Symposium : S
Organic photovoltaics: science and technology (OPV)
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| Device Physics I : Moritz Riede | ||
| 08:30 | Factors influencing photocurrent generation in polymer:fullerene blend solar cells Authors : Jenny Nelson Department of Physics, Imperial College London, Prince Consort Road, London, SW7 2AZ, UK. Affiliations : Resume : The efficiency of photocurrent generation in a polymer:fullerene blend solar cell depends on a number of factors, including the electronic energy levels of the donor and acceptor components, their specific chemical structure and the local microstructure. Until now, no agreed picture has emerged of the mechanism controlling charge pair generation. We use spectroscopic, structural and device studies and modelling to investigate the dependence of charge generation and recombination on the electronic and structural properties of the polymer donor and fullerene acceptor, using fullerenes with varying type and number of side chains. We show that both energetic and steric effects are important, and discuss the mechanism for charge generation in the light of these. | 4 1 |
| 09:00 | Efficient charge carrier generation in Polythiophene:Fullerene blends occurs via dissociation of the charge transfer state Authors : D.H.K. Murthy1,2, M.Gao1, M.J.W. Vermeulen1, L.D.A. Siebbeles1 and T.J. Savenije1 1 Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, 2628 BL Delft, The Netherlands. 2 Dutch Polymer Institute (DPI), P.O. Box 902,5600 AX Eindhoven, The Netherlands. Affiliations : Resume : Charge carrier formation in a polymer:fullerene bulk heterojunction (BHJ) solar cell starts with photoexcitation of the polymer followed by electron transfer to a fullerene molecule. To contribute to the photocurrent the initially generated electrostatically bound electron hole pair, also denoted as charge transfer (CT) state, must dissociate to yield free charge carriers. Despite considerable research effort, the factors determining the dissociation in these low dielectric blends are not clearly understood, including e.g. the role of the excess free energy of the electron transfer process and the presence of an electric field. In this work, we have investigated the effect of temperature and excitation wavelength on the formation of free charges in a P3HT:PCBM BHJ using the electrodeless time-resolved microwave conductance technique. The observation of free charge carrier formation on direct photoexcitation of the CT state at 830 nm unambiguously demonstrates that CT states are indeed the precursors in the generation of free charge carriers even under low electric field strength. Moreover, the similar charge carrier lifetimes and the identical temperature behavior (similar activation energy) found on excitation at 500 and at 830 nm suggest that the excess free energy is not affecting the dissociation process. In contrast, reducing the crystallinity of one of the constituents of the BHJ dramatically lowers the CT dissociation yield. | 4 2 |
| 09:15 | Direct detection of charge-transfer states in polymer:fullerene blends Authors : J. Behrends, T. Biskup, C. Teutloff, R. Bittl Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany A. Schnegg, K. Lips Institut für Silizium-Photovoltaik, Helmholtz-Zentrum Berlin, Kekuléstr. 5, D-12489 Berlin, Germany A. Sperlich, V. Dyakonov Experimentelle Physik VI, Julius-Maximilians-Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany Affiliations : Resume : Photoinduced charge transfer and subsequent charge separation are the key processes in organic solar cells. Prior to exciton separation into free charge carriers, bound polaron pairs (also referred to as charge-transfer states, CTS) form at the donor/acceptor interface. While the existence of CTS was confirmed by optical spectroscopy and electrical measurements, their exact role in the process of free charge carrier generation is subject to ongoing discussions. Thus, experimental techniques capable of detecting CTS and providing direct access to their dynamics are highly demanded.
Here we report transient electron paramagnetic resonance (trEPR) measurements with sub-µs time resolution performed on a P3HT:PCBM blend at low temperatures. We show that the trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs and thus decisively differs from the spectrum of separate polarons commonly observed in light-induced cwEPR. The pair partners (positive polarons in P3HT and negative polarons in PCBM) can be identified by their characteristic g values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that both constituents of each pair originate from the same exciton. We demonstrate that coupled polaron pairs are present even several microseconds after the charge transfer step and discuss their role in mediating the conversion from excitons into separate charge carriers as probed by trEPR. | 4 3 |
| 09:30 | Contribution of “hot” charge-transfer state to the performance of organic photovoltaic devices Authors : Artem A. Bakulin (1), Akshay Rao(1), Vladislav Pavelyev(2), Paul H.M. van Loosdrecht(2), Maxim S. Pshenichnikov(2), Richard H. Friend(1) (1) Cavendish laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK (2) Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Affiliations : Resume : The formation of free charge carriers in organic photovoltaic (OPV) devices is known to be mediated by a charge-transfer (CT) state, which is a bound pair of charges at the organic semiconductor interface. The mechanism of CT-state dissociation has not been fully elucidated, despite the fact that the understanding of this process is critical to optimal OPV cell design. Some studies [1] suggest that the CT state requires excess vibrational energy (“hot” state) to be converted into a pair of free charge carriers. An alternative viewpoint is that the initially hot CT state is not important because it rapidly relaxes to the lowest in energy (“cold”) CT state, from which the actual charge generation occurs. [2] Currently, there is no quantitative experimental evidence on the relevance and importance of these processes.
To address this issue, we apply a combination of the ultrafast optical 3-pulse and photocurrent spectroscopies, and their novel hybrid, optically-manipulated photocurrent spectroscopy. This allows us to selectively address the CT-state dynamics and, thereby, to elucidate the contributions of both cold and hot dissociation pathways to the photocurrent generation in various OPV devices. Our results suggest that the ratio of hot/cold dissociation is dictated by relative position of donor and acceptor energy levels which provides guidelines for the design of future OPV materials.
[1] H. Ohkita et al., JACS 130, 3030 (2008)
[2] J. Lee et al, JACS 132, 11878 (2010) | 4 4 |
| 09:45 | Imaging of Interfacial Charge Transfer State between Crystalline P3HT:PCBM Domains: a joint experimental and theoretical approach. Authors : Daniele Fazzi (a), Giulia Grancini (b), Dario Polli (b,a), Juan Cabanillas-Gonzalez (c), Giulio Cerullo (b) and Guglielmo Lanzani (a,b) (a) Center for NanoScience and Technology CNST-IIT@POLIMI, via Pascoli 70/3, 20133 Milano, Italy, (b) CNR-INFM Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy, (c) Madrid Institute of Advanced Studies, IMDEA Nanociencia, Faculdad de Ciencias, Av. Tomas y Valiente 7, 28049 Madrid, Spain Affiliations : Resume : A key challenge for improving BHJ solar cells performances, based on polymer: fullerene blends, is to develop a comprehensive understanding of the relationships between morphology and photophysics of charge transferred states (CTS).1 We investigate the dynamics of CTS at the border region between crystalline phases in P3HT:PCBM blend by transient absorption pump&probe spectroscopy combined with confocal microscopy, delivering simultaneously high temporal and spatial resolution. We identify CTS at the border between P3HT:PCBM crystallites. This CTS is long lived, free of geminate recombination. From polarization dependent measurements it results that CTS has a transition dipole moment highly tilted with respect to the P3HT chain, confirming strong interactions between donor and acceptor units. Ground and excited state calculations (ZINDO/S and TD-DFT) have been carried out on supramolecule complexes to understand the nature of CTS. According to experiments, we found that the character of the low lying excited state is a charge transfer from P3HT to PCBM, with small contribution from the so-called bridging configuration, favoring the electron transfer. Calculated CTS are polarized normal to P3HT chain supporting the experimental evidence of an off-chain component for interfacial CTS. Electronic and exciton couplings have been also evaluated for P3HT:PCBM clusters to correlate the electronic structure with local molecular configuration. 1.Lee K. et al., JACS,2010,132,11878 | 4 5 |
| 10:00 | Break | |
| Hybrid Devices : Kristofer Tvingstedt | ||
| 10:30 | High-Efficiency Hybrid Solar Cells Using a Blend of Quantum Dots and Nanorods Authors : Krischan F. Jeltsch1, Seyfullah Yilmaz2, Jörg-Bernd Bonekamp3, Frank Rauscher4, Phenwisa Niyamakom4, Ullrich Scherf2, Klaus Meerholz1 [1] K. F. Jeltsch, Prof. K. Meerholz Department für Chemie Universität Köln Luxemburger Straße 116, 50939 Köln (Germany) E-mail: klaus.meerholz@uni-koeln.de [2] S. Yilmaz, Prof. U. Scherf Institut für Polymertechnologie Bergische Universität Wuppertal Gauss-Strasse 20, 42097 Wuppertal (Germany) E-mail: scherf@uni-wuppertal.de [3] Jörg-Bernd Bonekamp Soluxx GmbH Greinst. 4, 50939 Köln (Germany) E-Mail: jb.bonekamp@soluxx.de [4] Frank Rauscher, Phenwisa Niyamakom Bayer Technology Services GmbH BTS-PT-PT-PDN Leverkusen, E 41 (Germany) E-mail: phenwisa.niyamakom@bayer.com Affiliations : Resume : In comparison with the organic solar cell technology the electron-accepting component in organic-inorganic hybrid solar cells is exchanged for an inorganic nano-structured material. Advantages are the size dependent absorption properties of inorganic semiconductor nano-materials. [1] The most crucial improvement in hybrid devices is the modification of the shape of the nanocrystals, which varies from spherical dot-like to hyperbranched particles. The use of the low band gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopen-ta[2,1-b;3,4-b′]di-thiophene)-alt-4,7-(2,1,3-benzothiadi-azole)] (PCPDTBT) and CdSe tetrapod-shaped particles led to an improved record efficiency of PCE = 3.2 %. [2] Nevertheless, no devices using blends of different shaped nano-particles are reported to our knowledge, so far.
Using PCPDTBT and pyridine-capped CdSe quantum dots we improved the efficiency from PCE = 2.0 % [3] to PCE = 2.8 %. In addition, the partial replacement of the photoactive polymer-QD blend by a vacuum-deposited C60-layer yields devices with PCE = 3.0 %. Finally, a new record efficiency for hybrid photovoltaic cells is obtained. By mixing PCPDTBT with QDs and NRs the devices give the highest ever reported efficiency of PCE = 3.5 % for hybrid solar cells.
[1] A. P. Alivisatos, Science 1996, 271, 933.
[2] S. Dayal, N. Kopidakis, D. C. Olson, D. S. Ginley, G. Rumbles, Nano Lett. 2010, 10, 239.
