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
Monday AM, November 28, 2011
9:30 AM - **I1.1
Lessons from Nature about Solar Light Harvesting.
Gregory Scholes 1 Show Abstract
1 Department of Chemistry, Univesity of Toronto, Toronto, Ontario, Canada
Sunlight is an abundant source of energy that plays the deciding role in net primary production on earth. Researchers must learn how to capture and store solar energy effectively because it is forecast to provide a significant fraction of the world’s energy needs over the next century. To utilize energy harvested from sunlight efficiently to promote photochemical reactions or to produce solar fuels, it is desirable to amplify the effective capture of photons at a reactive site. Concentrated excitation energy can promote high energy chemical transformations that otherwise proceed at negligible rates. Understanding how excitation energy moves in supramolecular assemblies will allow the design of molecular ‘circuits’ that can direct, sort, and respond in sophisticated ways to excitation energy. I will discuss the design principles, compromises, and challenges for antenna systems that capture light by drawing parallels to photosynthetic light-harvesting . Light-harvesting is important for the function of photosynthesis—for a variety of reasons—so exploring the operation of natural light harvesting complexes provides an excellent platform for understanding how photo-excitation can be directed and amplified using assemblies of light-absorbing molecules. How these ‘lessons’ might be put to use in organic excitonic solar cells will be highlighted. G.D. Scholes, G.R. Fleming, A. Olaya-Castro & R. van Grondelle, ‘Lessons from nature about solar light harvesting’, to be submitted.
10:00 AM - I1.2
Generation and Recombination of Polaron Pairs in Low-Bandgap Copolymers for Photovoltaics.
Enrico Da Como 1 , Raphael Tautz 1 , Jochen Feldmann 1 , Ullrich Scherf 2 Show Abstract
1 Department of Physics and CeNS, LMU Munich, Munich Germany, 2 Department of Chemistry, University of Wuppertal, Wuppertal Germany
The most recent advances in the power conversion efficiency of organic solar cells considered the design of low-bangap copolymers with an extended absorption in the near infrared. These materials, based on the donor-acceptor concept, i.e. with moieties of different electron affinity alternating on the chain, have attracted a considerable interest. While the improved coverage of the solar spectrum has been often considered as the origin for the improved performances, it has not been addressed the role of donor acceptor moieties in the process of charge generation and in particular if this differ from that observed in the most common homopolymers. In homopolymers the majority of free carriers is usually generated at the interface with a fullerene acceptor and only a few free carriers directly in the polymer. The generation of polaron pairs directly in the polymer domains is expected to be beneficial, since those would require smaller energy offsets with fullerene acceptors and therefore lower losses in the photovoltage. The question, of what the polaron pair generation-yield in pristine donor-acceptor copolymers actually is, remains largely unanswered while being of large relevance for understanding and improving materials for photovoltaics. Here, we report on how the chemical structure of donor and acceptor moieties controls one of the primary steps in organic photovoltaics, i.e. the photogeneration of polaron pairs. We have addressed this question by using different materials with systematic variations in the acceptor moiety and ultrafast infrared pump-probe spectroscopy. Such technique allows us to selectively probe the polaron absorption bands in the middle infrared. We determine that copolymers with adjacent donors and acceptors show yields of polaron pair formation up to 24% of the initial photoexcitations, a value three times larger than what observed in poly(3-hexylthiophene). This yield has a weak dependence, ±2%, on the difference in electron affinity, but a more pronounced one on the mutual separation between donor and acceptor, in which case it drops to ~16%. Spacers, used to separate the donor and acceptor center of masses, have the beneficial role of increasing the recombination time by almost an order of magnitude. The results provide useful input into the understanding of structure-property relationships in low-bandgap copolymers for photovoltaics.
10:15 AM - I1.3
Reciprocal Carrier Collection in Organic Photovoltaics.
Christopher Renshaw 1 , Cody Schlenker 2 , Mark Thompson 2 , Stephen Forrest 1 3 4 Show Abstract
1 Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Chemistry, University of Southern California, Los Angeles, California, United States, 3 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 4 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Buffer layers between the acceptor and cathode can perform several functions in organic photovoltaic devices, such as providing exciton blocking, protection of active layers against damage from cathode deposition, and optical spacing to maximize the electric field in the active device region. Here we study electron collection by replacing the common buffer layer, bathocuproine, with a series of six, substituted tris(β-diketonato)Ru(III) analogues in the structure: indium-tin-oxide/copper phthalocyanine/C60/buffer/Ag. These buffer layers enable collection of photogenerated electrons by transporting holes from the cathode to the C60/buffer interface, followed by recombination with photogenerated electrons in the acceptor.1 We use a model for free-polaron and polaron-pair dynamics2 to describe device operation and the observed inflection in the current-voltage (J-V) characteristics. Using non-adiabatic Marcus Theory for electron transfer,3 we correlate the highest occupied molecular orbital (HOMO) energy level of the buffer material to the observed J-V inflection and show that recombination between polaron pairs at the C60/buffer interface is the limiting rate that leads to this non-ideal behavior. The understanding of the exciton and charge dynamics afforded by this study provide guidance to the design of high performance exciton blocking and charge conducting buffers used in organic photovoltaics.
