9:00 AM - **I4.1
Size-Free Electronic Tuneability and Multiple Excitons in Alloyed Colloidal Quantum Dots and Rods.
Efrat Lifshitz 1 , Roman Vaxenberg 1 , Evgeniy Tilchin 1 , Georgy Maikov 1 , Diana Yanover 1 , Anna Brusilovsky 1 , Dikla Ahitov 1 , Alina Amel 1 , Aldona Sashchiuk 1
1 Schulich Faculty of Chemistry, Technion IIT, Haifa Israel
Show AbstractThis work introduces a new class of heterostructure (HS) quantum dots (QDs) and quantum rods (QRs), comprised of PbSe or CdTe cores (spheres or rods) coated by PbSe or CdSe shells, respectively, when either the core or the shell has an alloy composition with a general chemical formula PbSexS1-x or CdTexSe1-x (0
9:30 AM - **I4.2
Charge Transfer Excitons in Bulk Heterojunctions Solar Cells.
Maria Antonietta Loi 1
1 , University of Groningen, Groningen Netherlands
Show AbstractPhotovoltaic devices based on organic bulk heterojunctions have recently gained renewed interest because of the achievement of power conversion efficiencies up to 8%. These latest results showed the possible economical relevance of this class of solar cells. Nevertheless, it is also clear that not all the physical questions on their working mechanism are answered and that the use of new polymers, in particular the one with narrow band-gap, pose new interesting questions.I will show that narrow band-gap polymer bulk heterojunctions offer a new opportunity to shed light on the charge separation phenomena and on the photo-excitation involved in the working mechanism of organic solar cells. I will report different cases were charge transfer excitons play a main role in the photoexcitation landscape of bulk heterojunction solar cells.[1] [1]Dorota Jarzab, Fabrizio Cordella, Jia Gao, Markus Scharber, Hans-Joachim Egelhaaf, Maria Antonietta Loi, Advanced Energy Materials, DOI: 10.1002/aenm.201100083 (2011); Maria Antonietta Loi, Stefano Toffanin, Michele Muccini, Michael Forster, Ulrich Scherf, Markus Scharber, Adv. Funct. Mat. 17, 2111 (2007).
10:00 AM - I4.3
Understanding Charge Transport Characteristics in Hybrid Photovoltaic Devices Based on Controlled Donor-Acceptor Interfaces.
Beau Richardson 1 , John Bae 1 , Leize Zhu 1 , Qiuming Yu 1
1 Chemical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractIn order to reduce dependence on exhaustible energy sources and reduce carbon emissions, much scientific effort has been directed towards reducing the cost of renewable energy sources to make them competitive with conventional sources. Solar cells have great potential as a low-carbon, sustainable energy source but are still too expensive to compete. Searching for new materials to design low-cost and more efficient solar cells can reduce the cost per kilowatt-hour (kWh) of energy produced from these devices and bring them closer to grid parity. Specifically, pyrite FeS2 nanocrystals have several advantages for making solar cells but its use in thin-film, bulk-heterojunction type devices has not been thoroughly studied.Recently, our group has demonstrated controllable synthesis of pyrite nanocrystals in octahedral and cubic shapes which expose (111) and (100) facets, respectively. We are incorporating these nanocrystals into a hybrid, bulk heterojunction device with the semiconducting polymer Poly (3-hexylthiophene-2,5-diyl) (P3HT) and studying how the different nanocrystal sizes and shapes affect charge transport, light absorption, and overall device performance. We also used molecular modeling to understand the surface structures of different crystalline facets and the possible assembly of polymer donors. In conjunction with the fabrication of these hybrid devices, we are also fabricating organic devices with P3HT:PCBM active layers under the same conditions for direct comparison. The typical device is fabricated on an ITO coated glass substrate with a ~40 nm PEDOT:PSS hole conducting layer, followed by a ~100-200 nm P3HT/pyrite active layer and an ~85 nm Al electrode. The film thickness and the surface morphology of the hybrid thin films are measured by AFM. The optical properties of the film are studied using ellipsometry and UV-vis-NIR absorption spectroscopy. The power conversion efficiency (PCE) is determined by measuring the Isc and Voc under AM 1.5 Global Spectrum at 100 mW/cm2. A time-correlated single photon counting (TCSPC) system is used to analyze the relaxation of electrons in these films from an excited state to a lower energy state. By employing TCSPC to study P3HT, pyrite nanocrystals, PCBM, P3HT:PCBM, and hybrid P3HT:pyrite nanocrystal films, we can elucidate the charge transport characteristics between the n- and p-type materials with controlled interface structures.
10:15 AM - I4.4
Surface Nature of Photoconductivity and Long-Range Exciton Diffusion in Rubrene.
Vitaly Podzorov 1
1 Physics, Rutgers University, Piscataway, New Jersey, United States
Show AbstractWe demonstrate a surface nature of photoconductivity and a very large exciton diffusion length (L ~ 3-8 μm) in highly ordered and pure organic semiconductor - rubrene [Ref.]. These observations imply that energy can be transported over vast distances through the bulk to the surfaces of organic materials, most likely by triplet excitons produced by fission of singlets. This result is important because it opens a window into the fundamental understanding of the intrinsic optical properties of highly ordered organic semiconductors in hetero-structure devices. By performing polarization and wavelength resolved photocurrent excitation spectroscopy in high-purity rubrene crystals, we conclude that generation of triplet excitons by fission of singlets is very efficient in this material. In this process, a single high-energy singlet state undergoes fission into a pair of triplets. Triplet excitons would then diffuse over distances of the order of ~ 3-8 µm, as long as the material is highly ordered, which means that excitons have a high probability of reaching the surface/interface of the crystal and dissociate into free carriers.Ref.: H. Najafov, B. Lee, Q. Zhou, L. C. Feldman and V. Podzorov, "Observation of long-range exciton diffusion in highly ordered organic semiconductors", Nature Mater. 9, 938 (2010).
10:30 AM - **I4.5
Charge Transfer States, Photovoltages and Photocurrents in Donor/Acceptor Heterojunctions.
Koen Vandewal 1 , Kristofer Tvingstedt 1 , Olle Inganas 1
1 Biomolecular and organic electronics, IFM, Linkoping University, Linkoping Sweden
Show AbstractThe role of charge transfer states (CTS) in determining the photovoltaic properties of organic donor/acceptor heterojunctions has attracted attention in recent years. It has been shown that the CTS determine the photovoltage in polymer/fullerene blend based devices, and that the CTS can be detected though photoluminescence, electroluminescence and through optical absorption. The role of the CTS in formation of free charge carriers is still under debate, but it is clear that CTS may contribute one route for this photocurrent generation. We review present evidence in some polymer/fullerene systems, where the polymer comes from alternating copolymers of donor and acceptor elements, within main chain polymer families based on fluorene, phenylene and thiophene donors.
11:30 AM - **I4.6
Photoinduced Charge Separation Processes in Organic Photovoltaic Blends: High-Frequency EPR Spectroscopy.
Oleg Poluektov 1
1 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractCharged polarons in thin films of diverse polymer-fullerene composites are investigated by light-induced electron paramagnetic resonance (LEPR). When recorded with conventional X-band LEPR, the signals from positive and negative polarons (P) are overlapping, which does prevent its direct identification. Owing to the superior spectral resolution of HF (130 GHz) EPR spectroscopy we were able to separate light-induced signals from P+ and P- in number of different blends of polymers with C60- and C70-derivatives. The obtained g-tensor parameters are of importance as these are characteristics of structure, symmetry, and dynamics of the localized/delocalized unpaired spin states. Time-resolved EPR technique allows resolving long-lived spin-polarized charge separated states. These states were identified like spin-correlated radical pairs at distances of ~ 20-30 Â between radicals in the pair. In general, photochemistry of organic photovoltaic active materials is found to be similar to that in natural and artificial photosynthetic systems. Comparative analysis of charge separation processes in organic photovoltaic cells and natural photosynthetic proteins is discussed. The work at Argonne National is supported as part of the ANSER, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001059
12:00 PM - I4.7
Correlating Charge Creation with the Gibbs Free Energy Driving Force in Organic Photovoltaic Blends.
David Coffey 1 , Bryon Larson 2 1 , Olga Boltalina 2 , Steven Strauss 2 , Garry Rumbles 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Colorado State University, Fort Collins, Colorado, United States
Show AbstractThe field of organic photovoltaics currently lacks a fundamental understanding of how charge is created in blended organic semiconducting materials. Attempts to develop this understanding have been hampered by three primary obstacles: 1) distinguishing between changes in charge creation and changes in the other processes like optical absorption, exciton diffusion, charge transport, and charge extraction has proved challenging with most characterization techniques, 2) there exist few series of relevant materials that systematically vary in their energy levels without simultaneously varying other critical variables such as mobility, and 3) the morphology in typical blended devices shifts dramatically even for small changes in processing or chemical structure, severely complicating the interpretation of any results across series. Herein we describe a set of experiments whereby we are able to circumvent each of these obstacles and measure fundamental charge creation yields as a function of Gibbs free energy change. We achieve this capability by utilizing over a dozen fullerene acceptors to systematically change energy levels, by employing time-resolved microwave conductivity (TRMC), and by operating in the dilute morphological regime. Measurements performed on numerous polymer donors reveal correlations between the Gibbs free energy change and the charge creation yield that includes turn-on regimes, inverted regimes, and the importance of exciton energies, each of which have significant implications for attempts to improve device performance.
12:15 PM - **I4.8
Singlet Fission in Covalent Dimers and Solid-State Polyacene Materials.
Chris Bardeen 1
1 Chemistry, U. California Riverside, Riverside, California, United States
Show AbstractSinglet fission (SF), where an initially excited singlet state spontaneously splits into a pair of triplet excitons, has drawn interest as a possible way to boost the efficiency of organic solar cells. Although this phenomenon has been observed in several solid-state and covalent organic systems, its basic mechanism and requirements are still not well-understood. Our research focuses on several basic questions, including: 1) Does the existence of a delocalized singlet state enhance the SF process? 2) Does SF proceed in a single step where the singlet directly transforms into a triplet pair, or do intermediate states play a role? Recent theoretical work has suggested that states with charge-transfer character can enhance this process. 3) Can we find SF materials that will generate higher energy triplet states that will be easier to ionize? To answer these questions, we are investigating the photophysical dynamics of SF in different polyacene systems. We use picosecond luminescence and femtosecond transient absorption to measure the rate of singlet decay and triplet formation under different conditions, including samples with different morphologies (amorphous versus polycrystalline versus crystalline) and as a function of temperature and applied magnetic field. In tetracene, these experiments suggest that a bound triplet pair state is an important intermediate state that later dissociates into free triplets. The nature of this bound state in single crystals can be investigated by observing oscillations in the delayed fluorescence signals that can be attributed to quantum beats between different triplet pair states within the coherent pair state. The fact that coherent dynamics in the triplet manifold are directly mapped onto the singlet state appears to be evidence that the triplet pair state is directly coupled to the singlet state, without a third charge-transfer state acting as a real kinetic intermediate. Finally, we will describe investigations of anthracene and perylene derivatives as potential SF materials that can generate higher energy triplet excitons.
12:45 PM - I4.9
Charge Separation and Relaxation in Phthalocyanine-C60 Photovoltaic Systems.
Steven Robey 1 , Gregory Dutton 1
1 , NIST, Gaithersburg, Maryland, United States
Show AbstractOrganic photovoltaic (OPV) structures depend on charge transfer processes occurring within 10’s of nanometers of donor-acceptor interfaces. Charge generation involves successive steps of (1) optical absorption to create excitons, (2) diffusion of the exciton population to the donor-acceptor interface, (3) exciton dissociation and charge transfer at the interface and (4) charge transport and collection at electrodes. The charge separation step depends critically on electronic level alignment between the donor and acceptor. Exciton diffusion to the interface occurs on timescales of several to 10’s of picoseconds (ps) so relaxation processes occurring on sub-ps and ps timescales as the exciton diffuses can impact the energy available for charge separation. We investigated exciton relaxation and charge separation processes at phthalocyanine (Pc)-C60 interfaces by combining time-resolved two-photon photoemission (TR-2PPE) with organic MBE to form donor-acceptor interfaces layer-by-layer. Pc π→π* transitions were excited by a pump pulse to generate singlet (S1) excitons. The resulting population dynamics were probed with a time-delayed UV pulse to follow the relaxation and charge separation as a function of energy. We compared the decay dynamics as a function of CuPc thickness, and thus distance from the C60 interface, to allow determination of the rate of charge transfer at the interface. We find a charge transfer rate of ≈ 8 x 10 12 sec-1 for an initial exciton population formed at the interface with C60 immediately after pumping. This large rate of charge transfer is essentially limited to the CuPc\C60 interface. For greater distances from the interface, the CuPc exciton population undergoes vibrational relaxation and intersystem crossing (ISC) on a timescale of ≈ 1-2 ps, ultimately resulting in the production a triplet exciton population at significantly lower energy. We estimate that the charge transfer rate of this lower energy triplet population is reduced by at least a factor of ≈ 1000, giving charge transfer rates of about 8 x 10 9 sec -1. We will discuss the impact of interface morphology on dynamics and electronic structure and compare measurements for the case of H2Pc and C60 where ISC of the Pc singlet excitons to triplet levels is negligible. We will make connections with Marcus theory of charge transfer and recent calculations of charge transfer rates for donor-acceptor interfaces.
I5/H5: Joint Session: Materials
Session Chairs
Alex Jen
Christoph Lungenschmied
Tuesday PM, November 29, 2011
Grand Ballroom (Sheraton)
2:30 PM - **I5.1/H5.1
Charge Recombination and Transport Dynamics in Organic Solar Cells.
Alan Heeger 1
1 Center for Polymers and Organic Solids, Department of Physics and Materials Engineering, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractTransient photoconductivity measurements carried out on bulk heterojunction (BHJ) solar cells demonstrate the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination; see S. R. Cowan, R. A. Street, S. Cho, and A. J. Heeger, Transient photoconductivity in polymer bulk heterojunction solar cells: Competition between sweep-out and recombination. Phys. Rev. B 83, 035205 (2011). The transient photoconductance data imply the existence of a well-defined internal field; carrier sweep-out is proportional to the magnitude of the internal field and limited by the carrier mobility. At external voltages near open circuit where the internal field approaches zero, the photocurrent decays through recombination of photogenerated mobile carriers. Recombination of photogenerated charge carriers in polymer BHJ solar cells reduces the short circuit current (Jsc), the fill factor (FF) and the open circuit voltage (Voc). Light intensity and temperature-dependent current-voltage measurements on a variety of polymer BHJ cells show that the recombination kinetics are universally voltage-dependent and evolve from first-order recombination (with carrier sweep-out at short circuit) to bimolecular recombination at open-circuit as a result of increasing the voltage-dependent charge carrier density in the cell; see S. R. Cowan, A. Roy, and A. J. Heeger, Recombination in polymer-fullerene bulk heterojunction solar cells. Phys. Rev. B 82, 245207 (2010). The “missing 0.3 V” inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open-circuit voltage at room-temperature results from the temperature dependence of the quasi-Fermi levels in the polymer and fullerene domains—a conclusion based on the fundamentals of fermion statistics.Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device; see S.R. Cowan, W-L Leong, N. Banerji, G. Dennler and A.J. Heeger Adv. Functional Materials (in press). Identifying a threshold impurity level for organic solar cells: Enhanced first-order recombination via well-defined PC84BM traps in organic bulk heterojunction solar cells. Steady state studies show a dramatic increase in the trap-assisted recombination rate when [6,6]-phenyl C84 butyric acid methyl ester (PC84BM) is introduced as a trap site in polymer BHJ solar cells. The trap density dependent recombination can be described as a combination of bimolecular and Shockley-Read-Hall (monomolecular) recombination; the latter is dramatically enhanced by the addition of the PC84BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cells and gives insight into the reduction in Voc caused by trap induced monomolecular recombination.
3:00 PM - **I5.2/H5.2
All-Conjugated Block Copolymers and Thiophene-Based Conjugated Polyelectrolytes for Organic Photovoltaics.