[3] Y. Zhou, F. S. Riehle, Y. Yuan, H.-F. Schleiermacher, M. Niggemann, G. A. Urban, M. Krüger, Appl. Phys. Lett. 2010, 96, 013304. | 5 1 |
| 10:45 | Efficiency Enhancement of Hybrid Solar Cell Based on CdSe Nanocrystal and Conjugated Polymer Authors : Yunfei Zhou, Michael Eck, Michael Krüger, 1 Freiburg Materials Research Center (FMF), University of Freiburg, Germany, 2 Department of Microsystems Engineering (IMTEK), University of Freiburg, Germany. Affiliations : Resume : Inorganic semiconductor nanocrystals (NCs) such as CdSe NCs, with tunable bandgaps and high intrinsic charge carrier mobilities can act as good electron acceptors and be incorporated into conjugated polymers to form bulk-heterojunction hybrid solar cells. Although their power conversion efficiencies are still lagging behind that of the fullerene based organic solar cells, high potential for improvement has been shown and prospected [1]. Here we report on the efficiency enhancement of CdSe NC based devices due to an effective post-synthetic surface treatment of the NCs before their integration into hybrid solar cells [2] and the use of low-bandgap polymers [3], exceeding our recently reported state-of-the-art result for spherical CdSe NCs based hybrid solar cells. The optimizations of photovoltaic devices include the active layer thickness, the electrode materials, NC hybrid structures, and approaches to control the nanomorphology. Power conversion efficiencies approaching 3% and above are available and further enhancements can be expected by exploring and combining the above mentioned approaches. [1] Y. Zhou, M. Eck, M. Krueger, Energy Environ. Sci. 3, 1851 (2010). [2] Y. Zhou, F.S. Riehle, Y. Yuan, H. Schleiermacher, M. Niggemann, G.A. Urban, M. Krueger, Appl. Phys. Lett. 96, 013304 (2010). [3] Y. Zhou et al., Sol. Energy Mater. Sol. Cells (2010), doi:10.1016/j.solmat.2010.12.041 | 5 2 |
| 11:00 | Hybrid inorganic nanoparticle – polymer solar cells: new fabrication methods and towards a better fundamental understanding of device function. Authors : Saif A. Haque Department of Chemistry, Imperial College London, South Kensington campus, SW7 2AY Corresponding author: s.a.haque@imperial.ac.uk , Tel: +44 (0) 207 594 1886 Affiliations : Resume : Thin films comprising nanostructured organic and inorganic semiconductor nanioparticles are currently attracting considerable interest for the development of photovoltaic devices. The performance of such hybrid devices depends critically on the ability to control materials and interface structure at the nanometer length scale. For example, one of the major challenges to the design of photoactive layers for efficient inorganic nanoparticle / polymer solar cells is the compromise which must be made between efficient charge photogeneration and charge carrier transport, whilst ensuring high nanocrystal solubility within the film processing solution. Another challenge to the design of efficient hybrid devices is the ability to control the yields of charge photogeneration at the organic / inorganic heterojunction. In this talk we will present some of our recent work which is aimed at addressing these challenges. Specifically, in this talk we will focus on:
1) Our recent work on developing new strategies for the design of hybrid organic – inorganic photoactive layers [1].
2) Transient absorption studies of charge separation and recombination in hybrid inorganic / polymer heterojunctions. A key aim of this study is to develop a better mechanistic understanding of the parameters that control such interfacial charge transfer processes. Another key aim of this work is to develop quantitative structure- function relationships that can be used to guide the design and development of organic – inorganic heterojuntions for efficient hybrid solar cells.
References:
[1] Nano Letters (2010), 10, 1253-1258 | 5 3 |
| 11:15 | Sensitization of thin film silicon solar cells with small organic molecules: Interface experiments and hybrid structures of µc-Si:H and PA-PTCDI Authors : Andreas Decker[1], Sabin-Lucian Suraru[2], Sven Burdorf[3], Kerstin Lakus-Wollny[1], Gottfried Heinrich Bauer[3], Frank Würthner[2], Thomas Mayer[1] and Wolfram Jaegermann[1] [1] Technische Universität Darmstadt, Fachbereich Materialwissenschaft, Petersenstr. 32, 64289 Darmstadt [2] Universität Würzburg, Institut für Organische Chemie, Am Hubland, 97074 Würzburg [3] Universität Oldenburg, Institut für Physik, Carl-v.-Ossietzky Str. 9-11, 26129 Oldenburg Affiliations : Resume : In an attempt to improve thin film silicon solar cells, small organic molecules are embedded into intrinsic hydrogenated microcrystalline silicon (µc-Si:H). This approach is directed at combining the excellent transport properties of silicon and the high absorptivity of organic dyes in a functional composite. The aim is to decrease layer thickness, thus lowering deposition time and production cost.
PTCDI derivatives have been synthesized and optimized with respect to stability, reactivity with silicon and electronic lineup of the dye orbitals versus the silicon bands. The dyes were investigated by cyclovoltammetry (CV) and optical spectroscopy (UV/VIS). The silicon/dye interface has been analyzed with synchrotron induced photoelectron spectroscopy (SXPS). Hybrid structures of µc-Si:H/dye were produced in a HWCVD chamber which allows for sequential deposition as well as codeposition. The composites have been characterized by UV/VIS, Raman spectroscopy and opto-electrical measurements.
A symmetric electronic lineup suits device operation best as a driving force for both excited charge carriers from the dye to the silicon is needed. The electronic lineup was estimated by CV on dye in solution and specified by SXPS on the model system n-Si(111):H. The best match of the investigated molecules is achieved for Phenoxy-Allyl-PTCDI (PA-PTCDI). UV/VIS and Raman measurements showed that PA-PTCDI increases the absorptivity, tolerates the HWCVD process and still allows formation of µc-Si. | 5 4 |
| OPV Lifetime : Jenny Nelson | ||
| 14:30 | Organic Photovoltaics Research Guided by New Reactions and New Molecules Authors : Eiichi Nakamura Department of Chemistry, The University of Tokyo Bunkyo-ku, Tokyo 113-0033, Japan Affiliations : Resume : Natural products have given so much inspirations to chemists, and served for the development of drugs and biological science. An amazing diversity of molecules are being discovered in nature and being utilized for drug discovery. Natural products however are not very useful in materials science, except porphyrin--a crucial molecule in photosynthesis. Our strategy in the field of organoelectronic research capitalizes on the new classes of organic molecules obtained by newly invented reactions. This strategy has proven to generate remarkable array of molecules that have hitherto not been synthesized and examined in organic electronics research. Examples in the OPV research will be presented. The synthesis of new organofullerenes and polycondensed aromatics and the new device structures based on the new compounds will be discussed. | 6 1 |
| 15:00 | Using Förster Resonant Energy Transfer to Broaden the Spectral Response of Dye Sensitized Solar Cells Authors : Nicola Humphry-Baker 1, Kristina Driscoll 1, Akshay Rao 1, Tomas Torres 2, Henry Snaith 3, Richard Friend 1 1. Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, CB3 0HE, UK 2. Departamento de Química Orgánica (C-I), Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain 3. Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, OX1 3PU, UK Affiliations : Resume : Dye-sensitised solar cells (DSC) are promising third generation solar cells, yet they are limited by their ability to absorb only a small part of the solar spectrum. This issue has been tackled through the use of energy relay dyes (ERD). The excited ERD transfers its energy to the sensitizing dye, which injects electrons into the porous TiO2. This energy transfer process is believed to happen by Förster resonant energy transfer (FRET); however, there has so far been no direct evidence of how energy is transferred from one dye to the other. We have fabricated solid-state DSCs with poly(3-hexylthiophene) (P3HT) as both the hole conductor and the ERD and a zinc phthalocyanine dye (TT1) as the sensitizer. These DSCs show a broadening of 100 nm in the external quantum efficiency (EQE) and a near two-fold increase in the peak EQE. To directly monitor the energy transfer from the P3HT to the TT1 we use femtosecond transient absorption. The introduction of the TT1 dramatically reduced the lifetime of the ground-state bleach (GSB) of the P3HT from 400 ps (comparable to its photoluminescence lifetime) to 30 ps. The GSB of the TT1 rises on a comparable time scale, peaking as the P3HT GSB reaches a minimum, showing that little charge transfer occurs from the P3HT to the TT1 and that FRET is the main cause of the observed improvement in EQE. From the lifetimes of the GSB decays, we calculate an energy transfer rate of 30 ps with a corresponding FRET efficiency exceeding 90 %. | 6 2 |
| 15:15 | Nano-Morphology and its stability versus photovoltaic properties in bulk heterojunction solar cells Authors : Sabine Bertho (1), Koen Vandewal (2), Fortunato Piersimoni (1), Donato Spoltore (1), Bert Conings (1), Jan D’Haen (1), Laurence Lutsen (3), Hans-Gerd Boyen (1), Dirk Vanderzande (1,3) and Jean V. Manca*(1,3) (1)Institute for Materials for Research, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium. (2)Linköping University, Sweden (current affiliation) (3) IMEC vzw, associated lab IMOMEC (Diepenbeek), Belgium Affiliations : Resume : The nano-morphology in organic bulk heterojunction solar cells is of crucial importance for the resulting photovoltaic properties. It is well known that the morphology of the interpenetrating donor:acceptor networks has a determining impact on charge transfer, charge transport and charge collection and therefore on the resulting photocurrent. It is less known that the nano-morphology has also a systematic influence on the open circuit voltage (Voc) and on the corresponding effective band gap (Eg). Control of the nano-morphology is therefore of key importance in order to obtain state-of-the-art bulk heterojunction solar cells with high photovoltaic performance. However, when these state-of-the-art solar cells with tuned morphology are submitted to elevated temperatures, significant morphology changes can be induced with corresponding changes in photovoltaic properties. In this contribution the relation of morphology and its stability versus photovoltaic properties is investigated for different sets of donor:acceptor materials by using a variety of in-situ and ex-situ characterisation techniques. | 6 3 |
| 15:45 | Identification and quantification of loss processes in aged P3HT/PCBM blends "TOTAL YOUNG INVESTIGATOR AWARD" Authors : Enrico Da Como1, Felix Deschler1, Antonietta De Sio2, Elizabeth von Hauff2 and Hans-Joachim Egelhaaf3 1Photonics and Optoelectronics Group, Department of Physics and CeNS Ludwig-Maximilians-University, 80799, Munich, (Germany) 2Institute of Physics, Energy and Semiconductor Research Laboratory Carl von Ossietzky University, 26111 Oldenburg (Germany) 3Konarka Technologies GmbH, Nurnberg (Germany) Affiliations : Resume : The commercialization of organic solar cells depends on many factors related to production costs, installation, power conversion efficiency and cell lifetime. While during the last years efficiencies have reached values of up to 8% in single junction cells, the outdoor lifetime of these devices remains well below those for commercial inorganic solar cells. Many experiments have been performed on improving device performance, however, the mechanism by which degradation influences the fundamental steps of exciton diffusion and charge separation has not been addressed.
In this contribution we show how a combination of optical spectroscopy methods is capable of shining light on the most important loss processes in the active layer of regioregular poly[3-hexylthiophene] (P3HT) 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C61 (PCBM) solar cells. The aging is performed in ambient conditions upon illumination at AM1.5 conditions for different periods of time. By using steady state photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy we pinpoint the fundamental excitations of these bulk-heterojuctions. Aging results in a drastic reduction in the yield of polarons and as well a loss in excitons. In order to quantify the time scales for the different processes we performed femtosecond PIA revealing that exciton and polaron trapping occurs on an ultrafast time scale, comparable to rate of photoinduced electron transfer. | 6 4 |
| 16:00 | Total charge amount as indicator for the degradation of small-molecule organic solar cells Authors : Martin Hermenau¹, Sebastian Scholz², Karl Leo¹, Moritz Riede¹ 1: Institut fuer Angewandte Photophysik, Technische Universitaet Dresden, 01062 Dresden, Germany 2:Fraunhofer-Institut IKTS Dresden, Winterbergstraße 28, 01277 Dresden, Germany Affiliations : Resume : In addition to efficiency and cost, lifetime is an equally important factor for the commercialisation of small molecule organic solar cells. However, real time degradation experiments are not practical under normal circumstances. To quickly achieve results one has to perform accelerated measurements. Thus, knowledge about accelerating factors is necessary to relate these results with measurements under real working conditions.