10:30 AM - I1.4
Charge Carriers and Charge-Transfer Excitons in Realistic Organic Solar Cells Probed by Electrically Detected Magnetic Resonance.
Wolfgang Harneit 1 2 , Sebastian Schaefer 3 2 Show Abstract
1 Institut fuer Physikalische Chemie, Johannes Gutenberg-Univeritaet Mainz, Mainz Germany, 2 Institut fuer Experimentalphysik, Freie Universitaet Berlin, Berlin Germany, 3 Physik der Weichen Materie, Universitaet Potsdam, Potsdam-Golm Germany
Organic solar cells (OSCs) are still limited in both, efficiency and stability, due to the complex nature of charge carrier generation, transport, and extraction. Since OSCs are necessarily very thin and often dominated by interface and/or ambient effects, it is a formidable challenge to identify the major bottlenecks in real-world devices operating under realistic conditions (room temperature, white-light illumination, interface degradation, etc.). In this contribution, we show that electrically detected magnetic resonance, which measures the spin-dependent part of the photocurrent, is a particularly suitable method to assess trapped polarons and charge-transfer (CT) excitons at the various interfaces in OSCs.We have studied the small-molecule OSC system ZnPc/C60 thoroughly using IV/photocurrent analysis and electrically detected magnetic resonance using both cw and pulsed MW excitation. Starting from single ZnPc layer devices, we varied contact configurations and metals, to end up with complete solar cell structures both with planar or bulk hetero-junction (PHJ/BHJ) architecture. A central result is that a resonant decrease (quenching) or increase (enhancement) of the photocurrent can occur in ZnPc layers depending on whether it happens in the bulk of the layer or in an accumulation layer due to contact-induced extraction barriers. A similar accumulation of non-geminate P+/P- polaron pairs in ZnPc occurs in PHJ solar cells due to an oxygen-induced increase of the C60 resistance, leading to a positive signal. Finally, a quenching in PHJ cells with a characteristically strong dependence on illumination intensity is observed and tentatively attributed to self-trapping of CT excitons at the PHJ interface.The trapped carrier pairs show in all cases spin-coherent motion (i.e., Rabi oscillations) up to about half a microsecond. A spin-locking signal at twice the Rabi frequency clearly shows that the carrier pairs persist in a coherent quantum mechanical state on that time scale even at room temperature. An upper limit of about 1 MHz can be inferred for the spin-spin coupling constant from the detailed power dependence of the nutation patterns. This implies that the average distance of the coherent pair partners must be larger than about 3 nm.References: W. Harneit et al., Phys. Rev. Lett. 98 (2007) 216601 and ibid., 100 (2008) 199904. S. Schaefer et al., Phys. Stat. Sol. C 246 (2009) 2844. S. Schaefer, W. Harneit, in preparation
10:45 AM - I1.5
Localized State Spectroscopy in Organic Solar Cells.
Robert Street 1 , Katherine Song 1 , Alexa Krakaris 1 Show Abstract
1 , Palo Alto Research Center, Palo Alto, California, United States
Two complementary experimental techniques to measure the density of states (DOS) distribution in organic bulk heterojunction (BHJ) solar cells are presented, and data are obtained on various cells including P3HT:PCBM and PCDTBT:PCBM. One technique is long time transient photoconductivity, in which the photocurrent at increasing times is shown to be related to the density of states at increasing trap energy. The second technique is the photoconductivity spectral response at low photon energy, which arises from optical excitation from trap states to the charge transporting states. The data show that both types of BHJ cells have an exponential band tail of localized states extending from the polymer HOMO transport energy to a trap energy of about 0.4 eV, then changing to a broader distribution of deeper trap state, with density in the range 10e16-10e17 cm-3 eV-1 at a trap energy of 0.5-0.6 eV. The exponential band tails are attributed to the disorder in the semiconductors, but we cannot yet determine whether the deeper states are from disorder, defects or impurities. The existence of localized states provides further evidence for trap-related recombination being the dominant mechanism is the BHJ cells. The measurements are important because the DOS is the link between the atomic structure of the device and its electronic properties. An empirical measurement of the DOS provides necessary information to model the cell, and enables the structural changes due to annealing, atmospheric contamination and long light exposure etc, to be investigated. R. A. Street, K. W. Song J. E. Northrup and S. Cowan, Phys. Rev. B83, 165207 (2011) R. A. Street, M. Schoendorf, A. Roy and J. H. Lee, Phys. Rev. B81, 205307 (2010).