Ullrich Scherf 1
1 , Bergische Universitaet , Wuppertal Germany
Show AbstractThe talk reports synthesis, characterization and photovoltaic applications of all-conjugated donor-acceptor block copolymers and thiophene-based conjugated polyelectrolytes.
3:30 PM - **I5.3/H5.3
Are We There Yet? - Design of Better Conjugated Polymers for Polymer Solar Cells.
Luping Yu 1
1 Chemistry, The University of Chicago, Chicago, Illinois, United States
Show AbstractOver the last ten years, significant development in polymer solar cells has been achieved so that it is more likely that the OPV will be an attractive alternative to the traditional silicon photovoltaic devices in special applications. Remarkable progress has been made in polymer-fullerene solar cells and several polymers have shown power conversion efficiencies (PCE) greater than 7%. The most important recent development has been driven by the synthesis of new low bandgap polymers with optimal properties for the solar cells. Herein we provide an overview of the key strategies of optimization of HOMO and LUMO energy levels in the design of new low bandgap conjugated polymers. Detailed examples of representative donor polymers, the current state of the art in their solar cell performances and possible future directions for development of new polymers with improved efficiencies are discussed.
4:30 PM - **I5.4/H5.4
Materials for Photon Harvesting in Organic Solar Cells.
Paul Burn 1 , Paul Meredith 1
1 Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, Queensland, Australia
Show AbstractPolymers, 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 optimization and careful design of the absorber (donor) and acceptor electronic properties and control of their nanophase behavior. The overall aim is to achieve this optimization in materials that can be processed to create large area devices. In this presentation we will discuss recent progress in engineering the properties of materials that can be used in organic solar cells. We will present the development and our latest understanding of materials in terms of their structure, optical and electronic properties, processing, morphology, and device performance.
5:00 PM - **I5.5/H5.5
Non-Fullerene Acceptors for Organic Photovoltaics.
John Anthony 1
1 Chemistry, Univ of Kentucky, Lexington, Kentucky, United States
Show AbstractOrganic solar cells are a promising technology for low-cost power generation. To-date, impressive design rules and numerous functionalization approaches have been developed for donor materials in these devices - however, essentially all of these rules assume the use of fullerene derivatives as acceptor materials. The introduction of new classes of acceptors will allow optimization of this second important OPV component, and may yield systems that can be finely tuned to optimize performance with specific high-performance donor polymers. More importantly, new design rules for acceptors may be uncovered, which will allow further optimization of acceptor design, both for fullerenes and other small molecule materials. This talk will present our recent progress in the development of acene-based OPV acceptors, and describe the design rules that we have encountered to-date.
5:30 PM - **I5.6/H5.6
Solution Processable Small Molecules for High Efficiency Organic Solar Cells.
Gui Bazan 1 , Greg Welch 1 , Yanming Sun 1 , Wei Lin Leong 1 , Chris Takacs 1 , Alan Heeger 1
1 Materials, University of California, Santa Barbara, California, United States
Show AbstractResearch on bulk heterojunction organic solar cells has been dominated by the use of conjugated polymers as the donor phase. It is possible to tune the molecular structures to control the absorption profiles, for achieving proper overlap with the solar spectrum, and the orbital energy levels, for attaining high open circuit voltages with fullerene acceptors. However, the processing of polymeric systems presents some challenges, in particular because the statistical nature of polymerization reactions yields a distribution of molecular weights and because of batch to batch variations. Small molecule systems can circumvent these limitations but provide a different set of challenges. For example, film formation and wetting to underlayers is more problematic. In response, we have designed a new class of easy to prepare donor materials with intermediate molecular dimensions, which, if properly processed, can yield devices with power conversion efficiencies of 6.7 %. Molecular design is critical, as well as the use of solvent additives that influence the morphology of the bulk heterojunction active layer. These issues will be discussed in some detail, together with the techniques used for obtaining insight into the bulk organization and the internal order of the donor phases.
Symposium Organizers
Venkat Bommisetty South Dakota State University
Mario Leclerc Universite Laval
Vladimir Dyakonov Julius-Maximilians University of Wuerzburg
Garry Rumbles National Renewable Energy Laboratory
Niyazi Serdar Sariciftci Johannes Kepler University of Linz
I6: Microstructure and Interfaces
Session Chairs
Marc Baldo
Venkat Bommisetty
Wednesday AM, November 30, 2011
Liberty (Sheraton)
9:00 AM - **I6.1
Carrier Dynamics, Fluctuations and Imaging Studies of Polymer Based Bulk Heterostructure Blends.
K Narayan 1 , Monojit Bag 1 , Sabyasachi Mukhopadhyay 1
1 , JNCASR, Bangalore India
Show AbstractNear field scanning optical microscopy (NSOM) along with local photocurrent mapping is a unique technique that provides direct insight into evolution of optoelectronic properties in polymer blends with morphological (2D and 3D) variations. We demonstrate the utility of this method in Si-PCPDTBT/ PC71BM model devices. A steady state photocurrrent response is a net outcome arising from a sequence of charge generation, transport and loss processes. We study the accompanying fluctuations in the photocurrent which represents the discernible signature of the deviation from a time-independent steady current and attribute it to trapping events. The trapping process involves a capture, waiting and release stages of finite duration with a resulting 1/f type noise spectrum in a relevant frequency regime and is accompanied by additional features which provides considerable insight into the solar cell.
9:30 AM - **I6.2
The Influence of the Acceptor on the Photophysics of Organic Bulk Heterojunctions.
Nikos Kopidakis 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe efficiency of Organic Photovoltaics (OPV) with a polymer-based bulk-heterojunction active layer has steadily increased during the past few years. A major driver of this progress is the development of active materials specifically designed for OPV, namely conjugated polymer donors and molecular acceptors, typically fullerene derivatives. In this talk I will discuss how free charge carrier generation and transport depends on the choice of the acceptor in bulk heterojunction blends. Spectroscopy and contactless photoconductivity provide insight into the fundamental photophysics of bulk heterojunction films, which we correlate with OPV device performance. We also perform quantum chemical calculations to obtain information about the properties of the fullerenes at the molecular level and inform the analysis of experimental data. We have used various fullerenes to investigate the effect of the electron affinity on the efficiency of free carrier generation at the interface between the polymer and the fullerene and the effect of molecular structure on the mobility of electrons in fullerene domains. We model the photoconductivity dynamics taking into account trapping and recombination of carriers and show how the specific choice of fullerene affects these processes. While the focus of this talk is on fullerene acceptors, due to their success in OPV, comparison to other types of molecular acceptors is informative. I will discuss how the morphology of polymer-acceptor blends changes with the molecular structure of the acceptor and the corresponding changes observed in the dynamics of photoinduced carriers.
10:00 AM - **I6.3
Spin Engineering in Organic Solar Cells.
Marc Baldo 1
1 Center for Excitonics, MIT, Cambridge, Massachusetts, United States
Show AbstractSpin is essentially irrelevant to the operation of conventional solar cells, but it is crucial to the dynamics of organic semiconductors. Indeed, the impact of spin engineering in organic photovoltaics (OPVs) could rival the benefits achieved in organic light emitting devices. In this talk, we will describe the promise and current status of spin engineering within three fundamental OPV processes: exciton diffusion, charge recombination, and singlet exciton fission. Diffusion: Triplet excitons can theoretically be engineered to travel arbitrarily long distances. In contrast, the diffusion length of singlet excitons cannot be increased arbitrarily because increases in the near field energy transfer rate are cancelled by a shorter radiative lifetime. We report our recent measurements of triplet diffusion in single crystals of rubrene and tetracene, highlighting the importance of optical waveguiding to the apparent motion of triplets. Recombination: It is theoretically possible to turn off the recombination of CT states by engineering them in the triplet configuration. We report the spin dependence of charge transfer state recombination as a function of temperature and magnetic field. Fission: Singlet exciton fission is an efficient multi exciton process that occurs when the singlet has approximately twice the energy of the triplet. We will describe our recent characterizations of this effect in potential OPV materials and its application to generating higher efficiencies.
11:00 AM - **I6.4
Polarizing Organic Photovoltaics.
Rui Zhu 1 , Ankit Kumar 1 , Yang Yang 1
1 Materials Sci and Engineering, Univ. of California Los Angeles, Los Angeles, California, United States
Show AbstractLiquid crystal display (LCD) is the most prevalent information display technology. Unfortunately, most of the backlight energy (75%) is lost to the orthogonal polarizers. Here, we innovate on this energy loss component in the LCD by demonstrating a novel energy recycling technology called polarizing organic photovoltaics (ZOPVs), which can potentially boost the function of LCD by working simultaneously as a polarizer, a photovoltaic device and an ambient light or sunlight photovoltaic panel. To realize the ZOPV device, we firstly achieved the highly oriented P3HT films with dichroic ratio of up to 14.3. Then, we developed novel inverted quasi-bilayer device architecture. The structure gives the flexibility to process P3HT and PC60BM separately and provides the photovoltaic film with anisotropic optical property. Dichroic photovoltaic effect characterization has given a high Jsc ratio of 3.54. This concept of both recycling otherwise wasted energy and harvesting the ambient light energy will be significant towards the development of another promising green technology.
11:30 AM - I6.5
Controlling Recombination via Charge Transfer Excitons in Polymer:Fullerene Blends with Molecular Doping.
Elizabeth von Hauff 1 2 , Felix Deschler 3 , Enrico Da Como 3 , Jochen Feldmann 3 , Sybille Allard 4 , Ullrich Scherf 4
1 Physics, University of Freiburg, Freiburg Germany, 2 , Fraunhofer Institute for Solar Energy Systems, Freiburg Germany, 3 Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximillians-University Munich, Munich Germany, 4 Department of Chemistry and Institute of Polymer Chemistry, Wuppertal University, Wuppertal Germany
Show AbstractCharge separation at the polymer:fullerene interface is the primary process in organic photovoltaics. Following ultrafast photoinduced charge transfer, electrons and holes are still bound because of the Coulomb interaction, resulting in the formation of charge transfer excitons (CTEs)[1]. Those are known to play a crucial role in solar cells, limiting the open circuit voltage and the short circuit current[2].In this contribution, we propose a novel strategy to partially overcome the recombination channels due to CTEs. This new approach considers the modification of the electronic properties of the conjugated polymer by doping with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules. The free holes induced by F4-TCNQ on the low-bandgap 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) fill the tail of states in the highest occupied molecular orbital and in addition are expected to influence Coulomb interactions of Frenkel excitons in PCPDTBT. By the unique combination of time resolved photoluminescence and infrared photoinduced absorption spectroscopy, we demonstrate how doping results in a decreased population of CTEs and ultrafast enhanced formation of polarons[3]. The filling of the tail states is demonstrated by transport experiments, in which a five-fold increase in the hole mobility is observed. By modelling the optical transients according to a rate equations model we show how doping can reduce the probability of CTE formation from the initial Frenkel exciton resulting in a more efficient direct electron transfer. The results demonstrate a new approach, based on molecular doping, to achieve efficient charge separation and understand the photocarrier dynamics in organic semiconductor blends for solar cell applications. References: [1]M. Hallermann, S. Haneder and E. Da Como, Appl. Phys. Lett. 93, 053307 (2008); M. Hallermann, I. Kriegel, E. Da Como, J. M. Berger, E. von Hauff and J. Feldmann, Adv. Funct. Mater. 19, 3662 (2009).[2]M. Hallermann, E. Da Como, J. Feldmann, M. Izquierdo, S. Filippone, N. Martin, S. Juchter and E. von Hauff, Appl. Phys. Lett. 97, 023301 (2010).[3]F. Deschler, E. Da Como, R. Tautz, E. Von Hauff, U. Scherf and J. Feldmann, (submitted) (2010)
11:45 AM - I6.6
Enhancing the Efficiency of Bulk Heterojunction Solar Cells through Engineering of Metal Electrode - Active Layer Interface.
Mukesh Kumar 1 , Chong Chen 1 , Pavel Dutta 1 , Venkat Bommisetty 1
1 Electrical Engineering, South Dakota State University, Brookings, South Dakota, United States
Show AbstractEfficient carrier transport across metal-active layer interface is one of the critical parameters that significantly affect short circuit current, open circuit voltage, fill factor and overall efficiency of bulk hetrojunction (BHJ) solar cells. In this work, the effect of annealing treatment (prior to metal electrode deposition and post deposition) on aluminum-blend interface in poly(3-hexylthiophene) (P3HT) and phenyl-C61butyric acid methyl ester (PCBM) BHJ was investigated. The post-annealed device exhibits an efficiency of 2.3%, compared to 1.1% for pre-annealed and 0.5% for as-deposited cells. To understand the origin of increase in device performance, intensity modulated photocurrent spectroscopy (IMPS) under various DC light intensities (532 nm laser) and atomic force microscopy (AFM) at aluminum-blend interface was performed. The IMPS study suggests reduced interfacial recombination in post-annealed cells in comparison to as-deposited and pre-annealed cells. The DC light intensity dependent IMPS results indicate more pronounced increase of interfacial recombination with photon flux in as-deposited and pre-annealed devices than post-annealed device. The nanoscale topography and electrical measurements at aluminum-blend interface suggest fine donor-acceptor distribution due to post-annealing. This study reveals the importance of electronic quality of aluminum-blend interface in enhancing the efficiency of BHJ solar cell and helps to understand recombination processes occurring at this interface and carrier transport in the bulk of active layer.
12:00 PM - I6.7
Efficiency-Limiting Processes in Bulk Heterojunction Organic Solar Cells.
Frederic Laquai 1 , Ian Howard 1 , Ralf Mauer 1 , Fabian Etzold 1 , Michael Meister 1 , Don Cho 1 , Nam Seob Baek 3 , Tae-Dong Kim 2 , Kwang-Sup Lee 2 , Klaus Muellen 1
1 Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Mainz Germany, 3 IT Convergence Technology Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of), 2 Department of Advanced Materials, Hannam University, Daejeon Korea (the Republic of)
Show AbstractDespite significant study by many research groups, the efficiency-limiting processes that govern the performance of bulk heterojunction photovoltaic devices still remain ambiguous. In particular the role of interfacial charge-transfer (CT) states as potential intermediates of free charge carriers is diversely debated. In this contribution we directly observe charge generation and recombination processes in state-of-the-art polymer:methanofullerene photovoltaic blends by transient absorption (pump-probe) spectroscopy over the entire relevant timescale from femtoseconds to milliseconds and in a broad spectral range covering 500 to 2000 nm. We compare poly(3-hexylthiophene) (P3HT) of different regioregularity (i.e. regiorandom vs. regioregular) and low-bandgap polymers (PCDTBT, PCPDTBT) as electron donor materials. We observe a common feature of these blends is ultrafast (< 200 fs) exciton dissociation at the donor-acceptor interface. However, a certain fraction of excitons creates CT states that predominantly recombine geminately within a few nanoseconds. On the other hand the fraction of free charge carriers recombines non-geminately on a time scale competing with charge extraction and can thus be swept out of the device as photocurrent. In PCDTBT a pronounced relaxation of the charge carriers is observed indicating a rather broad density of states and large energetic disorder, which is reflected in a reduced fill factor of devices. PCPDTBT shows a distinct dependence of the photophysical properties on the presence of solvent additives during spin-coating. In general the results demonstrate for the different material systems the importance of i.) ultrafast free carrier generation, ii.) suppression of interfacial CT state formation, and iii.) high and balanced charge carrier mobilities to achieve high power conversion efficiencies.[1] F. Etzold, I.A. Howard, R. Mauer, M. Meister, T.-D. Kim, K.-S. Lee, N.S. Baek, F. Laquai, J. Am. Chem. Soc. 2011, 133 (24), 9469–9479.[2] I.A. Howard, R. Mauer, M. Meister, F. Laquai, J. Am. Chem. Soc. 2010, 132 (42), 14866-14876.[3] R. Mauer, I.A. Howard, F. Laquai, J. Phys. Chem. Lett. 2010, 1 (24), 3500-3505.
