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Berlin Oral Presentations and Poster Session Schedule We, 15. May Morning Session 1 Chair: Norbert Koch 09:00 Welcome Address: Anke Kaysser‐Pyzalla (HZB) 09:15 David Cahen, Weizmann Institute Inorganic, Organic and Hybrid Solar Cells: How different are they? 09:45 Lukas Schmidt‐Mende, U Konstanz Interface modification in hybrid TiO2:P3HT solar cells 10:15 Bruno Ehrler, U Cambridge Singlet Fission Sensitized Hybrid Solar Cells 10:30‐11:00 coffee break Morning Session 2 Chair: Lukas Schmidt‐Mende 11:00 Jean Manca, U Hasselt Hybrid polymer:metal oxide bulk heterojunction solar cells : bringing together the best of two worlds 11:30 Jürgen P. Rabe, HU Berlin Organic/Inorganic Hybrid Structures for Artificial Light Harvesting Based on Tubular J‐Aggregates 12:00 Pabitra K. Nayak, Weizmann Institute Role of structural order on solar cell parameters as illustrated in the hybrid SiC‐Organic junction model. 12:15 Patric Büchele, Siemens Erlangen Hybrid‐organic photodetectors for radiography 12:30 Wiebke Ludwig, HZB Electrochemically deposited ZnO nanorods for organic hybrid solar cells 12:45‐14:00 lunch break Afternoon Session 1 Chair: Jean Manca 14:00 Klaus Meerholz, U Köln Towards highly efficient solar cells based on merocyanine dyes 14:30 Dana Olson, NREL The influence of contact properties on device performance in organic solar cells 15:00 Andreas F. Bartelt, HZB/Sony Charge transfer between acyloin‐Type anchored organic sensitizer dyes and nanocrystalline oxides 15:15 Sergey Sadofev, HU Berlin Molecular beam epitaxy of Zn(Mg)O:Ga towards transparent conducting oxides and low‐damping plasmonic materials 15:30‐16:00 coffee break Afternoon Session 2 Chair: Klaus Meerholz 16:00 Oliver Monti, U Arizona The role of defects at hybrid organic/inorganic semiconductor interfaces 16:30 Michael Kozlik, U Jena Hybrid solar cells made of phthalocyanines and zinc oxide nanowires 16:45 Jan Behrends, FU Berlin Spins in organic solar cells: charge separation from an EPR perspective 2 17:00 Mohamed Haggui, FU Berlin Characterization of hybrid solar cells using scanning near‐field optical microscopy 18:30 Poster session Thu, 16. May Morning Session 1 Chair: Dana Olson 09:15 Karl Leo, TU Dresden Recent progress in small‐molecule organic solar cells 09:45 Antoine Kahn, Princeton U Interfaces of transparent metal oxides and other materials relevant to hybrid photovoltaics 10:15 Thomas Mayer, TU Darmstadt Bulk sensitization of inorganic semiconductors by organic dye molecules 10:30‐11:00 coffee break Morning Session 2 Chair: David Cahen 11:00 Feliciano Giustino, U Oxford Atomistic modelling of TiO2‐based interfaces for energy harvesting 11:30 Claudia Draxl, HU Berlin Describing, understanding, and discovering hybrid materials for photovoltaic applications from first principles 12:00 Patrick Rinke, FHI Bulk doping effects in hybrid organic/inorganic systems from quantum mechanical first principles 12:15 Carlo A. Rozzi, CNR‐NANO, Modena A time‐resolved view of artificial light harvesting. 12:30 Karsten Hannewald, HU Berlin Polaron transport in organic crystals: theory and modelling 12:45‐14:00 lunch break Afternoon Session 1 Chair: Oliver Monti 14:00 Akshay Rao, U Cambridge Ultrafast charge and energy transfer dynamics in hybrid OPVs 14:30 Natalie Stingelin, IC London Solution processable inorganic/organic photonic structures of low loss and tunable refractive index for use in photovoltaic devices 15:00 Stefanie M. Greil, HZB Organic‐inorganic hybrid solar cells using differently structured Silicon substrates 15:15 Jörg Rappich, HZB Degradation of hybrid heterojunction Solar cells P3HT/n‐Si in the Presence of humidity 15:30‐16:00 coffee break Afternoon Session 2 Chair: Claudia Draxl 16:00 Emil List, TU Graz A novel rout towards efficient polymer nano‐composite solar cells 16:30 Thomas Kirchartz, IC London Characterizing recombination in organic bulk‐heterojunction solar cells 3 16:45 Chittaranjan Das, TU Cottbus TiO2 thin layer on P‐Si for efficient and pH independent photo catalytic water splitting. 17:00 Mathieas Zellmeier, U Würzburg Charge transport in Organic Molecular Crystals of 9, 10‐Diphenylanthracene Fr, 17. May Morning Session 1 Chair: Natalie Stingelin Norbert Nickel, HZB Zinc Oxide and Silicon nanostructures for hybrid photovoltaic devices Konstantinos Fostiropoulos, HZB Effects of hybrid interface engineering on the device performance of small molecule organic solar cells Marin Rusu, HZB Engineering of interfaces in organic and hybrid photovoltaic cells coffee break Morning Session 2 Chair: Antoine Kahn Reinhard Carius, FZJ Nanoparticles for thin film photovoltaics Dieter Neher, U Potsdam On the photon energy and field dependence of charge generation across all‐organic and hybrid heterojunctions Closing Remarks & Farewell 09:15 09:45 10:15 10:45‐11:15 11:15 11:45 12:15 4 Locations Oral Presentations: Forum Adlershof Rudower Chaussee 24 12489 Berlin Germany Poster Session: BESSY II Albert‐Einstein‐Str. 15 5 Table of Contents Oral Presentations Inorganic, Organic and Hybrid Solar Cells: How different are they? * ............... 10 Interface modification in hybrid TiO2:P3HT solar cells ...................................... 11 Singlet Fission Sensitized Hybrid Solar Cells ..................................................... 12 Hybrid polymer:metal oxide bulk heterojunction solar cells: bringing together the best of two worlds ..................................................................................... 13 Organic/Inorganic Hybrid Structures for Artificial Light Harvesting Based on Tubular J‐Aggregates ....................................................................................... 14 Role of structural order on solar cell parameters as illustrated in the hybrid SiC‐
Organic junction model .................................................................................... 15 Hybrid‐organic photodetectors for radiography ............................................... 16 Electrochemically deposited ZnO nanorods for organic hybrid solar cells ......... 17 Towards highly efficient solar cells based on merocyanine dyes ....................... 18 The Influence of Contact Properties on Device Performance in Organic Solar Cells
........................................................................................................................ 19 Charge transfer between Acyloin‐Type Anchored Organic Sensitizer Dyes and Nanocrystalline Oxides .................................................................................... 20 The Role of Defects at Hybrid Organic / Inorganic Semiconductor Interfaces ... 21 Hybrid Solar Cells made of Phthalocyanines and Zinc Oxide Nanowires ........... 22 Spins in Organic Solar Cells: Charge Separation from an EPR Perspective ......... 23 Characterization of Hybrid Solar Cells using Scanning Near‐field Optical Microscopy ...................................................................................................... 24 Recent Progress in Small‐Molecule Organic Solar Cells ..................................... 25 Interfaces of Transparent Metal Oxides and Other Materials Relevant to Hybrid Photovoltaics ................................................................................................... 26 Bulk sensitization of inorganic semiconductors by organic dye molecules ........ 27 Atomistic modelling of TiO2‐based interfaces for energy harvesting ................. 28 Describing, understanding, and discovering hybrid materials for photovoltaic applications from first principles ...................................................................... 29 Bulk doping effects in hybrid organic/inorganic systems from quantum mechanical first principles ............................................................................... 30 A time‐resolved view of artificial light harvesting. ........................................... 31 Polaron Transport in Organic Crystals: Theory and Modelling .......................... 32 Ultrafast Charge and Energy Transfer Dynamics in Hybrid OPVs ....................... 33 Solution processable inorganic/organic photonic structures of low lossand tunable refractive index for use in photovoltaic devices .................................. 34 Degradation of hybrid heterojunction Solar cells P3HT/n‐Si in the Presence of humidity .......................................................................................................... 35 6 Organic‐inorganic hybrid solar cells using differently structured Silicon substrates ........................................................................................................ 36 A novel rout towards efficient polymer nano‐composite solar cells ................. 37 Characterizing recombination in organic bulk‐heterojunction solar cells .......... 38 TiO2 thin layer on P‐Si for efficient and pH independent photo catalytic water splitting. .......................................................................................................... 39 Charge transport in Organic Molecular Crystals of 9, 10‐Diphenylanthracene .. 40 Zinc Oxide and Silicon nanostructures for hybrid photovoltaic devices ............. 41 Effects of hybrid interface engineering on the device performance of small molecule organic solar cells ............................................................................. 42 Engineering of interfaces in organic and hybrid photovoltaic cells ................... 43 Nanoparticles for thin film photovoltaics ......................................................... 44 On the Photon Energy and Field Dependence of Charge Generation across All‐Organic and Hybrid Heterojunctions ........................................................... 45 Poster Session Dipole‐induced band‐bending in hybrid SiC\Diketopyrrolopyrrole junction and its effect on open‐circuit voltage ...................................................................... 47 Hybrid Lead Chalcogenide Nanocrystal Polymer Solar Cells .............................. 48 Thermally stimulated adjustment of the ZnPc:C60/MoOx hybrid interface in small‐molecule organic solar cells .................................................................... 49 In‐situ monitoring the growth of polyaniline at liquid/solid interface by combining the polarized infrared spectroscopy and reflectance anisotropy spectroscopy.................................................................................................... 50 Impact of Fluorination on Initial Growth and Stability of Pentacene on Cu(111) 51 PbS and PbSe‐nanocrystallites for applications in OPV ..................................... 52 Photo‐degradation in ladder‐type para‐phenylenes: Beyond fluorenone defects
........................................................................................................................ 53 Covalent Attachment of Porphyrin Dyes to Silicon (111) for Light Harvesting ... 54 Pulsed‐Laser Deposition of Silicon Nanowires .................................................. 55 Work function modification of ZnO poly‐crystalline films using short phenyl‐phosphonate layers .............................................................................. 56 Photovoltaic Applications of Silicon/PEDOT:PSS Hetero Structures .................. 57 Electric Field Distribution in Hybrid Solar Cells Containing Amorphous Silicon and Polymers ......................................................................................................... 58 Effect of dipole moment on energy level alignment and electronic structure of merocyanine dyes on Au(111) .......................................................................... 59 Charge Generation in ZnO:PCPDTBT Hybrid Photovoltaic Devices .................... 60 Time‐dependent elemental diffusion at the Mg:Ag electrode/ MePc:C60 organic interface and correlation to electrical properties of solar cells ......................... 61 7 In‐situ Synthesis of Semiconductor Nanoparticles Decorating Tubular J‐
aggregates ....................................................................................................... 62 Synthesis of Dipolar Terminally Substituted Sexiphenyl Derivatives ................. 63 Controlling nanocrystal quality and post synthetic treatment for creating efficient hybrid nanocrystal/polymer solar cells ............................................... 64 Work function modifications by self‐assembled monolayers: To the limits and beyond ............................................................................................................ 65 Device Physics of Tandem Hybrid Solar Cells Using Amorphous Silicon and PCPDTBT:PCBM with 6.7% Efficiency ................................................................ 66 A strong molecular acceptor for tuning the work function of ZnO .................... 67 Photoelectron Spectroscopy‐based Interface Analysis of ZnPc:C60/MoOx in Organic Solar Cells ........................................................................................... 68 ZnO nanostructures and PCPDTBT for efficient hybrid solar cells ..................... 69 Electrical and photoelectrical properties of hybrid heterojunction solar cells P3HT/n‐‐Si ....................................................................................................... 70 The Role of Intermolecular Hybridization in Molecular Electrical Doping .......... 71 8 Oral Presentations Location: Forum Adlershof Rudower Chaussee 24 Berlin 9 Inorganic, Organic and Hybrid Solar Cells: How different are they? * David Cahen, Pabitra K. Nayak 1
Dept. of Materials & Interfaces, Weizmann Inst. of Science, Rehovot, Israel 76100 While we know what are the solar cell efficiencies that we can hope to achieve for perfect single crystalline inorganic single crystal‐based cells, as demonstrated by the recent Alta results for GaAs p/n cells,1] applying the same criteria to other types of cells is pushing the Shockley – Queisser model “beyond its own limits“. So, while nothing is wrong with the S‐Q treatment, the question arises if, for the newer cells, so‐called 2nd and 3d generation ones, it tells the whole story. In other words, are the differences between cell types, that are easily gleaned from comparing S‐Q ‐ predicted, to actual performances, merely a matter of insufficient efforts or do basic scientific bounds, beyond those known today, exist [2]. We argue that additional limits exist for newer types of solar cells, including Organic PhotoVoltaics (OPV), Dye‐Sensitized Solar Cells and their siblings. Most strikingly, for organic material‐base cells, are the effects of disorder, static disorder, expressed via tail states, and dynamic disorder, apparent because of the importance of vibronic states [3]. This then leads us to a challenge for hybrid solar cells, which by “virtue” of combining organic with crystalline inorganic materials, introduce the extra limits of organics: Can we find ways where the combination of desirable characteristics overcomes the extra limits? Here it is likely that focusing solely on the pure science will be insufficient and we need to include factors related to ease, versatility and cost of fabrication, to meet the challenge. * Work done in part in collaboration with A. Kahn and with J. Bisquert. [1] E. Yablonovitch, O. D. Miller, S. R. Kurtz, The opto‐electronic physics that broke the efficiency limit in solar cells, Conf. Record 38th IEEE PVSC, art. no. 6317891 2012, 1556‐1559. [2] P. K. Nayak, J. Bisquert, D. Cahen, Assessing Possibilities & Limits for Solar Cells, Adv. Mater. 2011, 23, 2870‐6. http://www.weizmann.ac.il/materials/Cahen/images/Solar_cell_data_01_Jan_2013.pdf [3] P. K. Nayak, G. Garcia‐Belmonte, A. Kahn, J. Bisquert, D. Cahen, Photovoltaic Efficiency Limits and Material Disorder, En. Env. Sci. 2012, 5, 6022 – 6039 and P. K. Nayak, this conference. 10 Interface modification in hybrid TiO2:P3HT solar cells Jonas Weickert1, Eugen Zimmermann1, Lukas Schmidt‐Mende1 1
University of Konstanz, Dept. of Physics, 78457 Konstanz, Germany This research focusses on hybrid solar cells based on dye‐sensitized TiO2 electrodes infiltrated with conjugated polymers. This type of solar cell is promising since broad and tunable light absorption is possible if a good dye‐polymer combination is chosen. For efficiency improvements, however, it appears to be mandatory to enhance the contribution of the polymer to the photocurrent generation since often charges are mainly generated upon photon absorption in the dye. To foster charge injection from the polymer into the TiO2 and at the same time suppress charge carrier recombination interface modification is essential. Here we report on the interfacial modifier 4‐mercaptopyridine (4‐MP)[1], which induces controlled alignment of P3HT, the most widely used hole transporting polymer for hybrid solar cell. 4‐MP optimizes the charge separating interface between P3HT and a squarine dye‐decorated TiO2, inducing enhanced contribution to photocurrent generation by the polymer. In combination with tert‐butylpyridine (tBP), which enhances the open circuit voltage (VOC) in dye‐