11:30 AM - **I1.6
Charge Photogeneration and Recombination in Organic Solar Cells.
James Durrant 1 Show Abstract
1 Chemistry, Imperial College London, London United Kingdom
My lecture will focus on charge separation and recombination in organic solar cells and their impact upon device photovoltaic performance. The talk will be based around transient optical and optoelectronic studies of yields, lifetimes and densities of dissociated charges in blend films and solar cells. Comparative studies will be presented of different materials systems, including over 20 different donor polymers blended with fullerenes, as well as small molecule blend and bilayer devices. On the basis of these data, I will address the parameters which influence the impact of geminate and non-geminate recombination upon device efficiency, including the roles of molecular structure, film microstructure, interfacial energetics, and macroscopic electric fields.
12:00 PM - I1.7
Recombination Processes in Disordered Organic Bulk–Heterojunction Solar Cells.
Alexander Wagenpfahl 1 , Craig Peters 2 , Eric Hoke 2 , Zach Beiley 2 , Michael McGehee 2 , Carsten Deibel 1 , Vladimir Dyakonov 1 3 Show Abstract
1 Experimental Physics VI, Julius-Maximilians-University of Würzburg, Würzburg Germany, 2 Department of Material Science and Engineering, Stanford University, Stanford, California, United States, 3 Functional Materials for Energy Technology, Bavarian Centre for Applied Energy Research (ZAE Bayern), Würzburg Germany
In recent years the charge carrier recombination mechanisms in organic bulk–heterojunction solar cells have been controversially discussed. For blended organic semiconductors recombination orders between one—a monomolecular decay—and two —a bimolecular decay—or even higher values have been reported. The origin of these observations, the recombination pathways as well as their impact on the device performance, still need further investigations to be understood in detail.In an organic bulk-heterojunction solar cell two blended but spatially separated semiconductor phases are used to gain current from the incident light. Due to their spatial disorder organic semiconductors generally show a Gaussian distribution of the molecular orbitals. Introducing this disorder into our macroscopic numerical device simulation, we show the possible interaction pathways of conducting and trapped electrical charges within the multiple trapping and release model. Based on these results we illustrate the impact of various recombination pathways for charge carriers on the observed current-voltage characteristics under consideration of phase separation aspects. Finally we identify those predicted patterns in experimentally measured current-voltage characteristics. Our results clearly show that it is crucial for the performance which recombination pathway, either the recombination between two free or between a free and a trapped charge carrier, is dominant in organic bulk-heterojunction solar cells and which the energetic disorder plays in these processes.
12:15 PM - **I1.8
Understanding and Affecting the Mechanisms in Excitonic Solar Cells.
Nir Tessler 1 , Lior Tzabari 1 , Nir Nir Yaacobi-Gross 1 , Israel Ravia 1 Show Abstract
1 Electrical Engineering, Technion, Haifa Israel
In this talk we will discuss the charge recombination in amorphous organic materials and relate this mechanism to the charge transport. In this part we will combine a simple model along with experimental data based on relatively new methods we have developed. The charge recombination under applied electric field is studied using a recombination time of flight method which is an extension of the standard TOF method. The recombination in blends is studied using ultralow light intensities where the role of charge induced traps and its dependence on processing conditions would be described. If time permits we will discuss an interesting effect we have found in inorganic nanocrystal based cells. Namely, the ability to use the ligands to polarize the nanocrystal and enhance the charge generation efficiency.
12:45 PM - I1.9
Studies of Transport and Recombination in Polymer:Fullerene Bulk Heterojunction Systems.