12:15 PM - I6.8
Control of P3HT Ordering in P3HT/PCBM Photovoltaic Active Layers Formed from Initial P3HT:PCBM Bilayers.
Lesley Thompson 1 , Mark Dadmun 1 , Chris Milojevich 1 , Jon Camden 1
1 , University of Tennessee at Knoxville, Knoxville, Tennessee, United States
Show AbstractUnderstanding and controlling the morphology of organic solar cells is key to producing higher efficiencies to create devices that are economically competitive with inorganic devices. In this work, Raman spectroscopy is used to probe the electron-donor layer aggregation, while grazing incidence x-ray diffraction monitors the crystal size within the electron-donor layer. We focus on a 20/80 poly(3-hexylthiophene-2,5-diyl)/ [6,6]-penyl-C61-butyric-acid-methyl-ester (P3HT/PCBM) bilayer thin film spin-coated onto a Si wafer substrate. P3HT in ODCB was spun, and allowed to crystallize at 150 °C in a vacuum oven at times ranging from 0 minutes to 60 minutes. After this annealing procedure, PCBM in DCM was spun on top of the P3HT layer. Samples were then analyzed by Raman spectroscopy and GIXRD in the as-cast samples. The same samples were then annealed for 15 minutes at 150 °C in a vacuum oven and re-analyzed. Results yield useful information on how the modification of thermal history can change the morphology of thin films.
12:30 PM - I6.9
Enhanced Exciton Dissociation in Organic Photovoltaic Layers Doped with Ferroelectric Dipoles.
Kanwar Nalwa 1 , John Carr 1 , Rakesh Mahadevapuram 1 , Hari Kodali 2 , Sayantan Bose 3 , Yuqing Chen 1 , Jacob Petrich 3 , Baskar Ganapathysubramanian 2 , Sumit Chaudhary 1
1 Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa, United States, 2 Department of Mechanical Engineering, Iowa State University, Ames, Iowa, United States, 3 Department of Chemistry, Iowa State University, Ames, Iowa, United States
Show AbstractA key requirement for realizing efficient organic photovoltaic (OPV) cells is the dissociation of photogenerated electron-hole pairs (singlet excitons) (SE) in the donor polymer, and charge-transfer excitons (CTE) at the donor-acceptor interface. However, in modern OPVs, these excitons are typically not sufficiently harnessed due to their high binding energy. Here, we show that doping the OPV active-layers with a ferroelectric polymer leads to localized enhancements of electric field, which in turn leads to more efficient dissociation of SEs and CTEs. Bulk-heterojunction OPVs based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester were fabricated. When we incorporated a ferroelectric polymer in the active-layer, power conversion efficiencies increased by nearly 50%, and internal quantum efficiencies approached 100% – indicating complete exciton dissociation at certain photon energies. Similar enhancements in bilayer-heterojunctions, and direct influence of ferroelectric-poling on device behavior showed that improved dissociation was due to ferroelectric dipoles rather than morphological changes. Enhanced SE dissociation was also revealed by photoluminescence lifetime measurements, and predicted by simulations using a numerical device model.
12:45 PM - I6.10
Optical T-Matrix and Kinetic Monte Carlo Modeling of Nanostructured Planar Heterojunction Solar Cells.
Geraldine Paulus 1 , Moon-Ho Ham 1 , Michael Strano 1
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractDue to their simple geometry and design, planar heterojunction (PHJ) solar cells have advantages both as potential photovoltaics with more efficient charge extraction than their bulk heterojunction (BHJ) counterpart, and as idealized interfaces to study basic device operation. In this work, we combine for the first time both an optical T-matrix and a kinetic Monte Carlo model to investigate the photocurrent generation in two state-of-the-art PHJ photovoltaics. The combined model takes into account the rates of exciton generation, transport, recombination and dissociation using literature values. By including the optical, electronic and structural properties of the different materials, we are able to predict the short-circuit current of recently reported P3HT/SWNT PHJ and P3HT/PCBM PHJ solar cells from the literature. The experimental data for each of these devices show a maximum photocurrent output at a P3HT thickness of 60-65 nm, in contradiction to the expected value equal to the diffusion length of excitons in P3HT (8.5nm). The model demonstrates how a bulk exciton sink can explain this shifted maximum in the P3HT/SWNT case, whereas the maximum is mainly determined by PCBM interdiffusing in P3HT in the P3HT/PCBM case. Based upon the results of this model it will be possible to more intelligently design nanostructured photovoltaics and optimize them towards higher efficiencies.
I7: Microstructure - Carrier Dynamics
Session Chairs
Richard Friend
Serdar Sariciftci
Wednesday PM, November 30, 2011
Liberty (Sheraton)
2:30 PM - **I7.1
Singlet Fission and Interchromophore Coupling in Designed Molecular Chromophores.
Justin Johnson 1 , Akin Akdag 2 , Brian Stepp 2 , Millie Smith 2 , Josef Michl 2 , Arthur Nozik 1 2
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractWith the appropriate interchromophore geometry, organic compounds with designed singlet/triplet splitting formed into polycrystalline solids have yielded extremely high efficiency of triplet formation via singlet fission. Crystals, polymers, and covalently bound dimers that exhibit various interchromophore geometries have been investigated spectroscopically and theoretically in order to arrive at fundamental design criteria that will enable singlet fission to be efficient in a variety of compounds that may find use in photoconversion schemes. Recent advances in our investigations of the model chromophore 1,3-diphenylisobenzofuran will be discussed along with results on the industrial pigment Cibalackrot. Data from ultrafast transient absorption spectroscopy and related techniques will be compared with predictions of rates from calculations that employ either a direct (concerted two-electron process) or indirect (sequential one-electron processes) model for the formation of two triplets.
3:00 PM - **I7.2
Coherent Spin Spectroscopy of Excitonic Precursor States of Organic Thin Film Semiconductor Devices.
Christoph Boehme 1 , William Baker 1 , Sang-Yun Lee 1 , Seo-Young Paik 1 , Dane McCamey 1 , Kipp Van Schooten 1 , John Lupton 1
1 Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah, United States
Show AbstractThe optical and electronic properties of semiconductors with weak spin-orbit coupling are strongly influenced by spin-selection rules and consequently, experimental access to the qualitative and quantitative nature of these processes is crucial for materials and device design. In this presentation, an overview is given about the concepts of pulsed, electrically and optically detected magnetic resonance spectroscopy as techniques to manipulate and observe and thus, characterize these fundamental properties of electron and nuclear spin-dependent processes in organic semiconductors [1-4]. By coherent (pulsed) magnetic resonant perturbation of spin states one can cause the spins to coherently propagate. Spin-dependent charge carrier-transport or -recombination allow the observation of this coherent spin motion through electrical [2] or optical [4] measurements in working devices, such as organic light-emitting diodes or organic solar cells. The ubiquitous presence of hydrogen nuclei gives rise to strong hyperfine interactions, which appear to provide the basis for many of the magnetoresistive effects observed in these materials. Since hyperfine coupling influences resonantly driven quantum spin beating in electrically or optically detectable electron–hole pairs, an extraordinarily sensitive probe for hyperfine fields in such pairs is given [3,4]. This allows scrutinizing various existing models such as polaron pair and bipolaron processes, triplet-polaron- and triplet-pair processes or spin-dependent hopping. Qualitative as much as quantitative insights are gained into some of the physical intricacies of organic semiconductor device fabrication such as the influence of contact materials on spin-orbit coupling. References: [1] D. R. McCamey, et al. Nature Mat. 7, 723, (2008). [2] C. Boehme et al. Phys. Stat. Sol B. 246, 11-12, 2750 (2009).[3] D. R. McCamey, et al. Phys. Rev. Lett. 104, 017601 (2010).[4] S.-Y. Lee, et al. JACS 133, 072019 (2011).
3:30 PM - **I7.3
Dynamics of Light-Induced Charge Carriers in Poly(Thienothiophene) Derivatives Blended with PCBM.
Tom Savenije 1
1 Department of Chemial Engineering, Delft University of Technology, Julianalaan 136 Netherlands
Show AbstractBulk heterojunction solar cells based on conjugated polymers and fullerenes have shown great promise for photo-conversion through their synthetic variability, low temperature processing and possibility of producing lightweight, flexible and inexpensive solar cells. In this paper we discuss blends of three different thiophene copolymers incorporating fused thienothiophene units with PCBM. The morphology on thermal annealing of the pATBT:PCBM blend exhibits formation of phase segregated polymer and PCBM domains. Annealing of pBTTT:PCBM and pBTCT:PCBM yields a well-ordered structure with PCBM molecules intercalated between layers of the π-stacked polymers. In the intercalated systems the photoluminescence is almost completely quenched, in contrast to the phase segregated pATBT:PCBM blend. The higher degree of exciton quenching in the intercalated systems results in a higher initial yield of charge separation. However, on longer timescales (> 10 ns) the microwave photoconductance for the intercalated systems is an order magnitude lower than for pATBT:PCBM blend systems. This is likely due to restricted motion of charges in intercalated systems, which reduces the yield of free charge carriers. Using the technique of charge-carrier extraction by linearly increasing voltage (photo-CELIV) a rise of one order of magnitude in the initial charge-carrier concentration is observed on changing the pBTCT:PCBM weight ratio from 1:1 to 1:4. This change is attributed to an enhanced charge carrier geminate recombination in the 1:1 intercalated structures. In contrast, the extensive phase segregation in the 1:4 blend leads to efficient generation of free charge carriers resulting in higher short circuit current densities of the corresponding solar cells.
4:30 PM - **I7.4
Role of Polaron Formation in Limiting the Charge Generation Efficiency of Organic Bulk Heterojunction Solar Cells.
Richard Friend 1 , A. Bakulin 1 , A. Rao 1
1 Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom
Show AbstractCharge photogeneration in organic solar cells proceeds with high efficiency in a range of polymer-fullerene systems, in spite of the presence of strong Coulomb interactions that do stabilize a fraction of the charged photoexcitations as charge-transfer excitons across the donor-acceptor heterojunction. We propose here that charge separation can occur while electron and hole are in delocalized band-like states, but that they rapidly localize through polaron formation, with associated optical absorption bands below the gap. We find that optical excitation, ‘push’, into the lower of these bands, at 2.2 µm, causes a significant increase in short-circuit photocurrent for a range of polymer-polymer and polymer-fullerene solar cells. We attribute this increase to the separation of coulombically-bound electron hole pairs during the short-lived delocalization of the hole polaron on the donor polymer following the optical ‘push’ excitation.
5:00 PM - **I7.5
Influence of Acceptor Structure on Charge Separation Dynamics in Organic Photovoltaic Materials.
John Asbury 1 , Ryan Pensack 1
1 Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractThe dynamics of charge separation in photovoltaic polymer blends following photoinduced electron transfer from the conjugated polymer, regioregular-P3HT, to electron acceptors are observed with ultrafast vibrational spectroscopy. The investigators take advantage of a solvatochromic shift of the vibrational frequency of the carbonyl (C=O) stretch of the acceptors to directly measure the rate of charge transfer state dissociation to form charge separated states. Two acceptor classes are examined – functionalized fullerenes (PCBM) and perylene diimides (PDIs). Charge separation in rr-P3HT:PCBM blends occurs through activationless pathways whereas rr-P3HT:PDI blends exhibit activated charge separation. The variation in charge separation mechanism arises from differences in the degree of electron delocalization that the electron acceptors can support. The three-dimensional structure of fullerenes enables large electron delocalization giving rise to low barriers to charge separation. The pseudo-two-dimensional structure of PDIs causes localization of electrons in the acceptor phase and larger barriers to charge separation.
5:30 PM - I7.6
Charge Separation in Semicrystalline Polymeric Semiconductors by Photoexcitation: Is the Mechanism Intrinsic or Extrinsic?
Francis Paquin 1 , Maciej Sakowicz 1 , Gianluca Latini 2 , Paul-Ludovic Karsenti 1 , Linjun Wang 3 , David Beljonne 3 , Natalie Stingelin-Stutzmann 2 , Carlos Silva 1
1 Department of Physics, Universite de Montreal, Montreal, Quebec, Canada, 2 Department of Materials, Imperial College, London United Kingdom, 3 Chemistry of Novel Materials, University of Mons, Mons Belgium
Show AbstractWe probe charge photogeneration and subsequent recombination dynamics in neat regioregular poly(3-hexylthiophene) films over six decades in time by means of time-resolved photoluminescence spectroscopy. Exciton dissociation at 10K occurs extrinsically at interfaces between molecularly ordered and disordered domains. Polaron pairs thus produced recombine by tunnelling with distributed rates governed by the distribution of electron-hole radii. Quantum-chemical calculations suggest that hot-exciton dissociation at such interfaces results from a high charge-transfer character.
5:45 PM - I7.7
Surfactant-Assistant Supramolecular Control of Poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT): [6,6]-phenyl C61-butyric Acid Methyl Ester (PCBM[60]) Blend Microstructures.
Ester Buchaca-Domingo 1 , Fiona Jamieson 1 , Nikos Kopidakis 2 , Giuseppe Portale 3 , James Durrant 1 , Garry Rumbles 2 4 , Natalie Stingelin 1
1 Centre for Plastic Electronics (CPE), Imperial College London, London United Kingdom, 2 Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , ESRF Grenoble, Grenoble France, 4 Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractThe performance of organic bulk heterojunction solar cells is critically dependent on the morphology – the micro and nanostructure - of the photoactive layer. One promising route to control this structure is the use of surfactants.[1] In our presentation we will focus on poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM[60]) blends as these binaries (i) display high photoluminescent quenching (>99%; analogous to the high efficiency polymers such as poly[2,1,3-benzothiadiazole -4,7- diyl[4,4- bis(2-ethylhexyl)- 4H- cyclopenta[2,1- b:3,4- b']dithio-phene -2,6- diyl]], PCPDTBT) indicative of intermit mixing of essentially all the polymer in the film with PCBM, but (ii) also have the advantage that the intermixed phase comprises a well-defined pBTTT/PCBM[60]) co-crystal, thereby facilitating structural analyses of such blend films.[1] As surfactants, we used a set of simple, commercially available surfactants. We will demonstrate based on structural, physico-chemical and spectroscopy data that the surfactant successfully prevents intercalation of the fullerene between the polymer side chains – i.e. prevents formation of the pBTTT: PCBM[60]) co-crystal. This promises that the fullerene content possibly can be reduced in solar cells based on such systems as crystalline PCBM[60]) percolating pathways can be reached without the need to go to pBTTT:PCBM[60]) weight ratios of 1:4.[1]T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Guntner, U. Scherf, Nat Mater 2003, 2, 408.
Symposium Organizers
Venkat Bommisetty South Dakota State University
Mario Leclerc Universite Laval
Vladimir Dyakonov Julius-Maximilians University of Wuerzburg
Garry Rumbles National Renewable Energy Laboratory
Niyazi Serdar Sariciftci Johannes Kepler University of Linz
I8: Organic Photovoltaics: Materials Challenges
Session Chairs
Venkat Bommisetty
Mario Leclerc
Thursday AM, December 01, 2011
Liberty (Sheraton)
9:30 AM - **I8.1
Textured Donor Layers in Organic Solar Cells: Vacuum-Deposited and Solution Processed Small Molecules.