sensitized and hybrid solar cells but reduces the photocurrent, the two modifiers show a synergistic behaviour and both VOC and photocurrent can be simultaneously enhanced[2]. [1] E. V. Canesi, M. Binda, A. Abate, S. Guarnera, L. Moretti, V. D'Innocenzo, R. Sai Santosh Kumar, C. Bertarelli, A. Abrusci, H. Snaith, A. Calloni, A. Brambilla, F. Ciccacci, S. Aghion, F. Moia, R. Ferragut, C. Melis, G. Malloci, A. Mattoni, G. Lanzani, A. Petrozza, Energy Environ. Sci. 2012, 5, 9068. [2] J. Weickert, E. Zimmermann, A. Abrusci, L. Schmidt‐Mende, 2013, submitted. 11 Singlet Fission Sensitized Hybrid Solar Cells 1
Bruno Ehrler , Marcus L. Böhm1, Brian J. Walker1, Maxim Tabachnyk1, Richard H. Friend1, Neil C. Greenham1 1
Cavendish Laboratory, J.J. Thomson Avenue, University of Cambridge, Cambridge CB3 0HE, United Kingdom Hybrid solar cells utilizing singlet exciton fission [1] provide a means to overcome the limitations of conventional solar cells by harnessing two excitons for every high‐energy photon [2]. Infrared quantum dots can be used as the inorganic, broadband absorber to accompany the singlet fission sensitizer [3,4]. However, the solar cell performance so far lags behind its potential. Here we present insights into the loss mechanisms and suggest a future path to more efficient singlet fission sensitized solar cells. [1] M. B. Smith, J. Michl Chemical Reviews, 2010, 110, 6891–6936. [2] M. C. Hanna, A. J. Nozik, Journal of Applied Physics 2006, 100, 74510. [3] B. Ehrler, M. W. B. Wilson, A. Rao, R. H. Friend, N. C. Greenham, Nano Letters, 2012, 12, 1053–1057. [4] B. Ehrler, B. J. Walker, M. L. Böhm, M. W. Wilson, Y. Vaynzof, R. H. Friend, N. C. Greenham, Nature Communications, 2012, 3, 1019. 12 Hybrid polymer:metal oxide bulk heterojunction solar cells: bringing together the best of two worlds B. Conings, L. Baeten, G.K.V.V. Thalluri, W. Dierckx, D. Spoltore, J. D’Haen, M. Nesladek, A. Hardy, M. Van Bael, H.G. Boyen, J.V. Manca Universiteit Hasselt – Instituut voor Materiaalonderzoek / IMEC – Associated Lab IMOMEC, Wetenschapspark 1, B‐3590 Diepenbeek (Belgium) With the concept of hybrid polymer:metal oxide bulk heterojunction solar cells it can be aimed to bring together appealing benefits of the classes of fully organic bulk heterojunction solar cells and of Grätzel solar cells, e.g. nanostructuring for improved charge transport, environmentally friendly preparation and morphological stability. To improve the effectiveness of the charge carrier pathways in bulk heterojunction solar cells, the concept of (ordered) 1‐dimensional nanostructured interpenetrating networks through the use of (in)organic fibres is being explored in order to obtain highways for charge transport. Recent activities include the controlled growth of nanocolumnar ZnO for hybrid P3HT:ZnO‐nanorod solar cells [1] . A variety of advanced electro‐optical and morphological techniques have been used in order to investigate the relation between morphology, electro‐
optical and photovoltaic properties. From an environmental point of view, an important drawback of polymer based solar cells is the need for toxic organic solvents. It is however possible to develop fully ‘green’ solid‐state solar cells in which photosensitizer, electron and hole conductor are achieved from a water‐based preparation method [2]. Towards novel generation organic based solar cells, the concept of hybrid organic:inorganic solar cells could also be a promising route towards solar cells with an increased intrinsic thermal stability due to the thermally stable metal oxide template which eliminates the problem of thermally induced phase separation which is encountered in fully organic bulk hetero junction solar cells. [1] L. Baeten, B. Conings, H.G. Boyen, J. D’Haen, A. Hardy, M. D’Olieslaeger, J. V. Manca, M. K. Van Bael, Adv.Mat. 2011, 25, 2802. [2] L. Baeten, B. Conings, J. D'Haen, A. Hardy, J. V. Manca, M. K. Van Bael, Solar Energy Matials & Solar Cells 2012, 107, 230‐235. 13 Organic/Inorganic Hybrid Structures for Artificial Light Harvesting Based on Tubular J‐Aggregates Y. Qiao, F. Polzer, H. Kirmse, S. Kirstein, J.P. Rabe Humboldt‐Universität zu Berlin, Department of Physics, Newtonstr. 15, 12489 Berlin (Germany) Dye‐sensibilized semiconductor wires are most attractive components for large area excitonic solar cells [1]. We followed a synthetic strategy, based on suitable amphiphilic cyanine dyes, which have been shown to self‐assemble spontaneously in aqueous solutions into double walled tubular J‐aggregates with an extremely well defined structure [2], and which have been used successfully as chemically active nanotemplates to grow silverwires with a very homogeneous diameter of 6.5 nm [3]. Here we describe the in‐situ synthesis of quasi one‐dimensional (1D) nanohybrids based on the tubular J‐aggregates [2] and inorganic semiconductor nanocrystals. Different types of semiconductor nanocrystals (i.e., CdS, ZnS, and NiS) were successfully deposited on the surface of the supramolecular J‐aggregate nanotubes, giving rise to 1D nanohybrids. By varying the synthetic conditions, the thicknesses of the inorganic layers are rationally tuned. It is further demonstrated that the optical properties (e.g., large red‐shift, J‐band absorption and emission) of the J‐aggregates are retained in the J‐aggregate/CdS nanohybrids and the photoluminescence of nanosized CdS can be restored by photo‐bleaching the cyanine dye. We expect that these nanohybrids may be used for hybrid photovoltaics. [1] N. O. V. Plank, et al.; J. Phys. Chem. C 2009, 113, 18515. [2] D. M. Eisele, J. Knoester, S. Kirstein, J. P. Rabe, D. A. Vanden Bout, Nat. Nanotechn. 2009, 4, 658‐663. [3] D. M. Eisele, H. v. Berlepsch, C. Böttcher, K. J. Stevenson, D. A. Vanden Bout, S. Kirstein, J. P. Rabe, J. Am. Chem. Soc. 2010, 132, 2104‐2105. 14 Role of structural order on solar cell parameters as illustrated in the hybrid SiC‐
Organic junction model 1
Pabitra K. Nayak , Lee Barnea1, Soyoung Kim1, Andrew Shu2, Antoine Kahn2, David Lederman3and David Cahen1* 1
Dept. of Materials and Interfaces, Weizmann Institute of Science, Rehovot, 76100, Israel. 2
Dept. of Electrical Engineering, Princeton University, Princeton, NJ, 08544, USA 3
Dept. of Physics, West Virginia University, Morgantown, WV, 26506, USA Email: pabitra.nayak@weizmann.ac.il Interfaces, particularly the interface between a donor (D) and acceptor (A) material, play an important role in determining the photovoltaic parameters, if a hetero‐interface is used in a solar cell. It is mostly agreed in organic PV that the energetics at the interfaces i.e., the ionization potential of (D) and the electron affinity of (A) determine the open‐circuit voltage (Voc) and short circuit current density (Jsc) of the solar cell. However, there are other factors that can also play a crucial role in determining the Voc and Jsc of a cell. In order to look for other parameters that could influence solar cell parameters, we fabricated a model hybrid solar cell, where a crystalline wideband gap (~3 eV) inorganic semiconductor, 6‐H SiC, is used as acceptor and pentacene (optical gap ~ 1.8 eV) as donor. The interface between SiC and pentacene was modified by introduction of alkyl siloxane monolayers of different chain lengths. No significant change was found in the surface potential, and, thus, in the surface dipole, of the surface that resulted after MM formation, but a gradual change in static water contact angle, was observed. The Voc of the solar cell improves with increasing chain length, even though the Donor and Acceptor entities remain the same. The increase in Voc can be attributed to a combined effect of reduced recombination, due to the increased separation of D and A, and a decrease in the density of gap states. The latter results from an increase in long‐range ordering in pentacene, evident from XRD rocking curve analysis, which is due to the change in MMs, as the ordering and surface energy in MMs changes with chain length. The gap states, probed by Surface Photovoltage Spectroscopy (SPS) were found to be related to structural disorder in thin‐film of pentacene. The increase in Voc is accompanied by an increase in Jsc. The increment in Jsc reflects this increase in order as trap‐assisted recombination at short circuit is suppressed in a more ordered system. This model system illustrates the role of ordering in solar cell parameters in experimental conditions. Structural disorder translates into energy disorder which turn reduces solar cell performance. The above findings may help to find a way to reduce recombination and improve VOC and Jsc of a cell simultaneously. 15 Hybrid‐organic photodetectors for radiography 1,2
Patric Büchele , Sandro Tedde1, Katharina Poulsen3, Uli Lemmer2, Oliver Schmidt1 1
Siemens AG, Corporate Technology, Günther‐Scharowsky‐Str.1, Erlangen 2
Karlsruhe Institute of Technolgy, Light Technology Institute, Engesserstrasse 13, Karlsruhe 3
CAN GmbH, Grindelallee 117, Hamburg Medical imaging requires large area x‐ray detectors due to the limited ability to focus x‐ray radiation. Most detectors today are realized by stacking an amorphous silicon photodetector array and a scintillator which converts the incoming x‐ray photons into visible light. However, this concept provides limited image resolution as photons are emitted isotropically from the scintillator. Organic semiconductors provide the opportunity to easily process photosensitive elements on large areas. Similar as silicon, the intrinsic x‐ray absorption probability is low and therefore additional x‐ray absorbers are required for sensitive detectors. Here, we explore two different routes towards hybrid‐organic photodetectors which have been sensitized for x‐ray absorption. In a direct‐conversion approach lead sulfide (PbS) quantum dots have been processed in a matrix of a P3HT/PCBM bulk heterojunction. Dark current values down to 10‐5 mA/cm² have been realized and x‐ray sensitivity could be demonstrated for dose rates of 1 mGy/s. As a second route nanoscale scintillators are added to the organic photodiode in order to absorb photons as close to the szintillating emission center as possible and thereby minimize optical cross‐talk.
16 Electrochemically deposited ZnO nanorods for organic hybrid solar cells Wiebke Ludwig1,2, Wiebke Ohm1,2, Ümit Aksünger1, Sven Wiesner1, Volker Hinrichs1, Marin Rusu1, Sophie Gledhill1, M. Ch. Lux‐Steiner1,2 1
Helmholtz‐Zentrum für Materialien und Energie GmbH, Hahn‐Meitner‐Platz 1, 14195 Berlin (Germany) 2
Freie Universität Berlin, Fachbereich Physik, Berlin (Germany) Zinc oxide nanorod arrays (ZNA) have recently attracted much attention due to their special optical, morphological and electronic properties. Their application in thin film organic hybrid solar cells as highly structured transparent conductive oxide (TCO) or part of the p‐n junction is expected to offer both electronic and optical advantages. The anti‐reflective and light diffusing properties depending on the ZNA morphology may increase absorption and thus, charge generation in the thin organic absorber layer. Furthermore, the nanostructured, high surface area ZNA morphology is expected to allow for short exciton diffusion lengths which are beneficial for effective charge separation and collection in organic absorber materials. Efficient devices necessitate, however, ZNA with both suitable electronic and optical properties. Hence, the ZNA require a morphology yielding simultaneously appropriate optical properties, efficient charge separation and collection as well as a proper coating with subsequent absorber and contact layers. Industrial application demands, moreover, a cost‐effective and scalable fabrication of the ZNA. To address these challenges, we prepared ZNA at low temperature (75 °C) using a simple electrochemical process for which homogeneous deposition on large (100 cm2) substrates is demonstrated. Variation of reaction conditions and seed layer allows for obtaining a range of ZNA morphologies which are characterized using photoluminescence, optical transmission and reflection as well as X‐ray diffraction and X‐ray photoelectron spectroscopy measurements [1]. First devices, using ZNA as highly structured TCO in inverted organic solar cells with ZnPc:C60 bulk heterojunction absorbers displayed efficiencies of 2.8 % which is the best result for this kind of hybrid device [2]. [1] W. Ludwig, W. Ohm, Y. Zhao, M. Ch. Lux‐Steiner, S. Gledhill, Phys. Stat. Sol. (a), accepted. [2] W. Ludwig, V. Hinrichs, S. Wiesner, M. Rusu, M. Ch. Lux‐Steiner, in prep. 17 Towards highly efficient solar cells based on merocyanine dyes Klaus Meerholz1 1
Chemistry Department, University of Cologne, Luxemburger Straße 116, D ‐ 50939 Köln (Germany) Herein, we report on the latest results of our research on merocyanine (MC) based small‐molecule organic solar cells (SM‐OSC). We present results on tandem solar cells with complementary absorbing subcells in series connection, containing red and blue dyes, respectively. Due to the versatility of MCs, all possible combinations of solution‐ (SOL) and vacuum‐processed (VAC) active layers can be studied. Therefore, tandem solar cells with VAC/VAC, SOL/SOL, SOL/VAC and VAC/SOL active layer combinations are fabricated and characterized. The results are compared to optical simulations and the respective single‐junction solar cells. In the SOL‐devices the influence of the casting from solvent mixtures is investigated in detail. [1] M. D. Mustermann, O. V. Evermann, J. Am. Chem. Soc. 2010, 120, 5356−5359. [2] Tables, Electron spectroscopy: Cambridge University Press: Cambridge, 1995. 18 The Influence of Contact Properties on Device Performance in Organic Solar Cells Dana C. Olson1, 1
National Renewable Energy Laboratory (NREL), Golden, Colorado, USA Organic photovoltaics (OPVs) have become an attractive technology that offer a lower cost alternative to current commercial solar conversion technologies due to their potential for low temperature, large‐area, and high‐throughput manufacturing. Further barriers that must be overcome prior to commercialization lie in the development of OPV materials and device architectures to result in improved efficiency and stability. To achieve this, we are developing unique tools, design rules, and new materials for both active layers and selective contacts. While the active layer materials are important for determining the ultimate performance, interfacial contact layers must be optimized both electronically and chemically for new active layer components. Such interfacial contacts are believed to improve device performance by a variety of mechanisms such as improved energy level alignment and charge carrier selectivity leading to improved charge extraction and reduced recombination. We are investigating the influence of the electronic properties of electron and hole transport layer (ETL and HTL) contacts on device performance to gain a greater understanding of the relative contributions of contact properties such as work function and band alignment. In addition, we are studying the effects of interfacial chemistry on local band alignment and device performance, by showing the deleterious effects of protonation in high performance solution processed small molecule solar cells. The independent control of work function and band alignment is shown to result in increased performance in both standard and inverted device architectures. Through this approach we are able to actively tune the contact properties to the ever changing properties of new active layer materials. 19 Charge transfer between Acyloin‐Type Anchored Organic Sensitizer Dyes and Nanocrystalline Oxides 1
Andreas F. Bartelt , Robert Schütz1, Joachim Schaff1, Ivo Kastl1, Stephan Janzen1, Christian Strothkämper1, Rainer Eichberger1, Gabriele Nelles2, Lars‐Peter Scheller2, Gerda Fuhrmann2 1
Humboldt Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin (Germany) 2