We show that the number of charge carriers extracted from an organic solar cell is a suitable measure to compare lifetime measurements on organic solar cells at different intensities. In detail, we use pin-devices with active layers containing a bulk heterojunction of Zinc-Phthalocyanine (ZnPc) and C60. These devices are subjected to extended lifetime measurements under constant monochromatic or white illumination at defined temperatures of 50°C or 90°C.
Firstly, we show that for the same spectra but different intensities, the number of extracted charge carriers is the relevant parameter for the degree of degradation. Secondly, our results show that for comparable intensities the energy of irradiated photons is significant for accelerated measurements. Both issues have major advantages for the realisation of accelerated lifetime measurements. Additionally, we find that not single charge carriers, but rather excitons cause the degradation of the observed solar cells. | 6 5 |
| 16:15 | Influence of the hole extraction layer on the air stability of polymer:fullerene solar cells Authors : Eszter Voroshazi 1,2; Bregt Verreet 1,2; Andrea Buri 3; Claudio Girotto 1; Robert Mueller 1; Daniele Di Nuzzo 4; Paul Heremans 1,2 1. imec – Organic Photovoltaics, Kapeldreef 75, B-3001 Leuven (Belgium) 2. ESAT, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, B-3001 Leuven (Belgium) 3. Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129 Turin (Italy) 4. Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven (The Netherlands) Affiliations : Resume : In this study, we elucidate the role of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the degradation of polymer:fullerene solar cells. Unencapsulated cells were exposed to air in a humidity-controlled environment while in dark. Combined opto-electrical characterizations (photovoltaic and spectral response, UV-vis and photo-induced absorption spectroscopy) prove that cell degradation is caused by a reduced carrier extraction. Systematic investigations of the various interfaces within the cell confirm that this reduction is caused by the oxidation of the low workfunction metal cathode. We prove that this oxidation results from a spontaneous reaction of the reactive metal with water, accelerated by the hygroscopic nature of PEDOT:PSS. We observed a similar degradation when using neutral-pH PEDOT:PSS, thus excluding the role of acidity. By optical microscopy we tracked the diffusion of water from the edges of the device toward its central part, and show a quantitative correlation between degradation in cell efficiency and active area loss due to oxidation. We further confirmed this mechanism by replacing PEDOT:PSS by a versatile MoO3 layer, with a ten-fold increase in device lifetime even with highly reactive metal cathodes. Last, we establish an exponential trend between device lifetime and humidity levels in ambient condition, from which we extract an acceleration factor essential for lifetime tests of organic solar cells and their encapsulation. | 6 6 |
| Device Physics & Lifetime Posters : Gilles Dennler & Dana Olson | ||
| 16:30 | Peculiarities of extraction current of photogenerated charge carriers Authors : G. Juška , N. Nekrašas, K.Genevičius Departament of solid state electronics Vilnius university Affiliations : Resume : Charge carrier mobility and recombination are the main parameters causing energy conversion efficiency of organic solar cells. The charge extraction by linearly increasing voltage (CELIV) method allows independent measurements of both charge carrier mobility and recombination.
In this presentation we will demonstrate numerically modelled and experimentally measured dependencies of CELIV current transients on the light absorption coefficient, the light intensity and the recombination rate. We found that the duration necessary to reach maximum of CELIV current transient vary more than 10 times dependently on above-mentioned parameters. Thus, the latter causes the large errors of evaluated values of charge carrier mobility, which should be corrected by newly derived formulas and numerical modelling results.
Also, the simple method to determine the coefficient of bimolecular recombination will be presented. | 7 1 |
| 16:30 | Photogeneration and recombination of charge pairs and free charge carriers in polymer/fullerene bulk heterojunction films Authors : G. Sliauzys, V. Gulbinas Center for Physical Sciences and Technology, Savanorių av. 231, LT-02300 Vilnius, Lithuania, K. Arlauskas Vilnius University, Sauletekio al. 9, Build. 3, LT-10222 Vilnius, Lithuania. Affiliations : Resume : Photogeneration and recombination of charge pairs and free charge carriers in poly-3 (hexylthiophene) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction has been studied at different PCBM concentrations by means of fluorescence spectroscopy, time-of-flight (ToF) and extraction of photogenerated charge carriers by a linearly increasing voltage (photo CELIV) techniques. Quenching of P3HT fluorescence by PCBM, has been used for the evaluation of the exciton dissociation efficiency. The quantum efficiency of free carrier generation has been evaluated from ToF current transients at different electric fields. The free charge carrier recombination properties have been estimated from photo-CELIV current transients at different delay times between laser ant voltage pulses. Obtained results shows better carrier photogeneration quantum efficiency, and lower geminate recombination rate in samples with higher PCBM concentration. | 7 2 |
| 16:30 | Charge extraction measurements of PCDTBT:PCBM and PCPDTBT:PCBM Solar Cells: Extraction limited photocurrent Authors : Zhe Li and Chris McNeill Affiliations : Resume : Voltage-dependent transient photocurrent measurements are employed to study the dynamics of charge extraction in PCDTBT:CPCBM and PCPDTBT:CPCBM bulk heterojunction solar cells. By measuring the photocurrent decay as a function of applied bias following illumination turn off, the voltage-dependence of charge extraction dynamics and extracted charge can be observed. In contrast to the P3HT/PCBM system, where the extraction dynamics slow as the voltage is changed from short-circuit to the maximum power point with an associated increase in the amount of extracted charge, both PDCTBT:PCBM and PCPDTBT:PCBM systems show an insensitivity of extraction dynamics to applied field. Furthermore, the amount of extracted charge decreases for both PCDTBT:PCBM and PCPDTBT:PCBM systems as the working voltage is swept towards open-circuit and exhibits a similar voltage-dependence as photocurrent. These observations indicate that transport is more dispersive in PCDTBT:PCBM and PCPDTBT:PCBM devices compared to P3HT:PCBM devices, with the dispersive transport attributed to charge trapping. The similarity between the voltage dependence of extracted charge and photocurrent also suggests that charge extraction (ie the collection of charge following charge separation) is limiting photocurrent in these two systems. The influence of additives on charge extraction dynamics will also be discussed. | 7 3 |
| 16:30 | Voltage-dependent external quantum efficiency measurements of polymer:fullerene solar cells: Comparing the separation of charges following hole transfer and electron transfer Authors : Thomas J.K. Brenner, Zhe Li, and Christopher R. McNeill Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Ave., Cambridge CB3 0HE, United Kingdom Affiliations : Resume : Here we investigate the external quantum efficiency spectra of both P3HT:PC60BM (poly(3-hexylthiophene) : [6,6]-phenyl-C61-butyric acid methyl ester), a well-studied photovoltaic system and PCPDTBT:PC70BM (poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b']dithiophene-2,6-diyl]] : [6,6]-phenyl-C71-butyric acid methyl ester), a low-bandgap system. Specifically we examine how changing background light intensity and applied bias voltage change their EQE spectra.
For voltages closer to the open-circuit voltage we observe a drop in EQE for both systems, consistent with decreased charge separation and/or collection efficiency. Upon normalization P3HT:PC60BM spectra overlap very well, showing no spectral changes when changing bias voltage or background illumination. However, for PCPDTBT:PC70BM we see voltage-dependent (but light intensity independent) changes in the shape of the EQE spectra with a relative drop in EQE coinciding with the PC70BM absorption band with decreasing internal electric field. This observation is attributed to a stronger electric field dependence of charge separation for electron-hole pairs created via hole transfer from fullerene to polymer compared to those created via electron transfer. Furthermore we show that this setup can also be used for accurate light intensity dependence measurements with advantages over the common steady-state technique. | 7 4 |
| 16:30 | Controlling Charge Transfer in Polymer Photovoltaics Using Donor-Acceptor Block Copolymers Authors : Simon P. King Saif A. Haque Department of Chemistry, Imperial College London Affiliations : Resume : Block copolymers have recently attracted attention in the field of organic photovoltaics [OPV] as they display several attractive properties including: Phase separation of the component blocks on length scales commensurate with exciton diffusion lengths in these materials, and an ability to self assemble into a range of different morphologies. Based on these qualities, block copolymers have been used as both templating agents and photoactive layers in OPV devices.
Our studies have focussed on a number of promising semi-conducting block copolymers, especially with regard to photo-physical effects (1). Spectroscopic and microscopy studies undertaken on thin films have been compared to show how morphology relates to efficiencies of photo-induced processes. Understanding such processes is essential to the continuing development of high efficiency bulk heterojunction polymer PVs. Photo-induced charge separation at a donor-acceptor interface is one such important process that we have examined. Spectroscopic data, such as photoluminescence and transient absorption spectroscopy, show more than an order of magnitude increase in long lived charges in block copolymers compared to a similarly composed blend.
(1) Charge separation and recombination in self-organizing nanostructured donor–acceptor block copolymer films - Simon King, Michael Sommer, Sven Huettner, Mukundan Thelakkat and Saif A. Haque
J. Mater. Chem., 2009, 19, 5436-5441 | 7 5 |
| 16:30 | Large area surface profiling applied for thermally induced morphology degradation study in polymer:fullerene solar cells Authors : Eszter Voroshazi 1,2; Claudio Girotto 1; Tom Aernouts 1; Paul Heremans 1,2 1. imec – Organic Photovoltaics, Kapeldreef 75, B-3001 Leuven (Belgium) 2. ESAT, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, B-3001 Leuven (Belgium) Affiliations : Resume : One of the major degradation mechanisms in bulk heterojunction polymer:fullerene solar cells is the instability of the nanoscale morphology under thermal stress. The effects of temperature on the phase segregation in P3HT:PCBM blend has been mostly investigated by in-plane imaging techniques, thus neglecting the impact of height variations. In this study, we introduce surface profiling over mm2 areas as a complementary tool and correlate the topography of thermally aged P3HT:PCBM films with the photovoltaic response of the devices. The technique reveals the impact of temperature on the fullerene crystal formation. While below 100 °C circular, nanometer size particles are formed, at 150 °C the crystallization mechanism is strongly accelerated and a dense network of 3-5 μm high circular pillars is developed. Interestingly, a metal layer on top of the films acts as height confinement, thus changing the preferential crystal growth from out-of-plane to in-plane and increasing the particle density. The effect is visible at all the temperatures and is particularly pronounced at 150 °C, with a crystal growth constrained into branched, needle-like shapes with 1 μm average height. Cross-section SEM images confirms the important implications on the interfaces induced by these height changes. In conclusion, we demonstrate that the technique, which can even be applied to in-situ measurements, provides quantitative and statistically relevant information for the analysis of crystal growth. | 7 6 |
| 16:30 | Analysis of Density of States and its Influence upon the Performance of Organic Photovoltaics Authors : George F.A. Dibb, James R. Durrant & Jenny Nelson (Imperial College London) Affiliations : Resume : Organic solar cells present an opportunity for the production of affordable photovoltaics, yet if they are to become commercially viable their power conversion efficiencies (PCE) must be improved. Progress has been made in understanding the way in which loss mechanisms influence device parameters, but there is no consensus within the field upon the role of charge recombination, both geminate and non-geminate, on the PCE.
