@article{KniepertLangevanderKaapetal.2014, author = {Kniepert, Juliane and Lange, Ilja and van der Kaap, Niels J. and Koster, L. Jan Anton and Neher, Dieter}, title = {A conclusive view on charge generation, recombination, and extraction in As-prepared and annealed P3HT:PCBM blends: combined experimental and simulation work}, series = {dvanced energy materials}, volume = {4}, journal = {dvanced energy materials}, number = {7}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201301401}, pages = {11}, year = {2014}, abstract = {Time-delayed collection field (TDCF) and bias-amplified charge extraction (BACE) are applied to as-prepared and annealed poly(3-hexylthiophene):[6,6]-phenyl C-71 butyric acid methyl ester (P3HT:PCBM) blends coated from chloroform. Despite large differences in fill factor, short-circuit current, and power conversion efficiency, both blends exhibit a negligible dependence of photogeneration on the electric field and strictly bimolecular recombination (BMR) with a weak dependence of the BMR coefficient on charge density. Drift-diffusion simulations are performed using the measured coefficients and mobilities, taking into account bimolecular recombination and the possible effects of surface recombination. The excellent agreement between the simulation and the experimental data for an intensity range covering two orders of magnitude indicates that a field-independent generation rate and a density-independent recombination coefficient describe the current-voltage characteristics of the annealed P3HT: PCBM devices, while the performance of the as-prepared blend is shown to be limited by space charge effects due to a low hole mobility. Finally, even though the bimolecular recombination coefficient is small, surface recombination is found to be a negligible loss mechanism in these solar cells.}, language = {en} } @misc{LiuTkachovKomberetal.2014, author = {Liu, W. and Tkachov, R. and Komber, H. and Senkovskyy, V. and Schubert, M. and Wei, Z. and Facchetti, A. and Neher, Dieter and Kiriy, A.}, title = {Chain-growth polycondensation of perylene diimide-based copolymers}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-98724}, pages = {8}, year = {2014}, abstract = {Herein, we report the chain-growth tin-free room temperature polymerization method to synthesize n-type perylene diimide-dithiophene-based conjugated polymers (PPDIT2s) suitable for solar cell and transistor applications. The palladium/electron-rich tri-tert-butylphosphine catalyst is effective to enable the chain-growth polymerization of anion-radical monomer Br-TPDIT-Br/Zn to PPDIT2 with a molecular weight up to Mw ≈ 50 kg mol-1 and moderate polydispersity. This is the second example of the polymerization of unusual anion-radical aromatic complexes formed in a reaction of active Zn and electron-deficient diimide-based aryl halides. As such, the discovered polymerization method is not a specific reactivity feature of the naphthalene-diimide derivatives but is rather a general polymerization tool. This is an important finding, given the significantly higher maximum external quantum efficiency that can be reached with PDI-based copolymers (32-45\%) in all-polymer solar cells compared to NDI-based materials (15-30\%). Our studies revealed that PPDIT2 synthesized by the new method and the previously published polymer prepared by step-growth Stille polycondensation show similar electron mobility and all-polymer solar cell performance. At the same time, the polymerization reported herein has several technological advantages as it proceeds relatively fast at room temperature and does not involve toxic tin-based compounds. Because several chain-growth polymerization reactions are well-suited for the preparation of well-defined multi-functional polymer architectures, the next target is to explore the utility of the discovered polymerization in the synthesis of end-functionalized polymers and block copolymers. Such materials would be helpful to improve the nanoscale morphology of polymer blends in all-polymer solar cells.}, language = {en} } @article{LiuTkachovKomberetal.2014, author = {Liu, W. and Tkachov, R. and Komber, H. and Senkovskyy, V. and Schubert, M. and Wei, Z. and Facchetti, A. and Neher, Dieter and Kiriy, A.