed by Grätzel et al.  is the most prominent example with an efficiency in the 10% range. In this cell the light excites a dye absorbed on a mesoporous TiO2 layer. An electron is injected from the excited dye into the bulk of the TiO2 nanoparticles and transported to the front electrode. The remaining positive charge is exchanged to a redox couple in solution and discharged at the back electrode. However, recent research attention has been focused on replacing the liquid electrolyte because a fluid filled solar cell is difficult to maintain over long time periods under periodic solar illumination. The possibility of many side (redox) reactions leads to instability and toxic components are present. A solar cell is presented, which avoids some of the disadvantages of the liquid DSC. This cell consists also of sensitized TiO2 nanoparticles but the electrolyte is reduced to a stable film on the surface of the nanoparticles. It still includes a redox pair (Iodine-Iodide), specific chemicals and also strongly bound H2O molecules. This cell has still a low efficiency of about 3% but it promises improved stability and a significant potential for further optimisation. The efficiency of the cell is mainly limited by the hole transport via the surface of the nanoparticles to the back electrode. Fortunately, the absence of a liquid electrolyte in this cell allows the application of several highly performing transient techniques to the cell as transient photoconductivity in the microwave frequency range (TRMC) and transient absorption (TA) measurements. Knowledge on kinetic charge transfer behaviour is not only important for the improvement of this cell but also for the understanding of charge transport in nanosystems in general. Contactless TRMC measurements under short circuit conditions show a faster decay of photoinduced charge carriers, indicating either an increase of existing, or the presence of additional decay channels. Moreover, it is observed that the presence of the dye molecules at the surface quenches partially recombination of excess charge carriers and that the surface transport of holes to the back electrode takes at least partially place via a surface network of redox molecules. B. O`Regan and M. Grätzel, Nature, 353(1991) p.737.
6:00 PM - GG4.15
Three-dimensional Nanostructure of Bulk-heterojunction Imaged by Scanning Electrical Potential Microscopy and High-resolution Scanning Time-of-Flight Secondary Ion Mass Spectrometry.
Wei-Chun Lin 1 , Bonnie Yu 1 , Che-Hung Kuo 1 2 , Wei-Ben Wang 3 , Szu-Hsian Lee 1 2 , Wei-Lun Kao 1 2 , Guo-Ji Yen 1 2 , Chia-Yi Liu 1 , Yun-Wen You 1 , Hsun-Yun Chang 1 , Chi-Ping Liu 1 3 , Jwo-Huei Jou 3 , Jing-Jong Shyue 1 2 Show Abstract
1 Research Center for Applied Scienses, Academia Sinica, Taipei Taiwan, 2 Department of Materials Science and Engineering, National Taiwan University, Taipei Taiwan, 3 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan
Solution-processable fullerene and copolymer bulk-heterojunctions are widely used as the active layer of solar cells. It is known that the controlled phase-separation in the film provides a pathway for carrier transportation and is crucial to efficiency. Therefore, control of the nanostructure is crucial to the development of polymer solar cells. However, there are few methods can observe the nanostructure inside the bulk of polymer layer directly. In this work, Scanning electrical potential microscopy (SEPM) and scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) are used to examine and reconstruct 3D distributions of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and region-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk-heterojunction. A cluster ion beam (C60+) and low energy Cs+ ion sputtering are used respectively to remove the surface in order to determine the structure below. With the 2D SEPM and ToF-SIMS images acquired from different depth, a 3D volume image is obtained through the image stack. In the volume image, 3D network with features around 50 nm is clearly observed. The effects of post-annealing on the distribution of molecules and on the efficiency of solar cells with different architectures are also discussed. It is clear that the fabrication parameter affect the carrier transportation significantly.
6:00 PM - GG4.16
Characterization of Larger-area Dye-sensitized Solar Cells With ZnO/TiO2 Assorted Electrode.
Xu Wang 1 , Haiyou Yin 1 , Bao Wang 1 , Lifeng Liu 1 , Yi Wang 1 , Jinfeng Kang 1 Show Abstract
1 Novel Device Group, Institute of Microelectronics, Peking University, Beijing China
Dye-sensitized solar cells (DSSCs) are being widely studied as a promising solar cell technology because of its low-cost and environmental-friendship. Although conversion efficiencies exceeding 10% have been reported for small-area cell of DSSCs, there are still bottleneck problems to block DSSCs for practical application such as the relatively lower conversion efficiency and large-area integration process. In this paper, we proposed and demonstrated a new method to improve the cell conversion efficiency of larger-area DSSCs by using sol-gel grown ZnO/TiO2 assorted nanoparticle layer and current-collecting metal fingers between TiO2 and the conducting F-doped Tin Oxide (FTO) films. In this study, the current-collecting metal fingers were first fabricated on a 50×50mm cell of FTO by physical vapor deposition(PVD). Then the ZnO layer, which was expected to be a good candidate as electron acceptor and transport material in DSSCs, was deposited on FTO layer by sol-gel process followed by a furnace annealing treatment. The SEM pictures of ZnO layer showed that the average diameter was about 50~80nm. The measurements of the light transmission spectrum indicated that the rate of penetrating light through ZnO/FTO was higher than that through FTO only between the wavelength of 536nm and 800nm, which indicated that the additional ZnO enhanced the transmission of the effective light spectra for DSSCs. After deposition of ZnO layer, TiO2 nanoctrystalline layer was screen printed on ZnO layer to form the assorted layer. Then, 50×50mm DSSC cells were fabricated by using a novel larger-area integration process. The electrical characterization results of the large area DSSCs with and without a ZnO layer indicate that the intruduction of the ZnO layer will help to enhance the open circuit voltage Voc because of the inserted ZnO layer increasing the efficiency of electron transported to FTO. As a restult, a Voc increase of 50 mV and a conversion efficiency rise were achieved with about 250 nm ZnO thickness. Our studies also showed that the conductivity of the inserted ZnO layer was a key factor to influence the short circuit current of the DSSCs. By using Al or Ga doping into ZnO layer, the ZnO layer conductivity and the short circuit current as well as the conversion efficiency of the DSSCs were enhanced. The novel larger-area integration process mentioned above including the current-collecting metal fingers technique is critical to achieve higher conversion efficiency in the large-area DSSCs. Reference: Grätzel M., Journal Photochemistry and Photobiology A: Chemistry 2004, 164: 23–14.
6:00 PM - GG4.17
Improved Conversion Efficiency of Dye Sensitized Solar Cells by Using Novel Complex Nanostructured TiO2 Electrodes.
Tianshu Zhang 1 , Haiyou Yin 1 , Xu Wang 1 , Bao Wang 1 , Yan Wang 1 , Lifeng Liu 1 , Yi Wang 1 , Jinfeng Kang 1 Show Abstract
1 , Institute of Microelectronics, Beijing China
Dye-sensitized solar cells (DSCs) have great appeal for interest in academic studies and industrial applications due to its low-cost and environmental-friendship. DSC typically consists of transparent conductive oxide (TCO), a nanocrystalline TiO2 film covered by a monolayer of dye molecules, redox electrolyte, and counterelectrode. The electron-collecting layer is usually a TiO2 nanoparticulate film with a three-dimensional network of interconnected nanoparticles. However, there are imperfect factors to limit the current conversion efficiency (IPCE) including the recombination of I3+ ions in redox electrolyte with the injected electrons from the photon-excited dye to TCO and the poor contact interface between the TiO2 nanoparticulate film and TCO.In this work, a method to solve these problems by using a novel complex nanostructured TiO2 electrode consisting of a nanoporous and a nanoparticulate films to replace a single TiO2 nanoparticulate film was presented. This nanoporous TiO2 film was grown by anodizing the sputtering titanium film deposited on the cleaned fluorine-doped tin oxide (FTO) glass substrate by using HF-based electrolyte then annealed in oxygen. TiO2 nanoparticulate film was screen printed on the layer of nanoporous TiO2 film and then annealed in oxygen at 480°C for 1 hour. Fabrication variables crucial to obtaining the optimized nanostructured TiO2 electrodes include the originally amorphous nanoporous array, annealing temperature and time of the anodized, the number of screen printing and sputter deposition variables including the thickness of titanium film and anti-sputter effect. By adopting the optimized combination of these variables, we increased the IPCE by 17% under AM 1.5 sunlight, compared to those only using TiO2 nanoparticulate film as electrodes. Meanwhile, with the same thickness, this layer of nanoporous TiO2 film has a larger average refractive index than TiO2 nanoparticulate film. The enhanced performance of DSCs by using this novel complex nanostructured TiO2 electrode could be owed to its larger average refractive index than the single TiO2 nanoparticulate film and the suppressed recombination of I3+ ion and the electrons of TCO due to the blocking effect of the nanoporous TiO2 film as well as the improved contact interface between the nanoporous TiO2 film and TCO.