We have recently shown that both the open circuit voltage and fill-factor of polymer:fullerene solar cells are largely dependent upon the amount of non-geminate recombination. Therefore understanding the microscopic origin and mechanism of this process is vital to improving cell performance. With the results of experimental techniques such as charge extraction and transient photovoltage we can show that this recombination process is influenced by the density of electronic states (DoS) within the active layer material. By extending these techniques to different temperatures and using a new numerical model incorporating trap sates, we are able to determine the approximate form of the DoS and thereby explain features such as the observed super-2nd-order recombination. Furthermore, knowledge of the DoS using such techniques enables a deeper understanding of charge transport phenomena in a range of organic electronic device applications. | 7 7 |
| 16:30 | Ultrafast exciton dynamics and quenching in polymer solar cells Authors : Luke Reynolds, Department of Chemistry, Imperial College London Annalisa Bruno, Department of Chemistry, Imperial College London Jenny Nelson, Department of Physics, Imperial College London Saif Haque, Department of Chemistry, Imperial College London Affiliations : Resume : Organic photovoltaic (PV) cells are becoming increasingly attractive as power generation modules, with recent devices reaching efficiencies above 8%. Despite this progress, there is still much work to be done in order to understand the energy losses within devices. Critical to maximising the power generation efficiency is an understanding of the dynamics of photo-generated species, and the impact they have on the current and voltage a PV device can generate.
Here we present recent spectroscopic work performed in our group using a fluorescence upconversion technique. This system allows us to probe the ultrafast (<1ps) excited state dynamics of photo-generated species; the mechanistic processes occurring on these timescales are little understood. One focus of our research has been to study a series of polymer/fullerene blends in order to elucidate the relationship between energetic driving forces and exciton dynamics and quenching rates. From the time-resolved photoluminescence of these systems we are able to extract the exciton diffusion length of the polymer. A second focus has been to study polymer/inorganic semiconductor blends in order to understand the role the morphology of a blend system has upon the excited state dynamics. When coupled with other spectroscopic techniques the exciton recombination dynamics of such systems allow quantitative design rules to be formalised which is essential for the continued development of highly efficient organic PV devices. | 7 8 |
| 16:30 | Charge Generation in polymer:PCBM Blend Solar Cells Authors : Fiona Jamieson, Department of Chemistry, Imperial College London James Durrant, Department of Chemistry, Imperial College London Affiliations : Resume : Organic photovolataic solar cells (OPVs) potentially offer a cheap alternative to inorganic devices due to their lower cost and ease of fabrication. Polymer blend bulk heterojunction device efficiencies are now exceeding 8% and continue to increase and may provide this alternative.
Transient absorption spectroscopy (TAS), a pump-probe technique has been employed to probe the charge generation in OPVs, which has been correlated to short circuit currents in devices. This technique has been applied to both thin films and devices. This has been coupled with alternative analysis of non-geminate recombination across the current-voltage (J-V) curve.
The primary focus of this work has been upon understanding the processes that affect charge generation in devices including investigating the effects of the active layer morphology and also the application of an externally applied electric bias.
The application of an external bias, up to -4V in PCPDTBT:PC70BM devices indicates the charge generation in these devices is no dependent on the external field. The morphology of the active layer, especially considering the phase purity and domain size, affects the charge generation efficiency. | 7 9 |
| 16:30 | Influence of doping on performance of organic solar cells Authors : V.A.Trukhanov, V.V. Bruevich, D.Yu. Paraschuk. International Laser Center and Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow 119991, Russia. Affiliations : Resume : Organic semiconductors that constitute the active layer of organic solar cells contain numerous defects and impurities, which can be charged and act as dopants. However, in modeling organic solar cells, the active layer is usually assumed to be undoped. Conventional numerical models account for generation, recombination, drift and diffusion of charge carriers, space charge effects, etc. We have extended these models for planar and bulk heterojunction solar cells by introducing dopants in the active layer and studied their performance upon n- and p-doping.
For bulk heterojunction cell with optimized parameters based on the most studied material pair (P3HT and PCBM) we have found that the efficiency decreases from 4.5% to 1% upon doping due to decrease of the short-circuit current and fill factor; the open-circuit voltage changes slightly. However, for non-optimized cells doping can increase efficiency. For example, in cases of low charge carrier mobilities, unbalanced mobilities and non-ohmic contacts the efficiency can be increased by several times upon doping.
For bilayer organic solar cells we show that doping of layers by majority carriers (i.e. the donor/acceptor layer is p/n-doped) increases the short-circuit current, open-circuit voltage, fill factor and efficiency. Upon doping a P3HT/PCBM bilayer cell, the efficiency can be increased by more than three times. Reversely, i.e. upon doping by minority carriers, the efficiency decreases and the I/V curve becomes S-shaped. | 7 10 |
| 16:30 | Electrochemically doped organic solar cells Authors : Jacek Gąsiorowski, Helmut Neugebauer, Niyazi Serdar Sariciftci Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, Altenbergerstrasse 69, 4040 Linz, Austria Affiliations : Resume : We present a systematic study on applying electrochemical doping in an organic polymer bulk heterojunction solar cell. The goal of the work is to obtain a p-i-n structure which is expected to have the advantage of a more efficient electron/hole extraction. Devices containing poly (3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as an active layer, and P3HT:PCBM mixed with solid state polymer electrolyte poly(ethylene oxide) (PEO) and lithium hexafluorophosphate (LiPF6) as conduction salt were compared. In addition, cells with and without additional electron/hole blocking layers were fabricated. The solar cells were characterized using different techniques. Electrochemical properties were measured using cyclic voltammetry (CV). Doped and un-doped solar cells were characterized using different spectroscopic and electrical techniques to study their physical and optical properties. Morphology and surface imaging was done using scanning electron microscopy (SEM) and atomic force microscopy (AFM). All cells were characterized by I-V measurements in dark and under illumination with a solar simulator (AM1.5). From our result we show that reproducible and efficient electrochemically doped solar cells without the need of additional electron/hole blocking layers can be obtained. | 7 12 |
| 16:30 | Doping as a new approach in promoting charge separation in polymer:fullerene solar cells Authors : Enrico Da Como1, Felix Deschler1, Elizabeth von Hauff2 and Jochen Feldmann1 1 Photonics and Optoelectronics Group, Department of Physics and CeNS Ludwig-Maximilians-University, 80799, Munich, Germany 2 Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany Affiliations : Resume : Charge separation at polymer:fullerene interfaces is the primary process for organic bulk-heterojunction solar cells. Following the ultrafast photoinduced charge transfer, electrons and holes are still bound because of Coulomb interactions, resulting in the formation of charge transfer excitons (CTEs). Those are known to play a crucial role in solar cells being responsible for the open circuit voltage and the short circuit current.
In this contribution, we propose a novel strategy to overcome some of the recombination channels due to charge transfer excitons. 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 F4TCNQ on the low-bandgap polymer PCPDTBT fill the tail of states in the highest occupied molecular orbital (HOMO) and in addition are expected to screen Coulomb interactions of CTEs at the interface between PCPDTBT and the fullerene derivative PCBM. By the unique combination of time resolved photoluminescence and photoinduced absorption spectroscopy, we demonstrate how doping results in a decreased population of CTEs and enhanced formation of free carriers. The spectroscopy experiments are complemented by measurements on solar cells showing an overall increase of more than 30% in the power conversion efficiency. | 7 13 |
| 16:30 | 2D polarons in the electronic structure of P3HT Authors : M. Richter, D. Schmeißer Brandenburg University of Technology Cottbus, Applied Physics and Sensors, K.-Wachsmann-Allee 17, 03046 Cottbus, Germany Affiliations : Resume : In this contribution we report on investigations of the electronic structure and optical characteristics of poly(3-hexylthiophene) (P3HT) based films. X-Ray based measurements were done at the U49/2-PGM2 beam line at BESSYII in Berlin. The samples were produced by spin casting P3HT (regioregular and regiorandom) dissolved in chloroform and other suitable solvents. As substrates we used ITO (indium tin oxide) coated glass slides and silicon substrates. Regioregular-P3HT is known to form thin films with nanocrystalline lamellae. This is verified for our samples by XRD measurements. In photoinduced absorption measurements this lamellae structure gives rise to additional electronic excitations, which are attributed to 2D-polarons [1]. In the Total Electron Yield (TEY, surface sensitive) and Total Fluorescence Yield (TFY, bulk sensitive) NEXAFS spectra of regioregular-P3HT three additional features can be found prior to the first typical π* excitation which are absent in the spectra of regiorandom-P3HT. The energy differences relative to π*-peak maximum (0.54eV, 0.87eV and 2.24eV) suggest that these features are due to 2D-polarons. Further investigations performed by means of SRXPS, UPS, UV-VIS and FTIR measurements support the observation of the presence of polarons.
[1] Osterbacka et al., Synth. Met. 116 (2001) 317 | 7 14 |
| 16:30 | CHARGE CARRIER SEPARATION AND TRANSPORT DYNAMICS IN P3HT:PCBM BLENDS Authors : D. AmarasingheVithanage1, Y. Infahsaeng1, A.Yartsev1, V. Sundstrom1, A. Devizis2, P. Keivanidis3, S. Foster3, A. Bruno3, J. Nelson3, and V. Gulbinas2 1 Lund University Box 117, 221 00 Lund, Sweden. 2Center for Physical Sciences and Technology, Savanoriu 231, Vilnius 02300, Lithuania 3 Imperial College London, South Kensington Campus, London SW7 2AZ Affiliations : Resume : Blends of conjugated polymers and fullerene derivatives are the key materials for fabrication of plastic solar cells. Close to 100% efficiency of the exciton dissociation into charge pairs and reduced charge carrier recombination determine relatively high efficiency of the blend-based devices. Here we investigate carrier mobility and the corresponding drift dynamics in a well known blend combination, Poly (3-Hexylthiophene) (P3HT) and soluble fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We look at the carrier transport with a a TREFISH method previously used on a neat polymer devices []. This technique based on an electric field induced second harmonic (EFISH) generation allows to bridge the time scales
between measurements such as CELIV and time of fight (in the microsecond regime) to terahertz and transient absorption (to the picosecond regime). Initial mobility at 2 ps was of the order of 10-2 cm2/Vs, which reduced. to the order of 10-4 cm2/Vs at 2.5 ns for high voltages. The energy produced.due to excitation result in a high mobility immediately after excitation, which is reduced in time due to thermal dissipation. The recorded time of fight measurements show that the mobility continues to further reduce to the order of 10-8 cm2/Vs.
1. A. Devižis, A. Serbenta, K. Meerholz, D. Hertel and V. Gulbinas, Phys. Rev. Lett. 103, 027404 (2009). | 7 15 |
| 16:30 | Optical properties of P3HT and PCBM under pressure: Improving charge transfer properties in organic solar cells Authors : Malte SCHMIDT, M. Isabel ALONSO, Alejandro R. GOÑI, Mariano CAMPOY-QUILES Affiliations : Resume : Charge transfer and charge separation are two of the main processes involved in the generation of electricity in organic solar cells. This issue is commonly investigated by selecting a range of materials with different energy levels or by changing the stoichometry in bulk heterojunctions, both of which modify the microstructure of the blends and thus makes comparisons inaccurate.
Here, we propose the use of hydrostatic pressure as a "clean" way to investigate these important phenomena. By this technique solvatochromic effects, possible changes in the conjugation length and higher interchain interaction are all accessed through the optical properties. The long term goal is to understand the effects in such a way that optimisation of the charge transfer and charge separation under real working conditions can be performed systematically.