SONY Deutschland GmbH, Hedelfinger Straße 61, D‐70327 Stuttgart (Germany) In dye‐sensitized solar cells (DSC) the semiconductor/dye interaction is crucial for the overall efficiency of the photovoltaic device. The dye has to be stably attached to the semiconductor surface, the electron injection from the excited dye into the oxide must be efficient and the recombination slow. The interaction at the hybrid oxide/dye interface is mediated by the anchor group, which in most sensitizer dyes reported today are carboxylic acids. Recently, a new highly efficient acyloin‐type anchor group and a new class of metal‐free sensitizer dyes comprising this new anchor group were reported. DSCs fabricated with these semi‐squarylium dyes show remarkable performances particularly with regard to incident photon‐to‐current efficiencies. Here we report on the injection and recombination properties of these semi‐squarylium dyes. We will show ultrafast electron injection processes, which can be connected to the strong electronic coupling between the dye and the TiO2 surface due to the direct participation of the anchor group in the conjugated π‐system of the chromophore. The high performance of the new acyloin‐type anchor group is demonstrated for systematically varied donor‐acceptor characters of the dyes by comparison with a commercially available high‐performance indoline dye (D131) of similar optical characteristics, which has a conventional carboxylic acid anchor. By comparison with the TiO2/dye band alignments, the possibilities of minimizing energy offsets while retaining high electron injections rates will be discussed. The injection efficiency of the acyloin‐anchored dyes will further be demonstrated on nanostructured ZnO electrodes. Strongly coupled anchor groups often show detrimentally fast back electron transfer (BET). We will demonstrate that the solvent environment causes a beneficial slow‐down of the acyloin‐anchor mediated BET, which can be explained by passivation of TiO2 surface states. Ultrafast and photoemission spectroscopies were used. 20 The Role of Defects at Hybrid Organic / Inorganic Semiconductor Interfaces Leah L. Kelly1, David A. Racke1, Paul Winget,2 Hong Li,2 Ajaya K. Sigdel,3 Paul Ndione,3 Joseph J. Berry,3 David S. Ginley,3 Jaewon Shim,4 Bernard Kippelen,4 Jean‐Luc Brédas,2 Oliver L.A. Monti1 1
Department of Chemistry and Biochemistry, The University of Arizona, Tucson, AZ, 85721 (USA) 2
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, 30332‐0400 (USA) 3
National Renewable Energy Laboratory, Golden, Colorado, 80401 (USA) 4
School of Electrical and Computer Engineering and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, 30332‐0250 (USA) Hybrid organic / inorganic semiconductor interfaces present a range of novel phenomena different from the much better studied organic / metal interfaces. A critical role in establishing the interfacial electronic structure and likely also carrier dynamics is played by defects. Beyond band bending, defects may present sites of high reactivity or cause charge transfer across the interface. These factors are vital when attempting to design interfaces for efficient optoelectronic devices. I will discuss several different combinations of organic and inorganic semiconductors that illustrate the role of defects in defining the interfacial electronic structure. A combination of interface‐sensitive spectroscopies and electronic structure calculations helps unravel the specific interactions giving rise to hybrid interface states at oxide surfaces and strong interactions at layered semiconductor surfaces. 21 Hybrid Solar Cells made of Phthalocyanines and Zinc Oxide Nanowires Michael Kozlik, Marco Gruenewald, Sören Paulke, Roman Forker, Carsten Ronning, Torsten Fritz Friedrich‐Schiller‐Universität Jena, Max‐Wien‐Platz 1, 07743 Jena (Germany) In hybrid solar cells the inorganic material is often combined with organic polymers with increased interface area by using nanostructures. Our aim is to use small molecules, i.e., zinc phthalocyanine (ZnPc) [1], due to the better photo current performance compared to polymers. ZnPc exhibits a high absorbance in the visible range as derived by determination of the dielectric function from transmittance and differential reflectance spectra [2]. In combination with zinc oxide (ZnO) the interface forms a p‐n‐junction [3]. In our work, we investigate planar heterojunctions and nanostructured devices, the latter consisting of ZnO nanowires covered with ZnPc. The electronic ground state and the energetic level alignment were investigated using ultraviolet photoelectron spectroscopy (UPS). ZnO nanowires, synthesized by the vapor‐liquid‐solid route, are used due to the good conductivity and the transparency in the visible range [4]. A nearly perpendicular nanowire growth was achieved using a ZnO seed layer. The surfaceto‐ volume ratio can be varied by changing the growth parameters. The morphology of the samples was investigated with scanning electron microscopy, while the chemical composition was obtained from X‐ray analysis. The substrate and the front contact were optimized to remain the high conductivity during nanowire growth. We will also present first results of a hybrid solar cell made of the combination of ZnO nanowires and ZnPc. The results of optical spectroscopy and external quantum efficiency (EQE) measurements highlight the generation of excitons in ZnPc and link these with the photo current in the visible range. In combination with the derived parameters we show the performance of a simplified photovoltaic cell and identify the region of exciton dissociation and exciton diffusion length. [1] N. Papageorgiou, N. Salomon, T. Angot, J.‐M. Layet, L. Giovanelli, Prog. Surf. Sci. 2004, 77, 139‐170. [2] M. Kozlik, S. Paulke, M. Gruenewald, R. Forker, T. Fritz, Org. Electron. 2012, 13, 3291‐3295. [3] C. Ingrosso, A. Petrella, M. L. Curri, M. Striccoli, P. Cosma, P. D. Cozzoli, A. Agostiano, Electrochim. Acta 2006, 51, 5120‐5124. [4] C. Borchers, S. Müller, D. Stichtenoth, D. Schwen, C. Ronning, J. Phys. Chem. B. 2006, 110, 1656‐1660. 22 Spins in Organic Solar Cells: Charge Separation from an EPR Perspective Jan Behrends1, Felix Kraffert1, Robert Bittl1 1
Freie Universität Berlin, Arnimallee14,D‐14195 Berlin(Germany) Photoinduced charge transfer and subsequent charge separation are the key processes in organic and hybrid solar cells. Prior to exciton separation into free charge carriers, bound polaron pairs (also referred to as charge transfer states) form at the donor/acceptor interface. While the existence of charge transfer states 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 charge transfer states and providing direct access to their dynamics are highly demanded. Here we report transient electron paramagnetic resonance (trEPR) measurements with submicrosecond time resolution performed on a P3HT:PCBM blend. We show that the trEPR spectrum immediately following photoexcitation reveals signatures of spin‐‐‐correlated polaron pairs and thus decisively differs from the spectrum of separated 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 are geminate, i.e., originate from one exciton [1]. We discuss the role of coupled charge carrier pairs in mediating the conversion from excitons into separated charge carriers as probed by trEPR and compare the charge separation mechanisms found in purely organic systems with those encountered in purely inorganic and hybrid organic‐‐‐inorganic composites. [1] J. Behrends, A. Sperlich, A. Schnegg, T. Biskup, C. Teutloff, K. Lips, V. Dyakonov, R. Bittl; Phys. Rev. B 2012, 85, 125206. 23 Characterization of Hybrid Solar Cells using Scanning Near‐field Optical Microscopy 1
Mohamed Haggui , Patrick Andrä1,2 and Paul Fumagalli1 1
Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany 2
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Albert‐Einstein‐
Straße 15, 12489 Berlin, Germany Hybrid Solar Cells based on the combination of organic and inorganic materials are promising alternatives to conventional inorganic‐based solar cells. Exploiting the advantages of each material could increase the efficiency and reduce the production costs [1]. Various microscopies including Transmission Electron Microscopy (TEM) [2], Atomic Force Microscopy (AFM) [2] and Scanning Transmission X‐ray Microscopy [3] have been used to study the morphology of photovoltaic samples. Here, we report the application of Scanning Near‐field Optical Microscopy (SNOM) to study solar cells constituted of inorganic nanostructures embedded in organic matrix. SNOM is a powerful technique that scans a tapered optical fiber across the sample, illuminating only that area of the sample that lies directly under the tip aperture (50‐100nm). Kept within few nanometers of the surface, the tip illuminates the sample in the near field allowing the collection of optical information, better than the diffraction limit of the operating light, of the surface of the sample. Topographic information are equally obtained. We present preliminary results obtained with samples constituted of Silver nanodots and nanowires deposited on silicon substrates. The obtained measurements are compared to Scanning Electron Microscopy (SEM) measurements in order to ameliorate our SNOM system and to highlight the limit of its resolution. This system will be used, in the near future, to perform measurements and mapping of photocurrents in hybrid solar cells. [1] J. Conradt, Biophotonics: Spectroscopy, Imaging, Sensing, and Manipulation: Springer 2011. [2] H. Hoppe, T. Glatzel, M. Niggemann, W. Schwinger, F. Schaeffler, A. Hinsch, M. C. Lux‐Steiner, N. S. Sariciftci, Thin Solid Films 2006, 511, 587. [3] C. R. McNeill, B. Watts, L. Thomsen, W. J. Belcher, A. L. D. Kilcoyne, N. C. Greenham, P. C. Dastoor, Small 2006, 2, 1432. 24 Recent Progress in Small‐Molecule Organic Solar Cells Karl Leo1,2 1
Institut für Angewandte Photophysik, TU Dresden, 01062 Dresden (Germany) 2
Fraunhofer‐COMEDD, 01109 Dresden (Germany) Organic solar cells have recently achieved significant progress in terms of efficiency and lifetime and have crossed the 10% efficiency mark. However, they further significant improvements to achieve broad application, e.g. for the challenging power applications where inorganic PV technologies dominate. In my talk, I will present an overview over the key features of small‐molecule organic solar cells deposited by vacuum processes. Using this technology, one can easily deposit multilayer structures which allow to fine tune the optical and electrical properties. I will discuss recent experiments which relate the molecular structure of absorber materials with the layer morphology and the cell properties. It is shown that small changes of the molecular structure leaving the electronic properties of the individual molecular nearly unchanged can lead to larger changes in the crystal packing and molecular orientation, causing significant differences in electronic properties in the active layer. Furthermore, highly efficient tandem structures with optimized electrical and optical properties will be discussed. Very efficient recombination contacts can be realized by n‐ and p‐type doped transport layers. Structures based on these approaches have reached efficiencies of 12% and have the potential to reach approximately 20%. 25 Interfaces of Transparent Metal Oxides and Other Materials Relevant to Hybrid Photovoltaics Antoine Kahn Dept. of Electrical Engineering, Princeton University, Princeton, NJ 08544 (USA) This talk reviews some of the work done in our group to determine the energetics of key interfaces involving transparent metal oxides and other materials of importance for hybrid photovoltaics. We begin with requirements for carrier selective contacts, e.g. hole extractor / electron blocker, and look at the relevant energy parameters (ionization energy, electron affinity, work function) of materials such as NiO, ZnO, ZnMgO, MoO3 and TiO2 prepared via various deposition techniques (vacuum evaporation, sol‐gel, atomic layer deposition, chemical vapor deposition). The energetics of some oxide heterointerfaces, such as NiO/MoO3 and ZnO/MoO3, with application to hole‐extraction or charge recombination layers, are demonstrated. We review on‐going work on the electronic structure of electron‐ and hole‐blocking contacts on Si using P3HT and TiO2, respectively, for Si‐based solar cells [1]. Finally, the energetics and use of a “universal” electrode work function‐lowering polymer films, such as PEIE, are discussed [2]. [1] S. Avasthi et al. (in preparation). [2] Y. Zhou et al. Science 2012, 336, 327. 26 Bulk sensitization of inorganic semiconductors by organic dye molecules Thomas Mayer, Wolfram Jaegermann Technische Universität Darmstadt, Petersenstr. 32, 64287 Darmstadt (Germany) rd
Two concepts of 3 generation photovoltaic composite materials are proposed that both attempt to combine the excellent transport properties of inorganic semiconductors with strong absorptivity and energetic adjustability of organic dye molecules [1]. In concept one, we investigate bulk sensitization of thin film silicon with strongly absorbing dye molecules to reduce the absorber layer thickness. In concept two we attempt upconversion by two step absorption in organic dyade molecules that are incorporated into a wide band gap semiconductor matrix to provide for three different energy gaps within a single composite absorber layer. Thin film silicon was grown by hot wire CVD and perylenes or phthalocyanines were codeposited The molecules are incorporated intact as shown by photoemission, vibration and UV‐VIS spectroscopy [2]. Using substitution the band/state line up is adjusted to allow for dye exciton separation into injected electrons and holes in Si [3‐5]. ZnSe was grown using PVD and two molecules of different ionization energies and HOMO LUMO gaps were codeposited intact. Molecules have been adjusted to form dyads with the required band/state line up to allow for two step absorption and injection of the primary hole and upconverted electron into the valence respective conduction band of the ZnSe matrix. [1] T. Mayer, U. Weiler, E. Mankel, W. Jaegermann, W.; Proceeding of Fourth IEEE World Conference on Photovoltaic Energy Conversion WCPEC IV 2006 Waikoloa Hawaii. [2] T. Mayer, U. Weiler, E. Mankel, W. Jaegermann, C. Kelting, D. Schlettwein, N. Baziakina, D. Wohrle, Renewable Energy 2008 33, 262‐266. [3] U. Weiler, T. Mayer, w. Jaegermann, C. Kelting, D. Schlettwein, S. Makarov, D. Wohrle, Journal of Physical Chemistry B 2004, 108, 19398‐19403. [4] T. Mayer, U. Weiler, C. Kelting, D. Schlettwein, S. Makarov, D. Wohrle, O. Abdallah, M. Kunst, W. Jaegermann, Solar Energy Materials and Solar Cells 2007, 91, 1873‐1886. [5] A. Decker, S.‐L. Suraru, O. Rubio Pons, E. Mankel, M. Bockstedte, M. Thoss, F. Wuerthner, T. Mayer, W. Jaegermann, Journal of Physical Chemistry C 2011, 115, 21139‐21150. 27 Atomistic modelling of TiO2‐based interfaces for energy harvesting Feliciano Giustino1 1
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH (United Kingdom) Understanding and designing functional interfaces at the atomic scale has become a priority in research on nanostructured solar cells. A fundamental property of functional interfaces is the alignment of the electron energy levels between the donor and the acceptor materials. Such alignment underpins a variety of complex phenomena, from exciton dissociation to charge injection and recombination. In this talk I will review our activity [1‐3] in the computational modelling of TiO2‐
based interfaces of current interest for dye‐sensitized and semiconductor‐
sensitized solar cells, with a focus on the interfacial energy‐level alignment. In the first part of the talk I will discuss our progress towards a quantitative understanding of the electronic structure of the prototypical interface between anatase and the N3 dye. Emphasis will be placed on the direct comparison between calculations on realistic models of the interface and valence photoelectron spectra. In the second part of the talk I will consider TiO2 surfaces sensitized with stibnite (Sb2S3) nanoribbons. Here I will show how to construct plausible atomic‐scale models of the oxide/semiconductor interface in absence of detailed experimental characterization. I will also discuss strategies for the computational design of new functional interfaces based on semiconductors of the stibnite family. [1] C. E. Patrick, F. Giustino, Adv. Funct. Mater. 2011, 21, 4663‐4667. [2] C. E. Patrick, F. Giustino, J. Phys.: Condens. Matter. 2012, 24, 202201‐BLA. [3] C. E. Patrick, F. Giustino, Phys. Rev. Lett. 2012, 109, 116801. 28 Describing, understanding, and discovering hybrid materials for photovoltaic applications from first principles Claudia Draxl 1