6:00 PM - GG4.19
Origin of the Reduced Efficiency in Aged Organic Solar Cells.
Andreas Baumann 1 , Jens Lorrmann 1 , Julia Schafferhans 1 , Alexander Wagenpfahl 1 , Carsten Deibel 1 , Vladimir Dyakonov 1 2 Show Abstract
1 Experimental Physics 6, Julius-Maximilians-University of Würzburg, Würzburg Germany, 2 , Bavarian Center for Applied Energy Research e.V. (ZAE Bayern), Würzburg Germany
Organic solar cells are getting more and more in the focus of scientific research in view of applications for energy conversion. As the device efficiencies are increasing to a point where commercial aspects become tangible, the lifetime of organic solar cells - the stability versus degradation - deserves further attention. In order to improve the stability of organic solar cells against aging and oxygenation, a logical first step is to gain a detailed understanding of the processes responsible for the decreasing solar cell efficiency. We investigated the influence of synthetic air on organic solar cells based on the material combination P3HT:PCBM in dependence on exposure time. The power conversion efficiency of the solar cells of around 3.5 % in the unexposed case decreased with increasing time on synthetic air. We found that the observed efficiency drop is mostly due to a decrease in the short circuit current density J_sc by more than 50 % after 120 hours, whereas the open-circuit voltage V_oc and the fill factor FF remain almost unchanged. We observed a considerably faster degradation process when the solar cells are exposed to synthetic air and white light simultaneously. The experimental technique of Photo-CELIV (Charge Carrier Extraction by Linearly Increasing Voltage) was used by us to investigate the mobility and density of extracted charges. In the as-cast organic solar cells we determine the mobility to be around 2x10-3 cm2/Vs, showing almost no intrinsic charges in the dark. With increasing exposure time the mobility of charge carriers decreases only slightly. However, during the degradation cycle not only one, but two distinguishable extraction peaks can be observed in the Photo-CELIV signal, indicating charge carrier fractions with two different transit times. This is an indication that electrons and holes, having almost balanced mobilities in non-degraded samples , develop differently in degraded solar cells. Furthermore, the amount of intrinsic charge carriers increases when exposed to synthetic air and is even more increased by several orders of magnitude when exposed to air and light simultaneously. The recombination rates increase accordingly. Feeding the experimentally determined parameters into our macroscopic simulation program, we are able to model the experimental current-voltage characteristics well. We conclude that oxygen-induced doping of the active layer as well as imbalanced electron-hole mobilities are responsible for the decreasing short circuit current in organic solar cells exposed to synthetic air and light. A. Baumann, J. Lorrmann, C. Deibel, V. Dyakonov, Appl. Phys. Lett. 93, 252104 (2008)
6:00 PM - GG4.20
Tuning the Band Offset Between PbS Quantum Dots and Vertically Aligned TiO2 Nanorods in PEDOT/PbS/TiO2 Cell Structures.
Jason Rejman 1 , Dino Ferizovic 1 , Martin Munoz 1 , Pritish Mukherjee 1 , Sarath Witanachchi 1 Show Abstract
1 Physics, University of South Florida, Tampa, Florida, United States
The absorption of UV photons by both PbS and PbSe quantum dots in the size range of 2-10 nm can generate multiple excitons. When embedded in a second medium such as a TiO2, the dissociation of these excitons to produce free charge carriers is affected by the band offset at the interface. When TiO2 nanorods are used as the electron transport medium, efficient transfer of electrons from the PbSe QD to TiO2 takes place for QD sizes below 2.5 nm. However, much larger size QDs of PbS can satisfy the required band alignment for efficient electron transfer. In this study TiO2 nanorod-PbS QD structures were fabricated by a two-step process. The vertically aligned TiO2 nanorods were grown by a hydrothermal process on glass substrates that were coated with a conducting fluorine-doped tin oxide film. Nanorods have an average diameter of about 200-250 nm and a length of about 1 μm. Noaoparticles of PbS with average diameters of 4-6 nm were grown by a solvothermal method. The nanoparticles were dispersed in hexane and deposited on the TiO2 nanorods by a Laser Assisted Spray (LAS) process. This method allowed the growth of surfactant-free nanoparticle on the TiO2 nanorods, providing a direct contact between the two structures. Subsequently, a layer of the polymer PEDOT followed by Al electrodes were deposited to form a cell structure. We have investigated the photocurrent generated in these cells with different PbS QD sizes. The results relating the size of the QDs to the efficiency of photocurrent generation will be presented.
6:00 PM - GG4.3
Dye Sensitized TiO2 Nanotube Solar Cell With Markedly Enhanced Performance via Rational Surface Engineering.
Jun Wang 1 , Zhiqun Lin 1 Show Abstract
1 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Highly ordered anodic TiO2 nanotube arrays fabricated by electrochemical anodization were sensitized with ruthenium dye N-719 to yield dye sensitized TiO2 nanotube solar cell. Rational surface treatments on photoanode TiO2 nanotubes markedly improved the device performance. With TiCl4 treatment in conjunction with oxygen plasma exposure under optimized conditions, dye sensitized TiO2 nanotube solar cells produced by using 14 µm thick TiO2 nanotube arrays in backside illumination mode subjected to simulated AM 1.5 G irradiation of 100 mW/cm2 exhibited a pronounced power conversion efficiency, PCE of 7.37%. The improvement of PCE is believed to benefit from both TiCl4 treatment and oxygen plasma exposure on the TiO2 nanotubes.
6:00 PM - GG4.4
Studies of Charge Transfer-transport Processes at Donor-Acceptor Interface in a Bilayer Transistor Using Transient Current.
K Narayan 1 Show Abstract
1 , JNCASR, Bangalore India
Bilayer FET’s with n-channel transport were recently fabricated, with PCBM-C60 as the active transporting layer and P3HT polymer as the top layer. P3HT layer serves exclusively as a source for photocarriers which can be transferred to the PCBM-C60 layer.(1) The electrostatic dependent photophysical and charge transfer processes at the D-A interface can be followed and studied upon photoexcitation by measuring IDS transients. This device architecture and technique allows us to monitor charge transfer state formed at the D-A interface. We extend this approach of bilayer FETs with naphthalene-bis(dicarboximide) P(NDI20D-T2) as the acceptor polymer and P3HT or PCPDTBT as the donor polymeric layer. The features induced in the gate voltage dependence of the channel current upon photoexcitation can be traced to the spatial origin of the carriers. We correlate the charge diffusion length and life-time obtained from our measurements to the values obtained from conventional two terminal transverse measurements. 1) Studies of charge transfer processes across donor-acceptor interface using a field effect transistor geometry. Applied Physics Letter, 95, xxxx (2009) [In- Press]
6:00 PM - GG4.5
Analysis of Charge Transport and Recombination Studied by Electrochemical Impedance Spectroscopy for Dye-sensitized Solar Cells With Atomic Layer Deposited Metal Oxide TiO2 Surface Treatment.
Braden Bills 1 , Mariyappan Shanmugam 1 , Mahdi Farrokh Baroughi 1 , David Galipeau 1 Show Abstract
1 Electrical Engineering and Computer Science, South Dakota State University, Brookings, South Dakota, United States
The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the porous titanium oxide (TiO2) nanoparticle network to the electrolyte solution occurring almost exclusively through the interface. DSSCs have a much larger (~1000 times) interfacial area compared to conventional solar cells; thus making their performance greatly dependant on the density and activity of TiO2 surface states.Recent works have shown DSSC performance enhancements when a wide range of wet chemical processed materials were deposited onto the porous TiO2 network [1-2]. A new approach for depositing an interfacial layer onto the porous TiO2 network is by atomic layer deposition (ALD) [3-4], which is advantageous over wet chemical method for treating defective interfaces with metal oxides due to the higher purity of gas precursors, faster diffusion of gas precursors into the porous network, and better conformallity and controllability of metal oxide thickness.ALD aluminum oxide (Al2O3) and hafnium oxide (HfO2) ultra thin layer was grown on the surface of porous TiO2 and its effects on the performance of DSSCs were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy (EIS) measurements. It was found that these interfacial layers changed the surface properties of TiO2, resulting in the overall photoconversion efficiency being enhanced by 19 and 69% for 20 cycles of Al2O3 and 5 cycles HfO2 treatment respectively. Reasons for the improved photovoltaic performance were explained with charge transport, accumulation and transfer (recombination) EIS parameters. These impedance parameters strongly agree with dark and illuminated current-voltage measurements. The largest improvement in DSSC performance was observed by DSSCs with 5 cycles of HfO2 interfacial treatment, where the performance enhancement was explained by the reduced density of easily accessible electronic states and low recombination rate of electrons in TiO2. DSSCs with 20 cycles of Al2O3 showed a marginal increase in performance due to improved electron transport. The results also showed that electron transport occurs through surface states observed by the apparent tradeoff between worsened electron transport and reduced density of electronic states when using metal oxide interfacial layers. References Kay, A.; Grätzel, M. Chem. Mater. 2002, 14, 2930. Palomares, E. Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Chem. Commun. 2002, 14, 1464. Lin, C.; Tsai, F. Y.; Lee, M. H.; Lee, C. H.; Tien, T. C.; Wang, L. P.; Tsai, S. Y. J. Mater. Chem. 2009, 19, 2999. Hamann, T. W.; Farha, O. K.; Hupp, J. T. J. Phys. Chem. C 2008, 112, 19756.