The optical properties of the widely used conjugated polymer poly (3-hexylthiophene) (P3HT) and the fullerene derivative PCBM (often used as an electron acceptor material) were investigated. The photoluminescense (PL), absorption and Raman signal were measured between ambient pressure and about 8 GPa at room temperature using a diamond anvil cell. Pure samples and blends were measured to distinguish between the different contributions to effects like the shift to lower energies of PL peaks and the quenching of the PL intensity. Preliminary but very promising results will be presented | 7 16 |
| 16:30 | Single-chamber deposition of multilayer barriers by Plasma Enhanced and initiated CVD of organosilicones Authors : Anna Maria Coclite, Karen K. Gleason Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Affiliations : Resume : Protective coatings which prevent the ingress of water into OPV devices fabricated on flexible plastic substrates is essential to extend the device lifetime. Widely investigated barrier protective coatings are made of multilayer stacks where multiple dense, inorganic layers are alternated with soft, organic ones. The possibility of a single-chamber system may greatly simplify the production and allows the quicker and cheaper roll-to-roll deposition.
A new technique consisting of coupling initiated CVD (iCVD) and plasma enhanced CVD (PECVD) was investigated for the multilayer deposition, maintaining the same precursor and the same reactor configuration. Multilayer coatings of alternating silica-like and organosilicon layers were deposited using hexavinyldisiloxane (HVDSO) as precursor.
Chemical and morphological characterization of the organic layer showed that the iCVD of HVDSO resulted in a very crosslinked film with high Carbon content (79 % from XPS analysis) low roughness (0.7 nm) and planarizing properties. PECVD of the same monomer, highly diluted in Oxygen, gave inorganic coating with a low content of OH terminal groups. When the inorganic layer was deposited over the organic layer a graded interphase (around 40 nm) was detected, due to plasma ion bombardment. The C-rich interlayers effectively decoupled the defects of the C-depleted layers, therefore a barrier improvement factor of 100 over the single C-depleted barrier layer was obtained with a hexalayer. | 7 17 |
| 16:30 | Ultrafast Dynamics of Singlet Exciton Fission in Polycrystalline Pentacene Authors : Mark W.B. Wilson*(1), Akshay Rao(1), Jenny Clark(1), R. Sai Santosh Kumar(2), Daniele Brida(3), Giulio Cerullo(3) and Richard H. Friend(1) 1) Optoelectronics Group, Cavendish Laboratory, University of Cambridge, Cambridge, U.K. 2) Center for Nano Science and Technology, IIT@PoliMi, Milano, Italia 3) National Laboratory for Ultrafast and Ultraintense Optical Science—INFM-CNR, Dipartimento di Fisica, Politecnico di Milano, Italia Affiliations : Resume : We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet exciton pairs on a ~80 fs timescale independent of the excitation wavelength. The singlet population is unambiguously identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse. This violation of Kasha’s Rule that confirms that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes, and may proceed directly from an initial delocalized state. This is consistant with the spectral shape of the stimulated emission, which suggests superradiant emission from a state delocalized over several pentacene molecules. This direct demonstration that triplet excitons are efficiently produced via fission in polycrystalline pentacene films provides an attractive route to achieve multiple exciton generation in organic solar cells. | 7 18 |
| 16:30 | Numerical model and parameter fitting for more insight into BHJ organic solar cell operation Authors : Beat Ruhstaller, Benjamin Perucco, Evelyne Knapp, Felix Müller, Martin Neukom, Simon Züfle, Nils Reinke — Zurich Univ. of Applied Sciences, Institute of Computational Physics, Technikumstr 9, 8401 Winterthur, Switzerland; Fluxim AG, c/o ZHAW, 8401, Winterthur, Switzerland Affiliations : Resume : The characterization of solar cell parameters is an essential part in the process of improving the performance and lifetime of organic solar cells. Since many parameters are not easily accessible, dedicated measurement techniques in combination with numerical simulations are needed for their determination. We numerically simulate common characterization techniques such as charge extraction transients by linearly increasing voltage (CELIV), steady-state current-voltage curves and spectral photocurrent response with a fully-coupled opto-electronic model that seamlessly calculates light penetration, photocarrier generation as well as charge extraction [Häusermann et al., J. Appl. Phys.; 106, 10450 (2009), Neukom et al., Proc. SPIE 7722 30, (2010)]. The transient and steady-state model also considers the series resistance of the setup. For a couple of bulk heterojunction solar cells we use this model to fit several experimental datasets as a function of light intensity and temperature simultaneously. Thus we can estimate basic material parameters like the charge carrier mobility, recombination coefficient, photon-to-charge conversion efficiency and injection barriers. The mathematical quality of the fits is assessed by a covariance analysis and the determination of confidence intervals for the fitted parameters. | 7 19 |
| 16:30 | The mechanism of ultrafast exciton dissociation in neat regioregular poly(3-hexylthiophene) Authors : Gianluca Latini,(1,3) Francis Paquin,(2) Paul-Ludovic Karsenti,(2) Sakowicz Maciej,(2) Natalie Stingelin (3) and Carlos Silva (2) 1) Centre for Biomolecular Nanotechnologies, Italian Institute of Technology (IIT), Via Barsanti, 73010 - Arnesano (Lecce) – Italy 2) Departement de physique & Regroupement québecois sur les matériaux de pointe, Université de Montréal, C.P. 6128, Succursale centre-ville Montréal (Québec), H3C 3J7, Canada 3) Department of Materials and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom Affiliations : Resume : Semiconductor polymer films with high supramolecular order display high apparent intrinsic charge photogeneration yields by a mechanism that has not been unravelled by contemporary literature. We address charge photogeneration dynamics in neat regioregular poly(3-hexylthiophene) by means of transient photoluminescence (PL) and absorption spectroscopies at 10 K. Charges are generated over ultrafast timescales, but furthermore over timescales that compete with the exciton lifetime (~1 ns) by a mechanism that excludes diffusion-limited exciton dissociation. They recombine with distributed rates that lead to delayed PL over microsecond timescales, accounting for >12% of the time-integrated emission intensity in the studied lm microsctructure. By analyzing the spectral bansdhape of delayed PL, we conclude that charges recombine primarily at interfaces between photophysical aggregates comprised of lamellar polymer stacks and electronically uncoupled chains. This highlights the extrinsic role of boundaries between domains of contrasting microstructure in charge photogeneration processes in this important class of materials. | 7 20 |
| 16:30 | Photocurrent in bulk heterojunction solar cells Authors : Markus Mingebach 1 , Carsten Deibel 1, and Vladimir Dyakonov 1,2 - 1: Experimental Physics VI, Physical Institute, Julius-Maximilians-University of Würzburg, Am Hubland, D-97074 Würzburg - 2: Bavarian Centre of Applied Energy Research e.V. (ZAE Bayern), Am Hubland, D-97074 Würzburg Affiliations : Resume : In organic solar cells basic processes such as polaron pair dissociation and its influence on the voltage dependent photocurrent are not fully understood yet. We investigate solar cells from poly(3-hexyl thiophene) (P3HT) and 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)
blended with fullerene derivatives by means of temperature dependent pulsed photocurrent and charge extraction measurements. We identify a point of optimal symmetry (POS) that represents the case of quasi flat bands (QFB) in the bulk of the solar cell.[1] It differs from the built-in potential by diffusion-induced band-bending at the contacts. We model the photocurrent data by a modified Hecht equation, accounting for experimental voltage dependent charge carrier concentration. Thus, we are able to explain the impact of field dependent charge generation and recombination on the photocurrent of organic solar cells.
[1] M. Limpinsel, A. Wagenpfahl, M. Mingebach, C. Deibel and V. Dyakonov, Phys. Rev. B 81, 085203 (2010). | 7 21 |
| 16:30 | Effect of the active layer thickness on the device performance of polymer solar cells having [60]PCBM and [70]PCBM as electron acceptor Authors : P. Morvillo*, E. Bobeico, S. Esposito, R. Diana, ENEA, P.le E. Fermi, 1, 80055 Portici (NA) Italy Affiliations : Resume : Polymer solar cells have evolved as a promising cost-effective alternative to silicon-based solar cells. One of the problems for the use of such devices in commercial applications is the relatively small device efficiency that have been reached up to now. The short circuit current (Jsc) of these cells is strictly related to the absorption of the blend film. Recently it has been shown that the use of [70]PCBM as electron acceptor can improve the current output of such device because C70 derivatives have a stronger and broader absorption compared to C60 ones.
The aim of this work is to study the influence of the fullerene on the optical behaviour of the photoactive blend film of a polymer solar cell. We have determined the optical constants of polymer blend film based on P3HT and [60]PCBM or [70]PCBM. We also studied how different preparative conditions (solvent, annealing temperature, ratio between the components) influence the optical constants of such blend. We simulated the optical absorption of the active layer inside the device structure and we calculated the maximum achievable Jsc with the aim to correlate the variation of the optical constants to the output current of the device. Afterward, we realized several polymer solar cells (glass/PEDOT:PSS/P3HT:[60 or 70]PCBM/Al) varying the thickness of the blend film in order to prove the theoretical findings. | 7 22 |
| 16:30 | Probing charge transfer excitons in organic bulk heterojunction and bilayer solar cells Authors : Koen Vandewal, Kristofer Tvingstedt, Christian Muller, L. Mattias Andersson and O. Inganäs Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology, Linköping University, 58183 Linköping, Sweden Affiliations : Resume : Properties of the donor/acceptor (D/A) interface in organic bulk heterojunction and bilayer photovoltaic devices are probed directly by highly sensitive measurements of their photocurrent and electroluminescence spectra. For all material combinations exhibiting a decent charge carrier generation, weak absorption and emission involving a charge transfer (CT) state with energy ECT lower than the lowest optical gap of both donor and acceptor is observed [1,2]. We find that the internal quantum yields of photocurrent generation are similar for both pure phase excitations and direct excitation in the low energy part of the CT absorption band, indicating that properties of this relaxed CT state will determine the free charge carrier generation rate.
Also the open-circuit voltage (Voc) is related to ECT: when measured under solar illumination, we find an energetic difference between ECT and qVoc of ~0.6 eV. About 0.25 eV of this energetic loss is due to the radiative recombination through the CT state. The remaining ~0.35 eV is due to additional, non-radiative recombination mechanisms [4,5].
In order to optimize Voc, material combinations with ECT close to the optical gap of the main absorber (Eg) are desirable. A range of D/A combinations with different Eg-ECT values are investigated and possible detrimental effects on photocurrent generation are discussed.
[1] K. Vandewal et Al., Adv. Funct. Mater. 18, 2064 (2008). [2] K. Tvingstedt et Al., J. Am. Chem. Soc. 131, 11819 (2009). [3] J. Lee et Al., J. Am. Chem. Soc. 132, 11878 (2010) . [4] K. Vandewal et Al., Nature Mater. 8, 904 (2009). [5] K. Vandewal et Al., Phys. Rev. B 81, 125204 (2010). | 7 23 |
| 16:30 | Influence of Side-Chains on Geminate and Bimolecular Recombination in Organic Solar Cells Authors : Sylvain Massip(1), Philipp M. Oberhumer(1), Guoli Tu(2,3), Sebastian Albert-Seifried(1), Wilhelm T. S. Huck(2,4), Richard H. Friend(1), and Neil C. Greenham(1) (1)Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Ave., Cambridge, CB3 0HE, United Kingdom (2)Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge,CB2 1 EW, United Kingdom (3)Present address: Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road, Wuhan, 430074, China (4)Present address: Huygens building, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands Affiliations : Resume : We have recently reported the synthesis, photophysics and photovoltaic performances of a new family of polymers differing only by side-chain presence and position(1).