Jao van de Lagemaat 1 4 , Anthony Morfa 2 , Alexandre Nardes 2 , Sean Shaheen 3 1 , Nikos Kopidakis 1 , Brian Gregg 1 , Ziqi Liang 1 , Jian Li 5 Show Abstract
1 Chemical and Materials Sciences Center, NREL, Golden, Colorado, United States, 4 Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, Colorado, United States, 2 School of Chemistry & Bio21 Institute, University of Melbourne, Melbourne, Victoria, Australia, 3 Department of Physics and Astronomy, University of Denver, Denver, Colorado, United States, 5 National Center for Photovoltaics, NREL, Golden, Colorado, United States
Charged and uncharged defects are known to have a large influence on the transport and recombination of photogenerated charges in organic semiconductor materials. This contribution discusses measurements of charge transport and recombination in poly(3-hexylthiophene:[6,6]-phenyl- C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunctions in thick devices using electrical impedance, time-of-flight measurements and external quantum-efficiency measurements. We will also show results of the temperature activation of the current and photocurrent. The devices show Schottky-diode behavior with a large field-free region in the device owing to the presence of a large density of intrinsic acceptors, yielding an effective p-type doping. At high applied biases the depletion region spans the entire active layer and normal time-of-flight transients are observed from which the electron mobility is determined and Poole-Frenkel behavior is observed as a function of internal field. At lower applied biases, the depletion region only spans a small portion of the active layer, and diffusional transients are observed from which the Einstein coefficient (the thermodynamic factor) that relates drift and diffusion of carriers can be obtained. The thermodynamic factor differs significantly from one indicating that disorder or correlation between charges significantly impacts charge transport. The recombination kinetics are seen to follow a first-order rate law with a rate constant about two orders of magnitude lower than that predicted by Langevin recombination but in agreement with bimolecular recombination when one charge carrier density is large and controlled by doping instead of light intensity. The lower than Langevin recombination rate is consistent with results obtained by transient microwave methods in our laboratory and other reports in the literature. Consistent with the time-of-flight measurements, temperature activation experiments show that doping from a deep, intrinsic trap state controls transport and that Poole-Frenkel behavior both in the field dependence of the ionization of the dopants as well as the carrier mobilities explains the observed field-dependence of the current. A consistent picture therefore arises on the transport in these bulk heterojunction systems.
I2: Morphology I
Monday PM, November 28, 2011
2:30 PM - **I2.1
Origin of Open-Circuit Voltage in Organic Solar Cells Elucidated by Photoconductive Atomic Force Microscopy.
Thuc-Quyen Nguyen 1 , Xuan-Dung Dang 1 , Yuan Zhang 1 , Sarah Cowan 2 , Chunki Kim 1 Show Abstract
1 Chemistry & Biochemistry, UCSB, Santa Barbara, California, United States, 2 Materials, UCSB, Santa Barbara, California, United States
Open-circuit voltage (Voc) is a complex and critical parameter limiting the organic solar cell efficiency; however, its origin is under debate. In this work, a comprehensive picture of the nanoscale Voc of bulk heterojunction organic solar cells based on small molecule and polymer donors has been elucidated by photoconductive atomic force microscopy (pc-AFM). By changing the work function of anode and cathode electrodes, the maximum Voc can be explained by both optical band gap and metal-insulator-metal models. Our results indicate that Voc is dependent on electronic properties of bulk materials, electrode interfaces and energetic disorder of organic semiconductors, and on the vertical phase separation of photoactive layers. An empirical model is proposed to explain the nanoscale Voc of bulk heterojunction solar cells.
3:00 PM - I2.2
The Universality of Polymer-Fullerene Miscibility and Its Implications for Organic Photovoltaics.
Harald Ade 1 , Brian Collins 1 , Xiaoxi He 2 , Eliot Gann 1 , Christopher McNeill 3 Show Abstract
1 Physics, NC State University, Raleigh, North Carolina, United States, 2 Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom, 3 Materials Engineering, Monash University, Clayton, Victoria, Australia
The nature of interfaces is well known to play an important role in organic photovoltaic devices (OPVs), especially between the two common active components: polymers and fullerenes. Recently, the typical paradigm used for device morphology consisting of pure phases and discrete interfaces in the device active layer was challenged with the measurement of a finite molecular miscibility of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in poly(3-hexylthiophene) (P3HT) [1-3]. But with the field of materials continually expanding via breakthroughs in organic synthesis, it will be important to understand how common the phenomenon of miscibility in polymer-fullerene systems really is and what role it plays in device function. Using the same methods employed for the P3HT:PC61BM system , we have expanded the measurement of miscibility to two fullerenes and seven polymers including the silole-containing polythiophenes such as PCPDTBT and PSBTBT and the benzodithiophene polymer known as PTB7. The methods used include nanoscale and angle-resolved quantitative near edge x-ray absorption fine structure (NEXAFS) spectroscopy to measure composition on blend films brought to thermodynamic equilibrium at various temperatures as well as grazing incidence x-ray diffraction, which probes for crystallinity. The results suggest that regardless of the level of crystallinity of the polymer, polymer-fullerene miscibility in the amorphous portions of a device is likely universal. The overall implications of this finding on device performance as well as how the level of miscibility within different systems might affect the solar power conversion process will be discussed.B. Watts et al., Macromol. 42, 8392 (2009).B. A. Collins et al., J. Phys. Chem. Lett 1, 3160 (2010).N. D. Treat et al., Advanced Energy Materials 1, 82 (2010).