Neal Armstrong 1 , Diogenes Placencia 1 , Jeremy Gantz 1 , Xerxes Steirer 1 , Derek Manglesdorf 1 , Mayank Mayukh 1 , Weining Wang 1 , Mariola Macech 1 , Dominic McGrath 1
1 Chemistry & Biochemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractWhile polymer-based donor phases in organic solar cells (OPVs) have provided impressive gains in OPV efficiencies, they often lack the near-IR absorptivity that would provide for high responsivity over the entire solar spectrum. Certain small molecule donors, such as the trivalent and tetravalent metal phthalocyanines (e.g. TiOPc, ClInPc), when properly vacuum deposited and solvent annealed, undergo polymorphic changes which introduce charge-transfer (CT) excitonic features to their absorbance spectra, extending their photoelectrical activity out to ca. 950 nm. The donor layer is substantially textured providing a means to controllably enhance photocurrent yield and device efficiency in Pc/C60 OPVs. We present here an overview of the energetics of these heterojunctions, and estimates of the excited state energy differences (ELUMOPc – ELUMOC60 and ELUMOPc – ELUMOC70) which drive photo-induced electron transfer and significantly determine photocurrent yields at excitation wavelengths ≥ 800 nm. These studies have provided guidance in the creation of new solution-processable versions of TiOPc, which in either planar (Pc/C60) or bulk heterojunction (Pc/PCBM) OPVs demonstrate that the near-IR CT-excitonic feature seen in vacuum processed thin films can be retained, with disproportionately higher efficiencies for photocurrent production from the near-IR CT-excitonic feature. Moving to lower symmetry acceptors, such as PTCDA and perylene bisimides (PTCDI) we find that, unlike the fullerene acceptors, local molecular orientation appears to more strongly affect OPV behaviour than just the energy differences EHOMOPc – ELUMOC60 and ELUMOPc – ELUMOC60. For Pc/PTCDA OPVs, where cofacial molecular contacts are likely at the D/A interface, higher photocurrent yields (and lower VOC) are seen relative to Pc/C4-PTCDI OPVs, where cofacial contact is frustrated, JSC is lower, but VOC substantially higher. Not surprisingly there are large differences in reverse saturation current (JS) for these two heterojunction types, confirming the role of dark charge transfer and its contribution to JS and VOC.
10:00 AM - **I8.2
Efficient Broadband up-Conversion of near-IR Light.
Wenqiang Zou 1 , Cindy Visser 1 , Maxim Pchenitchnikov 2 , Jan Hummelen 1 2
1 Stratingh Institute for Chemistry, University of Groningen, Groningen Netherlands, 2 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
Show AbstractUp-conversion of infrared photons (i.e., photons with energy less than the bandgap of the absorber) is considered an interesting option for enhanced photovoltaic efficiencies as one way of going 'beyond the Shockley-Queisser limit'. The presently known lanthanide-based nanocrystalline up-converters have an extremely weak and narrow absorption around 975 nm, severely limiting any practical use for improving the efficiency of real solar cells. We now report on a simple and viable method to eliminate the limited absorption problem. We show that efficient broadband near-IR up-conversion can thus be obtained. Furthermore, we show that the method allows for tuning the spectral response of the up-converters. This tuning is essential for tailoring the up-conversion spectral response (input and output) with respect to the active layer PV semiconductor(s) bandgap(s).
10:30 AM - **I8.3
Field- and Temperature Dependence of Charge Photogeneration in Organic Bulk Heterojunction Solar Cells.
Markus Mingebach 1 , Stefan Walter 1 , Jens Lorrmann 1 , Carsten Deibel 1 , Vladimir Dyakonov 1 2
1 Experimental Physics VI, Julius-Maximilian University of Würzburg, Würzburg, Bavaria, Germany, 2 , Bavarian Centre for Applied Energy Research (ZAE Bayern), Würzburg, Bavaria, Germany
Show AbstractA topic of high interest is to understand the principles of charge carrier generation and recombination in organic bulk heterojunction (BHJ) solar cells. Fast generation of free charge carriers with high yield is a crucial step to ensure high power conversion efficiencies. The dissociation of singlet excitons into free charge carriers may occur either directly or via the intermediate step involving Coulomb bound charge transfer (CT) states. The field dependence of charge carrier generation can help to distinguish between these two channels. In the case of the poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC60BM) blends almost no field dependence of extracted charge carrier density under positive bias between 0V and open circuit voltage was observed at room temperature [1], thus indicating a direct generation. We point out that the binding energy of CT states in P3HT:PC60BM blends has not yet been determined. Recently we found it to be about 200 meV for poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] :[6,6]-phenyl-C61 butyric acid methyl ester (MDMO-PPV:PC60BM) blends [2], which is clearly beyond the thermal energy kBT and may imply the field dependence of charge carrier generation yield. In order to verify this hypothesis we performed field and temperature dependent Time-Delayed Collection Field (TDCF) measurements on MDMO-PPV:PC60BM BHJ solar cells and compared them to the P3HT:PC60BM case. In view of our results, we will discuss the connection between field dependent charge generation and CT binding energy. Accordingly, the impact of geminate recombination on the performance of BHJ solar cells, as well as the validity and applicability of Onsager-Braun theory for these material systems will be reconsidered.[1] J. Kniepert, M. Schubert, J.C. Blakesley and D. Neher, J. Phys. Chem. Lett., 2, 700-705 (2011)[2] J. Kern, S. Schwab, C. Deibel and V. Dyakonov, Phys. Status Solidi Rapid Res. Lett., accepted, DOI 10.1002/pssr.201105430 (2011).
11:30 AM - **I8.4
New Donor-Acceptor Copolymer Semiconductors for Highly Efficient Solar Cells.
Samson Jenekhe 1 2 , Selvam Subramaniyan 1 , Hao Xin 1 , Felix Sunjoo Kim 1
1 Chemical Engineering, University of Washington, Seattle, Washington, United States, 2 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractConjugated polymer semiconductors with donor-acceptor architecture can exhibit strong intramolecular charge transfer interaction that facilitates tunable charge transport, HOMO/LUMO energy levels, and optical band gap. They are thus very promising for developing more efficient polymer solar cells. We present a series of new donor-acceptor copolymer systems with thiazolothiazole (TT) acceptor units and various electron-donating moieties.[1,2] The optical band gap and HOMO energy levels of these copolymers were found to be 1.7-2.2 eV and -4.91 to -5.65 eV, respectively. They also showed good p-channel characteristics with field-effect hole mobility as high as 0.12 cm2/Vs and on/off current ratios of 105-106. The TT-copolymer/fullerene bulk heterojunction solar cells had a power conversion efficiency of up to 6%. Systematic investigation of the effects of the electron-donating units and the size of side chains on the performance of polymer photovoltaic cells reveal new insights and structure-photovoltaic property relationships useful for the design of optimum polymers for high performance solar cells.[1] Subramaniyan, S.; Xin, H.; Kim, F. S.; Shoaee, S.; Durrant, J. R.; Jenekhe, S. A. Adv. Energy Mater. 2011, Published online. DOI: 10.1002/aenm.201100215.[2] S. A. Jenekhe, S. Subramaniyan, E. Ahmed, H. Xin, F. S. Kim, International Patent Application No. PCT/EP2010/066179, October 26, 2010.
12:00 PM - I8.5
High Fill Factor and Open-Circuit Voltage in Organic Photovoltaic Cells with Diindenoperylene as Donor Material.
Julia Wagner 1 , Mark Gruber 1 , Alexander Hinderhofer 2 , Andreas Wilke 3 , Andreas Optiz 1 , Norbert Koch 3 , Frank Schreiber 2 , Wolfgang Bruetting 1
1 Institute of Physics, University of Augsburg, Augsburg Germany, 2 Institute of Applied Physics, University of Tuebingen, Tuebingen Germany, 3 Insitute of Physics, Humboldt-University of Berlin, Berlin Germany
Show AbstractSmall-molecule photovoltaic cells using diindenoperylene (DIP) as a new donor material in combination with the fullerene C60 as an electron acceptor are demonstrated. In addition to the successful application in planar and bulk heterojunction devices, a comprehensive analysis including structural studies, the determination of the energy level alignment and electrical transport investigations is given, stressing the correlation between growth conditions, film morphology, and device performance. Due to pronounced crystallinity and a large surface area of DIP films grown at elevated temperature, exceptionally high fill factors of almost 75% are achieved in planar heterojunction cells. Bulk heterojunctions exhibit large-scale phase separation forming a bicontinuous network of both molecular species, which enables efficient exciton dissociation and charge carrier transport. The high ionization potential of DIP and the favorable energy level alignment with the fullerene C60 yield large open-circuit voltages close to 1V and power conversion efficiencies of about 4% in both cell architectures.
12:15 PM - **I8.6
Organic Semiconductor Chemistry.
Seth Marder 1 , Lauren Polander 1 , Stephen Barlow 1 , Shree Tiwari 1 , Brian Seifried 1 , Bernard Kippelen 1 , Chad Risko 1 , Jean-Luc Bredas 1 , Tissa Sajoto 1 , Sang Bok Kim 1 , Swagat Mohapatra 1 , Song Guo 1 , Yabing Qi 2 , Wei Zhao 2 , Antoine Kahn 2
1 Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractOrganic semiconductors have attracted interest for electronic applications due to their potential for use in low-cost, large-area, flexible electronic devices. While many examples of organic semiconductors for p-channel and n-channel organic field-effect transistors (OFETs) have been reported in the recent literature, there is a paucity of high-performance, solution-processable, small-molecule materials for n-channel OFETs. In order to take advantage of the technological potential of organic semiconducting materials, solution-processable, ideally air-stable, electron-transport (ET) materials with low barriers for charge injection, high charge-carrier mobility values (> 1 cm2V-1s-1), large current on/off ratios (Ion/Ioff > 106), and low threshold voltage (< ±2.5 V) are still desirable.Here, we report that bis(NDI) derivatives with conjugated bridging groups based on fused heterocycle ring systems can be used to create solution-processed films that exhibit OFET electron mobility values of up to 1.5 cm2V–1s–1, which is among the highest yet reported for an n-channel OFET based on a solution-processed small molecule. In addition we will discuss the development of metal complexes that can be used to both n-dope or p-dope organic semiconductors.
12:45 PM - I8.7
Harvesting near-IR Irradiation Using Electron-Accepting Conjugated Polymers Based on Aza-Dipyrromethene Dyes.
Genevieve Sauve 1 , Lei Gao 1 , Cassie Daddario 1 , Zhenghao Mao 1 , Kenneth Singer 2 , Lei Zhu 3 , Saide Tang 3
1 Chemistry, Case Western Reserve University, Cleveland, Ohio, United States, 2 Physics, Case Western Reserve University, Cleveland, Ohio, United States, 3 Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractLow bandgap (<1.5 eV) conjugated polymers were synthesized via palladium-catalyzed Sonogashira coupling. These copolymers incorporate red-light absorbing azadipyrromethenes (Aza-DIPY) within the main chain, allowing for enhanced conjugation and absorption in thin films up to ~1000 nm. The new polymers displayed reversible reductions as ascertained by cyclic voltammetry experiments. Reaction with trifluoroboron etherate yielded a new series of polymers having BF2 incorporated into each azadipyrromethene unit of the polymer backbone. These materials displayed increased solubility, red shifts (optical bandgap of < 1.3 eV) and strongly stabilized LUMO energy levels. The resulting polymers thus have a unique combination of very low bandgap and high electron affinity. The absorption spectra of thin films were red-shifted compared to the corresponding solution spectra, consistent with extended conjugation and higher degree of structural order in films. Synchrotron X-ray diffraction studies of the annealed materials showed a strong effect of the substituents in the phenyl rings of aza-DIPY and of BF2 chelation on the molecular packing in the solid state, including π-π stacking of the main chain and lamellar ordering of the side chains. Preliminary results in organic solar cells and efforts to control their nanoscale morphology in blends with a donor material (P3HT) will also be presented.
I9: Nanocrystals and Quantum Dots
Session Chairs
Sumit Chaudhary
Svetlana Kilina
Garry Rumbles
Thursday PM, December 01, 2011
Liberty (Sheraton)
2:30 PM - **I9.1
Dynamics of Excitons and Charges in Organic Materials and Semiconductor Quantum Dots.
Laurens Siebbeles 1
1 , Delft University of Technology, Delft Netherlands
Show AbstractThe seminar will report studies of the behavior of electronic excited states (excitons) and excess charge carriers in conjugated polymers, covalent organic frameworks and semiconductor nanoparticles. These materials have fascinating optical and electronic properties that are little understood and are of interest for applications in e.g. solar cells, photodiodes, light-emitting diodes, field-effect transistors and nanoscale molecular electronics. The mechanism of charge carrier photogeneration in thin films of the polymer P3HT and the electron acceptor PCBM (C60 derivative) was studied by ultrafast optical pump-probe and terahertz spectroscopy. It is inferred that photoexcitation leads to formation of free electrons and holes, rather than Coulombically bound electron-hole pairs in close proximity. We studied the mobility of charges in covalent organic frameworks consisting of phthalocyanine units that are strongly coupled by pi-pi stacking in an eclipsed configuration. Charges were generated by irradiation with 3 MeV electrons from Van de Graaff accelerator, and were probed by microwave conductivity measurements. The virtually temperature independent charge mobility suggest a band-like mechanism of charge motion, rather than hopping via localized states. According to quantum mechanical simulations, eclipsed stacking of the phthalocyanine units can lead to a high charge mobility of ~100 cm2/Vs, which largely exceeds that for conventional organic semiconductors.The generation of two excited states for the absorption of a single photon in semiconductor quantum dots was studied. We established that this process of multiple exciton generation (MEG) occurs in PbSe quantum dots. Adding a PbS shell around a PbSe core quantum dot leads to delocalization of electron and hole wave functions into the PbS shell. Surprisingly, this does not affect the MEG efficiency. Possible causes for the anomalous independence of MEG on the presence of a PbS shell will be discussed. Using a combination of ultrafast terahertz photoconductivity and optical absorption measurements, it was found that photoexcitation of a film of strongly coupled PbSe quantum dots leads directly to formation of free charge carriers rather than excitons.
3:00 PM - I9.2
Exciton Recombination in Ternary Blends of Organic Semiconductors and near-Infrared Quantum-Dots.
Grigorios Itskos 1 , Andreas Othonos 1 , Tobias Rauch 2 , Sandro Tedde 2 , Oliver Hayden 2 , Maksym Kovalenko 3 , Wolfgang Heiss 3 , Stelios Choulis 4
1 Physics Department, University of Cyprus, Nicosia Cyprus, 2 , Siemens AG, Corporate Technology, Erlangen Germany, 3 Institute of Semiconductor and Solid State Physics, University of Linz, Linz Austria, 4 Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, Limassol Cyprus
Show AbstractSolution-processed photodetectors and photovoltaics based on bulk heterojunctions of organic semiconductors represent a promising route to devices that can be processed inexpensively at ambient conditions. However a significant limitation on the efficiency of such devices is that blend absorption is commonly limited to visible wavelengths so that a significant fraction of the longer wavelength solar emission energy cannot be harvested. To circumvent this limitation a significant effort has been carried out on the incorporation of near-infrared absorbing colloidal quantum dots (QDs) in organic blends. Recently efficient hybrid photodiodes based on blends of oleic-acid capped PbS QDs, poly(3-hexylthiophene) (P3HT) and methanofullerene derivatives (PCBM) materials with extended spectral sensitivity of up to 1.8 μm have been demonstrated[1]. The photodiode structures were investigated by contact techniques that probe their overall device performance. However it has been established that the presence of electrodes affects exciton recombination. To isolate the influence of the electrodes and thoroughly investigate the excitation dynamics in the active layer of such structures we report here a detailed non-contact optical investigation.A systematic series of samples that include pristine, binary and ternary blends of the materials are studied using absorption, steady-state and time-resolved photoluminescence (PL) and ultrafast transient pump-probe spectroscopy. Measurements show[2] a modest enhancement of the absorption strength in the near-infrared upon QD incorporation. PL quenching of the polymer and the QD exciton emission is observed and predominantly attributed to intermaterial photoinduced charge transfer processes. PL experiments and transient absorption measurements indicate that charge transfer occurs more efficiently at the fullerene/polymer and fullerene/QD interfaces compared to polymer/QD interfaces. Thus the inclusion of the fullerene seems to facilitate exciton dissociation in such blends. The experiments probe important and rather unexplored aspects of exciton recombination and charge transfer processes in ternary blend composites of organic semiconductors and near-infrared quantum dots for applications in solution-processed photodetectors and solar cells. [1]T. Rauch, M. Böberl, S.F. Tedde, J. Fürst, M.V. Kovalenko, G. Hesser, U. Lemmer, W. Heiss, O. Hayden, Nat. Photonics, 3, 332 (2009)[2]G. Itskos, A. Othonos, T. Rauch, S. F. Tedde, O. Hayden, M. V. Kovalenko, W. Heiss, S. A. Choulis, Adv. En. Mat published online: 16 Jun 2011
3:15 PM - I9.3
Solar Cells Using Quantum Funnels.