6:00 PM - GG4.6
Highly Efficient Liquid and Quasi-solid Dye Sensitized Solar Cells With Mesoporous Carbon Counter Electrodes.
Easwaramoorthi Ramasamy 1 , Jinwoo Lee 1 Show Abstract
1 , Pohang University of Science and Technology(POSTECH), Pohang Korea (the Republic of)
Dye-sensitized solar cells (DSSCs) have received widespread interest because of their potential for low cost solar to electric energy conversion. Recent advances in the energy conversion efficiency and thermal stability have further increased the prospects of DSSCs for practical applications. Usually, fluorine doped tin oxide glass substrate loaded with Pt employed as counter electrode for tri-iodide reduction in DSSCs. Since Pt is expensive and less stable in iodide/tri-iodide redox couple electrolyte, several attempts are made to develop inexpensive and stable counter electrodes. Here we present highly efficient liquid and quasi-solid DSSCs with ordered mesoporous carbon counter electrodes. Large size mesopores with an interconnected pore structure facilitated the diffusion of redox species and amorphous carbon wall act as catalytic site for tri-iodide reduction, thus carbon counter electrode DSSCs exhibit analogous charge transfer resistance and energy conversion efficiency in comparison with the conventional Pt counter electrode based DSSCs.
6:00 PM - GG4.7
Polymer Fullerene Solar Cells With Interpenetrating P3HT Network Structure via Solubility Induced Crystallization.
Ju Hyun Kim 1 , Jong Hwan Park 2 , Myungsun Sim 2 , Kilwon Cho 1 2 Show Abstract
1 School of environmental science and engineering, POSTECH, Pohang Korea (the Republic of), 2 Department of chemical engineering, POSTECH, Pohang Korea (the Republic of)
Thermal annealing is a widely used process to control the morphology and crystallinity of polymer solar cell based on poly(3-hexylthiophene) (P3HT) and ([6,6]-phenyl C61 butyric acid methyl ester) (PCBM). However, optimized morphology of the blend film is difficult to obtain, because both P3HT crystallization and PCBM diffusion occur simultaneously and interfere with each other during the thermal annealing process. In this study, we have demonstrated the controlled morphology of the blend film using two separate steps: P3HT crystallization by marginal solvent and subsequent phase separation by mild thermal annealing. Morphological changes of the active layers spin-cast from marginal solvent with varying annealing temperatures were studied systematically and compared to those of the films spin-cast from good solvent. With the help of interpenetrating network structure, power conversion efficiency reaches as high as 4.07 % due to enhanced charge transport. Hole and electron mobilities are increased substantially to 1.6×10-3 cm2V-1s-1 and 1.4×10-3 cm2V-1s-1, respectively in the process of P3HT crystallization and subsequent phase separation. Photovoltaic performances are improved with increased film thicknesses due to the interpenetrating donor/acceptor network structure formed by the proposed process.
6:00 PM - GG4.8
High Detectivity Infrared Organic Photodetectors.
Do Young Kim 1 , Dong Woo Song 1 , Franky So 1 Show Abstract
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Organic infrared photodetectors are attractive because of compatibility with flexible substrates, low cost process, and large area applications. An important figure of merit in photodetectors is detectivity which is related to the quantum efficiency and the dark current, and therefore low dark current in photodetectors is extremely important to high performance photodetectors. In this study, we report infrared organic photodetectors with different device architectures to reduce the photodetector dark current. Tin (II) phthalocyanine (SnPc) with an absorption band in the range of 600 ~ 1000 nm was used for infrared sensing. 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and bathocuproine (BCP) were used as electron blocking layer and hole blocking layers, respectively. We fabricated the following infrared photodetectors with different carrier blocking layers: (1) ITO/SnPc:C60/Al, (2) ITO/TAPC/SnPc:C60//Al, (3) ITO/SnPc:C60/BCP/Al, (4) ITO/TAPC/SnPc:C60/BCP/Al, and (5) ITO/MoO3/TAPC/SnPc:C60/BCP/Al. Both TAPC and BCP blocking layers decrease significantly the dark current density. The maximum detectivity in device (4) is 1.7 × 1012 cmHz1/2/W at 740 nm wavelength under an applied voltage of - 1.1 V. To further reduce the dark current and enhance the quantum efficiency, a MoO3 interlayer is inserted between the TAPC and SnPc:C60 layers. The maximum external quantum efficiency of the resulting device is 90 % at 740 nm under an applied voltage of - 5 V in device (5). The maximum detectivity is 2.3 × 10E12 cmHz1/2/W at 740 nm under an applied voltage of -1.1 V in device (5). A systematic study of these devices and the underlying device physics will be presented.
6:00 PM - GG4.9
Photoconductivity Measurements of Organic Polymer/Nanostructure Blends.
D. Black 1 , S Paul 1 Show Abstract
1 Emerging Technologies Research Centre, De Montfort University, Leicester United Kingdom
Hybrid organic/inorganic photovoltaic devices comprising a blend or blends of polymers and inorganic nanostructures offer the potential of flexible devices with a small number of steps in the fabrication process. Photovoltaic devices are excitonic in nature, when an organic photoconductive material absorbs a photon of an appropriate wavelength, an excited state is created. It has been demonstrated by other workers that the efficiency of photovoltaic devices is increased by incorporating nano-particulates in the organic photoconductive materials. A blend of photoconductive organic materials and ferroelectric nano-particles is prepared. An in-depth studies has been undertaken in our lab to understand photoconductivity of the blend. By increasing the concentration of ferroelectric nanoparticles in blend; the relative permittivity of the blend is increased above that of the pure photoconductive material, this in turn has the effect of increasing the photoconductivity of the material. The resulting material is particularly suitable for applications in organic photovoltaic cells. By incorporating ferroelectric nanoparticles we have demonstrated that there is an increase in photoconductivity of both poly(3-hexylthiophene) and dihexyl-sexithiophene. A blend of organic photoconductor and ferroelectric nano-particles was prepared in organic solvents and spin-coated onto a glass substrate coated with ITO ; a top Al contact was then evaporated onto the blend after drying. An electrical photoconductivity analysis of these structures was performed using an HP4140B picoammeter and a solar simulator. The spectroscopic studies (FTIR and UV-VIS) were carried out to understand the photoconductivity measurements.
Venkat Bommisetty South Dakota State University
Niyazi Serdar Sariciftci Johannes Kepler University of Linz
K. S. Narayan Jawaharlal Nehru Centre for Advanced Scientific Research
Garry Rumbles National Renewable Energy Laboratory
GG5: DSSC I/Hybrid Solar Cells
Wednesday AM, April 07, 2010
Room 3002 (Moscone West)
9:45 AM - GG5.1
Charge Transfer Studies of CdSe/Conjugated Dendrimer Blends to Study the Contribution of Inorganic Semiconductor Nanoparticles in Photovoltaic Device Performance.
Smita Dayal 1 , Nikos Kopidakis 1 , Dana Olson 1 , Benjamin Rupert 1 , David Ginley 1 , Garry Rumbles 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
We report on the charge transfer studies of solar cells composed of CdSe branched nanoparticles/phenyl-cored thiophene dendrimer composite active layer. High monodispersity and a high degree of molecular ordering make dendrimers ideal candidates to study the structure-property relationship in organic-inorganic blends for photovoltaic applications. A better understanding of the charge transfer of such composites is expected to guide their design and lead to higher efficiencies. In particular, the question whether each of the two components is photoactive has not yet been addressed in conjugated molecule / QD composites. Here we use active layers composed of a bulk heterojunction of CdSe QDs and phenyl-cored thiophene dendrimers to investigate the electron and hole transfer upon excitation of dendrimer or CdSe nanoparticles, respectively. The high bandgap dendrimers used here allow us to separate the absorption band of the dendrimer from that of the CdSe, so that we can selectively excite each component. The generation, transport and recombination dynamics of carriers is studied using the contactless flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique. We find that an efficient charge transfer upon exclusive excitation of nanoparticles occurs indicating the CdSe Nanoparticle contribution to the overall device performance. We have also used a low band-gap polymer 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) to study the electron transfer upon exclusive excitation of the organic component. We discuss the correlation of the TRMC results with the device efficiency measurements.