3:15 PM - I2.3
Connecting Morphology with Dominant Recombination Mechanisms in P3HT:PCBM Based Organic Bulk Heterojunction Solar Cells.
Pavel Dutta 1 , Mukesh Kumar 1 , Monika Rathi 2 , Scott Ahrenkiel 2 , David Galipeau 1 , Venkat Bommisetty 1 Show Abstract
1 Electrical Engineering, South Dakota State University, Brookings, South Dakota, United States, 2 Nanoscience and Nanoengineering Department, South Dakota School of Mines & Technology, Rapid City, South Dakota, United States
Recombination of photo-generated charge carriers is one of the primary carrier loss mechanisms that limit the photo-conversion efficiency of bulk heterojunction (BHJ) solar cells. Detailed study of carrier recombination is therefore important to understand the fundamental processes responsible for carrier losses in BHJ solar cells and to improve device performance by optimizing processing conditions, materials (donor and acceptor, DA) properties and device geometry. Moreover, morphology of the active blend also plays a critical role in determining charge transport and recombination. In this study, the role of nanomorphology of the DA blend in determining the nature of recombination dynamics in polymer-fullerene (P3HT:PCBM) based bulk heterojunction cells prepared using two halogenated spin casting solvents- chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB) has been investigated. Blend prepared from 1,2-DCB showed fine phase separated morphology (~ 15-20 nm phase separation) whereas CB resulted in a coarser phase separated morphology (~ 50-100 nm phase separation). Linear dependence of short circuit current (Jsc) vs. light intensity (I) plot (Jsc ∝ I^α) and slope α ~ 1 indicated the dominant role of monomolecular recombination in the fine phase separated devices (1,2-DCB cast). The carrier mobility (of both electrons and holes) was higher in for 1,2-DCB based blend and was attributed to the improved ordering and crystallinity of the donor-acceptor (DA) components, which was again substantiated by a red-shift in optical absorption spectra. Sub-linear dependence of Jsc with light intensity and slope α ~ 0.85 suggested dominant role of bimolecular recombination in coarse phase separated device (CB based blend). Moreover, low electron and hole mobility and an order of magnitude difference between them led to unbalanced charge transport and build-up of space charge in CB based device. Further, device characteristics measured under various light intensities showed a logarithmic dependence between open-circuit voltage (Voc) and light intensity with different slopes for CB and 1,2-DCB based devices. These results further indicated that bimolecular recombination was dominant in CB cast device while both monomolecular and bimolecular recombinations were present in 1,2-DCB cast device. Intensity modulated photocurrent measurements were performed under various light intensities and the results suggest enhanced carrier recombination in CB based devices, compared to 1,2-DCB cast devices at all the intensities. Improved device efficiency of 1,2-DCB based devices (η ≈ 2.5 %) compared to CB (η ≈ 1.3 %) was attributed to reduced bimolecular recombination at operational light intensities (1 Sun). Results also show a cross-over from monomolecular to bimolecular recombination in P3HT:PCBM based BHJ devices due to fine and coarse phase separation.
3:30 PM - I2.4
Charge Transport in Bulk Heterojunctions: The Influence of Morphology, Electric Field, and Charge Carrier Concentration.
L. Jan Anton Koster 1 2 Show Abstract
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands, 2 Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
Charge transport is one of the most important processes in a solar cell. Experimentally, charge carrier mobility is usually studied in devices where only one type of carrier is present. In neat materials the mobility is found to increase with increasing electric field. Recent experimental studies on bulk heterojunctions (BHJs), however, show that BHJ mobility can decrease with increasing electric field. Here we study the mobility in BHJs theoretically by numerically solving the Pauli master equation. In this model, charge carriers are restricted to one of the two constituent phases. The influence of morphology, disorder, electric field, and charge carrier concentration on BHJ mobility is assessed. Important differences between neat materials and BHJs are found. The dependence of mobility on charge carrier concentration is more pronounced in BHJs and it is influenced by the electric field strength. At low charge carrier densities, BHJ mobility is found to decrease with increasing field. Additionally, the impact of the volume ratio of the constituent materials and their domain size on the mobility is presented. Especially for strongly disordered materials charge transport is favored by relatively large domains. To compare these theoretical findings with existing experimental mobility data, the current density in a space-charge-limited BHJ device is computed. We find that, for the parameters and morphologies studied, the apparent mobility in such a device decreases with increasing bias voltage.
