@article{BlakesleySchubertSteyrleuthneretal.2011, author = {Blakesley, James C. and Schubert, Marcel and Steyrleuthner, Robert and Chen, Zhihua and Facchetti, Antonio and Neher, Dieter}, title = {Time-of-flight measurements and vertical transport in a high electron-mobility polymer}, series = {Applied physics letters}, volume = {99}, journal = {Applied physics letters}, number = {18}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.3657827}, pages = {3}, year = {2011}, abstract = {We investigate charge transport in a high-electron mobility polymer, poly(N, N-bis 2-octyldodecyl-naphthalene-1,4,5,8-bis dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene) [P(NDI2OD-T2), Polyera ActivInk (TM) N2200]. Time-of-flight measurements reveal electron mobilities approaching those measured in field-effect transistors, the highest ever recorded in a conjugated polymer using this technique. The modest temperature dependence and weak dispersion of the transients indicate low energetic disorder in this material. Steady-state electron-only current measurements reveal a barrier to injection of about 300 meV. We propose that this barrier is located within the P(NDI2OD-T2) film and arises from molecular orientation effects.}, language = {en} } @article{KniepertSchubertBlakesleyetal.2011, author = {Kniepert, Juliane and Schubert, Marcel and Blakesley, James C. and Neher, Dieter}, title = {Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments}, series = {The journal of physical chemistry letters}, volume = {2}, journal = {The journal of physical chemistry letters}, number = {7}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz200155b}, pages = {700 -- 705}, year = {2011}, abstract = {Time-delayed collection field (TDCF) experiments are performed on bulk heterojunction solar cells comprised of a blend of poly(3-hexylthiophene) and [6,6]-phenyl C-71 butyric acid methyl ester. TDCF is analogous to a pump-probe experiment using optical excitation and an electrical probe with a resolution of < 100 ns. The number of free charge carriers extracted after a short delay is found to be independent of the electric field during illumination. Also, experiments performed with a variable delay between the optical excitation and the electrical probe do not reveal any evidence for the generation of charge via field-assisted dissociation of bound long-lived polaron pairs. Photocurrent transients are well fitted by computational drift diffusion simulations with only direct generation of free charge carriers. With increasing delay times between pump and probe, two loss mechanisms are identified; first, charge-carriers are swept out of the device by the internal electric field, and second, bimolecular recombination of the remaining carriers takes place with a reduced recombination coefficient.}, 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{SteyrleuthnerDiPietroCollinsetal.2014, author = {Steyrleuthner, Robert and Di Pietro, Riccardo and Collins, Brian A. and Polzer, Frank and Himmelberger, Scott and Schubert, Marcel and Chen, Zhihua and Zhang, Shiming and Salleo, Alberto and Ade, Harald W. and Facchetti, Antonio and Neher, Dieter}, title = {The Role of Regioregularity, Crystallinity, and Chain Orientation on Electron Transport in a High-Mobility n-Type Copolymer}, series = {Journal of the American Chemical Society}, volume = {136}, journal = {Journal of the American Chemical Society}, number = {11}, publisher = {American Chemical Society}, address = {Washington}, issn = {0002-7863}, doi = {10.1021/ja4118736}, pages = {4245 -- 4256}, year = {2014}, language = {en} } @article{BangeSchubertNeher2010, author = {Bange, Sebastian and Schubert, Marcel and Neher, Dieter}, title = {Charge mobility determination by current extraction under linear increasing voltages : case of nonequilibrium charges and field-dependent mobilities}, issn = {1098-0121}, doi = {10.1103/Physrevb.81.035209}, year = {2010}, abstract = {The method of current extraction under linear increasing voltages (CELIV) allows for the simultaneous determination of charge mobilities and charge densities directly in thin-film geometries as used in organic photovoltaic (OPV) cells. It has been specifically applied to investigate the interrelation of microstructure and charge-transport properties in such systems. Numerical and analytical calculations presented in this work show that the evaluation of CELIV transients with the commonly used analysis scheme is error prone once charge recombination and, possibly, field- dependent charge mobilities are taken into account. The most important effects are an apparent time dependence of charge mobilities and errors in the determined field dependencies. Our results implicate that reports on time-dependent mobility relaxation in OPV materials obtained by the CELIV technique should be carefully revisited and confirmed by other measurement methods.