Humboldt‐Universität zu Berlin, Zum Großen Windkanal 6, 12489 Berlin (Germany) Hybrid materials are most exciting as one can expect new properties arising at the interface, which are absent in either of the building blocks. At the same time, they represent challenging cases for electronic‐structure theory. Methods that turned out useful for describing one side may not be applicable for the other one, and they are likely to fail for the interfaces. For selected examples of organic and inorganic semiconductors as well as hybrid interfaces, I will present structural properties, electronic bands, and optical excitation spectra as obtained from density‐functional theory and many‐body perturbation theory. They will highlight which properties can be reliably computed for such materials. It needs, however, also to be discussed, what is missing to reach predictive power on a quantitative level and, thus, open a perspective towards the discovery of new materials. 29 Bulk doping effects in hybrid organic/inorganic systems from quantum mechanical first principles 1
1
Patrick Rinke , Yong Xu , Oliver T. Hofmann1, Niko Moll2, and Matthias Scheffler1 1
Fritz‐Haber‐Institut der Max‐Planck‐Gesellschaft, Faradayweg 4‐6, 14195 Berlin (Germany) 2
IBM Research ‐ Zurich, 8803 Rüschlikon (Switzerland) The properties of hybrid inorganic/organic systems (HIOS) can be tuned by inserting dipolar layers at the interface between the two materials. For inorganic semiconductors in HIOS, the global effect of intrinsic or extrinsic dopants becomes important, because it determines the amount of charge transfer at the interface and gives rise to band bending. We present a quantum mechanical first principles approach that introduces excess charge in the unit cell by means of the virtual crystal approximation with fractionally charged nuclei [1,2] and includes the energy contribution of band bending explicitly. For the example of adsorbate layers of the strong organic acceptor tetrafluoro‐tetracyanoquinodimethane (F4TCNQ) on ZnO we investigate the global effects of doping on the interface as a function of the bulk dopant concentration and study the local effects at the interface (e.g. defects and dipole formation). For the bulk terminated ZnO(000‐1) surface covered with half a monolayer of hydrogen (2x1‐H) [3], we demonstrate that electrons from bulk dopants can stabilize deviations from this half monolayer coverage. Ambient hydrogen background pressures are therefore more conducive than ultra high vacuum conditions to form the defect free 2x1‐H surface, which would be a more controlled substrate in HIOS [4]. For the interface between ZnO(000‐1) 2x1‐H and F4TCNQ monolayers, we show that the adsorption energy and the charge transfer to the molecules depend strongly on the bulk dopant concentration. This affects the energy level alignment at the ZnO/F4TCNQ interface considerably. In the limit of low bulk doping concentrations charge transfer becomes vanishingly small in agreement with photoemission data [5]. [1] M. Scheffler, Physica B & C 1987, 146, 176. [2] N. Richter, S.Levchenko, M. Scheffler, to be published. [3] B. Meyer, Phys. Rev. B 2004, 69, 045416. [4] N. Moll, Y. Xu, O. Hofmann, and P. Rinke, under review. [5] R. Schlesinger et al, under review. 30 A time‐resolved view of artificial light harvesting. Carlo A. Rozzi1 , Sarah Maria Falke2, Nicola Spallanzani1,3, Angel Rubio4,5, Elisa Molinari1,3*, Daniele Brida6, Margherita Maiuri6, Giulio Cerullo6, Heiko Schramm7, Jens Christoffers7, Christoph Lienau2 1
Istituto Nanoscienze – CNR, Centro S3, via Campi 213a, I‐41125 Modena, Italy 2
Institut für Physik and Center of Interface Science, Carl von Ossietzky Universität, 26111 Oldenburg, Germany. 3
Dipartimento di Fisica, Università di Modena e ReggioEmilia. via Campi 213a I‐
41125 Modena, Italy 4
Nano‐Bio Spectroscopy Group and ETSF ScientificDevelopment Centre, Dpto. Física de Materiales, Universidad del País Vasco, Centro de Física de Materiales CSIC‐UPV/EHU‐MPC and DIPC, Av. Tolosa 72, E‐20018 San Sebastián, Spain 5
Fritz‐Haber‐Institut der Max‐Planck‐Gesellschaft, Berlin, Germany 6
IFN‐CNR, Dipartimento di Fisica, Politecnico di Milano,Milano, Italy 7
Institut für Reine und Angewandte Chemie and Center of Interface Science, Carl von Ossietzky Universität, 26111 Oldenburg, Germany. Nature has developed sophisticated and highly efficient molecular architectures to convert sunlight into chemical energy. It is known that the primary steps, specifically both energy and charge transfer, occur on extremely fast time scales. These processes have traditionally been interpreted in terms of the incoherent kinetics of optical excitations and of charge hopping, but recently signatures of quantum coherence were observed in energy transfer in photosynthetic bacteria and algae [1,2]. We have studied the early steps of photoinduced charge separation in an an organic donor‐bridge‐acceptor supramolecular assembly [3] by combining Time‐dependent Density Functional Theory simulations of the quantum dynamics and high time resolution femtosecond spectroscopy. We discuss the role of the electron‐nuclei coupling and of the linking group in the photoinduced charge separation process. Our results provide evidence that the driving mechanism of the charge separation process is a quantum correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds, thus establishing the role of quantum coherence in artificial light harvesting [4]. [1] G. S. Engel, et al. Nature 2007, 446, 782‐786. [2] E. Collini, et al. Nature 2010, 463, 644‐6470. [3] G. Kodis, et al. J. Phys. Org. Chem. 2004, 17, 724‐734. [4] C. A. Rozzi, et al., Nat. Comm. doi: 10.1038/ncomms2603. 31 Polaron Transport in Organic Crystals: Theory and Modelling Karsten Hannewald 1,2 1
Institut für Physik, Theoretische Festkörperphysik,Humboldt‐Universität zu Berlin (Germany) 2
European Theoretical Spectroscopy Facility & IFTO, Friedrich‐Schiller‐Universität Jena (Germany) The charge‐carrier mobility of organic semiconductors is a fundamental material property and one of the central quantities for the optimization of device performance in, e.g., organic transistors and solar cells. In order to investigate the intrinsic fundamental (i.e., not device‐specific) charge‐transport phenomena in organic solids, molecular crystals are ideal candidates because of their high degree of structural order. Nonetheless, even for such ultrapure organic crystals, the theoretical and numerical description of the charge transport is a highly nontrivial task due to the strong coupling between the electronic and vibronic degrees of freedom. Here, we present a theory for charge transport in organic crystals which generalizes Holstein's small polaron model to polarons of arbitrary size and allows to calculate the carrier mobilities using ab‐initio techniques (density‐functional theory). The generalized mobility expression includes both the coherent band transport as well as the thermally induced hopping on equal footing. As a prototypical example, the theory is applied to herringbone‐stacked crystals where the temperature dependence of the mobilities is simulated and compared to experimental data. Finally, the mobility anisotropy is analyzed by a novel 3D visualization technique for the relevant transport channels. [1] K. Hannewald et al., Phys. Rev. B 2004, 69, 075211 & 075212. [2] K. Hannewald and P.A. Bobbert, Appl. Phys. Lett. 2004, 85, 1535. [3] F. Ortmann, F. Bechstedt, and K. Hannewald, Phys. Rev. B 2009, 79, 235206. [4] F. Ortmann, F. Bechstedt, and K. Hannewald, New J. Phys. 2010, 12, 023011. [5] F. Ortmann, F. Bechstedt, and K. Hannewald, Phys. Stat. Sol. B 2011, 248, 511. 32 Ultrafast Charge and Energy Transfer Dynamics in Hybrid OPVs Akshay Rao1, Frederik S.F. Morgenstern1 and Neil C. Greenham1 1
Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom Hybrid systems based on organic semiconductors and inorganic nanocrystals are promising for photovoltaic applications, but the dynamics of energy and charge transfer in these systems are poorly understood. Here we use time‐resolved absorption and luminescence measurements that enable us to differentiate between energy‐ and charge‐transfer in a model system, based on blends of cadmium selenide nanoparticles (CdSe‐NP) with poly[2‐methoxy‐5‐(3’,7’‐
dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV). The use of various capping ligands such as oleic acid, pyridine and n‐butylamine as well as thermal annealing allowed tuning of the polymer‐nanoparticle interaction. We demonstrate that energy transfer from MDMO‐PPV to CdSe is the dominant exciton quenching mechanism in non‐annealed blends and occurs on ultrafast time scales (<1ps). Upon thermal annealing electron transfer becomes competitive with energy transfer. Hole transfer from CdSe to MDMO‐PPV occurs on nanosecond time scales and is strongly ligand‐dependent. We also study the dynamics of charge transfer using a newly developed technique, ultrafast pump‐
push photocurrent spectroscopy [1]. We find the presence charge‐transfer like states, akin to those in OPVs, indicating that charge separation can be inefficient even in hybrid systems. [1] Bakulin, et al., The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012. 33 Solution processable inorganic/organic photonic structures of low lossand tunable refractive index for use in photovoltaic devices Irene Votta,1,2 Andrew Strang,1,2 George Richardson,2,3Manuela Russo,2,3 Walter Caseri,4 Paul Stavrinou1,2 and Natalie Stingelin1,3 1
Department of Physics, Blackett Laboratory, Imperial College London, London, SW7 2AZ,UK 2
Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, UK 3
Department of Materials, Imperial College London, London, SW7 2AZ, UK 4
Department of Materials, ETH Zurich, CH‐8093 Zürich, Switzerland An ever increasing interest in the development and application of innovative optical and optoelectronic devices places greater emphasis for the advancement of new smart and functional materials that are readily processable. Significant progress has already been realised in the fields of organic light‐emitting diodes (OLEDs) and photovoltaic cells (OPVs) through development of novel semiconducting materials. Further developments in these areas are turning to the deployment of photonic structures to aid and improve light management in these systems, e.g. input‐/output‐coupling, enhanced absorption and waveguiding. In this work, results from a novel class of hybrid material systems that offer an outstanding set of optical and material properties, including tunable refractive index, low optical losses and solution process ability, are presented. We show that the attributes of these novel hybrid material systems can be controlled and manipulated by a range of means that include ‘alloying’ or suitable post‐
deposition treatments, such as thermal annealing and/or irradiation with UV‐light. As a consequence, these hybrid materials can exhibit refractive indices of up to 2.1 while also being highly transparent over the entire visible, near‐ and mid‐ infrared (N‐IR, M‐IR) wavelength regime [1]. Furthermore, the processing properties allow the realisation of solution‐based, optically low‐loss photonic structures that are straightforward to implement in structures, such as OPVs. Given that the readily achievable nature of high quality optical properties and the exceptionally low loss from a single high‐index up to several microns thick have already been demonstrated, the focus is here turned to the further development of this generic class of hybrid materials, which are based on metal oxide hydrates and bulk commodity polymers such as poly(vinyl alcohol). To this end, we highlight our recent efforts in introducing different metals, culminating in the successful development of mixed‐metal oxide hydrate hybrid materials. [1] M. Russo et al, J. Polym. Sci., Part B: Polym. Phys. 2012, 50 (1), 65. 34 Degradation of hybrid heterojunction Solar cells P3HT/n‐Si in the Presence of humidity 1,2
1
Victor Brus , Xin Zang , Marc A. Gluba1, Guoguang Sun3, Karsten Hinrichs3, Norbert H. Nickel1, Jörg Rappich1 1
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Institute für Si‐
Photovoltaik, Kekuléstr. 5, 12489 Berlin (Germany) 2
Yuriy Fedkovych Chernivtsi National University, Department of electronics and energy engineering, Kotsubinskystr. 2, Chernivtsi 58000 (Ukraine) 3
Leibniz‐Institut für Analytische Wissenschaften – ISAS –e.V., Department Berlin, Albert‐Einstein‐Str. 9, 12489 Berlin (Germany) P3HT/n‐Si heterojunction solar cells were fabricated by the deposition of P3HT thin films (70‐80 nm) onto oxygen passivated n‐Si single crystal substrates (ρ = 5 Ωcm) using the spin‐coating technique. A semitransparent gold layer (20 nm thick) was prepared by thermal evaporation and served as front contact. The effect of the degradation of P3HT/n‐Si heterojunc on solar cells was analyzed under laboratory condi ons (t=24 ͦC, RH=18%) during 20 days. The comparison of structural, optical, electrical and photoelectrical properties of the as‐deposited and degraded heterojunctions was carried out in order to describe the effect of the degradation process which is represented by a reduction in photovoltage and photocurrent and by an irreversible change in the chemical structure of the polymeric layer by breaking of C‐S bonds and possible formation of C‐S‐H in the presence of humidity. Degradation studies in literature are mostly described by cleaving of the hexyl‐groups and sulfoxide formation1‐3 when performing these experiments in diluted oxygen atmosphere or glove –box conditions. [1] Y. D. Park, S. G. Lee, H. S. Lee, D. Kwak, D. H. Lee, K. Cho, J. Mater. Chem. 2011, 21, 2338. [2] M. Manceau, A. Rivation, J. –L. Gardette, S. Guillerez, N. Lemaitre, Polym. Degrad. Stabil. 2009, 94(6), 898. [3] N. Ljungqvist, T. Hjertberg, Macromolecules 1995, 28(18), 5993. 35 Organic‐inorganic hybrid solar cells using differently structured Silicon substrates 1
Stefanie M. Greil , Matthias Zellmeier1, Victor V. Brus1, 2, Norbert H. Nickel1, Jörg Rappich1 1
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Kekuléstr. 5, 12489 Berlin (Germany) 2
Yuriy Fedkovych Chernivtsi National University, Department of electronics and energy engineering, Kotsyubynsky str. 2, Chernivtsi 58000 (Ukraine) We will present a screening of differently structured n‐type (100) Si substrates for application in organic‐inorganic hybrid solar cells [1]. As substrates, we used flat polished silicon wafers, Si nanowires prepared by metal assisted etching (MAE), and Si random pyramids that were produced by wet‐chemical etching. Different surface functionalizations like hydrogen‐ and methyl termination [2], as well as ultra‐thin oxide coverage are compared and discussed in terms of charge separation and charge transfer at the inorganic‐organic interface. All substrates contain an (n+) amorphous silicon back surface field (BSF) and a full area Al back contact. Solar cells were produced by spin coating the planar and nanostructured surfaces with different polymers. Finally, a transparent gold contact was evaporated on top of the polymer layers to act as a front contact. The device structures are characterized using photoluminescence, surface photovoltage and impedance measurements to gain insight into the surface defect‐density. Solar cell parameters were determined by measuring currentdensity voltage curves. [1] X. Shen, B. Sun, D. Liu, and S.‐T. Lee, Hybrid Heterojunction Solar Cell Based on Organic–Inorganic Silicon Nanowire Array Architecture, J. Am. Chem. Soc. 2011, 133 (48), 19408–19415. [2] F. Yang, K. Roodenko, R. Hunger, K. Hinrichs, K. Rademann, and J. Rappich, Near‐Ideal complete coverage of CD3 onto Si(111) surfaces using one‐step electrochemical grafting: An IR Ellipsometry, Synchrotron XPS, and Photoluminescence study, J. Phys. Chem. C 2012, 116, 18684−18690. 36 A novel rout towards efficient polymer nano‐composite solar cells Emil J.W. List1,2 1
NanoTecCenter Weiz Forschungsgesellschaft mbH, Franz‐Pichler‐Straße 32, A‐
8160 Weiz, Austria 2
Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A‐
8010 Graz, Austria Inorganic‐organic nano‐composite solar cells – also known as hybrid photovoltaic devices – are devices, composed of an organic conjugated polymer (e.g. P3HT, MDMO‐PPV, PCDTBT) acting as electron donor and inorganic semiconducting nanoparticles (e.g. CdSe, ZnO, PbS, CuInS2) used as electron accepting phase. A major advantage of such nanoparticles is that they may be tailored in size and shape to tune their electrical and optical properties. Moreover, the optical absorption of the inorganic nanoparticles leads to the benefit that light absorption not only takes place, appears in the polymer phase but also by the nanoparticle phase. Thus light harvesting may take place in both phases; this is a significant advantage in contrast to PCBM‐based solar cells, where typically only the polymer phase is involved in the light absorption process. By using the conjugated polymer poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐
2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl] (PCDTBT) [1] as electron donor material and novel precursor materials – copper and indium xanthates – for the generation of a CIS nanoparticle acceptor phase (generated in‐
situ from xanthates) as acceptor material, hybrid photovoltaic devices with efficiencies significantly above 2.5% have been achieved as will be discussed in this contribution. In particular we will report on the interplay of the attained morphologies and the corresponding photo and device physics in due detail. This work was partially funded by the Austrian Ministry of Economy, Family and Youth and Isovoltaic AG. [1] T. Rath, M. Edler, A. Fischereder, S. Moscher, A. Schenk, A. Pein, D. Meischler, K. Bartl, R. Saf, G. Trimmel, W. Haas, M. Sezen, F. Hofer, R. Trattnig, G. Mauthner, E. J. W. List, N. Bansal, S. Haque, A Direct Route Towards Polymer/Copper Indium Sulfide Nanocomposite Solar Cells, Advanced Energy Materials 2011, 1, 1046 ‐ 1050. 37 Characterizing recombination in organic bulk‐heterojunction solar cells Thomas Kirchartz Department of Physics and Centre for Plastic Electronics, Imperial College London, SW7 2AZ, London, UK Typical state of the art polymer:fullerene solar cells reach efficiencies of up to 9% as single junction devices [1] and show high internal quantum efficiencies [2]. However, the typical active layer thickness is in the range of 100 nm or less [1,3], implying that in these systems charge collection can only be very efficient at low device thicknesses [4]. Thus, the potential to increase efficiency by absorbing more light with thicker active layers cannot be harnessed in most cases. Characterizing and understanding charge recombination and transport is therefore crucial to improve single junction efficiencies beyond 10%. For a detailed understanding of transport and recombination, it is necessary to characterize the spatial and energetic distribution of charge carriers in the device. This involves an understanding of the density of states in the band gap as well as the distribution of the electric field which is affected by space charge caused by charged defect states and/or by asymmetric transport. I will present techniques and results on transient and frequency domain techniques to measure charge carrier lifetimes, mobilities, the density of states and the space charge and show how these parameters affect the device performance. In particular, I will show that the concentration of charged defects is much higher in polymer:fullerene solar cells than is usually assumed in literature, that the mobility and lifetime can be optimized at the same time, contrary to what would be expected from Langevin theory, and that surface recombination (rather than recombination at the internal donor:acceptor interface) is a relevant loss mechanism in many polymer:fullerene systems [5]. [1] Z. C. He, et al., Nat Photonics 2012, 6, 591‐595. [2] S. H. Park, et al., Nat Photonics 2009, 3, 297‐2U5. [3] T. Kirchartz, et al., Phys Chem Lett 2012, 3, 3470‐3475. [4] M. A. Faist, S. Shoaee, S. M. Tuladhar, G. F. A. Dibb, S. Foster, W. Gong, T. Kirchartz, D. D. C. Bradley, J. R. Durrant, J. Nelson, Advanced Energy Materials 2013, doi: 10.1002/aenm.201200673. [5] R. Xia, D. S. Leem, T. Kirchartz, D. D. C. Spencer, C. Murphy, Z. He, H. Wu, S. Su, Y. Cao, J. S. Kim, J. C. deMello, J. C.; D. D. C. Bradley, J. Nelson, Advanced Energy Materials 2013, doi: 10.1002/aenm.201200967. 38 TiO2 thin layer on P‐Si for efficient and pH independent photo catalytic water splitting. Chittaranjan Das, Ulrike I. Kramm, Massimo Tallarida and Dieter Schmeisser Brandenburg Technical University, Cottbus, Applied Physics and Sensors, K.‐Wachsmnn‐Allee 17, D‐03046, Cottbus Hydrogen fuel cells, being environmental friendly to produce energy, are a technology of future. The most efficient way to produce hydrogen is solar driven photocatalysis using semiconducting materials as photo electrodes. The choice of electrodes is a crucial factor and is done on the basis of photo corrosion stability, light absorption efficiency, and photocarrier lifetime. P‐type Si can be used as photo cathode to produce H2 by direct photocatalysis. Si cathodes can be used in acidic electrolytes to have efficient photo catalytic activity but they are unstable in alkaline electrolytes. Therefore, to use both Si electrodes in the same electrolyte, their chemical stability should be extended over a wide range of pH. To this purpose we modified the surface of a p‐type Si photocathode with very thin films of TiO2 grown by atomic layer deposition (ALD). We found that the modified Si cathode shows an increased photoresponse and a lower onset potential with respect to the pristine surface. Furthermore, in contrast to bare P‐Si our TiO2/P‐Si exhibits stability against photocorrosion at various pH values. 39 Charge transport in Organic Molecular Crystals of 9, 10‐Diphenylanthracene M. Zellmeier1, T. Schmeiler2, J. Pflaum2 1
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Albert‐Einstein‐
Straße 15, 12489 Berlin (Germany) 2
Julius‐Maximilians‐Universität Würzburg, Sanderring 2, 97070 Würzburg The need to find organic semiconducting materials that are thermally stable at operational temperatures and that offer sufficiently high charge carrier mobilities at the same time proves to be a challenge for the last couple of decades. In the past, highest mobilities for electrons as well as for holes were measured in low molecular weight single‐crystals of Naphthalene [1]. But the material itself proves not to be practical, due to the high vapor pressure at room temperature prohibiting the formation of stable thin films. Here, we focus on the organic semiconductor 9, 10‐Diphenylanthracene (DPA), which consists of two phenyl groups attached to the opposite (9, 10)‐positions of the Anthracene backbone. DPA has a low vapor pressure at room temperature and a high melting point (approximately 430K) [2] and is thermally stable well within the temperature range of a solar cell. Single‐crystals of this material have been grown from zone refined material from the gas phase and their temperature dependent electric transport behavior was analyzed. Structural characterization of the crystals has been carried out performing X‐ray diffraction measurements, showing Bragg reflexes up to the 14th order. Using the Scherrer formula, the crystallite grain size was estimated to be approximately 1200Å along the direction of charge transport. A charge carrier trap density of Nt=2.4x10‐13 cm‐3 was obtained by space charge limited current (SCLC) measurements. The observed hole transients in Time‐of‐Flight (TOF) experiments showed a clearly distinguishable transient time over the entire temperature range, with a high hole mobility of μ+=1.96 cm2/Vs at 150K. The electron transient kink is not distinguishable due to residual impurities causing dispersive transport. Finally, the obtained data will be discussed in the context of a multiple trapping and release model with regard to the limiting factor of charge transport. [1] W. Warta and N. Karl, Phys. Rev. B 1985, 32, 1172. [2] A.K. Tripathi, J. Pflaum, Adv. Mater. 2007, 19, 2097. 40 Zinc Oxide and Silicon nanostructures for hybrid photovoltaic devices N. H. Nickel, V. Brus, S. Greil, S. Käbisch, N. Karpensky, M. Zellmeier, X. Zhang, and J. Rappich Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Institut für Siliziumphotovoltaik, Kekuléstr. 5, 12489 Berlin (Germany) Hybrid photovoltaic devices consisting of inorganic and organic materials require a large surface area to maximize charge separation at the interface. Zinc oxide (ZnO) and silicon (Si) nanostructures were grown by pulsed laser deposition (PLD) at elevated temperatures. In addition to PLD metal induced etching of silicon was used to fabricate Si nanowires. The morphology and structural quality of the nanostructures were characterized by scanning electron microscopy, Raman backscattering spectroscopy, x‐ray photoelectron spectroscopy, and photoluminescence measurements. The nanostructures, and for reference planar samples, were spin‐coated with P3HT and PCPDTBT. To complete the solar cells, a top electrode consisting of transparent MoO3 and/or gold was deposited on the polymeric layer. The devices were characterized by I‐V measurements in the dark and under illumination. For planar silicon with a P3HT coating a conversion efficiency of 3.3 % was achieved. 41 Effects of hybrid interface engineering on the device performance of small molecule organic solar cells Konstantinos Fostiropoulos, 1
Helmholtz‐Zentrum für Materialien und Energie GmbH, Hahn‐Meitner‐Platz 1, 14195 Berlin, Germany Abstract: to be announced 42 Engineering of interfaces in organic and hybrid photovoltaic cells Sven Wiesner1, Wiebke Ludwig1,2, Dieter Greiner1, Volker Hinrichs1, Alexander Steigert1, Iver Lauermann1, Martha Ch. Lux‐Steiner1,2 and Marin Rusu1 1
Helmholtz‐Zentrum für Materialien und Energie GmbH, Hahn‐Meitner‐Platz 1, 14195 Berlin, Germany 2
Freie Universität Berlin, Fachbereich Physik, Berlin, Germany We engineer and investigate the interface electronic properties and chemistry at the front‐side TCO electrodes (e.g., indium tin oxide (ITO) and ZnO) as well as at the metal back‐side electrodes (e.g., Ag, Mg‐Ag) in metal phthalocyanine (MePc)/C60‐based solar cells. The absorber thin films were processed by a new Organic Vapor Phase Deposition. The device architectures were developed for conventional as well as for inverted structures. Conventional photovoltaic (PV) devices were prepared on planar ITO and ZnO on transparent glass substrates, while the inverted hybrid solar cells were additionally processed on TCO surfaces highly structured by ZnO‐nanorods (ZnO‐NRs). The work function of electrodes was adjusted in each specific case either by surface conditioning [1] or by use of alloy layers [2], or by application of new buffer layers from transition metal oxides [3]. The electrode/absorber interface was investigated by secondary ion mass spectrometry, grazing incidence x‐ray diffraction analysis and x‐ray photoelectron spectroscopy. As prepared solar cells with conventional architecture on planar TCO‐electrodes show power conversion efficiencies of up to 4.5% under an illumination of 100 mW/cm2 and 25°C. Efficiencies of 3.1% are demonstrated on inverted planar PV devices. First solar cells with ZnO‐NRs show efficiencies of 2.8%, which is the best result for this kind of hybrid devices [4]. The findings of this work are discussed against the background of the performance and electrical properties of respective MePc/C60‐based organic solar cells. [1] K. Fostiropoulos, M. Rusu, Sol. Energy Mater. Sol. Cells 2011, 95, 1489‐1494. [2] M. Rusu, S. Wiesner, I. Lauermann, Ch.‐H. Fischer, K. Fostiropoulos, J. N. Audinot, Y. Fleming, M. Ch. Lux‐Steiner. Appl. Phys. Lett. 2010, 97, 073504/1‐3. [3] M. Rusu, et al., AiF‐Abschlussbericht HEROS‐INNOSOL. Förderkennzeichen KF2002604HA0. [4] W. Ludwig, V. Hinrichs, S. Wiesner, M. Rusu, M.Ch. Lux‐Steiner. (in prep.) 43 Nanoparticles for thin film photovoltaics R. Carius, J. Flohre, M. Nuys, C. Leidinger, F. Köhler, S. Muthmann and U. Rau Institute für Energie und Klimaforschung 5 ‐Photovoltaik‐, Forschungszentrum Jülich GmbH, D‐52425 Jülich (Germany) Cost effective solar cells with high efficiency based on abundant non‐toxic materials is the long term target of present research and development. Multijunction/multibandgap thin film solar cells are considered as an important option for future solar cell technologies. Several binary compounds such as FeSi2, CuO, Cu2O, … have been identified fulfilling the above mentioned criteria but the optoelectronic properties demonstrated so far are not sufficient for photovoltaic applications. Nanoparticles may have advantages over thin film or bulk material regarding minimization of bulk defects. On the other hand the high surface to volume ratio and the respective demand for minimizing surface defects is challenging. We will present our strategy implementing optimized nanoparticles for charge generation into a matrix that serve as charge separation and/or charge transport material. This allows decoupling of the optimization of the nanoparticles which often requires processes incompatible with the embedding matrix or other functional material of the device. We show our progress to improve the quality of the structural and electronic properties of CuO, Cu2O and Fe2O3 nanoparticles by thermal annealing and the advantage of using micro‐Raman and micro‐
photoluminescence spectroscopy to monitor the optoelectronic properties at various stages of the processes, after annealing, passivation and embedding. 44 On the Photon Energy and Field Dependence of Charge Generation across All‐Organic and Hybrid Heterojunctions Koen Vandewal1, Steve Albrecht2, Alberto Salleo1, Dieter Neher2 1
Department of Materials Science and Engineering, 496 Lomita Mall, CA 94305, Stanford University, Stanford (USA) 2
Universität Potsdam, Institute of Physics and Astronomy, Karl‐Liebknecht‐Str. 24‐
25, D‐14476 Potsdam, Germany (Germany) In the past years, the power conversion efficiency of all‐organic and hybrid solar cells has been steadily increasing. Most of these high efficiency devices exhibit high fill factors (FF), suggesting that possible loss processes such as geminate recombination are either insignificant or dependent only weakly on the internal electric field. This insensitivity of free carrier generation to the internal electric field is rather counterintuitive, as the formation of free electrons and holes proceeds via Coloumbically‐bound interfacial charge‐transfer states (CT‐states). Several authors therefore proposed that non‐relaxed ‘hot’ CT states, thought to be more delocalized, must be involved in efficient photogeneration of charge [1‐
2]. In great contrast to this, it was reported that the internal quantum efficiency for charge generation in some devices is insensitive to excitation wavelength over a wide range, including photon energies that directly excite low‐lying CT‐states [3]. We have performed time‐delayed collection field experiments in combination with a detailed analysis of the steady‐state electroluminescence and EQE spectra to study the effect of photon energy and electric field on the efficiency of charge generation. We identify the fully relaxed charge transfer state (CT0) to be the precursor to free charges in a wide variety of organic donor‐acceptor couples of different nature, exhibiting a range of efficiency and energy offsets. Our study also proves that the field‐dependence and efficiency of free carrier formation is dictated by the location of the CT0 state energy with respect to the energy of the spatially‐separated charge pair. [1] G. Grancini, M. Maiuri, D. Fazzi, A. Petrozza,H. J. Egelhaaf, D. Brida, G. Cerullo, G Lanzani, Nat. Mater. 2013, 12, 29. [2] Y. Vaynzof, A. A. Bakulin, S. Gelinas, R. H. Friend, Phys. Rev. Lett. 2012, 108, 246605. [3] J. Lee, K. Vandewal, S. R. Yost, M. E. Bahlke, L. Goris, M. A. Baldo, J. V. Manca, T. V. Voorhis, JACS 2010, 132, 11878. 45 Poster Session We, 15. May Location: BESSY II Albert‐Einstein‐Str. 15 46 Dipole‐induced band‐bending in hybrid SiC\Diketopyrrolopyrrole junction and its effect on open‐circuit voltage Lee Barnea‐Nehoshtan1, Philip Schulz2, Pabitra K. Nayak1 , Frank Wuerthner3, Antoine Kahn2 and David Cahen2 1
Dept. of Materials & Interfaces, Weizmann Inst. of Science, Rehovot, 76100, Israel 2
Dept. of Electrical Engineering, Princeton University, Princeton, NJ, 08544, USA 3