Here, we study the influence of the side-chains on the loss processes in these three polymers blended with PCBM by comparing transient absorption spectroscopy and solar cell measurements.
It is shown that, after photoinduced charge transfer, charge pairs are the only significant species present in the blend and that their recombination can be followed by monitoring the PIA kinetics.
Their decay follows two regimes: the sub-nanosecond decay of the charge signal, responsible for the losses of 60% to 80% of the charges is well described by an Onsager-Braun geminate recombination model. The rates are found to be strongly affected by the side-chains.
After about 10 ns, the charges undergo bimolecular recombination. By modelling the time and intensity dependence of this process, we measure the rate constant and find that it is similar for the three systems and slightly lower than the theoretical Langevin rate.
By comparing these results with the intensity and bias dependence of the photocurrent, we conclude that under solar intensity, geminate recombination is the main loss mechanism and explains the difference between all the devices, their strong bias-dependence and their generally low performance.
(1) Tu, G. et al. J. Mater. Chem. 2010, 20, 9231 | 7 24 |
| 16:30 | Photo-induced charge carriers in intrinsic P3HT diodes Authors : S.Schlager, P.Stadler, H.Neugebauer and N. S. Sariciftci Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University of Linz, A-4040 Linz, Austria Affiliations : Resume : We show result on photo-induced charge carriers in intrinsic P3HT-diodes using a modified contact electrode. P3HT is well known for acting as electron donor in organic bulk-heterojunction solar cells with favourable absorption properties in the visible range of light. It can be doped, e.g. by simple air exposure or by molecular doping adding charge transfer complexes such as F4TCNQ [1]. These p-doped structures show the creation of a Schottky barrier, which has been utilized for the realization of photovoltaic devices [2].
In this study we show an alternative pathway to induce charge carriers in order to form a Schottky barrier between P3HT and the contact electrode. Instead of molecular bulk doping we apply fluorinated alkyl-chains to the ITO electrode and put purified intrinsic polymer on top. These samples are capped with different low work-function metals. Capacitance measurements at different electrical field and light intensities were recorded. The results from the dielectric spectroscopy of these structures show the creation of photo-induced carriers. It is expected that this idea can be applied to the elaboration of a photo-induced Schottky-barrier and further in photovoltaic devices as well. | 7 25 |
| 16:30 | Degradation effects related to the hole transport layer in organic solar cells Authors : Bernhard Ecker (1), Jairo Cesar Nolasco (2), Josep Pallarés (2), Lluis Francesc Marsal (2), Jörg Posdorfer (3), Jürgen Parisi (1), and Elizabeth von Hauff (1) (1) Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky University, 26111 Oldenburg (Germany) (2) Departament d’Enginyeria Electrònica Elèctrica i Automàtica, Universitat Rovira i Virgili, Avda. Països Catalans 26, 43007 Tarragona (Spain) (3) Enthone Nano Science Centre, Ormecon GmbH, Ferdinand-Harten-Str. 7, 22949, Ammersbek (Germany) Affiliations : Resume : We discuss the influence of the hole transport layer on the device stability in organic bulk-heterojunction solar cells. Two water based hole transport layers, poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) and polyaniline : poly(styrene sulfonate) (PANI:PSS), and one isopropyl alcohol based PANI:PSS transport layer were investigated. Solar cells were prepared with the three different hole transport layers and degraded under illumination. Current-voltage, capacitance-voltage, and capacitance-frequency data were collected at varying light intensities over a period of 7 hours. Solar cell performance and stability were compared between non encapsulated and encapsulated samples to obtain understanding about degradation effects related to oxygen and water as well as degradation mechanisms related to the intrinsic instability of the solar cell materials and interfaces. We show that the properties of the hole transport layer can have a significant impact on the stability of organic solar cells. | 7 26 |
| 16:30 | Mechanisms of Oxygen induced Degradation in Polymers and Fullerenes Authors : Andreas Sperlich 1, Hannes Kraus 1, Carsten Deibel 1, Vladimir Dyakonov 1,2 1, Experimental Physics VI, Julius-Maximilians University of Würzburg, 97074 Würzburg, Germany 2, Bavarian Center for Applied Energy Research (ZAE Bayern), 97074 Würzburg, Germany Affiliations : Resume : Understanding of degradation mechanisms in polymer:fullerene bulk heterojunctions on a microscopic level, aimed at improving their intrinsic stability, is crucial for the establishment of OPV technology. The blends of conjugated polymers and fullerenes are one of the most promising material approaches to date. These materials are vulnerable to simultaneous exposure to light and oxygen, thus involving the electronic excitations. To unambiguously probe the excited states of various multiplicity and their reactions with oxygen, we applied combined magneto-optical methods based on electron spin resonance (ESR), photoluminescence and optical detection of ESR (ODMR) to polymer P3HT and polymer-fullerene bulk heterojunctions (P3HT:PCBM). We studied the light-induced formation of polymer:oxygen charge transfer complexes in oxygen exposed P3HT and simultaneously monitored the evolution of the photoluminescence intensity. We were able to identify two separate photochemical reaction routes leading to degradation and corroborate them with ODMR measurements, which sensitively probe strongly bound triplet excitons as well as weakly bound charge transfer states. In this context we discuss the protective effect of PCBM on the polymer through efficient triplet quenching. | 7 27 |
| 16:30 | Triplet Exciton Generation and Electron Back Transfer in Bulk-heterojunctions with Endohedral Fullerenes Authors : M. Liedtke (a), A. Sperlich (a), C. M. Cardona (b), V. Dyakonov (a) (a) Faculty of Physics and Astronomy, Julius-Maximilian Wuerzburg University, Germany (b) Luna Innovations Incorporated, 521 Bridge Street, Danville, VA 24541, USA Affiliations : Resume : To increase the power conversion efficiency of organic solar cells it would be extremely advantageous to improve harvesting of singlet, triplet as well as charge transfer (CT) excitons. Decay of CT states into free charge carriers or, alternatively, electron back transfer reactions depends on the energy levels of constituting donor and acceptor molecules, on microscopic structure of blends but also on the relative spin orientation of charges carriers within the e-h pair. Organic bulk-heterojunctions and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 show an open circuit voltage 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PCBM) but slightly lower short circuit current. Here we address factors influencing the photocurrent in Lu3N@C80 blends by applying spin sensitive photoluminescence detected spin resonance technique. On the whole, the blends with Lu3N@C80 show the photo- and electrophysical behaviour resembling the P3HT:PCBM blends. Instead, triplet excitons were found in P3HT when blended with Lu3N@C80, which were completely absent in P3HT:PCBM blends. We show that electron back transfer to the triplet state of the polymer along with the lower exciton dissociation yield are responsible for the reduced photocurrent. | 7 28 |
| 16:30 | Charge carrier transport properties in derivative of triphenylamine Authors : G. Juška Jr., K. Genevičius, G. Juška, K. Arlauskas Department of Solid State Electronics, Vilnius University, Saulėtekio 9, build. 3, LT-10222 Vilnius Lithuania M. Daškavičienė, V. Getautis Department of Organic Chemistry, Kaunas University of Technology, Radvilėnų Rd. 19, LT-50254 Kaunas Affiliations : Resume : New hole transporting material - derivative of triphenylamine 4,4`-bis(2,2-diphenylethenyl)-4``-methyltriphenylamine (TPA) has been synthesized. Layers of TPA have been prepared by solution casting and thermal evaporation techniques; transport properties have been studied using time of flight (TOF) and charge extraction by linearly increasing voltage (CELIV) methods. Both deposition techniques demonstrates good quality films with hole drift mobilities approximately 3x10-3 cm2/Vs and 6x10-3 cm2/Vs for solution casted and thermal evaporation, respectively. Mobility dependencies on electric field and temperature have been established. Charge carriers transport properties of the samples of TPA and fullerene blend, TPA and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) blend and TPA with porous titanium dioxide coated with dyes as possible applications for solar cells have been studied. | 7 29 |
| 16:30 | S-shaped I-V Curves – Studies on Degraded APFO3:PCBM Solar Cells Authors : Mathias Nyman, Harri Aarnio and Ronald Österbacka Åbo Akademi University Department of Natural Sciences/Physics and Center for Functional Materials Sebastian Mühlbach and Uli Würfel University of Freiburg FMF and Fraunhofer ISE Affiliations : Resume : We present optical and charge transport studies on APFO3:PCBM solar cells. Using different charge extraction techniques we have compared the charge transport between APFO3:PCBM devices subject to fast degradation and pristine devices. The films have also been studied using photo induced absorption (PIA).
The degraded devices show a decrease in both the short circuit current and the fill factor. The open circuit voltage is not significantly affected. The decrease in short circuit is most probably correlated to the observed bleaching in the optical absorption around 500-630 nm. The PIA spectrum of a degraded film show a bleaching in the 1,8-2,0 eV range. A similar bleaching is also seen in pure APFO3 PIA spectra which might indicate that charge transfer to PCBM is limited in the degraded APFO3:PCBM device. The lowered fill factor is attributed to a screening of the electric field by a charge reservoir caused by either unbalanced charge transport or a lowered extraction rate at either contact. Photo-CELIV measurements show that the ambipolar mobility is lower by up to a factor of 2 in the degraded devices. The ambipolar mobility as measured by photo-CELIV is dominated by the faster charge carrier meaning that the slower charge carrier might have a significantly lower mobility in the degraded devices. Intensity dependent charge extraction studies show that both the pristine and degraded devices saturate at roughly the same concentration of charges (though slightly higher for the pristine case). The difference in the amount of extracted charge is larger at lower intensities (close to one order of magnitude).