}, title = {Chain-growth polycondensation of perylene diimide-based copolymers: a new route to regio-regular perylene diimide-based acceptors for all-polymer solar cells and n-type transistors}, series = {Polymer Chemistry}, volume = {5}, journal = {Polymer Chemistry}, number = {10}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1759-9954}, doi = {10.1039/c3py01707a}, pages = {3404 -- 3411}, year = {2014}, abstract = {Herein, we report the chain-growth tin-free room temperature polymerization method to synthesize n-type perylene diimide-dithiophene-based conjugated polymers (PPDIT2s) suitable for solar cell and transistor applications. The palladium/electron-rich tri-tert-butylphosphine catalyst is effective to enable the chain-growth polymerization of anion-radical monomer Br-TPDIT-Br/Zn to PPDIT2 with a molecular weight up to M-w approximate to 50 kg mol(-1) and moderate polydispersity. This is the second example of the polymerization of unusual anion-radical aromatic complexes formed in a reaction of active Zn and electron-deficient diimide-based aryl halides. As such, the discovered polymerization method is not a specific reactivity feature of the naphthalene-diimide derivatives but is rather a general polymerization tool. This is an important finding, given the significantly higher maximum external quantum efficiency that can be reached with PDI-based copolymers (32-45\%) in all-polymer solar cells compared to NDI-based materials (15-30\%). Our studies revealed that PPDIT2 synthesized by the new method and the previously published polymer prepared by step-growth Stille polycondensation show similar electron mobility and all-polymer solar cell performance. At the same time, the polymerization reported herein has several technological advantages as it proceeds relatively fast at room temperature and does not involve toxic tin-based compounds. Because several chain-growth polymerization reactions are well-suited for the preparation of well-defined multi-functional polymer architectures, the next target is to explore the utility of the discovered polymerization in the synthesis of end-functionalized polymers and block copolymers. Such materials would be helpful to improve the nanoscale morphology of polymer blends in all-polymer solar cells.}, language = {en} } @article{KraffertSteyrleuthnerAlbrechtetal.2014, author = {Kraffert, Felix and Steyrleuthner, Robert and Albrecht, Steve and Neher, Dieter and Scharber, Markus C. and Bittl, Robert and Behrends, Jan}, title = {Charge Separation in PCPDTBT : PCBM Blends from an EPR Perspective}, series = {The journal of physical chemistry}, volume = {118}, journal = {The journal of physical chemistry}, number = {49}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/jp509650v}, pages = {28482 -- 28493}, year = {2014}, language = {en} } @article{TremelFischerKayunkidetal.2014, author = {Tremel, Kim and Fischer, Florian S. U. and Kayunkid, Navaphun and Di Pietro, Riccardo and Tkachov, Roman and Kiriy, Anton and Neher, Dieter and Ludwigs, Sabine and Brinkmann, Martin}, title = {Charge transport anisotropy in highly oriented thin films of the acceptor polymer P(NDI2OD-T2)}, series = {dvanced energy materials}, volume = {4}, journal = {dvanced energy materials}, number = {10}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201301659}, pages = {13}, year = {2014}, abstract = {The nanomorphology of the high mobility polymer poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} P(NDI2OD-T2) in thin films is explored as a function of different annealing conditions and correlated to optical and electrical properties. While nanofibrils with face-on orientation in form I are obtained directly after spin-coating and annealing below the melt transition temperature, clear evidence of lamellar structures is found after melt-annealing followed by slow cooling to room temperature. Interestingly these structural changes are accompanied by distinct changes in the absorption patterns. Electron diffraction measurements further show clear transitions towards predominant edge-on oriented chains in form II upon melt-annealing. Large-scale alignment with dichroic ratios up to 10 and improved order is achieved by high temperature rubbing and subsequent post-rubbing annealing. These highly oriented morphologies allow anisotropic in-plane charge transport to be probed with top-gate transistors parallel and perpendicular to the polymer chain direction. Mobilities up to 0.1 cm(2) V-1 s(-1) are observed parallel to the polymer chain, which is up to 10 times higher than those perpendicular to the polymer chain.