Illan Kramer 1 , Larissa Levina 1 , Ratan Debnath 1 , David Zhitomirsky 1 , Edward Sargent 1
1 Electrical & Computer Engineering, University of Toronto, Toronto, Ontario, Canada
Show AbstractColloidal quantum dots offer broad tuning of semiconductor bandstructure via the quantum size effect. Devices involving a sequence of layers comprised of quantum dots selected to have different diameters, and therefore bandgaps, offer the possibility of funneling energy towards an acceptor. Here we report a quantum funnel that efficiently conveys photoelectrons from their point of generation towards an intended electron acceptor. Using this concept we build a solar cell that benefits from enhanced current and also fill factor as a result of this quantum funnel. This concept addresses limitations on transport in soft condensed matter systems and leverages their advantages in large-area optoelectronic devices and systems.We took the view that improved device architecture could help overcome transport limitations in this otherwise highly promising materials system. Our concept was to exploit bandgap engineering within the light-absorbing, charge-transporting quantum dot active layer to funnel performance-limiting photoelectrons towards the charge-collecting TiO2 electrode.Photovoltaic performance under 1 sun illumination shows that both short-circuit current density and fill factor are enhanced in the graded device compared to the ungraded. Under short-circuit conditions, the device is nearly fully depleted and therefore the electric field acts upon all photogenerated carriers throughout the thickness of the device. As the device is biased towards open-circuit conditions, the energy bands flatten out as the magnitude of the electric field is reduced. A quasi-neutral region grows further inward from the back of the device, degrading carrier extraction. The graded device ensures that minority carriers generated at these depths continue to be driven to the electron-accepting TiO2 even when reverse-bias is reduced under operating conditions. This translates into successful maintenance of a high current, more closely approaching the short-circuit current density, even as power is delivered to a load and the maximum power point is approached. This is manifest in a desirably increased shunt resistance that dominates the improvement in fill factor.
3:30 PM - **I9.4
Wave Function Engineering in Core/Shell Quantum Dots for Efficient Single and Multiexciton Dissociation.
Tianquan Lian 1
1 Dept. of Chemistry, Emory University, Atlanta, Georgia, United States
Show AbstractCharge transfer to and from quantum dots (QDs) is of intense interest because of its important roles in QD-based devices, such as solar cells and light emitting diodes. Recent reports of multiple exciton generation (MEG) by one absorbed photon in some QDs offer an exciting new approach to improve the efficiency of QD-based solar cells and to design novel multi-electron/hole photocatalysts. However, two main challenges remain. First, the efficiency of MEG process remains controversial and may need to be significantly improved for practical applications. Second, the utilizatoin of the MEG process requires ultrafast exciton dissociation prior to the exciton-exciton annihilation process, which occurs on the 10s to 100s ps time scale. In this presentation we report a series of studies of exciton dissociation dynamics in quantum dots by electron transfer to adsorbed electron acceptors. We showed that excitons in CdSe could be dissociated on the a few picosecond timescale to various adsorbates. As a proof of principle, we demonstrated that multiple excitons (generated by multiple photons) per QD can be dissociated by electron transfer to adsorbed acceptors. We will discuss the dependence of these rates on the size and the nature of the quantum dots and the approaches for optimizing single and multiple exciton dissociation efficiencies by wave-function engineering in QD heterostructures.
4:30 PM - I9.5
Neutron Scattering Provides a New Model for Optimal Morphologies in Organic Photovoltaics: Rivers and Streams.
Wen Yin 1 , Nathan Henry 1 , Kai Xaio 2 , John Ankner 2 , Mark Dadmun 1 2
1 , University of Tennessee, Knoxville, Tennessee, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe current model for the ideal morphology of a conjugated polymer bulk heterojunction organic photovoltaic (OPV) is a phase-separated structure that consists of two pure phases, one an electron donor, the other an acceptor, that form an interpenetrating, bicontinuous, network on the length scale of 10-20 nm. In this talk, neutron scattering experiments that demonstrate that this model is incorrect the archetypal conjugated polymer bulk heterojunction, poly[3-hexylthiophene] (P3HT) and the fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6]) C61 (PCBM) will be presented. These studies show that the miscibility of PCBM in P3HT approaches 20 wt%, a result that is counter to the standard model of efficient organic photovoltaics. The implications of this finding on the ideal morphology of conjugated polymer bulk heterojunctions will be discussed, where these results are interpreted to present a model that agrees with this data, and conforms to structural and functional information in the literature. Furthermore, the thermodynamics of conjugated polymer:fullerene mixtures dominate the formation of this hierarchical morphology and must be more thoroughly understood to rationally design and fabricate optimum morphologies for OPV activity.
4:45 PM - **I9.6
Exploring Interfacial Dynamics in Colloidal Nanocrystals.
Marcus Jones 1 , Kevin Major 1 , Gaurav Singh 1 , Danielle Woodall 1 , Edward Williams 1
1 Chemistry, University of North Carolina at Charlotte, Charlotte, North Carolina, United States
Show AbstractQuantum confinement of photo-generated electrons and holes in colloidal semiconductor nanoparticles is well understood and results in a wide array of potentially important properties. Application of nanocrystals in photovoltaics requires rapid and efficient generation of separated charges, which must be transported away from the chromophore to do useful work. These charge separation processes originate from highly delocalized exciton states and can be understood in terms of electron transfer reactions that can localize an electron or hole into a ligand orbital or a nanocrystal trap state.Electronic coupling of nanocrystal exciton states with states in the surrounding environment strongly influences relaxation dynamics. Nanocrystal charge transfer states typically have low absorption cross-sections, so it is hard to directly probe their role and relative impact on the underlying dynamics. Time resolved fluorescence spectroscopy reflects both radiative exciton recombination rates and non-radiative transitions rates to extrinsic surface or ligand states; however, interpretation of fluorescence transients is not trivial and typical multi- or stretched exponential decay models yield little specific photophysical insight.I will describe a quantitative analysis method for temperature-dependent time-resolved fluorescence data applied to a series of nanocrystal systems. I will demonstrate how solvent dielectric, ligand type and other nanoparticles influence fluorescence transients and illustrate how these data can be used to understand electron and energy transfer dynamics. With an emphasis on quantitative analysis, I will discuss what processes occur when quantum dots are dispersed in solution and how these processes affect the measured fluorescence.
5:15 PM - I9.7
Light Energy Conversion by Mesoscopic PbS Quantum Dots /TiO2 Heterojunction Solar Cells.
Lioz Etgar 1 , Thomas Moehl 1 , Stefanie Tscharntke 2 , Stephen Hickey 2 , Alexander Eychmuelerand 2 , Michael Grätzel 1
1 Laboratory of photonics and interfaces, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne Switzerland, 2 Physical Chemistry/Electrochemistry, TU Dresden, Dresden Germany
Show AbstractSolid state PbS Quantum Dots (QDs)/TiO2 heterojunction solar cells were produced by depositing PbS QDs on a 500nm thick mesoscopicTiO2 films using layer-by-layer deposition. Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO2 device produces a short circuit photocurrent (Jsc) of 13.04 mA/cm2, an open circuit photovoltage (Voc) of 0.55 V and a fill factor (FF) of 0.49, corresponding to a light to electric power conversion efficiency (η) of 3.5% under AM1.5 illumination. The electronic processes occuring during light energy conversion in the cell were investigated by electrical impedance spectroscopy (EIS) as well as transient photocurrent and photovoltage measurements. The resistivity of the PbS QDs was found to decrease exponentially with increasing forward bias in the dark while under illumination it stays nearly constant over the whole investigated potential range. Similarly, in the mesoscopic TiO2 film the conductivity as well as the recombination- and transport time of electrons show an exponential dependence on the applied potential in the dark whereas under illumination the observed changes of these parameters are small. The carrier transport time in the dark is longer then their lifetime due to fast recombination at the TiO2/QDs interface. Strikingly, under illumination the concentration of charge carriers increases by several orders of magnitude and the carrier transport time across the TiO2 film becomes much shorter than their lifetime, enabling efficient collection of photogenerated charge carriers.
I10: Poster Session
Session Chairs
Venkat Bommisetty
Vladimir Dyakonov
Mario Leclerc
Garry Rumbles
NiyaziSerdar Sariciftci
Friday AM, December 02, 2011
Exhibition Hall C (Hynes)
9:00 PM - I10.10
Selective Deposition of CdSe Nanoparticles on Reduced Graphene Oxide to Understand Photoinduced Charge Transfer in Hybrid Nanostructures.
Kehan Yu 1 , Ganhu Lu 1 , Shun Mao 1 , Kehung Chen 1 , Haejune Kim 1 , Zhenhai Wen 1 , Junhong Chen 1
1 Mechanical Engineering, University of Wisconsin – Milwaukee, Milwaukee, Wisconsin, United States
Show AbstractSemiconductor nanoparticles (NPs) are known for their size-dependent optical and electronic properties. Combining semiconductor NPs with graphene points to a new direction to design of optoelectronic devices such as solar cells and photodetectors with modulated performance. A linker-free reduced graphene oxide (R-GO)-CdSe NP hybrid nanostructure was synthesized using a chemical vapor deposition (CVD) method. CdSe aerosol NPs was selectively deposited on the surface of R-GO with controlled NP size and coverage. The distribution and morphology of CdSe NPs on R-GO were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The resulting hybrid nanostructure exhibited photoresponse to both laser and simulated sunlight AM 1.5G excitation. The hybrid structure with low CdSe NP coverage showed distinct photoresponse times in air, N2, NH3, and NO2, while high CdSe NP coverage led to nearly constant but three orders of magnitude smaller response time in all gases. Such a difference in photoresponse as a function of NP coverage is attributed to the energy band bending at the interface between the R-GO and the CdSe NP. The selective deposition of CdSe NPs on R-GO and the understanding of the subsequent photoinduced charge transfer can potentially lead to high-performance optoelectronic devices.
9:00 PM - I10.11
Effect of Cu-In-Ga Precursor Deposited at Different Temperature for CIGS Absorption Layer.
Jae-kwan Sim 1 , Yong-Ho Ra 1 , Ji-hyeon Park 1 , Bo-ra Yeom 1 , Ashok Karunanithi 1 , Cheul-Ro Lee 1
1 Semiconductor Materials Processing Laboratory, Chonbuk national university, Jeonju Korea (the Republic of)
Show AbstractCuInGaSe (CIGS) thin film solar cell is a promising material for the production of highly efficient and low cost Photovoltaic (PV) modules by its properties of appropriate direct band gap and higher absorption coefficient. The maximum efficiency of CIGS thin film solar cell is 20.0 % recorded by NREL.The composition of CuInGa (CIG) precursor layer is one of the important factors for the performance of solar cell. So, we have studied that composition of CIG precursor layer influence properties of CIGS absorber layer. In this work, we have controlled chemical composition of CIGS deposited on Soda Lime Glass (SLG) utilizing In target and Cu-Ga target with pre-determined compositions to deposit precursor using DC magnetron sputter. The constitution of Cu-Ga target was Cu:Ga=8:2. We have controlled working temperature from 100 °C to 500 °C to deposit CIG precursor layers with different texture. Also, we have varied sputtering power in order to maintain Cu/(In+Ga), Ga/(In+Ga) ratio in CIG precursor layer. CIG precursor layers deposited by sputtering were converted into CIGS absorber layers from post-selenization process. We have observed surface morphology, thickness, and grain size of CIGS absorber layer using FE-SEM, and composition of CIGS absorber layer is analyzed by energy dispersive X-ray spectroscopy (EDX). EDX results show Cu/(In+Ga), Ga/(In+Ga) ratio in CIGS absorber layer as constant by 0.9, 0.3, respectively. Cu-In, Cu-Ga alloys were observed in CIG precursor from X-ray Diffraction pattern. As a result of XRD, texture of CIG precursor exhibiting crystal phase of Cu11In9, Cu9In4 alloys were improved from increasing deposition temperature. Thereafter, we will convert into CIGS absorber layer under a controlled Se atmosphere. We will study the effect of deposition temperature of CIG precursor for CIGS absorption layer.
9:00 PM - I10.12
Redox-Active Radical Molecules for Organic Photovoltaic Devices.
Fumiaki Kato 1 , Takumi Okuyama 1 , Ken Muraoka 1 , Akitomo Kikuchi 1 , Takakazu Saito 1 , Chihiro Hayashi 1 , Kenichi Oyaizu 1 , Hiroyuki Nishide 1
1 Applied Chemistry, Waseda Unviersity, Tokyo Japan
Show AbstractRadical molecules such as 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) and galvinoxyl (Galvi) exhibi a rapid and reversible p- and n-type redox property, respectively. We have developed them to organic electronic devices such as a totally organic secondary based on their charge-propagation capability in gel state. In this paper, we describe that an electrochemically reversible one-electron redox couple of nitroxide radicals and galvinoxyl radicals provides a novel mediation system to produce a highly efficient photovoltaic effect. Dye-sensitized solar cells (DSSC) and the photovoltaic devices utilizing radical molecules were fabricated to show their performance.DSSC utilizing the TEMPO mediator achieved 5 % of energy conversion efficiency. TEMPO derivatives substituted with electron-withdrawing R groups gave significantly enhanced photovoltages in comparison with those obtained using iodides in DSSC. The photovoltaic cell using the p- or n-type polymer as the charge-separation and -propagation material, an organic dye, and a redox mediator were also fabricated to accomplish very high photovoltage of 800-1000 mV.
9:00 PM - I10.13
External Electric Field and Post-Annealing Induced Enhanced Efficiency in P3HT/PCBM Bulk Heterojunction Solar Cells.
Mukesh Kumar 1 , Chong Chen 1 , Pavel Dutta 1 , Venkat Bommisetty 1
1 Electrical Engineering, South Dakota State University, Brookings, South Dakota, United States
Show AbstractThe effect of external electric field during post-annealing on device characteristics of poly(3-hexylthiophene) (P3HT) and phenyl-C61butyric acid methyl ester (PCBM) bulk heterojunction solar cells was investigated and compared to those fabricated using traditional process. Presence of external electric field, in both forward (EF, negative bias to Al electrode relative to ITO) and reverse (ER) bias, resulted in significant enhancement in device performance metrics and overall efficiency. Devices post-annealed under electric field showed 2.49% efficiency (both EF and ER) verses 0.5% and 2.3% for as-deposited and post-annealed with no external bias, respectively. However, the devices post-annealed under forward and reverse bias showed improvements in different performance metrics: the devices annealed under forward bias showed higher VOC, with a decrease in JSC; whereas devices annealed under reverse bias showed higher JSC, with a decrease in VOC. The origin of the device performance improvement and difference between forward and reverse bias was probed using intensity modulated photocurrent spectroscopy (IMPS) and scanning probe microscopy. The frequency dependent IMPS under various AC and DC illuminations help differentiating bulk and interfacial charge transport and associated carrier recombination. Results indicated a significant reduction in the interfacial charge recombination in all post-annealed cells, compared to as-deposited cells. Also, the interfacial recombination in forward bias annealed devices was lower compared to those annealed under reverse bias. Moreover, application of external electric field (both forward and reverse bias) reduced the characteristic time of high frequency carrier relaxation in devices that also indicate efficient carrier transport through bulk of the active layer, as compared to traditional annealed devices. The results indicate that the post-annealing under external electric field can be effectively used to reduce recombination at the interfaces and efficient charge transport in the bulk of the active layer to improve the device performance.
9:00 PM - I10.14
Optimizing Charge Transfer Processes between Luminescent Polymers and TiO2 Nanoparticles.