We conclude that fast degradation under oxygen and light has caused bleaching of the optical absorption and unbalanced charge transport resulting in a weaker fill factor and a lower short circuit current. | 7 30 |
| 16:30 | Mechanism of photooxidation of C60 and PCBM Authors : Sylvain Chambon(1,2), Aurélien Tournebize(1,2), Aurélie Dupuis(1,2), Agnès Rivaton(1,2), Luc Gardette(1,2) (1) Clermont Université, Université Blaise Pascal, LPMM, BP 10448, F-63000 Clermont-Ferrand, France (2) CNRS, UMR 6505, LPMM, BP 80026, F-63171 Aubière, France Affiliations : Resume : Efficiency and stability targets are quoted as the threshold for commercialization of organic solar cells. The degradation of the active layer of the devices under outdoor conditions is well-known to be a pathway involved in the loss of the performances. The current state-of-the-art active layers of organic solar cells are based on conjugated polymers as electron-donors and soluble derivatives of fullerene (C60) as electron-acceptors. Over the past few years, the benchmark acceptor material for polymer solar cells has been PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). It is well established that the degradation of the conjugated polymer is mitigated by PCBM as it acts as a radical scavenger. Less known, the photodegradation of PCBM has been reported to be responsible for the deterioration of the photoconductivity. This work thus aims at elucidating the mechanism of photooxidation of PCBM. To identify chemical changes after aging, degraded samples were analyzed by X-ray Photoelectron Spectroscopy and by infrared spectroscopy associated derivatization treatments. In addition, in order to determine whether the mechanism of PCBM photo-oxidation is specific to PCBM or inherent to the fullerene cage, changes in the structure of PCBM were compared to those of irradiated C60 samples. Based on the photoproducts identification, this presentation will discuss about the main routes involved in the photodegradation of PCBM and C60. | 7 31 |
| 16:30 | Reproducibility issues of polymer solar cells to afford mass production: is it possible ? Authors : Aurélie Dupuis(1,2), Ghislain Boloma(3), Agnès Rivaton(1,2), Johann Bouclé(3), Luc Gardette(1,2), Bernard Ratier(3). (1)Clermont Université, Université Blaise Pascal, LPMM, BP 10448, F-63000 Clermont-Ferrand, France (2)CNRS, UMR 6505, LPMM, BP 80026, F-63171 Aubière, France (3)XLIM-UMR CNRS 6172, MINACOM, Faculté des Sciences et Techniques de Limoges, 87060 Limoges Cedex, France Affiliations : Resume : The most studied donor-acceptor and model couple of polymer solar cells is P3HT:PCBM able to afford devices with almost 5.5% power conversion efficiency. Knowing that the active layer morphology is a key factor for the efficiency of organic solar cells, there are on one hand dramatic effects of thermal annealing processes, between 75 and 230 °C, on the morphology of the P3HT:PCBM active layers. On the other hand, recent studies have evidenced that dispersity, molecular weight and regioregularity of P3HT strongly influence the active layer morphology and consequently the performances. The optimisation of the active layer morphology is time-consuming, and could be laborious if it has to be done each time a new batch of P3HT is used. We made a screening of four P3HTs (Mw ~ 50 000) in order to answer four questions: Can the optimised annealing temperature for one P3HT be transferred to the others? Can the optimized donor-acceptor blend formulation in the liquid phase be generalized to any P3HTs? Is there a good reproducibility of P3HT batches with time? Is the photostability of all the P3HTs the same ? This study allowed us to evidence which structural parameter of the P3HT must be controlled before the annealing treatment of the active layer, and which one is crucial for the stability of P3HT. This presentation will bring detailed answer to these basic questions and will be illustrated by results issued from both material and device characterizations. | 7 32 |
| 16:30 | Charge carriers trap distribution in organic photovoltaics explored by LESR. Authors : Lucia Bonoldi a,*, Claudio Carati a, Luciano Montanari a, Chiara Carbonera b, Riccardo Po’ b. a Eni, Refining and Marketing Division, Research Centre, via F. Maritano 26, I-20097 S.Donato Milanese, Italy b Research Center for non Conventional Energies Istituto Eni Donegani, ENI S.p.A, Via Fauser 4, 28100 Novara (Italy) * Corresponding author. Address: Refining and Marketing Division, Research Centre, via F. Maritano 26, I-20097 S.Donato Milanese, Italy Tel +02-520-46678; fax +02-520-35347; e-mail: lucia.bonoldi@eni.com Affiliations : Resume : The existence of a tail of trap states within the band gap is requested in the analysis of charge transport in organic photovoltaics, and information about traps concentration and energy distribution is needed. A method is proposed to study the binding energy distribution of charge traps by Light-induced Electron Spin Resonance (LESR). The persistent signal at temperature T after photo-excitation is due to the spins of the charges trapped after diffusion. By increasing T, traps with increasing binding energy are depleted so that informations about the traps binding energy distribution can be obtained.
Our method is applied to four poly(3-alkylthiophene) (P3AT) based film: a spin coated regio-regular poly(3-hexylthiophene)(rrP3HT):PCBM film (A) and drop casted poly(3-dodecylthiophene)(rrP3DDT:PCBM):PCBM (B), pure rr-P3HT (C) and pure rr-P3DDT (D) films, showing different spectroscopic behaviour. Films with lower persistent signals show faster decay kinetics of the prompt phase, as it occurs in the blend film deposited by casting with respect to the spin-coated one. In sample B the activation energy for detrapping was measured by kinetic measurements. The Arrhenius plot of the polaron decay rate in the temperature range 240-120 K is linear, yielding an activation energy of the order of 83 meV in fair agreement with the literature and of the same order as obtained by both fractional thermally stimulated current and PIA decay measurements in PAT:PCBM films. | 7 33 |
| 16:30 | The effect of ionic doping on the efficiency of polymer solar cells Authors : A. Kumar, G.J. Beirne, N. C. Greenham, Cavendish Laboratory, Department of Physics, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom. Affiliations : Resume : One of the main loss mechanisms limiting the efficiency of polymer solar cells is the geminate recombination of electrons and holes at the polymer/polymer interface. In fact, the low dielectric constant of typical organic materials is responsible for the strong Columbic attraction between the electron and hole. One possible strategy to improve photovoltaic efficiency is to enhance the charge separation by introducing ions as dopants in the polymer. The electrostatic field generated by the ions can then screen the Columbic attraction between electron and hole at the interface. Additionally, the mobile ions can migrate under the influence of an applied electric field and can furthermore give rise to n or p doping of the polymer.
We have investigated the F8BT:PFB donor-acceptor polymer system by introducing a lithium triflate: polyethyleneoxide mixture as the ionic dopant. The active layer was successfully doped via a solution processing method and bulk heterojunction and bi-layer photovoltaic devices were fabricated. The photovoltaic performance and optoelectronic properties were also characterized. Electro-absorption measurements indicated the presence of a lower built-in voltage in the doped devices, which may be attributed to the migration of ions. | 7 34 |
| 16:30 | Photoconductive-AFM study of bulk heterojunction organic solar cells Authors : G. Rivière, W. Vervisch, JJ. Simon, L. Escoubas, H. Derbal, F. Bassani Im2np Aix-Marseille Université & UMR CNRS 6242, Marseille, FRANCE Affiliations : Resume : Nanometric control of bulk heterojunction solar cells (BHJ OSC) active layer is important for organic solar cells efficiencies improvement [1]. Atomic Force Microscopy is a suitable tool for thin films study and moreover for local electrical characterization when the Conductive-AFM (CAFM) and Photoconductive-AFM (pC-AFM) modes are used. In this work, CAFM measurements were performed in air using a Veeco’s Dimension 3100 microscope equipped with a TUNA measurements electronics module and a green laser source (532nm). The investigated samples were solar cells based on a poly(3-hexylthiophene) P3HT as the donor blended with a C60 derivative (PCBM) as the acceptor.
In order to understand the electrical behavior of each layer, current-voltage measurements were performed on the active layer, on the ITO and PEDOT:PSS layers and also on test structures (PEDOT:PSS/ITO - P3HT/PEDOT:PSS/ITO – PCBM/PEDOT:PSS/ITO …).
In the dark and under positive bias, the local I-V curves obtained on P3HT/PCBM blend are heterogeneous. It is shown that this heterogeneity, in terms of conductivity, could not be related only to PCBM or P3HT domains (AFM maps) but to the distribution of the two materials in the blend thickness . Additionaly, the gains in current densities observed under illumination are also discussed. | 7 35 |
| 16:30 | Investigation of Field-dependent Charge Carrier Generation and Recombination in Polymer Based Solar Cells by Transient Extraction Currents Authors : Juliane Kniepert, Marcel Schubert, James Blakesley, Dieter Neher University of Potsdam, Institute of Physics and Astronomy, 14476 Potsdam, Germany Affiliations : Resume : There is an ongoing discussion as to whether photoinduced charge transfer in P3HT:PCBM solar cells leads to fully separated electrons and holes, independent of an electric field, or Coulombically bound interfacial charge pairs. While recent studies by R.A. Marsh et al. with transient absorption spectroscopy gave clear evidence for the formation and field-induced dissociation of bound polaron pairs, measurements by I.A. Howard et al. were in favour of hot exciton dissociation.
Here, we present the results of bias-dependent Time Delayed Collection Field (TDCF) measurements to access directly the density of free charge carriers in P3HT:PCBM blends coated from dichlorobenzene. Solvent annealing was applied to yield a phase-separated morphology and the corresponding solar cells exhibit high values for the external quantum efficiency and fill factor. Our setup allowed us to follow the generation and recombination of photogenerated charges with a so far unattained time resolution of 40 ns.
Our experiments show that the number of collected carriers is independent of the applied bias during pulsed illumination implying that extractable carriers in P3HT:PCBM blends are not generated by the field-assisted separation of bound polaron pairs. In addition, our experiments support the view that bimolecular recombination of free carriers is strongly suppressed in phase-separated P3HT:PBCM blends. | 7 36 |
| 16:30 | Multilayered composite barrier materials for cost-effective organic solar cells encapsulation Authors : Arnaud Morlier1, Stéphane Cros2, Nicole D. Alberola1 1INES-RDI LMOPS, UMR CNRS-UdS 5041, av. du lac d'Annecy 73376 Le Bourget du lac, France. 2INES-RDI CEA DRT/LITEN/DTS/LMPV, 50, av. du lac Léman 73377 Le Bourget du lac, France. Affiliations : Resume : Nowadays, polymer solar cells (OPV) on flexible substrates are emerging on the market. Nevertheless, lifetime of OPV devices remains a limiting factor. Operational stability could be considerably improved by using high barrier materials to protect devices from water and oxygen. Encapsulating materials have to follow driving aspects such as transparency, flexibility and low cost.
During the past decade, multilayer materials consisting of stacks of inorganic and organic layers have been successfully explored to achieve high barrier properties. However, currently used processes are still too costly for OPV applications.
In this study, we investigate the possibility of obtaining high gas barrier structures through exclusive use of in-solution layer deposition and fast curing processes. We explored different processes for curing perhydropolysilazane (in-solution deposited inorganic precursor) layers into silica on polycarbonates and PET. Kinetics and chemical structures obtained from these curing routes were compared and related to corresponding gas barrier properties.
Very good performances towards water permeation (WVTR of about 10-2 g.m-2.d-1) were obtained from single inorganic layers deposited on PET. Combination of such barrier structures with intercalating polymer layers has permitted to obtain high barrier properties (WVTR < 10-3 g.m-2.d-1), allowing OPV cells to reach several years of lifetime. | 7 37 |
| 16:30 | CONTACTLESS PHOTOCONDUCTANCE MEASUREMENTS ON C60/ZnPc BILAYERS FOR ORGANIC PHOTOVOLTAICS Authors : M.C. Fravventura1, T.J. Savenije1, J.H. Hwang2, P. Erk2 and L.D.A. Siebbeles1 1 Opto-electronic Materials Section, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands. 2 BASF SE, Specialty Chemicals Research, D-67056 Ludwigshafen. Affiliations : Resume : A heterojunction of the light absorbing electron donor zinc-phthalocyanine (ZnPc) and C60 as electron acceptor is of interest for application in potentially low-cost organic solar cells. Bilayer structures of C60/ZnPc were prepared by thermal vacuum deposition on quartz substrates. The dynamics of excitons and charges were studied by optical spectroscopy and contactless time-resolved microwave conductivity (TRMC) measurements. On gradually increasing the thickness of the acceptor layer from 5 to 50 nm the maximum photoconductance signal decreases and more importantly the charge carrier lifetime reduces from 23 ns to 12 ns. These observations are tentatively attributed to electron trapping sites formed in the first few nanometers of the C60 deposition on the quartz substrate. In addition, preferential excitation of the C60 layer in the UV results in a substantially higher photoconductance signal than selective excitation of the ZnPc layer at 627 nm. Possible explanations for these observations such as a longer exciton diffusion length within the C60 are discussed. | 7 38 |
| 16:30 | Dielectric properties of doped organic solar cells: oxygen induced degradation on P3HT:PCBM Authors : Oskar Armbruster and Siegfried Bauer: Department of Soft Matter Physics, Johannes Kepler University, Altenberger Strasse 69, 4040 Linz, Austria Christoph Lungenschmied: Konarka Austria F&E GmbH, Altenberger Strasse 69, 4040 Linz, Austria Hans-Joachim Egelhaaf: Konarka Technologies GmbH, Landgrabenstr. 94, 90443 Nuremberg, Germany Affiliations : Resume : A detailed understanding of the mechanisms of degradation of organic photovoltaic cells (OPV) is the basis for any improvement of module lifetimes which in turn will increase market opportunities. Since oxygen has been identified as a major cause for the degradation of OPV devices, a good understanding of the behavior of organic semiconductor devices exposed to oxygen may help to eventually overcome these negative effects.