}, language = {en} } @article{SchubertCollinsMangoldetal.2014, author = {Schubert, Marcel and Collins, Brian A. and Mangold, Hannah and Howard, Ian A. and Schindler, Wolfram and Vandewal, Koen and Roland, Steffen and Behrends, Jan and Kraffert, Felix and Steyrleuthner, Robert and Chen, Zhihua and Fostiropoulos, Konstantinos and Bittl, Robert and Salleo, Alberto and Facchetti, Antonio and Laquai, Frederic and Ade, Harald W. and Neher, Dieter}, title = {Correlated donor/acceptor crystal orientation controls photocurrent generation in all-polymer solar cells}, series = {Advanced functional materials}, volume = {24}, journal = {Advanced functional materials}, number = {26}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201304216}, pages = {4068 -- 4081}, year = {2014}, abstract = {New polymers with high electron mobilities have spurred research in organic solar cells using polymeric rather than fullerene acceptors due to their potential of increased diversity, stability, and scalability. However, all-polymer solar cells have struggled to keep up with the steadily increasing power conversion efficiency of polymer: fullerene cells. The lack of knowledge about the dominant recombination process as well as the missing concluding picture on the role of the semi-crystalline microstructure of conjugated polymers in the free charge carrier generation process impede a systematic optimization of all-polymer solar cells. These issues are examined by combining structural and photo-physical characterization on a series of poly(3-hexylthiophene) (donor) and P(NDI2OD-T2) (acceptor) blend devices. These experiments reveal that geminate recombination is the major loss channel for photo-excited charge carriers. Advanced X-ray and electron-based studies reveal the effect of chloronaphthalene co-solvent in reducing domain size, altering domain purity, and reorienting the acceptor polymer crystals to be coincident with those of the donor. This reorientation correlates well with the increased photocurrent from these devices. Thus, effi cient split-up of geminate pairs at polymer/polymer interfaces may necessitate correlated donor/acceptor crystal orientation, which represents an additional requirement compared to the isotropic fullerene acceptors.}, language = {en} } @article{GehrigRolandHowardetal.2014, author = {Gehrig, Dominik W. and Roland, Steffen and Howard, Ian A. and Kamm, Valentin and Mangold, Hannah and Neher, Dieter and Laquai, Frederic}, title = {Efficiency-limiting processes in low-bandgap polymer:Perylene diimide photovoltaic blends}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {118}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, number = {35}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/jp503366m}, pages = {20077 -- 20085}, year = {2014}, abstract = {The charge generation and recombination processes following photo-excitation of a low-bandgap polymer:perylene diimide photovoltaic blend are investigated by transient absorption pump-probe spectroscopy covering a dynamic range from femto-to microseconds to get insight into the efficiency-limiting photophysical processes. The several tens of picoseconds, and its efficiency is only half of that in a polymer:fullerene photoinduced electron transfer from the polymer to the perylene acceptor takes up to blend. This reduces the short-circuit current. Time-delayed collection field experiments reveal that the subsequent charge separation is strongly field-dependent, limiting the fill factor and lowering the short-circuit current in polymer:PDI devices. Upon excitation of the acceptor in the low-bandgap polymer blend, the PDI exciton undergoes charge transfer on a time scale of several tens of picoseconds. However, a significant fraction of the charges generated at the interface are quickly lost because of fast geminate recombination. This reduces the short-circuit current even further, leading to a scenario in which only around 2596 of the initial photoexcitations generate free charges that can potentially contribute to the photocurrent. In summary, the key photophysical limitations of perylene diimide as an acceptor in low-bandgap polymer blends appear at the interface between the materials, with the kinetics of both charge generation and separation inhibited as compared to that of fullerenes.