10:00 AM - GG5.2
A Unified Charge Transport Model for Bulk and Interfaces of Inorganic-Organic Mediums.
Mahdi Farrokh Baroughi 1 , Sadegh Mottagian 1 , Venkat Bommisetty 1 , Matt Bisecker 1 , Jun-Haung Kimn 1 Show Abstract
1 Electrical Engineering and Computer Science, South Dakota State University, Brookings, South Dakota, United States
Today, inorganic photovoltaic materials and devices are being challenged by organic and organic-inorganic hybrid solar cell technologies due to their low fabrication cost, easy integration and manipulation, flexibility, tailorability, and disposability. Unlike ordered inorganic semiconductors, inorganic and organic tissues in inorganic-organic solar cells, such as dye sensitized and bulk heterojunction devices, are made of nanoscle to microscale disordered mediums that seriously disturb charge transport in these mediums and particularly in the interface regions. The challenge of modeling of such structures not only lies upon the geometrical complexity of different medium but also on the diversity of charge carrying particles (electrons, holes, exitons, polarons, and bipolarons) in such systems. Furthermore, formulation of defects in the bulk and particularly at the organic-organic and organic-inorganic interfaces is very challenging. For example, dye sensitized and bulk heterojunction solar cells utilize very large surface area nanostructured heterointerfaces, often 1000 to 10000 times larger junction surface than their planar counterparts, to effectively trap and absorb sunlight. Large surface area is associated with large density of surface defects and often would lead to interface limited performance of solar cells. This article presents a unified model for transport of electric charges in inorganic and organic mediums as well as charge injection at the organic-organic and inorganic-organic interfaces. The model uses a generalized density of state model, adopted from disordered semiconductors, for representing bulk and surfaces of inorganic and organic material and includes drift and diffusion of charge particles (electrons, holes, polarons, and bipolarons), diffusion of excitons, dissociation of excitons at heterointerfaces, field assisted dissociation of excitons, and field emission of polarons at heterointerfaces. The model is composed of Poisson’s equation and continuity equations for electrons, holes, excitons, and polarons.
10:15 AM - GG5.3
A Novel Dye Sensitized Solar Cells With Enhancing Conversion Efficiency.
Bao Wang 1 , Ziqing Lu 1 , Haiyou Yin 1 , Bing Sun 1 , Lifeng Liu 1 , Jinfeng Kang 1 Show Abstract
1 Microelectronics, Peking University, Beijing China
Dye-sensitized solar cells (DSSCs) have attracted widely research interest as one of the major candidates for the third generation solar cell technology due to its low-cost and environmental-friendly. However, the most efficient Grätzel cell remained conceptionally unchanged and the maximum efficiency stagnated at approximately 11%. There are some processes such as the electron’s recombination with the oxidized dye and the interception by redox shuttle that block the efficiency increase of DSSCs . Although it has been proved that the interception plays the most important role in reducing cell efficiency, little improvement on cell performance can be achieved based on the traditional structure of DSSCs.In this article, a simulation program by taking into account four charge carriers (electron, redox pair and cation in electrolyte) coupled by the electrostatic potential and Drift-Diffuse Equations was developed to model the DSSCs behaviors. The simulation indicates that the quasi-Fermi level (chemical potentialφa) of the carriers and the generation term G is dependent on the intensity of light while the recombination rate R is greatly related to the electron interception by redox shuttle. The conversion efficiency can be effectively enhanced by increasing the generation term G and chemical potential φa.The recombination rate R is relatively decreased when the density of I3- is reduced and the density of I- in the electrolyte can be remained at a higher level. In this case, the short current of the cell is expectably increased and the open circuit voltage can also be improved. Based on this, a novel DSSCs by using a composite structure counter-electrode (cathode) is proposed to enhance the conversion efficiency of DSSCs. The stimulation shows that the new DSSCs can significantly enhance the ideal conversion efficiency. The practical conversion efficiency could be anticipated higher than 11% in the new structured DSSCs.  Thomas W. Hamann, Rebecca A. Jensen, Alex B.F.Martinson, Hal Van Ryswyk, Joseph T.Hupp, Advancing beyond current generation dye-sensitized solar cells, Energy & Environmental Science, July 66-78, 2008.
10:30 AM - GG5.4
Nanoscale Carrier Transport in TiO2 Thin Films by Current Sensing Atomic Force Microscopy.
Swaminathan Venkatesan 1 , Mariyappan Shanmugam 1 , Pavel Dutta 1 , Mahdi Baroughi 1 , David Galipeau 1 , Venkateswara Bommisetty 1 Show Abstract
1 , SDSU, Brookings, South Dakota, United States
Titanium dioxide (TiO2) is an important material for a wide variety of cost-effective photovoltaic technologies . Charge transport in TiO2 is strongly influenced by the density and distribution of defect states at grain boundaries. Distribution of grains and grain boundaries, defect density and bandgap play a vital role on carrier transport and lifetime in TiO2 [3, 4, 5]. Grain boundaries in TiO2 act as electron donors due to the presence of oxygen vacancies and titanium interstitials. Here, we analyze the carrier transport through grains and grain boundaries of nanocrystalline TiO2 deposited by RF sputtering using high resolution conducting atomic force microscopy (C-AFM) and scanning Kelvin probe force microscopy (KFM). TiO2 thin films were deposited on glass substrates were annealed at temperatures between 150° C and 450°C. Annealing caused significant changes in the surface morphology, nanoscale electrical conductivity and optical bandgap. Morphological changes include increased grain size (by ≈ 100%) and RMS surface roughness (by 30%). KFM measurements indicate gradual changes in the magnitude of surface potential with annealing temperature, specifically across grain boundaries. Annealing at 450C changed the crystallographic orientation of the thin films from rutile to anatase. This change was associated with a 200 mV increase in average potential and large change in grain sizes. Illuminated I-V measurements showed the resistivity of the TiO2 films varied in the range of 10 – 100 MΩ-cm. C-AFM measurements showed increased electrical conductivity with increased annealing temperature. In all cases, annealed TiO2 has higher conductivity compared to the as-deposited films. Temperature dependent I-V measurements are underway to estimate changes in the Fermi level and activation energy with annealing temperature. First-principles simulations will be used to explain the effect of local bandgap and defect distribution on nanoscale charge transport processes. References Journal of Physics D: Applied Physics 33 (2000) 2683–2686. Journal of Electroanalytical Chemistry 605 (2007) 151–156 J. Appl. Phys., 96(2004) 1556-1562 J. Phys. Chem. C (2007), 111, 9769-9778 J. Phys. Chem. 1994, 98, 11733-11738
10:45 AM - GG5.5
Understanding Charge Transport and Recombination in Dye-sensitized Solar Cells Using an Experimentally Validated Coupled Optical and Electrical Model.
Sophie Wenger 1 , Matthias Schmid 2 , Guido Rothenberger 1 , Michael Graetzel 1 , Juergen Schumacher 2 Show Abstract
1 Laboratory of Photonics and Interfaces, Ecole Polytechnique Fédérale de Lausanne, Lausanne Switzerland, 2 Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), Winterthur Switzerland
The mathematical description of charge transport and recombination in dye-sensitized solar cells requires equations specific to nanostructured electrochemical devices. Charge transport in the mesoporous TiO2 film is generally assumed to be purely diffusive (no drift component) due to the high ionic strength of the electrolyte permeating the pores. Experiments have repeatedly shown “electron lifetimes” in the ms range, which depend on illumination intensity, suggesting a trapping-limited transport. The electron diffusion length is in the order of the TiO2 film thickness (~10 µm) – there is still a debate on how to measure this value correctly though – which induces a photovoltage of about 0.8 V under full sun illumination. We have previously developed a validated optical model based on coherent and incoherent optics to accurately calculate the charge generation function in a complete device. The optical model is coupled to a linear electrical transport model via the charge generation function as source term. From the analytical solution, one can calculate the steady-state behavior, e.g. I-V curves. This paper presents our next steps in developing a spatially resolved time-dependent model to calculate the response of the cell to small perturbations (e.g. illumination or applied potential). Here, the trap distribution and the electron recombination route via the TiO2 conduction band and possibly via surface states come into play. From the numerical solution of this extended model one can extract electron lifetimes and electron diffusion coefficients from time-dependent measurements and further simulate the parameters for any defined experimental condition, e.g. at the maximum power point.S. Wenger, M. Schmid, G. Rothenberger, M. Grätzel, and J. Schumacher. Model-based optical and electrical characterization of dye-sensitized solar cells. Proceedings of the 24th European Photovoltaic Solar Energy Conference and Exhibition, September 21-25, Hamburg, Germany, 2009.
11:00 AM - GG5: DSSC-2
GG6/HH8/II4: Joint Session: Advancing Organic Photovoltaics I
Wednesday PM, April 07, 2010
Room 3001 (Moscone West)
11:30 AM - **GG6.1/HH8.1/II4.1
``Plastic” Solar Cells: Self-assembly of Bulk Heterojunction Nano-materials by Spontaneous Phase Separation.