3:45 PM - I2.5
Morphological Diversity in P3HT-Endohedral Fullerene Organic Solar Cells.
Lee Richter 1 , Andrew Herzing 1 , R. Joseph Klein 1 , Dean DeLongchamp 1 , Daniel Fisher 1 , David Gundlach 1 , Claudia Cardona 2 , John Wall 2 , Francis Swain 2 , Steve Joslin 2 Show Abstract
1 , NIST, Gaithersburg, Maryland, United States, 2 , Luna Innovations, Danville, Virginia, United States
Recently a soluble endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu:PCBEH) has been introduced for use in bulk heterojuction (BHJ) solar cell applications. The endohedral metal complex shifts the fullerene LUMO, enabling significant improvement in open circuit voltage with regioregular poly(3-hexylthiophene) (P3HT) as the absorber. Lu:PCBEH differs from the typical [6,6]-phenyl-C61-butyric methyl ester, PCBM, acceptor in both cage diameter (C80) and solubilizing adduct. We will present a detailed study of the morphology of P3HT/Lu:PCBEH BHJ films prepared with three deposition procedures: 1) rapid drying from chlorobenzene, 2) slow drying from 1,2-dichlorobenzene and 3) rapid drying from chlorobenzene with 1-chloronaphthalene additive. After thermal processing, all three procedures result in devices with power conversion efficiencies exceeding ≈ 4 %. However, the nanoscale morphologies of the BHJ films are not similar, having different domain shapes and different crystal orientation distributions. These results indicate that multiple nanoscale morphologies can adequately perform the essential BHJ functions of exciton diffusion, charge separation, and charge extraction. Unlike PCBM, neat films of the Lu:PCBEH exhibit no crystallization over the thermal range (23 to 160)C relevant to device processing, enabling clear characterization of the role of P3HT ordering in device performance.
Monday PM, November 28, 2011
4:30 PM - I3.1
Design and Characterization of Functional Materials for Highly Efficient Dye-Sensitized Solar Cells.
Eric Diau 1 Show Abstract
1 Department of Applied Chemistry, National Chiao Tung University, Hsinchu Taiwan
This lecture will give brief introduction for a series of push-pull porphyrin sensitizers  and novel heteroleptic ruthenium complexes  together with the improvement of the cell performance using one-dimensional (1D) TiO2 nanostructures . For the porphyrin sensitizers, extension of pi-conjugation was achieved by attaching various cyclic aromatic hydrocarbon substituents and/or electron-donating groups opposite to the anchoring group of the target porphyrin. We will demonstrate that devices made of such porphyrins have reached the power conversion efficiency above 10 % under one-sun irradiation, stimulating hope that porphyrin dyes might yield highly efficient DSSC applications . For the ruthenium complexes, we designed heteroleptic ruthenium complexes containing benzimidazole substituents in a series for which the the target compounds were easily synthesized according to a typical one-pot procedure with the corresponding heteroleptic ligands generated in only two simple steps. We found that the DSSC devices made of this series of dyes exhibit remarkable photovoltaic performance comparable to the device made of N719 dye (PCE > 10 %). The results obtained from transient photoelectric measurements (photocurrent and photovoltage decays) indicate that the electron lifetimes of the devices display a systematic trend which is consistent with the cell performance showing the same order. For the electron-transport layer, it has been pointed out that the electron transport in a traditional nanoparticulate (NP) DSSC device is a limiting factor to achieve higher light-to-electricity conversion efficiency due to the structural disorder at the contact between two TiO2 NPs. To improve the charge-collection efficiency by promoting faster electron transport and slower charge recombination, we have made the TiO2 films constructed of either 1D nanorods (NR) or oriented nanotube (NT) arrays for DSSC applications. Measurements of femtosecond fluorescence up-conversion were carried out to evaluate the electron-injection yields of the porphyrin/TiO2 films with control experiments conducted on porphyrin/Al2O3 films under the same experimental conditions. T. Bessho, S. K. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau, M. Grätzel, Angew. Chem. Int. Ed. 2010, 49, 6646. W.-K. Huang, C.-W. Cheng, S.-M. Chang, Y.-P. Lee, E. W.-G. Diau, Chem. Commun. 2010, 46, 8992. L.-L. Li, C.-Y. Tsai, H.-P. Wu, C.-C. Chen, E. W.-G. Diau, J. Mater. Chem. 2010, 20, 2753. C.-L.Wang, Y.-C. Chang, C.-M. Lan, C.-F. Lo, E. W.-G. Diau, C.-Y. Lin, Energy Environ. Sci. 2011, 4, 1788.