}, language = {en} } @article{VandewalAlbrechtHokeetal.2014, author = {Vandewal, Koen and Albrecht, Steve and Hoke, Eric T. and Graham, Kenneth R. and Widmer, Johannes and Douglas, Jessica D. and Schubert, Marcel and Mateker, William R. and Bloking, Jason T. and Burkhard, George F. and Sellinger, Alan and Frechet, Jean M. J. and Amassian, Aram and Riede, Moritz K. and McGehee, Michael D. and Neher, Dieter and Salleo, Alberto}, title = {Efficient charge generation by relaxed charge-transfer states at organic interfaces}, series = {Nature materials}, volume = {13}, journal = {Nature materials}, number = {1}, publisher = {Nature Publ. Group}, address = {London}, issn = {1476-1122}, doi = {10.1038/NMAT3807}, pages = {63 -- 68}, year = {2014}, abstract = {carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold viaweakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer: fullerene, small-molecule:C-60 and polymer: polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90\% without the need for excess electronic or vibrational energy.}, language = {en} } @article{AlbrechtGrootoonkNeubertetal.2014, author = {Albrecht, Steve and Grootoonk, Bjorn and Neubert, Sebastian and Roland, Steffen and Wordenweber, Jan and Meier, Matthias and Schlatmann, Rutger and Gordijn, Aad and Neher, Dieter}, title = {Efficient hybrid inorganic/organic tandem solar cells with tailored recombination contacts}, series = {Solar energy materials \& solar cells : an international journal devoted to photovoltaic, photothermal, and photochemical solar energy conversion}, volume = {127}, journal = {Solar energy materials \& solar cells : an international journal devoted to photovoltaic, photothermal, and photochemical solar energy conversion}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0927-0248}, doi = {10.1016/j.solmat.2014.04.020}, pages = {157 -- 162}, year = {2014}, abstract = {In this work, the authors present a 7.5\% efficient hybrid tandem solar cell with the bottom cell made of amorphous silicon and a Si-PCPDTBT:PC70BM bulk heterojunction top cell. Loss-free recombination contacts were realized by combing Al-doped ZnO with either the conducting polymer composite PEDOT:PSS or with a bilayer of ultrathin Al and MoO3. Optimization of these contacts results in tandem cells with high fill factors of 70\% and an open circuit voltage close to the sum of those of the sub-cells. This is the best efficiency reported for this type of hybrid tandem cell so far. Optical and electrical device modeling suggests that the efficiency can be increased to similar to 12\% on combining a donor polymer with suitable absorption onset with PCBM. We also describe proof-of-principle studies employing light trapping in hybrid tandem solar cells, suggesting that this device architecture has the potential to achieve efficiencies well above 12\%. (C) 2014 Elsevier B.V. All rights reserved.}, language = {en} } @article{RolandSchubertCollinsetal.2014, author = {Roland, Steffen and Schubert, Marcel and Collins, Brian A. and Kurpiers, Jona and Chen, Zhihua and Facchetti, Antonio and Ade, Harald W. and Neher, Dieter}, title = {Fullerene-free polymer solar cells with highly reduced bimolecular recombination and field-independent charge carrier generation}, series = {The journal of physical chemistry letters}, volume = {5}, journal = {The journal of physical chemistry letters}, number = {16}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz501506z}, pages = {2815 -- 2822}, year = {2014}, abstract = {Photogeneration, recombination, and transport of free charge carriers in all-polymer bulk heterojunction solar cells incorporating poly(3-hexylthiophene) (P3HT) as donor and poly([N,N'-bis(2-octyldodecyl)-naphthelene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) (P(NDI2OD-T2)) as acceptor polymer have been investigated by the use of time delayed collection field (TDCF) and time-of-flight (TOF) measurements. Depending on the preparation procedure used to dry the active layers, these solar cells comprise high fill factors (FFs) of up to 67\%. A strongly reduced bimolecular recombination (BMR), as well as a field-independent free charge carrier generation are observed, features that are common to high performance fullerene-based solar cells. Resonant soft X-ray measurements (R-SoXS) and photoluminescence quenching experiments (PQE) reveal that the BMR is related to domain purity. Our results elucidate the similarities of this polymeric acceptor with the superior recombination properties of fullerene acceptors.}, language = {en} }