Gustavo Valente 1 , Angelo Faceto 1 , Francisco Gontijo Guimaraes 1
1 , University of São Paulo, São Carlos, SP, Brazil
Show AbstractConjugated polymers combined with TiO2 nanoparticles are used for the development of organic photovoltaic cells. In these devices, the TiO2 acts as an electron acceptor and conducting material. The efficiency of the photovoltaic process depends on the properties on polymer/TiO2 interface, the yield of exciton dissociation as well electron transfer and TiO2 nanoparticles organization responsible for the carrier transport to the anode. In this work we investigate the charge transfer processes through luminescence quenching of a thin (10 nm) polymer films deposited by casting and Layer-by-Layer (LbL) methodologies on the TiO2 nanoparticles. A wide range of material have been used for this study, including polyfluorene (PFO), poly(p-phenylene vinylene) (PPV), poly(3-hexylthiophene) (P3HT), poly[2-methoxy,5-(2’-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and the PPV precursor poly(xylyliden tetrahydrothiophenium chloride) (PTHT). We observed that, beside the strong luminescence suppression supporting the efficient charge separation, the spectrum shape is also modified. This spectral change indicates the existence of different competing transfer rates during the electron energy relaxation. Using an inert polyelectrolyte spacer between the polymer donor and the TiO2 nanoparticles, we notice that charge transfer takes places through distances much longer than that predicted in the Marcus (1985) formulation for electron transfer. The results will be compared with Monte Carlo simulation. We hope that understanding the electro-optical interface processes may contribute to an optimization of the organic photovoltaic cells.
9:00 PM - I10.15
Exciton Diffusion in PbS Quantum Dot Thin Films.
David Strasfeld 1 , Gautham Nair 2 , Scott Geyer 3 , Liang-Yi Chang 1 , Moungi Bawendi 1
1 Chemistry, Massachusetts Institute of Technology, Cambridge, MA, Massachusetts, United States, 2 Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Physics, Stanford University, Palo Alto, California, United States
Show AbstractSolution processable photovoltaics are of great interest, due to their potential for generating low cost,large area solar cells. Organic polymer based solar cells have shown early promise in generating high efficiencies at low cost; however, these photovoltaic materials absorb mainly in the visible region of the spectrum, leaving the infrared photons generated by the sun unused. Small bandgap semiconductor nanocrystals, in particular the lead chalcogenides (PbS and PbSe), absorb throughout the visible region of the spectrum and well into the infrared (< 4.1 μm), making them ideal candidates for incorporation into photovoltaic devices. The band edge absorption of these nanocrystals can be tuned to optimize absorption efficiency. This is fortuitous, considering that photon absorption along with exciton diffusion to a donor/acceptor junction, charge transfer, and carrier collection are the four physical processes that dictate device efficiency. We seek to improve device efficiency by understanding exciton diffusion in PbS nanocrystal thin films. We have made efforts towards this goal by depositing PbS nanocrystal thin films of varying thicknesses on a quenching surface, in this case gold. The small PL lifetimes (<500 ps) observed in the ethane dithiol (EDT) treated nanocrystal films necessitate the use of a method that can resolve such time scales. For these experiments we utilize a transient photoluminescence upconversion set-up. The average photon arrival time is determined as a function of film thickness. The photon arrival time saturates above thicknesses of 110 nm, which indicates an exciton diffusion length of roughly half this thickness for EDT treated thin films deposited on gold.
9:00 PM - I10.16
Conjugated Starburst Macromolecules for Optoelectronic Applications.
Feng Liu 1 2 3 , Chao Tang 1 , Hongbin Wu 2 , Wei Wei 1 , Wei Huang 1
1 Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, nanjin, 0, China, 2 Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 0, China, 3 Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States
Show AbstractMonodisperse conjugated star-shaped macromolecules with well defined structure are of high scientific interest due to their unique physical properties and various applications in organic electronic devices. For a molecule reaches the size of nanometers, the physical properties of the single molecule as well as the intermolecular aggregation induced superamolecular structures are of high scientific interest. At the same time, due to the monodisperse nature of the starburst, it is possible to study the structure-topology-property relationship for these materials. In this work, we focus on the structure-property relationship of a few conjugated starburst macromolecules and their applications. In general, we observed, by changing the core of these starbursts, the physical properties largely changed. The truxene centered starburst are generally good blue light emitting materials and amorphous in nature. By using pyrene as the center, the intermolecular aggregation of these materials increases. And they are crystalline by DSC measurement. By changing the arms of starbursts, the physical properties also change a lot. These two ways offers great flexibility of tuning the property of starbursts. The HOMO/LUMO energy level, light absorption and emission as well as superamolecular aggregation can be controlled, which will be shown in our presentation. We also demonstrated these starburst works well in light emitting diodes and organic bulk heterojunction solar cell devices.
9:00 PM - I10.17
The Organic/Inorganic Heterojunction as Model for Excitonic Solar Cells.
Pabitra Nayak 1 , Lee Barnea Nehoshtan 1 , Antoine Kahn 2 , David Cahen 1
1 , Weizmann Institute of Science, Rehovot Israel, 2 , Princeton University, Princeton, New Jersey, United States
Show AbstractMaterials for Organic Photovoltaics (OPV) are of interest, per se, and are being studied extensively for their potential use for large area, cost-effective solar cells[1]. However, even with the impressive gains in efficiency of the past years, it is clear that present cells suffer higher conversion losses than most of their inorganic counterparts[2]. Thus, understanding the loss mechanisms in OPV is of more than academic interest. A first issue is the relationship between the maximum achievable open circuit voltage (Voc) and the absorber’s optical absorption edge (band gap). A voltage loss of 0.5 V or more is observed for cells with reasonable photocurrents and should be accounted for. To study this problem, we use a heterojunction between a wide band gap inorganic semiconductor, SiC, and an organic semiconductor. Because of the ~3 eV SiC bandgap, experiments can be arranged so that all the excitons are created in the organic semiconductor. In this way we can modify the inorganic semiconductor and, to a first approximation also the junction’s interface energetics without changing the exciton population or transport properties in the organic absorber. The surface of SiC was modified with monolayers of different molecular silanes and phosphonates. These monolayers fulfill a dual role, namely electrically passivating the surface and providing a controllable interfacial dipole for changing the interface energy level/band alignment. Metal-phthalocyanine organic absorbers were deposited by thermal vacuum evaporation. Tuning the inorganic/organic interface by different dipole layers showed effects on the Voc that can be attributed to the relative change in energetics at the interface. The energy alignment (at the free surface) was studied separately by electron spectroscopies and contact potential difference measurements. In a series of cells, we observed an increase of the Voc from 0.3V to 0.9 V, using a 1.7eV band gap absorber. These and more recent results will be discussed and analyzed for insights into the question”where does all the photon energy go?”References:[1] B. Kippelen, J.-L. Brédas, Energy & Environmental Science,2,(2009) 251-261; J.-L. Brédas, J. Norton, J. Cornil & V. Coropceanu, Acc. Chem. Res. 42,(2009) 1691-1699.[2] Pabitra K. Nayak, J. Bisquert and David Cahen, Adv. Mater. 2011, DOI: 10.1002/adma.20110087
9:00 PM - I10.18
Direct near–Field Optical Imaging of P3HT Nanostructures.
Mina Baghgar 1 , Sibel Yalcin 2 , Joelle Labastide 2 , Irene Dujovne 2 , Kevin Early 3 , Harihara Venkatraman 2 , Yipeng Yang 1 , Michael Barnes 1 2 , Dhandapani Venkataraman 2 , Anthony Dinsmor 1
1 Physics, University of Massachusetts Amherst, Amherst, Massachusetts, United States, 2 Chemistry, University of Massachusetts Amherst, Amherst, Massachusetts, United States, 3 JILA, University of Colorado Boulder, Boulder, Colorado, United States
Show AbstractUnderstanding structural and optical properties of Poly(3-hexylthiophene) nanostructures is fundamental to the development of new generation organic photovoltaic devices. While many P3HT nanostructures have been studied in far field using single molecule fluorescence spectroscopy, new evidence suggests that there are many more structures with large absorption sections that are not fluorescent, making them difficult to study via the afore-mentioned methods. Non- fluorescent nanostructures have very efficient charge transfer mechanisms, and are therefore important to characterize and study in the context of photovoltaic materials.Here, we employ near-field scanning optical microscopy (NSOM) techniques in reflection mode to correlate size and morphology with optical properties of single P3HT nanostructures. We perform NSOM optical absorbance and scattering ratio measurements using laser irradiation sources with different wavelengths. Optical properties of these nanostructures will provide information on recombination pathways for excited electronic states. Further, the technique of absorbance/scattering ratio measurements allows us to gain insight into the types of internal structures that semicrystalline P3HT systems adopt, and how their photophysical properties affect their viability as photovoltaic materials.
9:00 PM - I10.19
Size and Internal Structure Dependent Photoluminescence in Semicrystalline Polythiophene (P3HT) Nanoparticles.
Joelle Labastide 1 , Mina Baghgar 2 , Irene Dujovne 1 , Harihara Venkatraman 1 , David Ramsdell 1 , Dhandapani Venkataraman 1 , Michael Barnes 1 2
1 Chemistry, University of Massachusetts Amherst, amherst, Massachusetts, United States, 2 Physics, University of Massachusetts Amherst, Amherst, Massachusetts, United States
Show AbstractSecond to the materials themselves, the internal structural order of semiconducting polymer nanostructures is the most important determinant of their viability as efficient photovoltaic materials. Understanding the effects of structure and internal order on the photophysical attributes of semiconducting polymers is essential to achieving the level of control over these properties that is necessary to break the efficiency barrier of existing organic photovoltaic systems. Polymer nanoparticles show remarkable promise as a possible solution to many mesoscale morphology problems, but their photophysical properties are not well described by P3HT thin film studies.Using single molecule spectroscopic techniques, we report on a remarkable size and internal structure dependence on time- and polarization-resolved photoluminescence (PL) from individual rr-P3HT regioregular (poly-3-(hexyl thiophine)) nanoparticles. Time and polarization resolution of the PL from individual particles gives us access to information about internal crystal structure and its effect on the competition between radiative recombination and charge transfer rates within the nanoparticles. The evolution of the polarization contrast parameter in time, reporting on real time exciton dynamics, is measured directly here. For the smallest particles (≈34 nm) with relatively low crystallinity (40%), the time-evolution of polarization contrast is nearly stationary; for intermediate sized particles (≈ 65 nm), depolarization occurs on a 1- 2 nanosecond timescale. The largest, and most crystalline particles studied (118 nm, 70%), show a PL depolarization on a timescale < 50 ps. In every time regime, we observe P3HT nanoparticle PL dynamics that are qualitatively different from extended films, highlighted by intriguing differences in power-law dynamics in the PL intensity at long times. This work may support the hypothesis that hierarchical assemblies of conducting polymer nanoparticles could offer a route to higher efficiency in organic PV systems.
9:00 PM - I10.2
Controlling Charge Transfer in Hybrid Organic/Nanocrystals Semiconductor Blends.
Neha Bansal 1 , Saif Haque 1
1 , Imperial College London, London United Kingdom
Show AbstractUnderstanding the fundamental science governing the behaviour of hybrid materials consisting of π- conjugated polymer and semiconductor nanocrystals are of great importance in the quickly growing field of hybrid organic/inorganic electronics. The use of hybrid materials is particularly attractive due to their superior charge mobilities and robustness of inorganic semiconductors along with the design flexibility, tunability and processability offered by the organic materials. For this reason, such hybrid materials are now investigated as active layers in hybrid solar cells. The function of a typical hybrid solar cell is based on photoinduced electron transfer exciton dissociation at the organic-inorganic heterojunctions. Therefore, understanding the processes that controls the charge photogeneration yield at such interfaces are crucially important to the design of efficient hybrid solar cells.The main focus of this work is to gain an understanding of how changing organic-inorganic interface structure influences the charge transfer processes that occur at these heterojunctions. Specifically, a key part of this work is to study the effect of interfacial energetics, and morphology upon the interfacial charge transfer in hybrid organic/ nanocrystal semiconductor blend films. In particular, in this work we employ the application of laser based optical spectroscopy and steady state spectroscopy to study the yield of dissociated polarons (ΔOD) and free-energy difference driving charge separation (ΔGCS) at hybrid organic/inorganic heterojunction.
9:00 PM - I10.20
Efficient Multi-Electron Transfer from Multi-Exciton States in Singlet Fission.
Wai-Lun Chan 1 , Manuel Ligges 1 , Askat Jailaubekov 1 , Xiaoyang Zhu 1
1 Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas, United States
Show AbstractSinglet fission refers to the creation of two electron-hole pairs (triplets) from the absorption of one photon. This process holds great promise in increasing the efficiency of organic solar cell. In this work, we use time-resolved two-photon photoemission (TR-2PPE) spectroscopy to study the singlet fission in pentacene and tetracene. This technique unambiguously follows the singlet and triplet population, and the energy relaxation of the immediate state on a femtosecond time scale. We found that in both molecular solids, singlet fission starts instantaneous with laser excitation, due to coherent quantum coupling between the singlet and the multiexciton intermediate state. Surprisingly, this multi-exciton state can also be key to the efficient harvesting of the fission process for solar energy conversion. By using fullerene (C60) molecules as an electron acceptor, we find that the electron transfer rate from the multi-exciton state to C60 is an order of magnitude higher than that from the triplet state. This result shows that one of the effective ways to harvest electrons from the fission process (or more generally, multi-exciton generation in other quantum confined systems) is through the multi-exciton state.
9:00 PM - I10.21
Optimization of Carrier-Multiplication for More Efficient Solar Cells: The Case of α-Sn Quantum Dots.
Christophe Delerue 1 , Guy Allan 1
1 , IEMN - ISEN, Lille Cedex France
Show AbstractCarrier-multiplication (CM) is a process in which more than one electron-hole pair is created as the result of the absorption of a single photon. CM has recently received considerable attention both experimentally and theoretically because it has the potential to improve the performance of solar cells. The main origin of CM is the relaxation of photo-excited carriers by impact ionization which, above a certain energy threshold, becomes more efficient than the intraband relaxation by emission of phonons. In this context, the main challenge is to find the materials with most efficient CM. In this work, we present calculations of impact ionization rates, carrier-multiplication yields and solar-power conversion efficiencies in solar cells based on quantum dots (QDs) of a semi-metal, α-Sn. Using these results and previous ones on PbSe and PbS QDs, we discuss a strategy to select QDs with the highest carrier-multiplication rate. We suggest to use QDs of materials with a close-to-zero bandgap and a high multiplicity of the bands in order to favor the relaxation of photo-excited carriers by impact ionization. The gain in solar-power conversion efficiency is discussed.
9:00 PM - I10.23
Study of Multi-Excitons in Silicon Clusters for Efficient Solar Cell.
Yi He 1 , Taofang Zeng 1
1 , M. I. T. , Cambridge, Massachusetts, United States
Show AbstractThe semiconductor nanostructures may enhance the limit efficiency of a single-junction photovoltaic system from 31% to 45% in an economic way via the multiple exciton to multiple photon (MEMP) process. In this investigation, the physics underlying the MEMP including the photons, electrons, excitons, phonons and interactions between them is investigated through the state-of-the-art quantum mechanical approaches. The inelastic scattering rates of electrons and excitons are calculated by the many-body Green’s function method. The non-radiative and radiative relaxation rates are simulated using the Fermi’s golden rule. The study finds that the reverse Auger process occurs for an exciton with energy larger than a threshold, around twice the optical bandgap. The relaxation lifetime of an exciton through the reverse Auger process ranges from several hundred femto-second (fs) to several fs, and decreases monotonically with increasing excitonic energies. Most hot electrons and holes relax quickly through electron-phonon interaction, with a lifetime on the order of sub-picosecond. However, the relaxation rate can be very slow, when there are a larger energetic gap and fewer symmetry-allowed normal modes between two electronic states. Most spontaneous radiation lifetimes of excitons are on the order of microsecond, and a few radiation lifetimes are on the order of nanosecond for excitons with large absorption cross section.
9:00 PM - I10.24
The Role of Optical Waveguiding on Triplet Exciton Transport in Tetracene and Rubrene Crystals.