Alan Heeger 1 Show Abstract
1 Center for Polymers & Organic Solids, UC Santa Barbara, Santa Barbara, California, United States
The achievement of 6% power conversion efficiency and the demonstration of quantum efficiencies approaching 100% have been demonstrated: Each photon absorbed leads to a (positive and negative) pair of mobile charge carriers, and all the photo-generated charge carriers are collected at the electrodes. Higher efficiencies will come from improved harvesting of the photons from the solar spectrum using new semiconducting polymers designed and synthesized for use in “plastic” solar cells.TEM studies of thin films, of cross-sectional images “sliced” from this films have revealed the details of the morphology in BHJ solar cells.Progress during recent months will be summarized.
12:00 PM - **GG6.2/HH8.2/II4.2
Bulk Heterojunction PV from the Acceptor Perspective.
Jan Hummelen 1 Show Abstract
1 Stratingh Institute for Chemistry, University of Groningen, Groningen Netherlands
There is ample room for power conversion efficiency improvement of plastic solar cells. Progress on improved donor polymer ingredients recently led to substantial efficiency enhancement. At the other side, much can be gained at the acceptor side as well. We will give an overview of recent developments on tailored and electronically optimized fullerene-based acceptor materials for bulk heterojunction application. We will address recent investigations on the dynamics of hole transfer in various polymer:methanofullerene blends. For the first time, time constants for hole transfer in these blends have been determined using ultra-fast optical spectroscopy. Furthermore, we report on our recent reconnaissance studies concerning advanced photon management (i.e. photon up-conversion) inside bulk heterojunction composites.
12:30 PM - **GG6.3/HH8.3/II4.3
The Effect of Three-dimensional Morphology on the Efficiency of Hybrid Polymer Solar Cells.
Rene Janssen 1 , Stefan Oosterhout 1 , Martijn Wienk 1 , Jan Gilot 1 , Jan Anton Koster 1 , Ralf Thiedmann 2 , Volker Schmidt 2 , Svetlana Van Bavel 3 , Joachim Loos 3 Show Abstract
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands, 2 Institute for Stochastics, Ulm University, Ulm Germany, 3 Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, Eindhoven Netherlands
Hybrid conjugated polymer – metal oxide photovoltaic devices have been prepared with AM1.5 energy conversion efficiencies of about 2% via an in-situ approach in which a metal oxide network is formed in the semiconducting polymer layer via reactive spin coating and moderate thermal annealing. High resolution three-dimensional experimental data on the morphology and phase separation of these bulk heterojunction solar cells will be presented that provide unprecedented insights into the actual heterojunctions present in the films. The phase separation will be related to the device performance via exciton diffusion and exciton quenching, and the presence of suitable continuous percolation pathways for photogenerated charge carriers to reach the respective electrodes in both phases. The data provide a unique new insight in the operation of bulk heterojunction devices and provides directions to further improvements.
Venkat Bommisetty South Dakota State University
Niyazi Serdar Sariciftci Johannes Kepler University of Linz
K. S. Narayan Jawaharlal Nehru Centre for Advanced Scientific Research
Garry Rumbles National Renewable Energy Laboratory
GG8: Carrier Dynamics
Thursday AM, April 08, 2010
Room 3002 (Moscone West)
9:30 AM - **GG8.1
Dynamics of Light-induced Charge Carriers in Conjugated Poly(alkylthiophenes) Blended With PCBM.
Tom Savenije 1 , Wojciech Grzegorczyk 1 , Tieneke Dykstra 1 , Jorge Piris 1 , Juleon Schins 1 , Laurens Siebbeles 1 Show Abstract
1 Faculty of Applied Sciences, Delft University of Technology, Opto-Electronic Materials Section, Delft Netherlands
The morphology, time-resolved transient optical absorption and photoconductance of blends of poly(3-hexyl thiophene) (P3HT) or the poly(thienothiophene) derivatives PATBT, pBTTT and pBTCT with PCBM were studied. For P3HT:PCBM blends the quantum yield for charge carrier photogeneration was found to be virtually independent of temperature for timescales up to tens of nanoseconds after photoexcitation of P3HT. The decay of charges due to recombination and/or trapping on longer times becomes faster at higher temperature, as a result of thermally activated electron and hole mobilities. The quantum yield increases by an order of magnitude on going from a bend film spin-coated from chloroform to an annealed film prepared from dichlorobenzene. Implications of theses observations for the mechanism of free charge carrier generation are discussed. On thermal annealing of PATBT:PCBM blend extensive phase segregation of the polymer and PCBM domains is observed. Annealing of pBTTT:PCBM and pBTCT:PCBM yields a bilayer structure with PCBM molecules intercalated between successive polymer stacks. In the bilayer systems the photoluminescence is almost completely quenched, in contrast to the phase segregated PATBT:PCBM blend. The higher degree of exciton quenching in the bilayer systems likely results in a higher initial yield of charges. However, on longer timescales (> 10 ns) the microwave photoconductance for the bilayer systems is lower than for PATBT:PCBM. This is likely due to the restricted motion of charges in bilayer systems, which impedes escape from recombination.
10:00 AM - GG8.2
Tailoring Carrier Transport in Low Bandgap Donor-acceptor Copolymers for Photovoltaic Applications.
Kaushik Roy Choudhury 1 , Jegadesan Subbiah 1 , Chad Chad Amb 2 , Pierre Beaujuge 2 , John Reynolds 2 , Franky So 1 Show Abstract
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States
Polymer solar cells (PSCs) have attracted much attention due to their potential in low-cost solar energy harvesting. To realize high efficiency devices, several crucial parameters like the absorption range, the photon-to-electron conversion efficacy and the carrier transport of the light-harvesting species need to be considered. An important strategy is to harvest sunlight over a broad spectral range by reducing the optical bandgap of polymers. At the same time, to ensure effective carrier transport and reduce the photocurrent loss, high carrier mobility is vitally important for PSCs. Recently, solar cells from a new low-bandgap donor-acceptor polymer PSBTBT, based on dithieno[3,2-b:2′,3′-d]silole (DTS) and 2,1,3-benzothiadiazole (BTD), were reported with remarkably uniform photovoltaic response over a broad wavelength range (300-800 nm) leading to power conversion efficiencies (PCE) well exceeding 5%. In this work we study how structural modifications to this polymer allow us to control and enhance the charge carrier transport in order to improve the PCE. We also examine the role of molecular weight and solubilizing side-chain groups on photocurrent generation and carrier transport in PSCs.Structural modifications to the polymer PSBTBT were made by varying the concentration of electron-donating and -withdrawing segments along the polymer backbone to yield four variations of the copolymer. By reducing the concentration of solubilizing groups and increasing the co-planarity of the backbone, which increases ordering and molecular packing, we were able to increase the hole mobility by more than two orders of magnitude. On mixing with PC70BM to form photovoltaic blends, the hole transport in the polymer phase is affected. Interestingly, while the two copolymers lacking co-planarity show significant increase in hole mobility in the blends, the other two with a greater tendency to π-stack display a slightly reduced hole mobility on mixing with PC70BM. Other than modifying the polymer backbone, charge transport is also significantly altered by varying the molecular weight of the polymer and by changing the solubilizing side-chains. For example, changing the side-chains from octyl to ethylhexyl groups improved the hole mobility by more than an order of magnitude. Modifying these parameters also led to a difference in the photogeneration efficiency in solar cells made from PSBTBT leading to a significant variation of efficiency. Interestingly, the effect of modifying the mobility in the polymer phase shows up as a difference in the nature of photocurrents, which affects the fill-factor and the overall PCE of solar cells. Our work shows that synthetic fine-tuning of the structure of semiconducting polymers can render them with desirable electronic properties for photovoltaic applications.
10:15 AM - **GG8.3
Why is the Open Circuit Voltage of Polymer-Fullerene Solar Cells So Much Lower Than the Energy of the Charge Transfer State?