Inst. für Organische Chemie, Univ. Wuerzburg, D‐97074 Wuerzburg, Germany Energy level alignment is a key parameter controlling the performance of a solar cell. Predicting this alignment, based on measurements of the isolated components may be misleading, though, because of interactions between them. To study the importance of energy level alignment for the electrical behavior of junctions involving organic semiconductors, we use a heterojunction between a wide band gap inorganic semiconductor, SiC, and Diketopyrrolopyrrole (DPP) derivatives as organic semiconductor. This simplified system allows us to manipulate the energy alignment at the donor\acceptor interface with minimal effect on the organic material properties themselves. Due to the 3 eV bandgap of the SiC, optical absorption is mainly limited to the organic semiconductor. Thus, we can modify the interface without changing the exciton population or transport properties. We modified the surface of a single‐crystal SiC with self‐assembled alkyl silane monolayers. Attaching electron‐donating or ‐withdrawing groups to the molecules allows systematic variation of the molecular dipole, leading to a concomitant change in the surface electrostatic potential (and the local vacuum level). Deposition of the DPP on the modified SiC results in charge transfer between the organic and inorganic components, and formation of an interface dipole. Depending on the direction of the dipole, the energy difference between the DPP’s highest‐occupied molecular orbital (HOMO) and the bottom of the SiC’s conduction band (CB) changes. As a result, the open‐circuit voltage (Voc) of a SiC\DPP based solar cell increases with increasing interface dipole. At the same time, the electron and hole transport barriers change, and the short‐circuit current (Isc) changes accordingly. Consequently, we are able to demonstrate an interface dipole‐assisted scheme for improving Voc of a hybrid organic\inorganic junction. 47 Hybrid Lead Chalcogenide Nanocrystal Polymer Solar Cells Marcus L. Böhm1 Bruno Ehrler1, Alain Voucher1, Neil C. Greenham1 1
Cavendish Laboratory, J. J. Thomson Avenue, University of Cambridg,(Cambridge) CB3 0HE, United Kingdom Hybrid bulk heterojunction solar cells show a promising path to low cost solar cells. Due to their large interface and the high dielectric constant in the inorganic component, charge separation yield is more favorable than in all‐organic bilayer devices [1]. Up to present the most successful devices consists of bulk heterojunctions from a blend of CdSe nanocrystals and low‐bandgap polymers [2, 3]. However, these devices do not allow harvesting photons in the infrared region. Changing the inorganic component to infrared absorbing lead chalcogenide nanocrystals permits conversion of additional photons in this spectral region. The major bottleneck for an implementation of lead chalcogenide nanocrystals in a solar cell device, however, is their initial surface coverage that consists of long, insulation organic molecules. In order to provide efficient charge transfer between the inorganic and organic domain these original ligands need to be replaced with short capping groups. This process is well‐established in CdSe nanocrystals but there are only few examples of lead chalcogenide nanocrystals with effective solution process able capping groups that enable high charge transfer rates between the donor and acceptor domain, but also stabiles the colloid in solution. Solution processable infrared nanocrystals would ultimately allow to fabricate bulk heterojunction films from blends of soluble organic molecules capable of singlet fission with infrared nanocrystals. [1] N. C. Greenham, Wiley‐VCH Verlag GmbH & Co. KgaA 2009 54, 17628. 48 Thermally stimulated adjustment of the ZnPc:C60/MoOx hybrid interface in small‐molecule organic solar cells 1,2
1
C. Stenta , S. Wiesner , V. Hinrichs1, A. Steigert1, J. P. Veiga2, I. Lauermann1, M. Rusu1 and M. Ch. Lux‐Steiner1,3 1
Institut für Heterogene Materialsysteme, Helmholtz‐Zentrum Berlin für Materialien und Energie, Lise‐Meitner Campus, Hahn‐Meitner‐Platz 1, 14109 Berlin, Germany 2
CENIMAT‐I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus de Caparica, 2829‐516 Caparica, Portugal 3
Freie Universität Berlin, Fachbereich Physik, Arnimallee14, 14195 Berlin, Germany Recently, we have demonstrated efficient organic solar cells based on blend organic absorbers consistiong of metal‐phtalocyanine (Me‐Pc) donor and fullerene C60 acceptor 4.5 % efficient conventional and 3.2 % efficient inverted solar cells were demonstrated on devices with indium tin oxide and ZnO window layers, respectively. In both cases MoOx was used as a hole transport (electron blocking) layer. In this contribution, we report on the optimization of the ZnPc:C60/MoOx hybrid interface by an in‐situ annealing in UHV. The surface work functions (WF) of the MoOx thin films on ZnPc:C60 were determined in the temperature range 25‐200°C by ultraviolet photoelectron spectroscopy: a change from 4.75 eV to 5.20 eV was observed. In addition, annealing experiments in an N2 atmosphere were performed on complete ZnO/ZnPc:C60/MoOx/Ag device structures in the temperature range 25‐320°C. Whereas the morphology of the absorber films is only slightly affected by the annealing temperature, the efficiency of complete devices is significantly improved. In particular, some device efficiencies were increased twofold compared to initial values. At the same time, three slopes are observed on photoelectric curves. The observed slopes correlate well with the Ones detected on the WF graphs. Therefore we conclude that thermally activated processes take place predominantly at the ZnPc: ZnPc:C60/MoOx interface. The mechanisms of the latter processes are correlated to interface chemistry. 49 In‐situ monitoring the growth of polyaniline at liquid/solid interface by combining the polarized infrared spectroscopy and reflectance anisotropy spectroscopy 1
Guoguang Sun , Xin Zhang2, Christian Kaspari3, Kolja Haberland3, Jörg Rappich2, and Karsten Hinrichs1 1
Leibniz‐Institut für Analytische Wissenschaften ‐ ISAS ‐ e.V., Department Berlin, Albert‐Einstein‐Str. 9, 12489, Berlin, Germany. 2
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium Photovoltaik, Kekuléstr. 5, 12489, Berlin, Germany 3
LayTec GmbH, Seesener Str. 10‐13, 10709, Berlin, Germany An experimental set‐up combined polarized infrared spectroscopy with reflectance anisotropy spectroscopy (RAS) [1] was applied to in‐situ monitor the growth process of a polyaniline (PANI) film synthesized by electrochemical pulse potentiostatic method from aqueous solution. The composition of the as‐
deposited films can be determined by the IR signature. An acceleration effect on the growth was found in the presence of PSS dopant based on the IR and RAS results. The financial support of the European Union through the EFRE program and Investitionsbank Berlin (IBB) through the ProFIT program (LiquiRAS, contract Nr.10144387) is gratefully acknowledged. [1] G. Sun, X. Zhang, C. Kaspari, K. Haberland, J. Rappich, K. Hinrichs, J. Electrochem. Soc. 2012, 159 H811‐H815. 50 Impact of Fluorination on Initial Growth and Stability of Pentacene on Cu(111) H. Glowatzki1,3, G. Heimel2, A. Vollmer3, S. L. Wong1, H. Huang1, W. Chen1, A. T. S. Wee1, J. P. Rabe2, N. Koch2 1 National University of Singapore, Department of Physics, 2 Science Drive 3, 117542, Singapore 2 Humboldt ‐Universität zu Berlin, Institut für Physik, Newtonstr. 15, D‐12489 Berlin, Germany 3 Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH Bessy II, Albert‐
Einstein‐Str. 15, 12489 Berlin, Germany In this study, the organic semiconductor pentacene (PEN) and its fully fluorinated analogue perfluoropentacene (PFP) were investigated on Cu(111). The structure, growth, thermal stability, and electronic properties of thin films of PFP were determined by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and X‐ray photoelectron spectroscopy (XPS). In contrast to PEN, where at room temperature molecules could only be imaged by STM at full monolayer coverage, PFP was found to stabilize in disordered clusters already in the sub‐monolayer regime. Moreover, PFP formed only a disordered first wetting layer, whereas for PEN long‐
range order was already observed for closed molecular monolayers. However, highly ordered domains of PFP molecules were found for the second layer. In this layer the molecular planes are inclined to the surface, as supported by additional STM measurements on graphite and theoretical modeling. We conclude that careful consideration of the structural details in the transitional growth regime from molecular mono‐ to multilayers is a key factor to achieving a deeper understanding of metal/organic hybrid interfaces relevant for organic electronic devices. 51 PbS and PbSe‐nanocrystallites for applications in OPV Juliane Kristen, Burkhard Elling, Armin Wedel Fraunhofer Institut für Angewandte Polymerforschung (IAP), Geiselbergstrasse 69, 14476 Potsdam‐Golm Nanoparticles have become a popular field of research because of their unique properties. Among them, quantum dots are of special interest and offer a broad range of applications in various fields. They can be described as inorganic semiconductors having a size between 1 and 10 nm with different shapes such as platelets, stars and many more [1], which depends on the chemical synthesis procedure. The nanocrystallites can be characterized by their composition, band gap, crystal phase, crystallinity, crystal structure, size distribution and surface stabilization. Colloidal chemistry offers a smart and controlled access to these particles while simultaneously enabling efficient and easy cleaning of the end product [2]. Tuning of the particles’ emission over the whole visible and IR‐
spectrum becomes possible leading to materials with tailor made properties prepared from one substance that can be embedded in different matrices. Of special interest is the application of IR‐active nanocrystallites which offers new and interesting features like quantum dot based solar cells [3,4] or LEDs, so called Q‐LEDs [5,6], LDS (luminescent down‐shifting)‐photodetectors [7] or applications in biology and medicine [8,9]. We synthesized PbS and PbSe QDs of high quality for successful incorporation in solar cells and fluorescent concentrators. [1] Y. Xia, et al., Angewandte Chemie 2009, 121, 62‐108. [2] D. V. Talapin, et al., Chemical Reviews 2010, 110, 389‐458. [3] H. J. Lee, P. Chen, S.‐J. Moon, F. Sauvage, K. Sivula, T. Bessho, D. R. Gamelin, P. Comte, S. M. Zakeeruddin, S. I. Seok, M. Gräzel, Md. K. Nazeeruddin, Langmuir 2009, 25, 13, 7602‐7608. [4] W. Lee, S. K. Min, T. Park, W. Yi, S.‐H. Han, Materials Science and Engineering 2009: Part B, 156, 48‐51. [5] E. H. Sargent, Advanced Materials2005, 17, 5, 515‐522 [6] J. S. Steckel, S. Coe‐Sullivan, V. Bulovic, M. G. Bawendi, Advanced Materials 2003, 15, No. 21, 1862‐1866. [7] S. M. Geyer, J. M. Scherer, N. Moloto, F. B. Jaworski, M. G. Bawendi, ACS Nano 2011, 5, 7, 5566–5571. [8] T. Jamieson, R. Bakhshi, D. Petrova, R. Pocock, M. Imani, A. M. Seifalian, Biomaterials 2007, 28, 4717‐4732. [9] H. M. E. Azzazy, M. M. H. Mansour, S. C. Kazmierczak, Clinical Biochemistry 2007, 40, 917‐927. 52 Photo‐degradation in ladder‐type para‐phenylenes: Beyond fluorenone defects Björn Kobin1, Francesco Bianchi2, Simon Halm2, Fritz Henneberger2, and Stefan Hecht*1 Humboldt‐Universität zu Berlin, Institut für Chemie, Brook‐Taylor‐Str. 2, 12489 Berlin (Germany) Humboldt‐Universität zu Berlin, Institut für Physik, Newtonstr. 14, 12489 Berlin (Germany) Ladder‐type para‐phenylenes, such as the corresponding quarterphenyl L4P, are promising candidates for hybrid inorganic‐organic optoelectronic devices due to their narrow and intense optical transitions, small Stokes shift, and high fluorescence quantum yield.[1] However, after extended UV‐illumination, a decreasing L4Pluminescence accompanied by the appearance of a low‐energy (green) emission was observed. In polyfluorenes, such effect is commonly attributed to energy‐trapping fluorenone defects.[2] In the case of L4P as a small molecule, kinetic studies of the degradation process indicate that at least two photochemical reaction steps are involved in the formation of the green emitting defect species. Comparison of the optical properties and mass spectrometry data of degraded L4P with reference compounds indicate that the mainproduct associated with the green emission does not contain a fluorenone defect. In addition, we observe that the degradation kinetics of p‐quarterphenyl 4P is very similar to L4P, which underlines that fluorenone formation is not the favored process. Time‐resolved photoluminescence spectra clearly prove a dominant energy‐transfer from L4P towards the green emitting defect. [1] B. Kobin, L. Grubert, S. Blumstengel, F. Henneberger, S. Hecht, J. Mater. Chem. 2012, 22, 4383‐4390. [2] One of the first examples: V. N. Bliznyuk, S. A. Carter, J. C. Scott, G. Klärner, R. D. Miller. D. C. Miller, Macromolecules, 1999, 32, 361‐369. 53 Covalent Attachment of Porphyrin Dyes to Silicon (111) for Light Harvesting Nicholas Alderman1,2, Lefteris Danos 1,3, Martin Grossel2, Tom Markvart1 1 Solar Energy Laboratory, University of Southampton, Highfield, Southampton, SO17 1BJ (UK) 2
Faculty of Natural and Environmental Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ (UK) 3
Department of Chemistry, Lancaster Univeristy, Lancaster, LA1 4YB (UK) A technique for the covalent attachment of porphyrins to a silicon (111) surface has been developed to enhance the output of thin‐silicon solar cells. Thin film crystalline silicon solar cells have been produced in our laboratory at Southampton which have a device thickness in the order of 200 nm. The substantial reduction in the quantity of silicon, however, brings with it a low efficiency due to poor light absorption, and defines a fundamental challenge which represents one of the aims of our work: improving photoexcitation by ‘light harvesting’. Well known in photosynthesis, this process involves the sensitisation of solar cells by the near‐field interaction of dyes with silicon, and by analogy with Förster energy transfer between molecules, stimulating energy transfer and generating electron‐hole pairs in the silicon. The chromophore‐silicon distance dependency of the chromophore‐silicon distance on the fluorescence quenching was investigated through changing of the linker‐group chain length. A decrease in the fluorescence lifetime was observed at distances down to 6 Å, where the decay was in the order of the decay of the laser. The porphyrin‐functionalized silicon surfaces were characterised by external reflection infrared spectroscopy and X‐ray photoelectron spectroscopy. 54 Pulsed‐Laser Deposition of Silicon Nanowires Nicole Karpensky1, Sven Käbisch1, Marc Gluba1, Norbert Nickel1 1 Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Albert‐Einstein‐
Straße 15, 12489 Berlin (Germany) Nanostructuring of silicon surfaces opens a new field of various applications. For that, nanoscaled wires are especially desirable since their high aspect ratio enables large surface area at small interwire distances. This is very important for hybrid photovoltaic device since a larger surface will result in an increase of the conversion efficiency. Commonly, such structures are produced by a vapor‐liquid‐
solid (VLS) mechanism employing prepatterned gold templates. However, the use of gold is undesirable because it gives rise to the formation of intrinsic defects that limit the conversion efficiency of the solar cell. In contrast to VLS, it is possible to grow self‐organized nanostructures by pulsed laser deposition without the need of a catalyst. To elucidate this promising approach, silicon nanowires were grown on quartz substrates by pulsed laser deposition (PLD). The morphology and structural quality of the samples were characterized by scanning electron microscopy and Raman backscattering spectroscopy. The most To foster nanostructured growth different process gases were used. The most promising results were obtained with Helium. In addition, the influences of growth temperature and laser fluence on the morphology of the nanostructures were investigated. At elevated temperatures and a laser fluence of about 20 J/cm2 silicon nanostructures were obtained. Optimization of the growth parameters resulted in silicon nanowires with a diameter of about 20 nm and a height of ca. 170 nm. Raman measurements showed that the nanostructures a LO‐TO phonon mode was observed at 517‐520 cm‐1 55 Work function modification of ZnO poly‐crystalline films using short phenyl‐phosphonate layers 1
Nir Kedem , Sylke Blumstengel2, Fritz Henneberger2, Gary Hodes1, David Cahen1 1
Weizmann Institute of Science, Rehovot, 76100 (Israel) 2
Humboldt‐Universität zu Berlin, Brook‐Taylor‐Str. 6, 12489 Berlin (Germany) Transparent electrodes are a major part of future electronic technology development in general and of photovoltaic technology in particular. ZnO is a highly studied semiconductor due to its high carrier mobility, highly versatile morphology and transparency in the visible range. Modification of ZnO electronic properties at its surface can potentially extend its applicability and improve device performances beyond what is determined by bulk properties such as doping level and the work function of pure ZnO. In this work we demonstrate work function modifications of ZnO crystals of various morphologies. The crystals are grown either by Chemical Bath Deposition (CBD) as wires or polycrystalline film, Molecular Beam Epitaxy (MBE) as a single crystalline film and Atomic Layer Deposition (ALD) as polycrystalline film. Work function modification was done using dipole‐bearing molecular layers, chemisorbed onto the surface via a phosphonate group. Three types of molecules were used: cyanophenyl (PO3‐CN), methoxyphenyl (PO3‐OCH3) and phenyl (PO3‐