We report on an analysis of the dielectric response of oxygen-doped inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) heterojunction solar cells. The experimental data are modelled with a density-of-states (DOS) approach that describes the contribution of extrinsic charge carriers. The DOS-approach is compared to established models such as Debye relaxation and equivalent circuit representation. Not only do we obtain superior agreement between model and experimental data over the latter two, our model also yields important device parameters such as the dopant concentration. | 7 39 |
| 16:30 | Contribution of “hot” charge-transfer state to the performance of organic photovoltaic devices Authors : Artem A. Bakulin (1), Akshay Rao(1), Vladislav Pavelyev(2), Paul H.M. van Loosdrecht(2), Maxim S. Pshenichnikov(2), Richard H. Friend(1) (1) Cavendish laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK (2) Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Affiliations : Resume : The formation of free charge carriers in organic photovoltaic (OPV) devices is known to be mediated by a charge-transfer (CT) state, which is a bound pair of charges at the organic semiconductor interface. The mechanism of CT-state dissociation has not been fully elucidated, despite the fact that the understanding of this process is critical to optimal OPV cell design. Some studies [1] suggest that the CT state requires excess vibrational energy (“hot” state) to be converted into a pair of free charge carriers. An alternative viewpoint is that the initially hot CT state is not important because it rapidly relaxes to the lowest in energy (“cold”) CT state, from which the actual charge generation occurs. [2] Currently, there is no quantitative experimental evidence on the relevance and importance of these processes.
To address this issue, we apply a combination of the ultrafast optical 3-pulse and photocurrent spectroscopies, and their novel hybrid, optically-manipulated photocurrent spectroscopy. This allows us to selectively address the CT-state dynamics and, thereby, to elucidate the contributions of both cold and hot dissociation pathways to the photocurrent generation in various OPV devices. Our results suggest that the ratio of hot/cold dissociation is dictated by relative position of donor and acceptor energy levels which provides guidelines for the design of future OPV materials.
[1] H. Ohkita et al., JACS 130, 3030 (2008)
[2] J. Lee et al, JACS 132, 11878 (2010) | 7 40 |
| 16:30 | Modelling charge carrier dynamics in disordered semiconducting materials Authors : Jens Lorrmann 1, Andreas Baumann 1, Julien Gorenflot 1, Carsten Deibel 1 and Vladimir Dyakonov 1,2 1: Experimental Physics VI, Julius-Maximilians-University of Würzburg, 97074 Würzburg, Germany 2: Bavarian Center for Applied Energy Research e.V. (ZAE Bayern), 97074 Würzburg, Germany Affiliations : Resume : Charge carrier recombination in organic solar cells is a loss mechanism which limits the power conversion efficiency. We present a multiple trapping model describing these charge carrier dynamics and how its rate is affected by various physical parameters involved in organic solar cells, e.g. energetic disorder and temperature. In this model the tail of a gaussian density of states is approximated by an exponential exp (−E/E_0 ) distribution and we assume, that this tail states act as the trapping centers. While here recombination is considered bimolecular at least one of the recombining species has to be mobile for this process to take place. We found a strong dependence of the recombination rate (R ∝ n^(λ+1) ) on the energetic disorder and temperature, which reproduces the experimentally observed deviations from the Langevin theory, which predicts R_L ∝ n^2 . The order of the charge carrier dynamics λ + 1 is found to be λ + 1 > 1.5. Furthermore the model is capable to fit the temperature dependent data from transient absorption (TA) and charge extraction by linearly increasing voltage (CELIV) measurements on pure and fullerene blended poly(3-hexyl thiophene-2,5-diyl). The results clearly show, that recombination in the blend is trap-limited with recombination orders higher than λ + 1 > 2 for low temperatures, while the recombination in the pure material is mainly rendered by the temperature dependent mobility and a constant recombination order λ + 1 = 2. | 7 41 |
| 16:30 | Role of drift and diffusion in organic solar cells measured by transient photocurrents Authors : Wolfgang Tress 1 Zhe Li 2 Chris McNeill 2 Karl Leo Moritz Riede 1 Institut für Angewandte Photophysik, TU Dresden, Dresden, Germany 2 Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom Affiliations : Resume : The role of the electric field and the contribution of diffusion to the photocurrent in organic solar cells are controversially discussed. We investigate flat heterojunction solar cells based on evaporated small molecules with systematically varied energy barriers at the contacts. These barriers are created by an offset in the ionization potential between donor and hole transport layer and cause injection and extraction barriers for holes. Such barriers lead to S-kinks in the IV curve and to strong imbalanced and even reversed forces on charge carriers, resulting either from the field or the concentration gradient. The reason is that a contact barrier manipulates the built-in field of the active material stack and the concentration of charge carriers at the contact.
This behavior is investigated in more details using photocurrent transient measurements under illumination with square pulses of light of varying intensity. By systematically varying the bias voltages of the solar cell, the current response in the µs domain is monitored at every point of the IV curve.
Comparing these data to electrical simulations based on a drift-diffusion model we can explain the observed overshoots in the current response caused by barriers.
Thus, the transient current data of our experimental series and simulation data prove the influence of barriers on the spatial charge carrier density distribution within a solar cell and give new insights into the working principle of organic solar cells. | 7 42 |
| 16:30 | Analytic framework for transient current experiment - Charge Carrier Extraction by Linearly Increasing Voltage Authors : Jens Lorrmann 1, Bekele Homa Badada 2, Olle Inganäs 2, Carsten Deibel 1 and Vladimir Dyakonov 3 1: Experimental Physics VI, Julius-Maximilians-University of Würzburg, 97074 Würzburg, Germany 2: Biomolecular and Organic Electronics, IFM, Center of Organic Electronics, Linköping University, 5813 Linköping, Sweden 3: Bavarian Center for Applied Energy Research e.V. (ZAE Bayern), 97074 Würzburg, Germany Affiliations : Resume : Charge extraction by linearly increasing voltage (CELIV) has attracted much interest for investigating the charge transport and recombination in organic semiconductors, due to its ability to simultaneously measure the charge carrier mobility and density. Up to now the basic theoretical description of CELIV is solved for a low conductivity approximation only. We present the full analytical solution, thus generalizing the theoretical framework for this method. [1] The comparison of the general solution and the approximated theory showed that especially for typical organic solar cell materials the latter approach has a limited validity and results in deviations. Therefore, we present parametric equations for the mobility and for the first time for the charge carrier density, which we derived from simulated current transients within the general CELIV framework. These equations can be applied over the entire range of experimentally parameters, like e.g voltage slope or material conductance. Furthermore, Photo-CELIV measurements on poly(3-hexyl thiophene-2,5-diyl) :[6,6]- phenyl-C61 butyric acid methyl ester blends were evaluated by fitting the current transients to the analytical solution. The fit results are in a very good agreement with the experimental observations, if ambipolar transport is taken into account.
[1] J. Lorrmann, B. H. Badada, O. Inganäs, V. Dyakonov, and C. Deibel, J. Appl. Phys. 108, (2010) | 7 43 |
| 16:30 | Hot carrier mobility in porphyrin polymers monitored with photoinduced transient Stark spectroscopy Authors : J. Cabanillas-Gonzalez,1 M. Mroz2, G. Lanzani2,3, and H. L. Anderson4 1Madrid Institute for Advanced Studies in Nanoscience (IMDEA), UAM, 28049 Madrid, Spain. 2Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano 20133 Italy. 3Center for NanoScience and Technology CNST-IIT@POLIMI, via Pascoli 70/3, 20133 Milano, Italy. 4Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA (UK) Affiliations : Resume : Geminate recombination is one of the mechanisms limiting the efficiency of organic solar cells . The probability for a photogenerated e-h pair to escape from geminate recombination depends on the ability for both e and h to move away the donor-acceptor interface before thermalization occurs. Accordingly, hot carrier mobility is a property of paramount importance for the final efficiency of the solar cell. Photoinduced transient Stark spectroscopy is an electro-optical technique which provides mobility of carriers without current extraction with typical time resolution of 100 fs . The information obtained with this technique concerns microscopy mobility in very small domains of the film. Characterization of these intrinsic material properties is important not only to identify possible efficiency losses but also to understand how structural / morphological properties affect operation. In this work we investigate picosecond charge transport in a porphyrin polymer. Two regimes are observed. We found that initial transport is characterized by large mobility values with weakly t-dependent mobility. At longer delays a large drop in mobility is likely associated to e and h encountering swallow traps within their corresponding density of states. | 7 44 |
| 16:30 | Electrical modeling of polymer based solar cells Authors : L.H. Slooff, B. Brockholz, W.J.H. Verhees, S.C. Veenstra, E.M. Cobussen-Pool, J.M. Kroon and E.E. Bende, ECN, PO Box 1, 1755ZG, The Netherlands, tel: +31 224 56431, slooff@ecn.nl Affiliations : Resume : Electrical modeling of solar cells is carried out to optimize the grid pattern in polymer solar cells. The basic input for this type of modeling are the resistivity of the materials, film thicknesses and the diode parameters of the solar cell. The diode parameters are often determined by fitting the experimental current-voltage measurements to a one-diode model. This gives the well-known dark saturation current density (J0), diode ideality factor (n), photon current density (JL), shunt resistance and series resistance. However, the fitted parameters do not always correspond with the intrinsic solar cell parameters, i.e. those that correspond to an infinitessimally small diode, but they are actually lumped parameters containing information of the heterogeneity of the system. For this reason, two one-diode fits corresponding to two different systems (in size and geometry) can yield different intrinsic diode parameters. The reason for this can be found in the heterogeneity of the system, which is due to the following.
Firstly, the distributed series resistance effect results in a voltage distribution across the solar cell in the lateral direction, which causes that all notional diodes are operated at different voltages. Secondly, illumination masks are used when IV measurement of organic solar cells are carried out. In that case, the intrinsic diode parameters might be different for the illuminated and shaded areas and, moreover, the shaded area will deprive current from the illuminated zone, resulting in lower open-circuit voltages. We will show an approach to determine the so-called intrinsic diode parameters, by fitting the experimental IV curve against a simulated IV curve that is obtained from a model in which the experimental solar cells are explicitly modeled in 3D. | 7 45 |
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