Jiye Lee 1 , Carlijn Mulder 1 , Hiroshi Mendoza 1 , Marc Baldo 1
1 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States
Show AbstractMicrometer-range triplet exciton transport has recently been reported in single crystals of rubrene [1]. Such long-range exciton transport should benefit the power conversion efficiency of organic solar cells by eliminating the exciton diffusion bottleneck that presently restricts the thickness of the absorbing material. In this work we investigate the role that optical waveguiding and exciton diffusion play in transporting triplets within crystalline tetracene and rubrene. We create triplet excitons by fission of singlet photoexcitations and we study the time- and spatially resolved triplet distribution by monitoring delayed fluorescence, which results when two triplets collide to create a singlet exciton. Our technique allows us to directly image the triplet transport in the a, b and c crystallographic directions of crystalline tetracene and rubrene with sub-micron resolutions. Pump-intensity-dependent measurements reveal that at higher initial singlet densities (above 10^18 cm-3), transport of triplets through crystalline tetracene and rubrene is dominated by optical waveguiding, a relatively inefficient process that is not likely to be practical in solar cells. Here, two triplets fuse to form a singlet exciton that emits a photon that is waveguided through the crystal and re-absorbed after several microns. We support our measurements by an optical model that simulates the triplet and singlet densities as a function of pump intensity. [1] H. Najafov et al, Nature Materials 9, pp. 938 (2010).
9:00 PM - I10.26
Perturbing Excited-State Dynamics Using Nanoplasmonic Substrates.
Andrew Ferguson 1 , Natalia Azarova 3 1 , Ross Larsen 2 , Christopher Chang 2 , Jao van de Lagemaat 1 , Won Park 3 , Justin Johnson 1
1 Chemical & Materials Science Center, NREL, Golden, Colorado, United States, 3 Department of Electrical & Computer Engineering, University of Colorado at Boulder, Boulder, Colorado, United States, 2 Computational Science Center, NREL, Golden, Colorado, United States
Show AbstractPhotocurrent generation in excitonic solar cells is typically assumed to proceed from the initially generated exciton to free carriers via an ‘intermediate’ state of differing character, depending on the precise system under investigation. As such, a thorough investigation of the photophysical properties, and strategies to modify them, of excitonic systems is required to understand, and ultimately control, the solar photoconversion efficiency. Although the compounds 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and pentacene are considered model excitonic semiconductors, they also exhibit somewhat atypical excited-state properties. The lowest excited state of PTCDA possesses significant charge-transfer character, which may facilitate exciton dissociation into free carriers. Pentacene has been shown to undergo singlet fission to produce two triplet states with high efficiency, which has been touted as a next-generation solar photoconversion strategy. We will show results from recent work probing the effect that various nanoplasmonic substrates have on the photoinduced excited-state dynamics of both model compounds, and will discuss the impact these perturbations have on their observed optoelectronic properties.
9:00 PM - I10.28
Fullerene Crystallisation as a Key Driver of Charge Separation in Polymer/Fullerene Bulk Heterojunction Solar Cells.
Fiona Jamieson 1 , Ester Buchaca Domingo 2 , Thomas McCarthy-Ward 1 , Martin Heeney 1 , Natalie Stingelin 2 , James Durrant 1
1 Chemistry, Imperial College London, London United Kingdom, 2 Materials, Imperial College London, London United Kingdom
Show AbstractSolution processed polymer/fullerene blend films are receiving extensive attention as the photoactive layer of organic solar cells.1,2 We report a range of photophysical (transient absorption spectroscopy and photoluminescence), electrochemical (cyclic voltammetry), physicochemical (X-ray diffraction and differential scanning calorimetry) and structural data (absorption spectroscopy and optical microscopy) which provide evidence that formation of a relatively pure, molecularly ordered phase of the fullerene component, phenyl-C61-butyric acid methyl ester (PCBM), may be the key factor driving the spatial separation of photogenerated electrons and holes in many of these devices. PCBM crystallisation is shown to result in an increase in its electron affinity, providing an energetic driving force for spatial separation of electrons and holes. Based upon our observations, we propose a functional model applicable to many organic bulk heterojunction devices based upon charge generation in a finely intermixed polymer/fullerene phase followed by spatial separation of electrons and holes at the interface of this mixed phase with crystalline PCBM domains. This model has significant implications for the design of alternative acceptor materials for organic solar cells. (1)Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. Journal of the American Chemical Society 2011, 12, 4625–4631.(2)Chu, T.-Y. , et al., Journal of the American Chemical Society 2011, 271-278
9:00 PM - I10.29
Hierarchical Data Mining to Link BHJ Structure with Properties of Organic Solar Cells.
Olga Wodo 1 , Hari Kodali 1 , Baskar Ganapathysubramanian 1 2
1 Mechanical Engineering Department, Iowa State Univeristy, Ames, Iowa, United States, 2 Electrical and Computer Engineering Department, Iowa State University, Ames, Iowa, United States
Show AbstractThe last decade has seen significant progress in enhancing the power conversion efficiency of organic solar cells through various strategies. One such approach is based on morphology control. This is because morphology affects all phenomena involved in solar conversion. Obtaining detailed knowledge of the structure–property relationship is thus necessary for optimizing morphology to achieve high-efficiency solar cells. In this work, this is done by using a data-driven approach that uses ideas from data-mining, homology and graph theory. This is based on construction of a set of physically meaningful morphology descriptors with respect to each subprocesses of photovoltaic effect: light absorption, exciton generation, exciton dissociation, charge transport and charge recombination. These descriptors are used to interrogate representative BHJ morphologies. We show comprehensively the relationship between morphology characteristics (feature size, fraction of domain within exciton diffusion length to the interface, fraction of useful domains, interfacial area, path tortuosity, degree of structure balance) and photovoltaic subprocess characteristics of device (overall exciton generation, dissociation and relaxation rate; charge recombination; charge collection at each electrodes). Such interrogation allows for hierarchical, quantitative comparison between morphologies with respect to each subprocess in the photovoltaic effect. Furthermore, it allows identification of bottlenecks in the photovoltaic process.
9:00 PM - I10.3
Efficient Organic Solar Cells Based on Aromatic Spacer Bridged Diketopyrrolopyrrole Derivatives.
Yoon-suk Choi 1 , Yoonkyoo Lee 1 , Wonho Jo 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractIn recent years, organic molecules with donor-acceptor type alternating architecture have attracted extensive interest because their electronic properties can be easily manipulated by controlling the electronic push-pull interaction between donor and acceptor units. Particularly, since it has generally been accepted that the open-circuit voltage (VOC) in organic photovoltaics (OPVs) is proportional to the energy difference between the highest occupied molecular orbital (HOMO) of donor and the lowest unoccupied molecular orbital (LUMO) of acceptor, it is indispensable for achieving high performance OPVs to tune LUMO and HOMO energy levels of donor and/or acceptor materials. Also, the offset of LUMO between the donor-acceptor is the driving force for the charge separation, which is related to short-circuit current (Jsc).Electron-deficient and strongly absorbing diketopyrrolopyrrole (DPP) unit is an attractive building block due to facile synthesis of low band gap push-pull type materials. The planarity of the DPP unit encourages π-π stacking resulting in high charge carrier mobility. Introduction of extended side chain such as 2-ethylhexyl on DPP affords high solubility and thin film uniformity required for solution processed small-molecule OPVs. Herein, we report the synthesis and characterization of new organic molecules based on DPP as a donor material in OPVs. To optimize photovoltaic properties as a donor component, both sides of the aromatic spacer are terminated by thiophene-flanked DPP units with branched 2-ethylhexyl alkyl chains. Furthermore, in order to precisely tune the energy level of the donor compound, we have introduced cyano groups onto aromatic spacers and onto the end of molecule. Through electron withdrawing properties of cyano groups, we expect that the molecule has a deep HOMO level and thus a high Voc. The molecules are highly soluble in common organic solvents, such as chloroform, toluene, and THF at room temperature. The molecules exhibit a broad absorption band covering visible region. The optical bandgap of the compounds as determined from the onset of the absorption spectra is ~1.6 eV which is very close to the ideal bandgap (1.5 eV) for high performance OPVs. The LUMO and HOMO energy levels as estimated from the cyclic voltammetry are -3.5 eV and -5.2 eV, respectively. The frontier orbitals of the material are well aligned with those of fullerene acceptor, allowing the open-circuit voltage of 0.7~0.8 V. The energy levels of DPP-based organic molecules, correlated with VOC and the power conversion efficiency of OPVs, are systematically analyzed in terms of molecular structure.
9:00 PM - I10.30
Carrier Transport Behavior in Single Crystalline Rutile Nanorod Based Excitonic Solar Cell.
Mengjin Yang 1 , Ding Bo 1 , Jung-Kun Lee 1
1 Mech. Eng. & Mater. Sci., University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show Abstract1-D nanostructure materials have been proposed as a promising alternative to spherical nanoparticles for dye-sensitized solar cells because of its unique properties, such as fast carrier transport and less recombination. However 1-D structure based device currently suffers inferior performance compared to particle-based system, which is attributed to the dramatic decrease of interface area. It is of importance to treat nanowire/nanorod to form hierarchical or composite materials in order to obtain higher efficiency. Nevertheless, systematic investigation of carrier transport in these systems is lacking so far. In this presentation, authors investigate the fundamental carrier transport (diffusion coefficient and life time) in single crystalline rutile nanorod with various surface treatment using stepped light induced-transient measurement of photocurrent and voltage (SLIM-PCV). The most common strategy of surface treatment is TiCl4 treatment. The hydrolysis of TiCl4 in acidic solution results in rutile nanoparticles, and the contact between precipitated rutile particle and rutile nanorod is a homogeneous contact. It is found that surface diffusion is dominant in this homogeneous contact. During the surface diffusion, carriers in treated nanorod have similar chance of recombination as those in nanoparticle based device, which means that the potential of nanorod (i.e. fast carrier extraction) may not be not fully exploited. In order to overcome the limit of short carrier life time, another type of surface treatment was explored. The basic concept of new approach is to build an energy confining layer using energy band matched heterogeneous junction. The higher conduction in outer shell works as a fence to prevent back diffusion of carriers to electrolyte. For example, carriers in Nb2O5, which has 100meV higher conduction band than that of rutile, roll down to rutile nanorod with this driving force coming from the band bending. The adoption of heterogeneous structure shows a longer lifetime, which manifests the successful confinement of carriers in nanorod.
9:00 PM - I10.31
Low Bandgap Polymer Tailored for High Performance Tandem Polymer Solar Cells.
Letian Dou 1 , Jingbi You 1 , Yang Yang 1
1 Materials Science, University of California, Los Angeles, Los Angeles, California, United States
Show AbstractTandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combining two or more solar cells with different absorption ranges. However, for polymer solar cells (PSC), the performance of tandem devices lags behind single layer solar cells due to the lack of a proper combination of low and high bandgap polymers. Here, we demonstrate a highly efficient tandem PSC with a novel low bandgap conjugated polymer (Eg ~ 1.44 eV) specifically designed to satisfy the requirements of the tandem structure. In the single layer device, it showed power conversion efficiencies (PCE) around 6-7%. When applied to tandem solar cells, the PCE of the devices achieved 9.5%, which is the highest reported efficiency to date for organic photovoltaic devices. This study opens a new direction for materials design and demonstrates great potential for polymer tandem solar cells for practical use.
9:00 PM - I10.32
Charge Carrier Generation and Transport in Hybrid P3HT/Silicon Nanocrystal Solar Cells.
Sabrina Niesar 1 , Daniel Herrmann 2 , Rui Pereira 3 , Wolfgang Fabian 1 , Christina Scharsich 4 , Anna Koehler 4 , Hartmut Wiggers 5 , Martin Brandt 1 , Eberhard Riedle 2 , Martin Stutzmann 1
1 Walter Schottky Institut, Technische Universität München, Garching Germany, 2 Lehrstuhl für BioMolekulare Optik, Ludwig-Maximilians-Universität, München Germany, 3 Department of Physics and I3N, University of Aveiro, Aveiro Portugal, 4 Lehrstuhl Experimentalphysik II, Universität Bayreuth, Bayreuth Germany, 5 Institut für Verbrennung und Gasdynamik und CeNIDE, Universität Duisburg-Essen, Duisburg Germany
Show AbstractOne current approach addressing the demand for new routes of energy conversion are hybrid solar cells made from organic semiconductors and inorganic nanoparticles. Silicon nanocrystals (Si-ncs) have emerged as a particularly promising inorganic base material that offers the opportunity for cost-effective solution processing as well as a manifold of tunability possibilities in terms of doping, particle size and band gap. In this work, the synthesis of freestanding Si-ncs with mean particle diameters between 4 nm and 50 nm was performed in a low-pressure microwave plasma reactor, which allows for a future upscaling of the fabrication volume to an industrial level. As organic counterpart in the hybrid devices, we have chosen the widely used polymer poly(3-hexylthiophene) (P3HT) due to the favorable energy band alignment. The crucial issues for the understanding and improvement of P3HT/Si-ncs based bulk heterojunctions are the light absorption, the charge separation at the hybrid interface, and the subsequent transport of the separated charge carriers towards the electrodes. Initial studies demonstrating the feasibility of such hybrid solar cells have shown that the efficiencies achieved were limited by transport barriers and a high concentration of silicon dangling bond (Si-db) defects in the Si-nc network, which act as recombination centers for photo-generated charge carriers. Therefore, in the first part of this work, we focus on the reduction of the Si-db density in the Si-ncs employing different post-growth treatments. Etching of the Si-ncs with hydrofluoric acid (HF) in combination with low-temperature vacuum annealing (200°C) was identified as a method leading to the lowest Si-db concentrations. Moreover, this treatment significantly enhances the conductivity of Si-nc films and improved current-voltage characteristics of P3HT/Si-ncs bulk heterojunctions are obtained. In the second part, the primary photo-excitation, charge separation and recombination mechanisms in P3HT/Si devices are investigated using a novel broadband (UV-Vis-NIR) transient absorption spectroscopy setup. Comparing P3HT of different regioregularities, we find that the polymer structural order is a key criterion for an efficient generation of free charge carriers. The time resolution of 40 fs allows the observation of a delayed dissoziation of singlet-excitons into free polarons with a time constant of 140 fs in regioregular P3HT. Further, we have studied the influence of different parameters of the P3HT/Si-ncs blends on the current-voltage characteristics as well as on the charge transfer processes.
9:00 PM - I10.36
Making Molecular Multilayers Using ``Click" Chemistry: Growth, Characterization, and Applications in p-Type Dye-Sensitized Solar Cells.
Peter K.B. Palomaki 1 2 , Peter Dinolfo 1 2
1 Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Rensselaer Polytechnic Institute, Baruch ‘60 Center for Biochemical Solar Energy Research, Troy, New York, United States
Show AbstractBottom-up approaches to creating molecular multilayer assemblies using layer-by-layer (LbL) techniques give one the ability to tailor the surface properties of an interface through molecular control. LbL assembly methods can provide molecular level control of structure in one dimension from simple solution deposition processes. We have developed a versatile LbL fabrication method using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) or “click” chemistry in the construction of multilayer assemblies on oxide surfaces.1,2 These reactions occur in minutes at room temperature allowing for the facile creation of complex, covalently attached multilayer assemblies. Multilayers containing synthetic porphyrins, perylenes, and mixtures of the two have been constructed in order to highlight the versatility of this method. The fabrication of this new type of molecular multilayer thin film will be discussed along with characterization of the photophysical, electrochemical, and structural properties. Initial results pertaining to the use of these films as light harvesting systems in p-type dye sensitized solar cells will be presented.1. Palomaki, P.K.B. and Dinolfo, P.H. Langmuir 2010, 26 (12), 9677-9685.2. Palomaki, P.K.B.; Krawicz, A.; Dinolfo, P.H. Langmuir 2011, 27 (8), 4613-4622.
9:00 PM - I10.37
Study of Interfacial Processes in Dye Sensitized Solar Cells (DSSC) via Thin Film Approach: Effect of Oxygen Vacancy Concentration.