Phen) phosphonic acid. Kelvin probe measurements show up to 1.2 eV difference in work function, i.e. 4.2 vs. 5.4 ± 0.1 eV, with the methoxyphenyl and cyanophenyl surface terminations, respectively. Upon illumination with supra‐band gap light (365 nm), a work function reduction of 300 – 400 meV was measured for all samples, including samples, which were not covered by a molecular layer. The lower work function is stable for days after the illumination has been stopped. Similar phenomena occur under X‐irradiation, as a result of XPS analysis. When comparing a quick, low irradiation intensity scan, with a long, high irradiation intensity scan, there is a shift of the core levels (specifically of the Zn 2p one) towards higher binding energy, with similar magnitude as the work function shift. A 150‐200 meV offset towards higher binding energy of the Zn2p peak is also seen, when comparing PO3‐CN and other samples. This offset was constant at low and high x‐ray irradiation. A model in which the light‐induced work function change is decoupled from the molecular dipole effect is proposed. 56 Photovoltaic Applications of Silicon/PEDOT:PSS Hetero Structures Matthias Pietsch1, Florian Schechtel1, Silke Christiansen1,2 1
Max Planck Institute for the Science of Light Günther‐Scharowsky‐Str. 1, 91058 Erlangen (Germany) ² Helmholtz‐Zentrum Berlin, Kekuléstrasse 5, 12489 Berlin (Germany) Organic/inorganic hetero junctions using high conductive polymers have the potential to provide higher solar cell efficiencies and better long‐term stability than their all‐organic counterparts [1]. It combines the excellent electrical properties of inorganic materials with the easiness of processing of solution based conductive polymers [2]. In our present studies hetero junctions between Silicon and Poly(3,4‐ethylenedi‐
oxy‐thiophene):poly(styrenesulfonate) (PEDOT:PSS) were investigated on three‐
dimensional nanoscale architectures of silicon nanowires (SiNWs). These structures allow enhanced absorption and an enhanced junction area between the SiNW surface and the conducting polymer [3]. To improve the performance of the solar cell the influence of different factors as the electrical properties of SiNW surfaces, the conductivity of PEDOT:PSS and the substrate doping concentration were analyzed in detail. Optimized planar hybrid Si/PEDOT:PSS solar cells show efficiencies of 7.5% and open circuit voltages of 600 mV. The enhancement is attributed to a combination of bulk and interface property changes of Si and PEDOT:PSS that were studied by means of photoresponse and dark‐current I‐V‐measurements and various spectroscopic characterization techniques. A detailed picture of the charge carrier injection process from Si into PEDOT:PSS was provided by Transient absorption spectroscopy (TPIA). The advantages of SiNW/PEDOT:PSS hybrid solar cells will be discussed. [1] S. Jeong, E. C. Garnett, S. Wang, Z. Yu, S. Fan, M. L. Brongersma, M. D. McGehee, Hybrid Silicon Nanocone‐Polymer Solar Cells, Nano Lett., 2012, 12 (6), 2971–2976. 57 Electric Field Distribution in Hybrid Solar Cells Containing Amorphous Silicon and Polymers 1
1
1
S. Schäfer , S. Albrecht , D. Neher , T. Schultze2, C. Erhard2, L. Korte2, B. Rech2, J. Wördenweber3, A. Gordijn3, U. Scherf4, I. Dumsch4 1
Universität Potsdam, Institute of Physics and Astronomy, Karl‐Liebknecht‐Str. 24‐
25, D‐14476 Potsdam, Germany (Germany) 2
Department of Silicon Photovoltaics, Helmholtz Center Berlin for Materials and Energy 1, Kekuléstr. 5, D‐12489 Berlin (Germany) 3
IEF5‐Photovoltaics, Research Center Jülich, 52425 Jülich (Germany) 4
Bergische Universität Wuppertal, Macromolecular Chemistry and Institute for Polymer Technology, Gauss‐Strasse 20, D‐42097 Wuppertal (Germany) We present a study on the performance and analysis of hybrid solar cells comprising a planar heterojunction between hydrogenated amorphous silicon (a‐
Si:H) and two different conjugated polymers, P3HT and PCPDTBT. A comparison of the modeled absorption spectra of the layer stack with the measured external quantum efficiency is used to investigate the contribution of the two materials to the photocurrent generation in the device. Although both materials contribute to the photocurrent, the devices exhibit poor quantum efficiencies and low short circuit currents. Bandstructure simulations of the hybrid layer structure reveal that an unfavorable electric field distribution within the planar multilayer structure limits the performance. By using electroabsorption measurements we can show that the electric field is extremely weak in the amorphous silicon but strong in the organic material. The situation changes drastically when a p‐doped polymer layer is used instead of pristine material. Doping not only increases the conductivity of the organic material, but also restores the electric field in the amorphous silicon layer. Optimized hybrid devices comprising thin doped P3HT layers exhibit power conversion efficiencies up to 2.84 % [1]. [1] S. Schäfer et al., submitted to Phys. Rev B. 58 Effect of dipole moment on energy level alignment and electronic structure of merocyanine dyes on Au(111) 1
S. Krause , M. Stolte2, F. Würthner2, and N. Koch1,3 1
Helmholtz‐Zentrum für Energie und Materialien GmbH, Hahn‐Meitner‐Platz 1, 14109 Berlin (Germany) 2 Institut für Organische Chemie & Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg (Germany) 3
Institut für Physik, Humboldt‐Universität zu Berlin, Brook‐Taylor‐Str. 6, 12489 Berlin (Germany)
Merocyanine dyes show good performance as active layers in organic thin film transistors [1] (OTFT) and high efficiencies in bulk‐heterojunction organic solar cells (OSC) of up to 4.8% in a tandem cell arrangement [2]. Furthermore, they also feature high absorption cross‐section allowing to keep the active layer in OSCs thin. This makes them also ideal candidates for dye‐sensitized solar cells. Such devices have an inverted solar cell arrangement with a high work function material as their top electrode. Because of its high work function and due to its weak chemical reactivity, allowing to study the molecules with minimal substrate influence, Au(111) was chosen as substrate in our study. The facts that merocyanine dyes have a permanent dipole moment and are highly asymmetric contradict the notion of avoiding strong variations of the electrostatic potential in organic thin films to improve charge carrier transport. In our photoelectron spectroscopy study four merocyanines with increasing dipole moment were chosen to investigate the effect of the dipole moment on the energy level alignment with the Au(111) substrate and the electronic structure of thin films. For the latter, the tendency to form dimers in the bulk crystal, as known from x‐ray diffraction measurements, is of particular interest. A clear splitting of the HOMO due electronic coupling of the dimer partners is observed for all investigated molecules. [1] L. Huang, et al., High‐Performance Organic Thin‐Film Transistor Based on a Dipolar Organic Semiconductor, Advanced Materials, 2012, 24(42), 5750‐5754. [2] V. Steinmann, et al., A simple merocyanine tandem solar cell with extraordinarily high open‐circuit voltage, Applied Physics Letters, 2011, 99(19). 59 Charge Generation in ZnO:PCPDTBT Hybrid Photovoltaic Devices Thomas J.K. Brenner1, Wiebke Ludwig2, Ümit Aksünger2, Sophie Gledhill², Martha Lux‐Steiner² and Dieter Neher1 1
Universität Potsdam, Institute of Physics & Astronomy, Karl‐Liebknecht‐Str. 24‐ 25, 14476 Potsdam (Germany) 2
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Hahn‐Meitner‐ Platz 1, 14109 Berlin (Germany) We present a study on hybrid photovoltaic devices consisting of nanostructured zinc oxide with varying nanowire aspect ratio and the low‐bandgap polymer PCPDTBT. Zinc oxide nanowires are grown entirely from solution at low temperatures, being compatible with flexible substrate technology. We investigate how nanowire length and morphology impact device performance. In particular, we are interested in the efficiency of charge generation at the organic/inorganic heterojunction. Therefore, we employ a technique called timedelayed collection field (TDCF), which allows studying charge generation via fast charge extraction from the device after applying a short laser pulse, and has proven to be very useful for organic solar cells [1]. We also look at how the presence of a molecular layer of PCBA, which has shown potential for improved charge generation in devices with planar ZnO films [2], influences device performance in cells with nanostructured zinc oxide as electron acceptor. [1] S. Albrecht et al., J. Phys. Chem. Lett. 2012, 3, 640. [2] Y. Vaynzof et al., Appl. Phys. Lett. 2010, 97, 033309. 60 Time‐dependent elemental diffusion at the Mg:Ag electrode/ MePc:C60 organic interface and correlation to electrical properties of solar cells S. Wiesner1, J.N. Audinot2, M. Rusu1 and M.Ch. Lux‐Steiner1 1
Institut für Heterogene Materialsysteme, Helmholtz‐Zentrum Berlin für Materialien und Energie, Lise‐Meitner Campus, Hahn‐Meitner‐Platz 1, 14109Berlin, Germany 2
Département Science et Analyse des Matériaux, Centre de Recherche Public ‐ Gabriel Lippmann, 4 rue du Brill, 4422 Belvaux, Luxembourg The most efficient vacuum‐processed small‐molecular‐weight photovoltaic devices have been prepared using a heterojunction based on metal (e.g., Cu, Zn) phthalocyanine (MePc) donor and fullerene (C60) acceptor materials. Recently, we have demonstrated 4.5%‐efficient single heterojunction glass/ITO/PEDOT/ MePc:C60 (1:1 by weight)/Mg:Ag (20:80 at. %) photovoltaic (PV) devices. The stability tests of the prepared PV devices show in the first weeks a rise of all the PV parameters. After a slight decrease to values that are still higher than the initial ones, all the PV parameters remain stable for thousands of hours. The observed behaviour of PV parameters is attributed to changes of the device fill factor. The latter PV parameter is directly influenced by such electric device parameters as series and parallel resistances. Since the electrodes are in direct contact with both species of the absorber, the variations of the latter parameter should point to variations of the electrodes work function and consequently to directly influence the open circuit voltage. The variations of the interfacial electrode work function, in particular of the Mg:Ag alloy cathode, may occur as a result of the redistribution of the constituent elements. The elemental distribution in investigated devices was studied by nano depth profiling using secondary ion mass spectrometry. The data were recorded as a function of time. The analysis shows Mg diffusion into MePc:C60 absorber whereas Ag retracts from the cathode/absorber interface. Mg diffusion was observed earlier to be beneficial for the device PV and electric parameters [1]. We will discuss our findings against the background of the performance and electrical properties of corresponding MePc:C60‐based organic solar cell devices. [1] M. Rusu, S. Wiesner, I. Lauermann, Ch.‐H. Fischer, K. Fostiropoulos, J. N. Audinot, Y. Fleming and M.Ch. Lux‐Steiner, Appl. Phys. Lett. 2010, 97, 073504. 61 In‐situ Synthesis of Semiconductor Nanoparticles Decorating Tubular J‐
aggregates Yan Qiao, Frank Polzer, Holm Kirmse, Egon Steeg, Stefan Kirstein, Jürgen P. Rabe Institut für Physik, Humboldt‐Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany Organic/inorganic hybrid nanomaterials, that combine the strengths of the individual organic and inorganic materials while compensating for their deficits, have recently attracted more and more attention. Quasi one‐dimensional (1‐D) nanomaterials, such as wires and tubes, have been intensively investigated owing to the benefit of dimensionality on electronic and optical properties [1]. Based on our previous results on silver [2], this work focuses on the fabrication of quasi 1‐D organic/inorganic hybrid nanostructures consisting of nanotubular J‐aggregates self‐assembled from amphiphilic cyanine dyes, and semiconductor nanoparticles such as CdS decorating the surface of the nanotubes. Electrostatic interaction between the negative surface potential of the aggregates and the Cd2+ leads to the enrichment of Cd2+ within the ionic double layer close to the aggregate surface. Addition of thioactamide leads to the formation of CdS nanocrystals with diameters less than 6 nm. With investigation of cryo‐TEM and UV‐Vis spectroscopy, It is found that CdS nanoparticles are exclusively formed at the surface of the aggregates. This opens a route to the formation of efficiently electronically coupled organic/inorganic systems. [1] Y. N. Xia et al., Adv. Mater. 2003, 15, 353. [2] D. M. Eisele et al., J. Am. Chem. Soc. 2010, 132, 2104. 62 Synthesis of Dipolar Terminally Substituted Sexiphenyl Derivatives Yves Garmshausen1, Jutta Schwarz1, Michael Pätzel1, Stefan Hecht1 1
Humboldt‐Universität zu Berlin, Brook‐Taylor‐Str. 6, 12489 Berlin (Germany) In the burgeoning field of organic electronics there are few small molecules that have sparked and dominated innovation. Most of these molecules are planar, rigid, and consist of an extended π‐system [1]. Among these one of the best studied examples is para‐sexiphenyl (6P), which consists of six phenyl moieties connected via single bonds in a rod like manner. 6P is known to grow in well‐
ordered thin films on surfaces, such as KCl(001), GaAs(001), mica(001), TiO2(110), Au(111) or Al(111). The crystal structure and therefore properties of the first layers can be controlled using different deposition rates or sample temperatures. Furthermore, surface termination plays a central role for structure formation [2]. An interesting “bottom‐up” approach is aiming at controlling the self‐assembly on the surface by varying the molecular building block rather than the surface. Therefore, we wanted to introduce a dipole into 6P by substitution with a donor or acceptor group on one terminus only. Although the first synthesis of 6P was reported by Pummerer und Bittner as early as in 1924 [3], no end‐substituted 6P derivatives with a net dipole are known. Hence, the challenge is to synthesize such a terminally substituted, non‐symmetric 6P derivative yet without solubilizing side chains that would cause a distortion and consequently decoupling of the phenyl units. It is important to note that the desired optical and electronic properties go hand in hand with their low solubility, which significantly challenges the synthetic chemist in their preparation and purification. Here, we describe a new modular synthetic route to meet this challenge by providing access to a versatile, well soluble precursor that is planarized in the ultimate step of the synthesis. Using this method we have been able to prepare a series of terminally substituted sexiphenyls by introducing various electron withdrawing or electron donating groups. The self‐assembly of these new taylor‐
made 6Ps on several substrate surfaces and the optoelectronic properties of the resulting hybrid organic/inorganic systems have been studied. [1] G. Witte, C. Wöll, J. Mater. Res. 2004, 19, 1889‐1916. [2] R. Resel, J. Phys.: Condens. Matter 2008, 20, 1‐10. [3] R. Pummerer, K. Bittner, Chem. Ber. 1924, 57, 84‐88. 63 Controlling nanocrystal quality and post synthetic treatment for creating efficient hybrid nanocrystal/polymer solar cells Michael Eck1,2, Simon Einwächter1,2, Michael Krüger1,2 1
Freiburg Materials Research Center (FMF), Albert‐Ludwigs‐Universität Freiburg, Stefan‐Meier‐Str. 21, 79104 Freiburg (Germany) 2
Department of Microsystems Engineering (IMTEK), Albert‐Ludwigs‐Universität Freiburg, Georges‐Köhler‐Allee 103, 79110 Freiburg, Germany (Germany) For achieving optimal power conversion efficiencies in hybrid nanocrystal/polymer solar cells, the quality of the nanocrystals (NCs) is of crucial importance. In fact, both NC quality and post synthetic NC treatment are influencing the solar cell performance. In this work we utilized a CdSe NC synthesis with hexadecanol (HDO) as a weakly coordinating ligand, to induce a less stable synthesis and a resulting reduced NC ligand sphere, for providing us with a system that can easily be utilized for our investigations. Moreover, this approach allows for a low temperature synthesis, which is enabling the use of in‐