}, language = {en} } @article{SchubertSteyrleuthnerBangeetal.2009, author = {Schubert, Marcel and Steyrleuthner, Robert and Bange, Sebastian and Sellinger, Alan and Neher, Dieter}, title = {Charge transport and recombination in bulk heterojunction solar cells containing a dicyanoimidazole-based molecular acceptor}, issn = {1862-6300}, doi = {10.1002/pssa.200925312}, year = {2009}, abstract = {Carrier transport and recombination have been studied in single component layers and blends of the soluble PPV- derivative poly[2,5-dimethoxy-1,4-phenylenevinylene-2-methoxy-5-(2-ethyl-hexyloxy)- 1,4-phenylenevinylene] (M3EH-PPV) and the small molecule acceptor 4,7-bis(2-(1-hexyl-4,5-dicyanoimidazole-2-yl)vinyl) benzo[c][1,2,5]-thiadiazole (HV-BT). Measurements on single carrier devices show significantly smaller electron mobility in the blend compared to the pure HV- BT layer, which is suggestive of the formation of isolated clusters of the acceptor in a continuous polymer matrix. The significant change in fill factor (FF) with increasing illumination intensity is consistently explained by a model taking into account bimolecular recombination and space charge effects. The decay of the carrier density after photoexcitation has been studied by performing photo-CELIV measurements on pure and blend layers. It is found that the decay at long delay times follows a power-law dependence, which is, however, not consistent with a Langevin-type bimolecular recombination of free charges. A good description of the data is obtained by assuming trimolecular recombination to govern the charge carrier dynamics in these systems.}, language = {en} } @article{SchubertYinCastellanietal.2009, author = {Schubert, Marcel and Yin, Chunhong and Castellani, Mauro and Bange, Sebastian and Tam, Teck Lip and Sellinger, Alan and Hoerhold, Hans-Heinrich and Kietzke, Thomas and Neher, Dieter}, title = {Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells}, issn = {0021-9606}, doi = {10.1063/1.3077007}, year = {2009}, abstract = {The authors present organic photovoltaic (OPV) devices comprising a small molecule electron acceptor based on 2- vinyl-4,5-dicyanoimidazole (Vinazene (TM)) and a soluble poly(p-phenylenevinylene) derivative as the electron donor. A strong dependence of the fill factor (FF) and the external quantum efficiency [incident photons converted to electrons (IPCE)] on the heterojunction topology is observed. As-prepared blends provided relatively low FF and IPCE values of 26\% and 4.5\%, respectively, which are attributed to significant recombination of geminate pairs and free carriers in a highly intermixed blend morphology. Going to an all-solution processed bilayer device, the FF and IPCE dramatically increased to 43\% and 27\%, respectively. The FF increases further to 57\% in devices comprising thermally deposited Vinazene layers where there is virtually no interpenetration at the donor/acceptor interface. This very high FF is comparable to values reported for OPV using fullerenes as the electron acceptor. Furthermore, the rather low electron affinity of Vinazene compound near 3.5 eV enabled a technologically important open circuit voltage (V-oc) of 1.0 V.}, 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{SchubertDolfenFrischetal.2012, author = {Schubert, Marcel and Dolfen, Daniel and Frisch, Johannes and Roland, Steffen and Steyrleuthner, Robert and Stiller, Burkhard and Chen, Zhihua and Scherf, Ullrich and Koch, Norbert and Facchetti, Antonio and Neher, Dieter}, title = {Influence of aggregation on the performance of All-Polymer Solar Cells containing Low-Bandgap Naphthalenediimide Copolymers}, series = {dvanced energy materials}, volume = {2}, journal = {dvanced energy materials}, number = {3}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201100601}, pages = {369 -- 380}, year = {2012}, abstract = {The authors present efficient all-polymer solar cells comprising two different low-bandgap naphthalenediimide (NDI)-based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4\%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70\%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near-field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large-scale phase-separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donoracceptor heterojunction.}, language = {en} } @article{FrischSchubertPreisetal.2012, author = {Frisch, Johannes and Schubert, Marcel and Preis, Eduard and Rabe, J{\"u}rgen P. and Neher, Dieter and Scherf, Ullrich and Koch, Norbert}, title = {Full electronic structure across a polymer heterojunction solar cell}, series = {Journal of materials chemistry}, volume = {22}, journal = {Journal of materials chemistry}, number = {10}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {0959-9428}, doi = {10.1039/c1jm14968g}, pages = {4418 -- 4424}, year = {2012}, abstract = {We correlate the morphology and energy level alignment of bilayer structures comprising the donor poly(3-hexylthiophene) (P3HT) and the acceptor polyfluorene copolymer poly(9,90dialklylfluorene-alt-4,7-bis(2,5-thiendiyl)-2,1,3-benzothiadiazole) (PFTBTT) with the performance of these bilayers in organic photovoltaic cells (OPVCs). The conducting polymer poly(ethylenedioxythiophene): poly (styrenesulfonate) (PEDT:PSS) was used as the bottom electrode and Ca as the top electrode. Ultraviolet photoelectron spectroscopy (UPS) revealed that notable interface dipoles occur at all interfaces across the OPVC structure, highlighting that vacuum level alignment cannot reliably be used to estimate the electronic properties for device design. Particularly the effective electrode work function values (after contact formation with the conjugated polymers) differ significantly from those of the pristine electrode materials. Chemical reactions between PEDT: PSS and P3HT on the one hand and Ca and PFTBTT on the other hand are identified as cause for the measured interface dipoles. The vacuum level shift between P3HT and PFTBTT is related to mutual energy level pinning at gap states. Annealing induced morphological changes at the P3HT/PFTBTT interface increased the efficiency of OPVCs, while the electronic structure was not affected by thermal treatment.}, language = {en} }