Carsten Deibel 1 , Alexander Wagenpfahl 1 , Andreas Baumann 1 , Alexander Foertig 1 , Daniel Rauh 1 2 , Vladimir Dyakonov 1 2 Show Abstract
1 Experimental Physics VI, Julius-Maximilians-University of Würzburg, Würzburg Germany, 2 , Bavarian Centre for Applied Energy Research (ZAE Bayern), Würzburg Germany
The maximum open-circuit voltage of organic bulk-heterojunction solar cells is given by the energy of the crucial intermediate state between photogenerated excitons and free charge carriers: the charge transfer (CT) state, also called polaron pair . However, the CT energy of P3HT:PCBM solar cells is around 1.1eV, whereas the open-circuit voltage is often only between 550 and 650 mV. Photovoltaic devices based on different donor-acceptor combinations behave similarly. The authors of Ref. correctly name non-radiative recombination as the source of this discrepancy, although without going into detail. In our paper, we are concerned with the questions (a) which type of recombination–a mechanism in the bulk or at the surface–is responsible for the half Volt of open circuit voltage lost, (b) to which relative extent each of these processes is involved, and (c) how to reduce this loss.In order to consider these questions adequately, we perform macroscopic device simulations with input parameters resulting from a comprehensive experimental characterization in terms of charge generation, transport, and recombination properties. Based on our recent investigations on charge transfer states  and bulk recombination  by using advanced simulation techniques, we model state-of-the-art organic photovoltaic devices in view of the origin of the loss in open circuit voltage. On basis of the experimentally determined parameters, we can draw conclusions about the magnitude and importance of surface recombination, and relate its relative contribution to the total non-radiative. We stress the importance of a simulation based approach for this task, as no experimental work on surface recombination in organic solar cells can be found in literature, so that extensive insights into the entire non-radiative losses can only be gained on a theoretical basis as of yet.In addition to discussing the detailed origin of the loss of open circuit voltage relative to the CT energy in view of non-radiative recombination, we consider which optimization strategies can be applied to enhance the present device architectures.  K. Vandewal et al., Nat. Mater. 8, 904 (2009) C. Deibel, T. Strobel, V. Dyakonov, Phys. Rev. Lett. 103, 036402 (2009) C. Deibel, A. Wagenpfahl, V. Dyakonov, Phys. Rev. B. 80, 075203 (2009)
10:45 AM - GG8.4
General Features in Light Intensity Modulated Photocurrent Response of Bulk-hetero-junction Polymer Solar Cells.
Monojit Bag 1 , K. Narayan 1 Show Abstract
1 Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, Karnataka, India
Limiting factors for efficiency in organic/polymer solar cells are known to arise from carrier generation and carrier transport processes. We select a set of systems with a wide range of photo-induced charge-separation efficiencies. We observe a common feature in these systems from Intensity-Modulated-Photocurrent-Spectroscopy (IMPS) studies of bulk-hetero-junction polymer solar cells, in form of a photocurrent maxima I(ω)max in the 5 KHz < ωmax< 9 KHz range. This photocurrent peak position at ωmax remains invariant over a wide range of temperature (100 K – 300 K), modulated light intensity, wavelength (400 nm < λ < 930 nm) and bias (for a wide range of efficiency in all the systems studied). The only apparent factor which dictates the position of ωmax is the CW background white light. In the lower frequency range (<0.1 KHz) however, we observe variations in the response which can be correlated to the fill-factor magnitude of the devices. We speculate on the origin of this behavior using simplistic microscopic kinetic model transfer function to predict ωmax and introduce a macroscopic electrical equivalent circuit to represent the various processes.
11:00 AM - GG8.5
Fully Solution-cast P3HT/PCBM Bilayer Photovoltaics: Toward an Understanding of Efficient Charge Transport and Long-range Exciton Quenching.
Alexander Ayzner 1 , Christopher Tassone 1 , Sarah Tolbert 1 , Benjamin Schwartz 1 Show Abstract
1 Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California, United States
Recently we have shown that we can produce fully solution-cast bilayer P3HT/PCBM photovoltaics with a relatively sharp, planar interface between donor and acceptor with power conversion efficiencies exceeding 3.5%. By controllably varying the thickness of the two layers, we were able to show that the best bilayer devices are produced when the transport of holes on the polymer and electrons on the fullerene is roughly balanced, even when the polymer thickness is greater than 100 nm. We also find that the optimum devices have a ~ 4/1 polymer/fullerene thickness ratio, indicating that it is the conduction and extraction of electrons that limits the solar cell performance. In addition, we have worked on trying to understand the nature of the highly-efficient long-range exciton quenching in our bilayer devices. We find that the high quenching efficiency likely stems from multiple factors, including long-range energy transfer and potentially the presence of in-gap charge-transfer exciton states at the polymer/fullerene interface.
11:15 AM - GG8: Carrier
GG9: Novel Structures
Thursday PM, April 08, 2010
Room 3002 (Moscone West)
11:30 AM - **GG9.1
Defects, Doping and Transport in Excitonic Semiconductors.
Brian Gregg 1 Show Abstract
1 , NREL, Golden, Colorado, United States
Most excitonic (organic) semiconductors, XSCs, have a surprisingly high charge density, often 10 - 12 orders of magnitude higher than expected for an intrinsic material. They are doped by their defects. This is not presently included in most models of XSCs and organic photovoltaic cells. Because of the low dielectric constant, an unshielded charge shifts the energy levels of all species within a volume of ~10^5 Å^3. Thus, even at relatively low charge concentrations (~10^17 cm^-3), there are expected to be essentially no electrostatically unperturbed energy levels remaining in the semiconductor. Obviously, the presence of charges in this magnitude, and the electrostatic fluctuations they cause, will have a powerful influence on the transport of both excitons and charge carriers. For example, they provide an obvious explanation for both the "correlated disorder" and the low-field Poole-Frenkel (PF) mobilities often observed in π-conjugated polymers. As in all low-dielectric media, most charge carriers in XSCs will remain electrostatically bound near their counterions. Thus the free carrier density is just a small fraction of the total charge density. This is also observed in studies of low-defect, purposely doped organic semiconductors in which only 1 free carrier is created per every ~100 added dopants. The PF mechanism, although simplistic, accounts naturally for the interaction between a charge bound in a Coulomb well and an applied electric field. Together with a field-dependent mobility (not included in the original PF model), it provides a remarkably accurate description of the doping efficiency and charge transport in many XSCs.Thin films of π-conjugated polymers such as poly(3-hexylthiophene), P3HT, exhibit a free hole density of pf ≈ 10^15 – 10^17 cm^-3, with the lower range approachable only in extensively purified materials. The charged defects from which these free carriers originate almost certainly control much of the observed (photo)electrical behavior of π-conjugated polymers. In first attempt at defect engineering, we introduce chemical treatments that beneficially modify or remove the defects in these polymers. For example, treating P3HT with lithium aluminum hydride decreases the apparent p-type defect density by a factor of ~400. This results in doubling the exciton diffusion length to 14 nm, increasing the hole mobility more than ten fold to 2 x 10^-3 cm^2/Vs and partially protecting the polymer against photodegradation. Apparently, the charged defect density in P3HT has a large and mostly deleterious influence on the optoelectronic properties and chemical stability of the polymer. One beneficial aspect of charged defects, however, is that they act as dopants which can improve both the conductivity and the electric field distribution in OPV cells.
12:00 PM - GG9.2
Origin of Photocurrent Generation in ZnO:P3HT Bulk Heterojunction Solar Cells.
Guillaume Poize 1 , Pedro Atienzar 2 , Johann Boucle 2 , James Durrant 2 , Jenny Nelson 2 , Cyril Martini 1 , Frederic Fages 1 , Joerg Ackermann 1 Show Abstract
1 Centre Interdisciplinaire de Nanoscience de Marseille, CINAM UPR-CNRS 3118, CNRS, Marseille France, 2 Centre for Electronic Materials and Devices, Imperial College London, London United Kingdom
Bulk heterojunction based on blends of donor polymers and inorganic acceptor nanoparticles are promising alternatives to all organic solar cells. Although the use of shape tunable semiconductor nanoparticles held great promise for high electron mobility and thus higher device performance, hybrid bulk heterojunction solar cells show still much lower efficiency than the all organic devices. Blends of poly (3-hexylthiophene ) (P3HT) and zinc oxide (ZnO) nanoparticles have been studied intensively over the last years. Recently the highest efficiency of 2 % has been reported1, which is still more than twice lower than the performance of those devices made of P3HT:fullerene blends. In order to understand the limitation of such hybrid bulk heterojunctions, we present here a detailed study of the photovoltaic properties of blends based on P3HT and ZnO nanospheres as a function of solvent and ZnO concentration. We combined current-voltage characterization in air and under vacuum with corresponding steady state absorption and fluorescence as well as transient absorption spectroscopy to study the photogeneration at the hybrid interface. Furthermore the blend morphology was analyzed by HRTEM and AFM. First by optimizing the blend morphology, we show that solar cells with external quantum efficiency values up to 50% could be produced. The increase in efficiency was attributed to a very well dispersion of ZnO within the blends with average distances between nanoparticles of 5 nm all over the blend. The TAS analysis show that intensities and recombination characteristics of the hybrid blend are comparable with P3HT:PCBM systems. Surprisingly the corresponding fluorescence spectroscopy of the blends reveals that P3HT fluorescence is not quenched by incorporation of ZnO nanoparticles, but increases by a factor 5. As the average distance between ZnO nanoparticles is smaller than the exciton diffusion length in the P3HT, such an absence of quenching is surprising in the context of standard models of photocurrent generation via exciton dissociation at the donor-acceptor interface. Furthermore by measuring the performance of P3HT:ZnO solar cells in air and under vacuum, we observe a strong increase in photocurrent up to a factor of 9 under vacuum depending on the ZnO concentration.By combining our results, alternative mechanisms for photocurrent generation in P3HT:ZnO blends will be discussed. 1: S. D. Oosterhout, M. M. Wienk, S. S. van Bavel, R. Thiedmann, L. J. A. Koster, J. Gilot, J. Loos, V. Schmidt, R. A. J. Janssen, Nature Materials 2009, 8, 818-824
12:15 PM - GG9.3
Enhanced Exciton Harvesting and Charge Collection in P3HT/TiO2 Solar Cells With a Fullerene Derivative Convalently-bound to TiO2 at the Organic/Inorganic Interface.