Onkar Game 1 , Vishal Thakare 2 , Abhimanyu Rana 2 , Satishchandra Ogale 2
1 , University of Pune, Pune India, 2 Physical and Materials Chemistry Division, National Chemical Laboratory, Pune India
Show AbstractDye Sensitized Solar Cell technology is rapidly emerging as an attractive alternative to the conventional silicon based solar cells due to the fairly high projected ratio of efficiency to cost in the case of DSSC. Contrary to conventional p-n junction solar cells the light absorption and charge transport functions in Dye Sensitized Solar Cells are well separated by the use of dye loaded nano-porous TiO2 film. However, the presence of multi-component (FTO-TiO2-Dye-Electrolyte) architecture with interdependent roles of the constituents, a large number of functional interfaces, and inherent complexities of nanoscale systems make it difficult to analyse the effects of individual components in the system on the overall performance of DSSC. Although much of the action in DSSC occurs at the interfaces between different media in the architecture, such interfaces are embedded in the film thickness and are hardly accessible to the normal surface/interface characterization tools. In order to tackle this limitation we have performed the explorations of the formation and properties of the attendant key interfaces on a thin film model. Thus thin films of TiO2 were grown on Fluorine Doped Tin Oxide (FTO) substrates by Pulsed Laser Deposition (PLD) and the same were characterized not only for solar energy conversion parameters but also by other surface characterization techniques. Specifically non-porous thin films (150 nm) of TiO2 were deposited under different oxygen partial pressures and were characterized to elucidate the possible role of oxygen vacancies. Even though less than 1% light is absorbed by such non-porous sensitized TiO2 films it is a good starting point (model system) to evaluate the systematic effect of oxygen vacancies on the performance of Dye Sensitized Solar Cells. Three cases deposited at 10-5 mbar, 10-3 mbar and 10-1 mbar were studied. Change in the oxygen vacancy concentration was determined by XPS, UV-Vis spectroscopy and STM. The film with lowest oxygen vacancy concentration (10-1 mbar) showed the highest photo-conversion efficiency, while TiO2 film deposited at 10-5 mbar showed the lowest photo-conversion efficiency. The fabricated DSSCs with different oxygen vacancies in TiO2 were studied for interfacial charge transfer, charge transport within TiO2 and electron lifetime by Electrochemical Impedance Spectroscopy (EIS). Based on the characterizations, the effect of oxygen vacancy concentration on various parameters of DSSC viz. Electron transfer resistance from Dye to TiO2, TiO2-Dye-Electrolyte interface recombination, Charge transport resistance and electron lifetime etc. is discussed.
9:00 PM - I10.38
Understanding at the Atomic Scale the Microstructure of Polymer-Metaloxide Hybrid Nanomaterials for Photovoltaics.
Claudio Melis 2 1 , Maria Ilenia Saba 1 2 , Claudia Caddeo 2 1 , Giuliano Malloci 1 , Luciano Colombo 2 1 , Alessandro Mattoni 1
2 Dept of Physics, University of Cagliari, Monserrato , Cagliari, Italy, 1 SLACS Cagliari, CNR-IOM SLACS, Monserrato , Cagliari, Italy
Show AbstractPolymer metaloxide hybrids have emerged as promising systems for photovoltaics combining the formability of polymers and the good trasport properties and thermal stability of the inorganic component. The actual microstructure of the polymer close to the metaloxide substrate (where electrons are accepted) critically controls the properties of the system. We adopt molecular dynamics (MD) simulations to generate models of poly3hexylthiophene (P3HT) interacting with inorganic nanostructured substrates, such as titanium dioxide (TiO2), zinc oxide (ZnO). We provide evidence that the polymer organization at the interface strongly affects the properties of the hybrid system. It is found that the polymer adhesion depends on the curvature at the nanoscale and on the local charge of the metaloxide[1]. Furthermore, the polymer assembling[2] at the interface critically affects the transport properties[3]. Wrapping phenomena of P3HT on carbon nanotubes [4] and ZnO nanoneedles are discussed as well. This work is funded by the Italian Institute of Technology (IIT) under Seed Project “POLYPHEMO” and Regione Autonoma della Sardegna under Project “Design di nanomateriali ibridi organici/inorganici per l’energia fotovoltaica” L.R.7/2007.[1] C. Melis, A. Mattoni and L. Colombo, "Atomistic Investigation of Poly(3-hexylthiophene) Adhesion on Nanostructured Titania" J. Phys. Chem C 114 3401 (2010)[2] C. Melis, L. Colombo, and A. Mattoni,"Self-Assembling of Poly(3-hexylthiophene)" J. Phys. Chem. C115 576 (2010)[3] M. I. Saba et al., "Polymer crystallinity at the P3HT/Zinc Oxide Hybrid interface" in preparation (2011)[4] C. Caddeo, C. Melis, L. Colombo, and A. Mattoni,"Understanding the Helical Wrapping of Poly(3-hexylthiophene) on Carbon Nanotubes" J. Phys. Chem. C114 21109 (2010)
9:00 PM - I10.4
Charge Generation and Dissociation Mechanisms in PbS-ZnO Heterojunction Colloidal Quantum Dot Photovoltaics.
Cheng Cheng 1 , Shawn Willis 1 , Hazel Assender 1 , Andrew Watt 1
1 Department of Materials, University of Oxford, Oxford, Oxfordshire, United Kingdom
Show AbstractThe power conversion efficiencies of solution-processed semiconductor nanocrystal quantum dot photovoltaic devices have been growing rapidly. Understanding the nature of charge generation, transport, dissociation and recombination at the quantum dot heterojunction interface is key to further device optimisation. There is some debate as to whether devices behave as conventional p-n junctions, excitonic photovoltaic devices or some combination of both. In this paper we study a heterojunction photovoltaic structure based on lead sulphide and zinc oxide quantum dots. The best performing devices generate ~3% power conversion efficiency under AM1.5 illumination with an open circuit voltage of 0.56 V. Using external quantum efficiency studies we find that devices optimised for thickness have limited performance due to the formation of a carrier extraction dead zone, such that photo-generated carriers cannot make it to the interface and recombine. One possible approach is to eliminate this dead zone and develop an interconnected structure in which the size of each layer is optimised for photon absorption and carrier extraction. We consider the approach to designing such a system based around depletion region and excitonic extraction mechanisms. Using temperature-dependent electrical characterisation we further probe the nature of charge separation and recombination across the junction.
9:00 PM - I10.5
Singlet Excitons in Highly Ordered Poly(3-hexylthiophene) Crystals Studied by Transient Absorption Spectroscopy.
Yasunari Tamai 1 , Hideo Ohkita 1 2 , Hiroaki Benten 1 , Shinzaburo Ito 1
1 Department of Polymer Chemistry, Kyoto-university, Kyoto Japan, 2 , PRESTO, Japan Science and Technology, Saitama Japan
Show AbstractRegioregular poly(3-hexylthiophene) (P3HT) is the most widely studied conjugated polymer for organic electronic devices, such as organic photovoltaic devices and field-effect transistors. In thin films, P3HT self-organizes into two-dimensional π-stacked lamellar structures. Excitonic and electric natures of thin films correlate closely with crystalline quality. Although previous studies have revealed the formation and decay dynamics of various photoexcitations in P3HT, the influence of crystalline quality on those dynamics is not fully understood. Previously we found highly ordered P3HT fibril formation in P3HT/PMMA blend films. The surface of P3HT/PMMA blend film was almost covered with P3HT because of the lower surface free energy of P3HT, and P3HT fibril networks were formed on the blend surface. In this study, we determined crystalline quality of these fibrils from the absorption spectrum of the blend film using weakly interacting H-aggregate model. The crystallinity in the blend film was evaluated to be 80% which was higher than in P3HT neat film. From the ratio of the 0-0 and 0-1 peak absorbance, the exciton bandwidth, W was found to be quite lower in P3HT fibrils formed on the blend surface than in P3HT neat film. A recent quantum chemical calculation has shown that the extension of conjugation length results in narrow W. Therefore, these results indicate that the fibril networks consist of highly ordered crystals with longer conjugation and fewer traps of excitons. Furthermore, we study the dynamics of singlet exciton and charged species generated in highly ordered crystals by femtosecond transient absorption spectroscopy.
9:00 PM - I10.6
Light-Harvesting Efficiency of Polymer Solar Cells Incorporating near-IR Dye with Bulky Axial Substituents.
Huajun Xu 1 , Takaaki Wada 1 , Hideo Ohkita 1 2 , Hiroaki Benten 1 , Shinzaburo Ito 1
1 Department of Polymer Chemistry, Kyoto University, Kyoto Japan, 2 PRESTO, Japan Science and Technology Agency (JST), Saitama Japan
Show Abstract Polymer/fullerene bulk heterojunciton solar cells have attracted great interest because of their potential to be lightweight, flexible, solution processable, and hence cost-effective. Among them, the blend of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]methanofullerene (PCBM) has been most widely studied as the active layer in polymer/fullerene solar cells. This polymer solar cell exhibits high external quantum efficiency (EQE) and fill factor (FF), which are still the highest in organic solar cells. However, P3HT can harvest a part of the solar light up to ~650 nm, which corresponds to only a quarter of the total photons in the solar light. Recently, we demonstrated that the light-harvesting efficiency of P3HT/PCBM solar cells can be improved by incorporating silicon phthalocyanine bis(trihexylsilyl oxide) (SiPc6). This is probably because the bulky trihexylsilyl oxide group can effectively suppress dye aggregation even in solid films. In this study, we synthesized a series of SiPc derivatives with different axial groups to study the size effect of bulky axial groups on the dye sensitization in polymer solar cells. Seven SiPc derivatives were synthesized with different axial groups: -OSi(CnH2n+1)3 (SiPcn, n = 2, 3, 4, 6) and -OSi(iBu)2C18H37 (SiPcB18). In solution, absorption spectra of these dye molecules were almost the same, suggesting that axial groups have little impact on the electronic state in the phthalocyanine unit. In P3HT/PCBM blend films, absorption spectra were different between SiPc2 and the others. The absorption band of SiPc2 decreased in intensity and was split into two bands, suggesting the formation of π stacked aggregates in the blend films. The absorption bands of the other dyes were almost the same even in blend films although the peak wavelength was slightly red-shifted compared to that in solution. This finding suggests that propyl chain (SiPc3) in the axial ligand is large enough to effectively suppress the dye aggregation even in solid films. On the other hand, EQE at the dye absorption band was as low as ~6% for SiPc2, increased to 40% for SiPc3, 42% for SiPc4, and 43% for SiPc6, and 45% for SiPcB18. The short-circuit current density (JSC) also showed the same tendency as EQE. The open circuit voltage was almost independent of dye molecules. The fill factor was almost constant at ~0.68 except for SiPcB18 (FF = 0.65). As a result, power conversion efficiency (PCE) reached the maximum at SiPc6 (PCE = 4.2%). This PCE is larger by 10% than that of P3HT/PCBM control cells without dye. Longer axial chains can be effectively suppress the dye aggregation but too long chains such as dodecyl are likely to hinder the charge transport in the blend film. We therefore conclude that hexyl chain in the axial ligand is the most appropriate for the dye sensitization in P3HT/PCBM solar cells.
9:00 PM - I10.7
Synthesis and Characterization of Low-Band-Gap Poly(Thienylenevinylene) Derivatives for Polymer Solar Cells.
Dong-Yu Kim 1 2 3 , Soo-Young Jang 1 3 , Bogyu Lim 4 , Byung-Kwan Yu 1 3 , Juhwan Kim 1 3 , Kang-Jun Baeg 1 3 , Dongyoon Khim 1 3
1 School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 4 Department of Material Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractRecently, many different types of conjugated polymers such as polythiophenes, polyfluorenes and polycarbazoles have been used for photo-active layers in organic solar cell devices. In order to enhance photocurrent in these photovoltaic devices, the ability to absorb large area of solar spectrum is essentially demanded. For this reason, a class of conjugated polymers with low band gap has attracted much attention to many researchers and most of them have a structure of donor-acceptor type copolymers. This concept can stabilize the quinoid form of a polymer since the alternation of donors and acceptors increases the double bond character between repeating units, thereby reducing bond length alternating energy and reducing the band gap. In this presentation, we demonstrate the synthesis and characterizations of a homopolymer and two copolymers of thienylenevinylene derivatives, (E)-poly[2,2’-(1,2-ethenediyl) bisthiophene] (PEBT), (E)-poly[2,2’-(1,2-ethenediyl) bisthiophene-alt-4,7-(2,1,3-benzothiadiazole)] (PEBTBT) and (E)-poly[2,2’-(1,2-ethenediyl)bisthiophene-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PEBTTBT). Two copolymers with electron-accepting benzothiadiazole units showed higher photocurrents than homopolymer because of their larger absorption of solar spectrum especially in near-IR region, and their increased hole mobilities. Also we found that the morphology of each blended system with PCBM plays an important role in its photovoltaic performance.
9:00 PM - I10.8
Synthesis and Characterizations of TriphenylAmine Based Low Band Gap Polymer for Organic Solar Cell.
Nam-koo Kim 1 3 , Bogyu Lim 4 , Jun-suk Yeo 2 3 , Dong-Yu Kim 1 2 3
1 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 4 Department of Materials Science and Engineering , Stanford University , Stanford, California, United States, 2 School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractOne of important obstacle for commercialization of organic solar cells (OSCs) is high efficiency. To produce high efficiency OSC, wide range of absorption and high mobility of charge carrier are needed for the improvement of short-circuit current (Jsc) and also low Highest Occupied Molecular Orbital (HOMO) level is needed for the improvement of open-circuit voltage (Voc). Moreover, good morphology is needed for the improvement of fill factor(FF). Generally, triphenyl amine (TPA) derivatives are often used as a hole transport layer because of their hole mobility. Compared with Poly(3-hexylthiophene) (P3HT), TPA units also have lower HOMO level. However, TPA has large bandgap and absorb short wavelength. For wide range of absorption, we need to tune the bandgap to be smaller. In this work, diketo-pyrrolo-pyrrole (DPP) is used as an electron withdrawing group to make a push-pull structure for the control of bandgap and implementation of broad absorption. Poly(triphenyl amine oxy dodecyl-diketo-pyrrolo-pyrrole hexyl decyl) (PTODDHD) and Poly(fluoro triphenyl amine oxy dodecyl-diketo-pyrrolo-pyrrole hexyl decyl) (PFTODDHD) were synthesized and characterized for OSC. As a result, PTODDHD and PFTODDHD showed Voc of 0.58 and 0.73V, Jsc of 8.94 and 3.87 mA/cm2, fill factor of 58 and 56%, power conversion efficiency of 2.76 and 1.60%, respectively. We expect that the device performance can be further improved by the optimization of the material and the device fabrication.
9:00 PM - I10.9
Synthesis, Characterization, and Photovoltaic Properties of Dithienothiophene-Based Donor-Acceptor Conjugated Polymers.
Dong-Yu Kim 1 2 3 , In-Bok Kim 1 3 , Byung-Kwan Yu 2 3 , Dongyoon Khim 2 3
1 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Hegger Center for Advanced Materials, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractPolymer solar cells (PSC) have attracted attention as a renewable energy source due to their low cost, ease of manufacture, and compatibility with flexible substrates. To improve efficiency of PSC, donor polymers which have adequate energy level related to acceptor materials should be developed. Donor polymers which have deep HOMO level enhance VOC that is an important factor for determining the efficiency of PSC and the absorption band of donor polymer should also be matched with the solar spectrum. Based on this motivation, we synthesized donor polymers containing dithieno[3,2-b;2’,3’-d]thiophene(DTT) unit which can induce deep HOMO level. Thermal properties of donor polymers were measured by thermal gravimetric analysis and differential scanning calorimeter. To estimate energy level of polymers electrochemical property of polymers was measured by cyclic voltammetry. UV-vis absorption spectrum of polymers was measured to confirm the absorption range of polymers. PSCs were fabricated using the polymer as an electron donor material and [6,6]-phenyl-C61 butylic acid methyl ester (PC61BM) and [6,6]-phenyl-C71 butylic acid methyl ester (PC71BM) as an electron acceptor material.