situ synthesis monitoring microwave based setup [1]. Firstly, by monitoring the PL signal during the NC synthesis we could map the NC size and quality development, allowing for determining synthesis time points significant for our NC quality comparison. Secondly, the development of NC dispersivity after different post synthetic treatment times, whereas the organic ligand sphere is reduced to increase the effective NC conductivity [2] (which is also leading to NC aggregation), was mapped by using dynamic light scattering (DLS). We thereby found that the NC quality has an influence on the solar cell performance as well as the optimal post synthetic treatment time, which in turn depends on the stage in which the NC synthesis is stopped. Thus, we could determine criteria for both stopping the NC synthesis in its optimal time and determine criteria for finding the optimal post synthetic treatment time to allow for the creation of highest possible efficiency from each NC synthesis approach. Conclusively, we proved these findings to be valid also for other CdSe NC synthesis methods using a different ligand system (i.e. hexadecylamine/trioctylphosphine oxide) and/or a different cadmium precursor. [1] S. Einwächter, M. Krüger, MRS Fall Meeting Proceedings 2010, 1284. [2] Y. Zhou, F. S. Riehle, Y. Yuan, H. Schleiermacher, M. Niggemann, G. A. Urban, M. Krüger, Appl. Phys. Lett. 2010, 96, 013304. 64 Work function modifications by self‐assembled monolayers: To the limits and beyond 1
2
Oliver T. Hofmann , David A. Egger , Ferdinand Rissner², Yong Xu1, Patrick Rinke1, Egbert Zojer2, and Matthias Scheffler1 1
Fritz‐Haber Institut der Max‐Planck‐Gesellschaft, Faradayweg 4‐6, 14195 Berlin (Germany) 2
Institut für Festkörperphysik, TU Graz, Petersgasse 16. 8010 Graz (Austria) Controlling the work function of semiconductor crystals is of critical importance for inorganic/organic hybrid devices, including photovoltaic cells. There, the relative offset of the Fermi level and the electronic levels of the active organic material determines important properties like charge injection and ‐extraction barriers. The adsorption of dipolar molecules forming self‐assembled monolayers (SAMs) is a common pathway to tune the work function of electrode materials. In the present contribution, we explore the limits of this approach and identify pathways to overcome them. In particular, we show that the dipole of a molecule in a self‐assembled monolayer is not directly correlated to the dipole in the gas phase, but rather limited by its band gap [1]. Moreover, for Fermi‐level pinned SAMs, the adsorption induced bond‐dipole and the dipole moment of the monolayer are closely related and typically oppose each other [2,3]. Based on these considerations, we deduce that molecules with a negative electron affinity and a large gap do not should be able to reduce the work‐
function of substrates essentially down to zero. As a proof of concept, we study the interaction between the ZnO‐surface and pyridine using a combination of density functional theory augmented with van‐der‐Waals interactions, and experimental thermal desorption and photoelectron spectroscopy. We observe an extraordinarily large pyridine induced work function change of up to ‐2.9 eV under ultrahigh vacuum conditions. The combination of experimental and theoretical methods allows us to validate the predictive power of our calculations, which then gives us microscopic insight into the interface properties and the mechanism that leads to the large work function change. [1] F. Rissner, et al., Organic Electronics 2012, 12, 3165‐3176. [2] D. A. Egger, et al., Phys. Chem. Chem. Phys. 2010, 70, 4291‐4294. [3] O. T. Hofmann, et al., Nano Letters 2010, 10, 4369‐4374. 65 Device Physics of Tandem Hybrid Solar Cells Using Amorphous Silicon and PCPDTBT:PCBM with 6.7% Efficiency 1
Steve Albrecht , Sebastian Schäfer1, Jan Wördenweber2, Björn Grootoonk2, Sebastian Neubert3, Steffen Roland1, Bernd Stannovski3, Bernd Rech3, Thomas J.K. Brenner1, and Dieter Neher1 1
Universität Potsdam, Institute of Physics & Astronomy, Karl‐Liebknecht‐Str. 24‐ 25, 14476 Potsdam (Germany) 2
Forschungszentrum Jülich GmbH, Institut für Energie‐ und Klimaforschung (IEK‐ 5), 52425 Jülich (Germany) 3
Helmholtz‐Zentrum Berlin für Materialien und Energie, Kekuléstraße 5, 12489 Berlin (Germany) Single layer solar cells with either amorphous silicon (a‐Si) or conjugated materials forming the active component now exceed 10 % efficiency. Interestingly, highlyefficient all‐organic solar cells usually comprise low bandgap polymers while the absorption range of a‐Si cells is limited to photon energies above around 2 eV. An attractive approach towards high efficiency cells is, therefore, to combine both material systems to hybrid tandem cells. Up to now, reports on such hybrid tandem cells are rare and their efficiencies remained below 6% [1,2]. One reason is that the organic sub‐cells need to combine sufficient absorption at longer wavelengths, efficient photon‐to‐current conversion and a high open circuit voltage; conditions which are difficult to realize at the same time with a single organic system. We have recently published an extensive study on the photovoltaic properties of organic solar cells with the low‐bandgap polymer PCPDTBT [3]. The identification of main loss processes allowed for a substantial improvement of the efficiency of these cells. This enabled us to form hybrid tandem devices with a record PCE of 6.7%. In this contribution, we will present the main characteristics of these cells and address possible ways of improving the efficiency, which includes light trapping mechanisms and the development of hybrid triple junction tandem devices. [1] Kim et al., Appl. Phys. Lett. 2011, 98, 183503. [2] Seo et al., Adv. Mater. 2012, 24, 33, 4523. [3] Albrecht et al., J. Am. Chem. Soc. 2012, 134, 14932. 66 A strong molecular acceptor for tuning the work function of ZnO Raphael Schlesinger1, Yong Xu2, Oliver T. Hofmann2, Stefanie Winkler3, Johannes Frisch1, Jens Niederhausen1, Antje Vollmer3, Sylke Blumstengel1, Fritz Henneberger1, Patrick Rinke2, Matthias Scheffler2, Norbert Koch1,3 1 Humboldt‐Universität zu Berlin, Institut für Physik, 12489 Berlin, Germany, 2 Fritz‐Haber‐Institut der Max‐Planck‐Gesellschaft, 14195 Berlin, Germany, 3
Helmholtz‐Zentrum Berlin ‐ BESSY II, 12489 Berlin, Germany Inorganic/organic semiconductor heterojunctions have opened up new opportunities for (opto‐) electronic devices due to their potential for combining the favorable properties of two distinct material classes. Being able to tune the alignment of the frontier energy levels of hybrid inorganic/organic systems is essential to control their function, e.g., to achieve energy or charge transfer across the interface. Employing molecular acceptor interlayers to tune the work function (φ) of a metal and thus the energy level alignment between the Fermi‐
level of the metal and the energy levels of organic semiconductors was reported previously [1]. In this contribution we extend this concept to ZnO by using the strong molecular acceptor 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ). We investigate the energy level alignment and interaction mechanism of F4TCNQ deposited on ZnO surfaces with photoemission. We find an extraordinarily large adsorption‐induced Φ increase (ΔΦ) of up to 2.8 eV. However, neither noticeably filled molecular states at or near EF in UPS, nor the emergence of shifted core level signals in X‐ray photoelectron spectroscopy (XPS), indicative of molecular anion formation, are observed. This implies minute electron transfer to F4TCNQ, in contrast to what was observed for the same mo lecule on metals for even smaller ΔΦ [2, 3]. We present a simple electrostatic model for the adsorption of acceptors on idealized semiconductor surfaces, which differentiates between the potential drop within the substrate due to internal band bending ΔΦBB [4] and the electrostatic potential drop between the surface and the organic acceptor due to charge transfer (i.e., interface dipole, ΔΦID). [1] N. Koch, S. Duhm, A. Vollmer, J. P. Rabe, R. L. Johnson, Phys. Rev. Lett. 2005, 95, 237601. [2] L. Romaner, G. Heimel, J. L. Brédas, A. Gerlach, F. Schreiber, R. L. Johnson, J. Zegenhagen, S. Duhm, N. Koch, and E. Zojer, Phys. Rev. Lett. 2007, 99, 256801. [3] G. M. Rangger, O. T. Hofmann, L. Romaner, G. Heimel, B. Bröker, R.‐P. Blum, R. L. Johnson, N. Koch, and E. Zojer, Phys. Rev. B 2009, 79, 165306. [4] W. Chen, D. Qi, X. Gao, and A. T. S. Wee, Prog. Surf. Sci. 2009, 84, 279. 67 Photoelectron Spectroscopy‐based Interface Analysis of ZnPc:C60/MoOx in Organic Solar Cells 1
Caterina Stenta , Alexander Steigert2, Iver Lauermann2, Volker Hinrichs2, Swen Wiesner2, Martha Ch. Lux‐Steiner2,3, and Marin Rusu2 1 Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus Universitário Monte de Caparica, Setúbal 2829‐516 (Portugal) 2
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Albert‐Einstein‐
Straße 15, 12489 Berlin (Germany) 3Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin (Germany) Control over the electronic structure of organic/inorganic semiconductor interfaces is a key step for the development of hybrid PV devices. In the present study this issue is addressed for an organic solar cell (OSC) with an inverted structure: an organic absorber layer of phthalocyanine:Fullerene heterojuntion, sandwiched between a front electrode of zinc oxide doped with aluminium, an intermediate buffer layer of molybdenum oxide and a silver electrode. The objective of this work has been the investigation of the electronic properties of the MoOx/ZnPc:C60 interface. For this purpose photoelectron spectroscopy (UPS and XPS) was used. Zinc phthalocyanine (ZnPc) was used as photoactive donor and Buckminsterfullerene (C60) as the electron acceptor material. Its role as an electron transport in polymer blend devices is well established. In this work glass is used as substrate and zinc oxide doped with aluminium as n‐type front electrode. Absorber/electrode interfaces in OSCs are here optimized by insertion of an intermediate buffer layer of molybdenum oxide to decrease the probability of charge recombination. Results will be presented on the interface analysis carried out by UPS for the study of the valence electron levels and by XPS to probe the core electron levels. Valence band offsets and work function values at MoOx/ZnPc:C60 interfaces have been characterized as a function of MoOx thickness. The work function changes with MoOx layer thickness from 4.1±0.1 eV (no MOx) to 4.9±0.1 eV (5nm MOx). At the same time the distance between the valence band and the Fermi level increased from 0.5±0.1 eV to 0.7±0.1 eV. These data were used to construct a band diagram of the interface energetics of these devices. 68 ZnO nanostructures and PCPDTBT for efficient hybrid solar cells Sven Käbisch 1,2 , Marc A. Gluba2, Thomas Brenner3, Dieter Neher3, Norbert H. Nickel2, and Norbert Koch1 1
Humboldt‐Universität zu Berlin, Brook‐Taylor‐Str. 6, 12489 Berlin (Germany) 2
Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium‐Photovoltaik, Kekuléstr. 5, 12489 Berlin (Germany) 3
Universität Potsdam, Hybrid Photovoltaics & Optoelectronics Group, Karl‐
Liebknecht‐Straße 24–25, 14476 Potsdam‐Golm (Germany) Zinc oxide (ZnO) is a transparent wide‐gap semiconductor forming a rich diversity of nano‐ and microstructures. Commonly, it is used as a transparent conductive wide band‐gap semiconductor in photovoltaic devices. Hybrid devices consisting of inorganic and organic materials benefit from nanostructured ZnO by maximizing the interface area between the two materials. In this study, planar and nanostructured ZnO films are used in conjunction with the small band‐gap polymer Poly[2,6‐(4,4‐bis‐(2‐ ethylhexyl)‐4H‐cyclopenta [2,1‐b;3,4‐
b′]dithiophene)‐alt‐4,7(2,1,3‐ benzothiadiazole)] (PCPDTBT) to form a hybrid solar cell. Catalystfree undoped ZnO nanowires are grown by pulsed laser deposition (PLD) on top of O‐terminated Al‐doped ZnO seed layers on coriented sapphire. The nanostructures are produced at a temperature of 700°C and possess a diameter of about 20 nm. Subsequently, the PCPDTBT is spin‐coated onto the nanostructures followed by a post‐deposition solvent annealing to effectively infiltrate the nanostructures. A top electrode consisting of MoO3 and gold completes the hybrid solar cell. The devices were characterized with a sun simulator under AM1.5 conditions. 69 Electrical and photoelectrical properties of hybrid heterojunction solar cells P3HT/n‐‐Si 1,2
1
Victor V. B rus , Xin Zhang , Stefanie M. Greil1, Matthias Zeellmeier1, 1
Jörg Rappich1, Norbert H. Nickel1 Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Institut für Si‐Photovoltaik, Kekuléstr. 5, 12489 Berlin (Germany) 2
Yuriy Fedkovych Chernivtsi National University, Department of electronics and energy engineering, Kotsubinskystr. 2, Chernivtsi 58000 (Ukraine) Hybrid organic‐inorganic P3HT/n‐Si heterojunction solar cells were prepared by the deposition of P3HT thin films (70‐80 nm) onto oxygen passivated n‐Si single crystal substrates (ρ =5 Ωcm) using the spin‐coating technique. The front electrical contact was a semitransparent gold layer with a thickness of about 20 nm prepared by means of thermal evaporation method. Dark and light current‐voltage characteristics as well as capacitance‐voltage characteristics of the hybrid heterojunctions under investigation were measured within the temperature range of 283 to 333 K and at different frequencies of the ac signal, respectively. The spectral distribution of the apparent quantum efficiency of our P3HT/n‐Si heterojunctions was measured at the wavelengths of monochromatic light from 400 to 1150 nm. The detailed analysis of electrical and photoelectrical properties of the fabricated hybrid solar cells was carried out and lead to the following conclusion. The tunneling‐recombination and space charge limited current mechanisms at forward biases as well as the hopping charge transport mechanism at reverse bias were determined to be the dominating current transport mechanisms in such structures. 70 The Role of Intermolecular Hybridization in Molecular Electrical Doping Ingo Salzmann,1 Henry Méndez,1 Georg Heimel,1 Katrein Sauer,1 Andreas Opitz,1 Patrick Barkowski,1 Martin Oehzelt,2,1 Junshi Soeda,3 Toshihiro Okamoto,3 Jun Takeya,3 Jean‐Baptiste Arlin,4 Jean‐Yves Balandier,4 Yves Geerts,4 and Norbert Koch1,2 1
Humboldt‐Universität zu Berlin, Department of Physics, 2
Helmholtz Zentrum Berlin für Materialien und Energie ‐ BESSY II 3
Osaka University, Institute of Scientific and Industrial Research (ISIR) 4
Chimie des Polymères CP/206 01, Université Libre de Bruxelles Molecular electrical doping of functional organic semiconductor (OSC) films is typically done by admixing strong molecular donors/acceptors as dopants. For p‐type doping, electron transfer between the highest occupied molecular orbital (HOMO) of the OSC and the lowest unoccupied molecular orbital (LUMO) of the p‐dopant is assumed to occur, leading to a localized electron on the dopant and a mobile hole in the OSC matrix. Recently, we reported this perception to be at odds with the absence of polarons even at considerable dopant ratios [1]. Instead of mutual ionization, frontier molecular orbital hybridization between the OSC‐
HOMO and the dopant‐LUMO occurs forming a doubly occupied bonding and an empty anti‐bonding supramolecular hybrid orbital with a reduced fundamental gap. As all available states are occupied following Fermi‐Dirac statistics, only a fraction of the complexes is ionized at room temperature rationalizing the high dopant concentrations used in practical applications. We extend this study by applying TCNQ‐derivatives with increasing degree of fluorination and, therefore, increasing acceptor strength as p‐dopants in solution‐processed OSC films. The systems are structurally well defined and show a decrease of the hybrid gap with increasing electron affinity (EA) of the dopant, which allows systematically assessing the requirements for efficient molecular dopants: In addition to the general prerequisite of the OSC ionization energy to be in the range of the dopant EA, reducing the degree of hybridization in order to reduce the energy‐level splitting in the complex emerges as design strategy for improved molecular dopants in organic electronic devices. [1] I. Salzmann, G. Heimel et al., Phys. Rev. Lett. 2012, 108, 035502. 71