Bertrand Tremolet de Villers 1 , E. Joseph Nemanick 1 , Sarah Tolbert 1 , Benjamin Schwartz 1 Show Abstract
1 Chemistry and Biochemistry, UCLA, Los Angeles, California, United States
Offering real potential to aid in the increasing global demand for sustainable energy generation, solution-processed organic and hybrid organic-inorganic excitonic solar cells are the focus of intense research at the moment. Here, we show that exciton harvesting and charge collection can be dramatically improved in P3HT/TiO2 photovoltaics by incorporating a small molecule mediator derived from C60 onto the surface of TiO2. Steady-state and time-resolved photoluminescence quenching measurements reveal that the surface-modified TiO2 quenches many more excitons from P3HT compared to bare TiO2. The LUMO of the fullerene mediator lies between those of P3HT and TiO2 thus, once electrons reach the fullerene, it is energetically favorable for them to continue on to the titania and toward the cathode. Efficient charge transfer from the surface fullerene mediator to TiO2 is confirmed by AM1.5 solar illumination current-voltage measurements which show a fourfold enhancement of the short-circuit current density in devices with the surface-modified titania. In addition, surface modification increases the open-circuit voltage by changing the energetics of the interface between the polymer and titania as well as possibly introducing an interface dipole. Electronic states at the interface due to the fullerene may also reduce charge recombination as surface-modified TiO2 devices also showed higher fill-factors.
12:30 PM - **GG9.4
Single-wall Carbon Nanotube/Polymer Excitonic Photovoltaics.
Nanditha Dissanayake 1 , Zhaohui Zhong 1 Show Abstract
1 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States
Single-wall carbon nanotubes (SWNTs) hold the promise for boosting organic photovoltaic efficiency by offering high-mobility conducting channels for low-loss charge transport. To harvest the potential of SWNT/polymer hybrid photovoltaics, fundamental studies of the photoinduced charge transfer between polymers and SWNTs are essential. Thus far, measurements of this nature were carried out primarily in polymer-SWNT blends, and the results can not distinguish the difference between metallic (m-) and semiconducting (s-) SWNTs. In fact, m-SWNTs are thought to be detrimental for the photovoltaic performance by providing charge recombination centers. To this end, we present detail photocurrent studies on SWNT/poly[3-hexylthiophene-2,5-diyl] (P3HT) hybrid photovoltaics fabricated using both individual and array of SWNTs grown on quartz substrate. Significantly, measurements on individual m-SWNT/P3HT junction reveal efficient photocurrent originated by the hole transfer from P3HT to m-SWNT. The direction of the photocurrent is independent of the choices of metal contacts. Furthermore, devices containing arrays of SWNTs show dramatic increase of photocurrent compared to the polymer-only reference, and the overall photocurrent is again found to flow from P3HT to SWNTs. The results suggest the formation of a type II heterojunction at the SWNT/polymer interface, enabling efficient dissociation of excitons. The results also hint for new types of SWNT/polymer hybrid photovoltaics utilizing m-SWNTs for improved performance. Studies on carrier dynamics of individual SWNT/P3HT hybrid junction and the external quantum efficiency of bulk SWNT/P3HT excitonic photovoltaics will also be presented.
GG10: Charge Transport
Thursday PM, April 08, 2010
Room 3002 (Moscone West)
2:30 PM - **GG10.1
Correlations Between Contactless Photoconductivity and Device Performance: Insight into the Mechanisms that Govern the Efficiency of Nanostructured Bulk Heterojunctions.
Nikos Kopidakis 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Contactless photoconductivity probes allow one to measure the time-dependent photoconductivity in photovoltaic active layers without the need to collect the charges at metal contacts. The advantage of these techniques is that they can provide much more information on the dynamics of electron and hole photogeneration and loss than one can obtain solely by photovoltaic device characterization. Of these contactless probes, Time-Resolved Microwave Conductivity (TRMC) has the sensitivity required to study nanostructured composites of low-mobility materials like conjugated organic molecules and inorganic nanoparticles. Over the last few years, our group at NREL has combined TRMC with complete photovoltaic device characterization in several variations on the Organic Bulk Heterojunction theme. In this presentation I will discuss what our TRMC studies have revealed, and how we have used these to understand device characteristics. In these studies, we have used composites of conjugated organic polymers and dendrimers with fullerene derivatives, single-walled carbon nanotubes, conducting oxides and quantum-confined semiconductor nanoparticles. The supply of high quality, tunable materials, many of them synthesized in-house, has allowed us to address various aspects of the influence of interfaces, morphology and energetics on photocarrier dynamics and, ultimately, on device efficiency.
3:00 PM - GG10.2
Using Bulk Heterojunction Field Effect Transistor Measurements to Understand Charge Transport in Solar Cell Materials.
Christopher Lombardo 1 , Ananth Dodabalapur 1 Show Abstract
1 Microelectronics Research Center, The University of Texas at Austin, Austin, Texas, United States
There is great promise for organic photovoltaic cells to reduce the cost of solar energy generation by enabling millions of people to affordably purchase photovoltaic panels. To attain this goal efficiently, we need to learn how to manufacture these devices, electrically and optically characterize their performance, and understand how charge carriers move throughout these devices. Ambipolar organic field effect transistors (FETs) have been used to study the transport of these carriers in bulk heterojunction (BHJ) organic photovoltaic devices [1,2]. Active layers of phase separated conjugated polymer : fullerene, most notably poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), were chosen due to their use in performance BHJ organic photovoltaic devices  as well as ease of device fabrication. To determine the effect on electron and hole mobilities, carrier injection, and other FET parameters, we have systematically studied variations in device fabrication and structure. These parameters include solvent selection, annealing temperature, annealing time, electrode workfunction, device geometry, etc. In addition to varying the fabrication parameters and the device structure, we have studied modes of charge carrier transport through drain current methods that yield carrier mobility as a function of temperature in devices where electron and hole mobilities are approximately equal. Studying the temperature dependence of mobility has resulted in a lot of information on charge transport mechanisms of both electrons and holes in bulk heterojunctions and has also allowed us to correlate processing conditions such as solvent choice, annealing temperature, and annealing time with transport properties in more detail. We will report on measurements in the 77-330K range for P3HT:PCBM devices with different annealing temperatures and solvents. References1. W. Geens, S. Shaheen, C. Brabec, J. Poortmans, and N. Serdar Sariciftci, AIP Conf. Proc. 544, p. 516, 20002. D. Gundlach, S. Karg, and W. Riess, J. Appl. Phys. 95, p. 5782, 20043. J. Kim, K. Lee, N. Coates, D. Moses, T. Nguyen, M. Dante, and A. Heeger , Science 317 p. 5835, 2007
3:15 PM - GG10.3
Sub-nanosecond Geminate Charge Recombination in Polymer: Polymer Photovoltaic Devices.
Justin Hodgkiss 1 2 , Andrew Campbell 1 , Alex Marsh 1 , Akshay Rao 1 , Richard Friend 1 Show Abstract
1 Physics, University of Cambridge, Cambridge United Kingdom, 2 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand
Considerable efforts are being invested in the development of new materials for use in organic photovoltaic devices. A common approach is the development of copolymers that comprise alternating electron-donor and -acceptor groups along the chain, thereby inducing a charge-transfer absorption band. The strong optical absorption of these materials and their tunable energy levels also make them attractive candidates for electron acceptors to blend with hole transporting polymers in a combination that conserves maximal open-circuit voltage. However, the modest photon-to-charge conversion efficiencies observed in such blends have thus far prevented them from fulfilling these expectations. Here, we present direct spectroscopic evidence for substantial sub-nanosecond geminate charge recombination in organic photovoltaic devices that contain the alternating donor-acceptor copolymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2,2-diyl) (F8TBT) blended with the electron donor poly(3-hexylthiophene) (P3HT). Transient absorption spectroscopy is used to resolve prompt and delayed charge transfer on timescales spanning femtoseconds to 100 picoseconds. 30% of the charge population is found to decay via geminate charge recombination within just 2 nanoseconds, while the remaining phase recombines geminately with a half-life of approximately 100 ns. Picosecond dynamics are only exposed upon undertaking a careful series of measurements to remove the effect of exciton-charge interactions, which are found to complicate pulsed excitation measurements. The sub-nanosecond recombination phase is considerably more rapid and more severe than previously supposed. The invariance with blend morphology suggests that the origin of fast recombination is inherent to the molecular structure at the donor–acceptor interface - consistent with the wide range of recombination times suggested in quantum chemical models of heterojunction interfaces. We believe that such rapid geminate charge recombination poses a challenge to the development of organic photovoltaics.