4:45 PM - **I3.2
Enhancing and Understanding Charge Generation in Mesoscopic Hybrid Solar Cells.
Henry Snaith 1 Show Abstract
1 , Oxford University, Oxford, OXON, United Kingdom
Organic based and dye-sensitized solar cells require a nano-to-meso scale phase separated structure to deliver the required internal surface area to enable effective solar light capture and charge generation. In some specific systems, particularly for dye-sensitized solar cells, charge generation can occur rather efficiently at a hybrid organic absorber – metal oxide electron accepting interface. However, this is the exception rather than the rule and in the majority of instances, particularly for polymer – metal oxide interfaces, charge generation is far less than ideal. In the first part of this talk we will focus on charge generation in solid-state dye-sensitized solar cells. The photoexcited state on most push-pull sensitizers is strongly influenced by the polarity of the medium around it, with the charge-transfer nature to the excited state being significantly enhanced in a polar medium. We present two very different examples where the charge transfer exciton is responsible for ultra-fast electron transfer into TiO2, which is only significant when ionic additives such as Li-TFSI are added to the hole-transpoter matrix (spiro-OMeTAD, and P3HT). Without the ions, the non-polar environment of the hole-transporter results in slow injection into the TiO2 which does not compete with other decay channels.In the second part of the talk I will present an investigation into the charge generation in polymer- oxide solar cells. Despite what appears to be a good energy level offset between a host of semiconducting polymers and TiO2, the historical poor performance of hybrid solar cells constructed from semiconducting polymers infiltrated into mesoporous metal oxide electrodes, is almost entirely due to ineffective exciton quenching and charge generation at the polymer metal oxide interface. In order to overcome this, we have chemically modified the surface of the mesoporous TiO2 with a molecular electron acceptor to improve the “electronic coupling”, between the polymer and the oxide. We demonstrate that this enables efficient charge generation from excitons formed in the low-band gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT). This polymer also infiltrates the mesostructured TiO2 film suitable well to enable efficient charge collection and ensuring external quantum efficiencies of over 30% at 700nm and solar power conversion efficiencies of over 1% are achieved. Though this is relatively low compared to solid-state dye-sensitized solar cells, it is a clear identification of the critical challenge for polymer-oxide hybrid solar cells, and will lead to significant advances in the near future with improved and tailored materials.
5:15 PM - I3.3
Photo-Induced Dynamics of Electronic Excitations in RuO2 Nano-Catalyst at a TiO2 Substrate.
Talgat Inerbaev 2 , James Hoefelmeyer 1 , Dmitri Kilin 1 Show Abstract
2 , South China Normal University, Guangzhou China, 1 Chemistry, University of South Dakota, Vermillion, South Dakota, United States
Dye-sensitized TiO2 nanorods with adsorbed RuO2 and Pt nano-catalysts are inspected for photo-catalytic splitting of water. Charge transfer from a photo-excited TiO2 nanorod to an adsorbed nano-catalyst triggers the photo-catalytic event (such as water oxidation half-reaction). We perform ab-initio computational modeling of the charge transfer dynamics on the interface of TiO2 nanorod and RuO2 nano-catalyst. A slab of TiO2 represents a fragment of TiO2 nano-rod in the anatase phase. RuO2 planar cluster is adsorbed in a way to match the symmetry of TiO2 substrate. A mix of H2O, OH-, and H3O+ ions, in different proportions, compensates dangling bonds at TiO2 and RuO2. The modeling is performed by reduced density matrix method in the basis of Kohn-Sham orbitals . RuO2 nano-catalyst contributes energy levels near the top of the valence band of TiO2 nano-rod. A photo-excitation in nano-rod is dissipating due to interaction with lattice vibrations, treated through non-adiabatic coupling. The electron part of an excitation relaxes to the bottom of conduction band while the hole part is experiencing a transfer to the RuO2 nano-catalyst. We systematically investigate the dependence of the rate of the charge transfer on the thickness of substrate slab and surface coverage by water, hydroxyl and hydronium ions. These results are of the importance for an optimal design of nano-materials for photo-catalytic water splitting and solar energy harvesting.References D. S. Kilin, D. A. Micha, J. Phys. Chem. Lett. 2010, 1, 1073-1077.
5:30 PM - I3.4
Interplay of Three-Dimensional Morphologies and Photocarrier Dynamics of Polymer/TiO2 Bulk Heterojunction Solar Cells.