3:30 PM - GG10.4
The Effect of Charge Trapping on Organic Solar Cell Efficiency: A Monte Carlo Simulation Study.
Chris Groves 1 2 , Neil Greenham 2 Show Abstract
1 School of Engineering and Computing Sciences, Durham University, Durham United Kingdom, 2 Cavendish Laboratory, Cambridge University, Cambridge, Cambridgeshire, United Kingdom
In recent years it has been shown that the local environment in which charge transport and recombination occurs has a dramatic effect upon the performance of organic solar cells. One such parameter that changes on a length scale of nanometres is the energy of a particular conjugated polymer segment. How the site energies are distributed in energy is described by the term energetic disorder, the standard deviation of which is fairly straightforward to measure experimentally. What is more challenging however is to measure how the site energies are distributed spatially throughout the material. One may have a situation where one polymer chain may have little influence on another, leading to uncorrelated site energies, which in turn means that the site-to-site energy experienced by a travelling carrier will be undulating and populated by a significant number of traps. Conversely, the energy of one site may be strongly correlated to its neighbours, perhaps through the polymer structure and packing. In this case the site-to-site energy will be less subject to abrupt changes and the number of traps will be smaller. It requires fairly detailed analysis to be able to discern whether a material shows spatially correlated or uncorrelated disorder. Thus it is not clear to what extent the site energies are locally ordered in conjugated polymers, especially at the hetero-interface where geminate recombination occurs. Here we examine the effects that local variation in site energies has upon geminate separation efficiency and ultimately solar cell performance using a Monte Carlo model. Device performance is predicted in devices with spatially uncorrelated and correlated disorder. It is found that the efficiency of correlated disorder devices outperform those with uncorrelated disorder by a factor of 2.6 and 3.1 for a bilayer and a blend respectively. This significant difference in performance arises because there are fewer isolated traps in the device with correlated disorder. The result is interesting as it shows that poor separation efficiency in conjugated polymers is not necessarily inescapable, whilst also pointing to the fact that designing less disordered materials should give rise to better performance. We also discuss what implications this has for the interpretation of device characteristics in terms of bulk parameters. One may expect as a consequence of correlating the energetic disorder that the mobility will rise, and that this in turn gives rise to the improvement in performance seen. However, we artificially raise the mobility to that seen in the correlated device whilst keeping energetic disorder uncorrelated (i.e. retaining the trap sites at the interface) and we do not see the same improvement in device performance. This implies that it is not the bulk mobility that determines the performance in this case, rather it is the local variation of energetic disorder, which is extremely challenging to measure.
3:45 PM - GG10.5
Novel Spectroscopic Technique for Defect Characterization in Organic Semiconductors Relevant to Exciton Diffusion Processes.
Hikmet Najafov 1 , Vitaly Podzorov 1 Show Abstract
1 Physics Department, Rutgers University, Piscataway, New Jersey, United States
Organic semiconductors are very promising materials for photovoltaics. One of the poorly explored issues in this area is the nature of electronic defects and their influence on performance of OPV devices. Until recently defects have been mostly studied in polycrystalline and amorphous organic films, where disorder is typically very significant, leading to a complete domination over the intrinsic excitonic and polaronic properties. In this work, we present novel optical spectroscopic technique for characterization of impurities and defects relevant to exciton diffusion in highly ordered organic semiconductors with relatively small density of defects that allows observation of intrinsic properties and their evolution with disorder.
4:00 PM - GG10: Charge
4:30 PM - **GG10.6
Device Physics of Organic PV Devices.
Nir Tessler 1 , Nir Yaacobi 1 , Eran Avnon 1 , Lior Levy 1 , Ariel Ben-Sasson 1 Show Abstract
1 Electrical Engineering, Technion, Israel institute of technology, Haifa Israel
Semiconducting polymers and small molecules form an extremely flexible class of amorphous materials that can be used in a wide range of applications some of which are display, RF-tags, and solar cells. The rapid progress towards functional devices is occurring despite the lack of sufficient understanding of the physical processes and not much experience in device engineering. In this talk we will address the physical mechanisms governing organic devices and the role of structure engineering. We will focus on the physics underlying the operation of PV cells with an emphasis on the issue of spatial inhomogeneities and the concept of mobility distribution function. If time permits we will also briefly describe how better understanding of the device physics may lead to new device designs as the patterned electrode vertical field effect transistor.
5:00 PM - **GG10.7
Recombination in Organic Solar Cells.
Robert Street 1 , Megan Schoendorf 1 Show Abstract
1 , Palo Alto Research Center, Palo Alto, California, United States
For the continued improvement in the efficiency of organic bulk heterojunction (BHJ) solar cells, it is important to identify and understand the primary loss mechanisms. The BHJ cell is a complex structure and there are several possible mechanisms to limit the efficiency. The talk will discuss the transport and recombination processes in BJH cells, based on measurements made on polycarbazole/fullerene blend solar cells which have efficiency of about 5% . We analyze the different recombination mechanisms that determine the shape of the current-voltage characteristics and the fill factor. The mechanisms considered are recombination of the charge transfer exciton, Auger recombination and recombination through localized states at the polymer-fullerene interface. The measured monomolecular recombination kinetics, the temperature dependence of the current-voltage characteristics, the dark forward bias diode current, and modeling studies, all indicate that the dominant recombination is through interface states between the polymer and fullerene domains. From the measurements we estimate that the interface state density is of order 1011 cm-2, but the experiments do not yet reveal the origin of these states. Modeling studies also indicate that a 10-fold reduction in the interface state density could potentially double the solar cell efficiency.1. The solar cells were fabricated by the Heeger group at UC Santa Barbara.
5:30 PM - **GG10.8
Comparison of Charge Carrier Transport in Diodes and Planar Structures.
Almantas Pivrikas 1 , Niyazi Serdar Sariciftci 1 Show Abstract
1 Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, Linz Austria
Charge carrier transport mechanisms in organic semiconductors are of great importance since the charge carrier mobility and lifetime are the main parameters governing the device performance and efficiency. Therefore, full understanding of charge carrier transport properties in organic electronic devices provides a possibility to learn and develop new organic compounds with required properties through chemical engineering.The charge carrier transport studies in solution processed devices fabricated from π-conjugated polymers or other organic materials are problematic. Measuring carrier mobility from Space Charge Limited Current in diode configuration from current-voltage characteristic is indirect due to required assumptions and the integral nature of the method. Strongly time-dependent charge carrier mobility imposes a limitation of well know and used in the past Time-of-Flight (TOF) technique. Even though the non-dispersive charge transport has been observed in solution processed polymeric films, the disordered hopping transport usually dominates. Moreover, organic films often demonstrate too high level of thermally generated charge carrier concentration rendering TOF inapplicable. Often the Organic Field Effect Transistor (OFET) structures are used to study the carrier mobility since they operate in steady state. However, much larger, sometimes even a few orders of magnitude, carrier mobility is reported in OFET configuration, which is explained by high carrier concentration in the channel and different properties of DOS distribution at the interface between semiconductor and insulator.Charge Carrier Extraction by Linearly Increasing Voltage (CELIV) is one a few techniques able to directly measure charge transport and recombination in disordered films. Apart from many advantages the greatest benefit of CELIV is that it allows the relaxation of photogenerated charges to be measured within Density-of-States (DOS) distribution. The method opens up new possibilities to study the charge carrier mobility dependence on time, carrier concentration, electric field, temperature, film thickness and morphology directly in the operational devices. As it has been shown in the past, the film morphology plays a crucial on the device performance. Combining the transport studies with morphology characterizing methods (AFM, TEM, SEM etc.) allows better understanding of underlying photophysical phenomena in the organic electronic devices with a final goal to improve the performance.We have studied the charge transport and recombination in various well known and novel organic compounds as well as in bulk-heterojunction and solid state dye-sensitized solar cells using CELIV and OFET methods.[2,3] The comparative charge transport analysis will be presented allowing the conclusions to be made about the relation between the carrier mobility/lifetime and the film structure/nanomorphology. G. Juska et al., PRL 84, 4946, 2000. A. Pivrikas et al., PRL 94, 176806, 2005. A. Pivrikas at el., Prog. Photovolt: Res. Appl. 15, 677, 2007.