Chun-Wei Chen 1 , Shao-Sian Li 1 , Ching-Pin Chang 1 2 , Chih-Cheng Lin 1 , Ming-Wen Chu 2 Show Abstract
1 Materials Science and Engineering, National Taiwan University, Taipei Taiwan, 2 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan
Recently, the photovoltaic device based on poly(3-hexylthiophene) (P3HT) and/TiO2 nanorod hybrid bulk heterojunctions has demonstrated a promising efficiency over 2 % through interface modification. (JACS, 131,3644,2009). In this work, we would like to present the interplay of three-dimensional morphologies and the photocarrier dynamics of (P3HT)/TiO2 nanocrystals hybrid photoactive layers consisting of TiO2 nanoparticles (NPs) and nanorods (NRs). Electron tomography based on scanning transmission electron microscope using high-angle annular dark-field imaging (STEM-HAADF) was performed to analyze the morphological organization of TiO2 nanocrystals in P3HT in optimal solar cell devices. The three-dimensional morphology of these hybrid films were correlated with the photocarrier dynamics of charge separation, transport and recombination which were comprehensively probed by various transient techniques including time-resolved photoluminescence (TRPL) spectroscopy, transient open-circuit voltage decay (TOCVD) measurement, time of flight (TOF) technique, and photo assisted charge extraction with a linearly increasing voltage (photo-CELIV) measurement. The results support the establishment of a favorable morphology for polymer/ TiO2 hybrid solar cells due to the presence of the dimensionality of TiO2 nanocrystals as a result of more effective mobile carrier generation and more efficient and balanced transport of carriers.(JACS, 2011, in press) In addition, the grazing incidence X-ray diffraction (GIXRD) measurements were also performed to reveal the polymer structure in these hybrid materials to support the correlation between the three-dimensional morphology, carrier dynamics and device performance in P3HT/ TiO2 hybrid solar cells.
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Energy-Level Alignment at the ZnO/P3HT Photovoltaic Interface.
Feliciano Giustino 1 , Keian Noori 1 Show Abstract
1 Materials, University of Oxford, Oxford United Kingdom
Hybrid organic-inorganic nanostructured photovoltaics are currently the focus of intenseresearch efforts as they hold promise for large area and low cost solar energy conversionusing nontoxic and Earth-abundant elements. During the past five years hybrid solar cells based on poly(3-hexylthiophene) (P3HT) and ZnO have been investigated in great detail, with research being conducted into bilayer devices as well as bulk heterojunctions based on ZnO nanowires or nanoparticles [1-3]. Despite substantial progress in this area, these devicessuffer from low short-circuit currents (~2 mA/cm2) and small open-circuit voltages (~0.5 V),and the resulting power conversion efficiency does not exceed 2%. While low short-circuit currents have been attributed to insufficient interface contact area, the origin of the small open-circuit voltages remains poorly understood. In particular it is not clear yet whether this bottleneck is an intrinsic limitation of the ZnO/P3HT interface or is related to extrinsic effects such as defects or contaminant molecules. In order to clarify this aspect we performed a first-principles computational study of the interface between ZnO and P3HT at the atomic scale. We addressed the structure and energetics of the semiconductor/polymerinterface using density-functional theory (DFT) calculations, and we investigated the interfacialelectron energy-level alignment using post-DFT techniques based on hybrid functionals. Our calculations show that, even when P3HT is physisorbed on the ZnO surface, there issignificant charge transfer between the polymer and the semiconductor, resulting in an electrostatic potential offset as high as 0.4 V at the interface. This result calls fora critical revision of simplified energy-level diagrams where each component is aligned to the vacuum level. More importantly our calculations show that for an ideal ZnO/P3HT interface the energy difference between the highest occupied molecular orbital of P3HT and the conduction band bottom of ZnO is as high as 2 V. This result indicates that there are no intrinsic limitations to the open-circuit voltages in ZnO/P3HT solar cells. In general our study suggests that the power conversion efficiency of ZnO/P3HT nanostructured solar cells could be substantially increased by optimizing the ZnO surface preparation and by controlling the regioregularity of the polymer blend. P. Ravirajan, A. M. Peiro, M. K. Nazeeruddin, M. Graetzel, D. D. C. Bradley, J. R. Durrant, J. Nelson, J. Phys. Chem. B 110, 7635 (2006). W. J. E. Beek, M. M. Wienk, R. A. J. Janssen, Adv, Funct. Mater. 16, 1112 (2006). D. C. Olson, J. Piris, R. T. Collins, S. E. Shaheen, D. S. Ginley, Thin Solid Films 496, 26 (2006).