TY - JOUR A1 - Gross, M. A1 - Müller, David C. A1 - Nothofer, Heinz-Georg A1 - Scherf, Ullrich A1 - Neher, Dieter A1 - Bräuchler, C. A1 - Meerholz, Klaus T1 - Improving the performance of doped p-conjugated polymers for use in organic light-emitting diodes Y1 - 2000 ER - TY - JOUR A1 - Joshi, Siddharth A1 - Pingel, Patrick A1 - Grigorian, Souren A1 - Panzner, Tobias A1 - Pietsch, Ullrich A1 - Neher, Dieter A1 - Forster, Michael A1 - Scherf, Ullrich T1 - Bimodal temperature behavior of structure and mobility in high molecular weight p3ht thin films N2 - We report a temperature dependent crystalline structure of spin-coated thin films of high molecular weight regioregular poly(3-hexylthiophene) (P3HT) (M-n similar to 30000 g/mol) and its correlation with charge carrier mobility. These investigations show a reversible change of the crystalline structure, where the interlayer lattice spacing (100)along the alkyl side chains continuously increases up to a temperature of about 220 degrees C; in contrast, the in-plane pi-pi distance reduces with increasing temperature. These changes in structure are reversible and can be repeated several times. The temperature-induced structural properties differ for thick and thin films, pointing to a surface/interface role in stabilization of the layer morphology. In contrast to the structural changes, the carrier mobility is rather constant in the temperature range from room temperature up to 100-120 degrees C, followed by a continuous decrease. For thick layers this drop is significant and the transistor performance almost vanishes at high temperature, however, it completely recovers upon cooling back to roorn temperature. The drop of the charge carrier mobility at higher temperatures is in contrast with expectations front the structural studies, considering the increase of crystalline fraction of the polycrystalline layer. our electrical measurements Underscore that the reduction of the macroscopic mobility is mostly caused by it pronounced decrease of the intergrain transport. The thermally induced crystallization along(100) direction and the creation of numerous small crystallites at the film-substrate interface reduce the number of long polymer chain, bridging crystalline domains, which ultimately limits the macroscopic charge transport. Y1 - 2009 UR - http://pubs.acs.org/journal/mamobx U6 - https://doi.org/10.1021/Ma900021w SN - 0024-9297 ER - TY - JOUR A1 - Schubert, Marcel A1 - Steyrleuthner, Robert A1 - Bange, Sebastian A1 - Sellinger, Alan A1 - Neher, Dieter T1 - Charge transport and recombination in bulk heterojunction solar cells containing a dicyanoimidazole-based molecular acceptor N2 - 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. Y1 - 2009 UR - http://www3.interscience.wiley.com/cgi-bin/jhome/40000761 U6 - https://doi.org/10.1002/pssa.200925312 SN - 1862-6300 ER - TY - JOUR A1 - Schubert, Marcel A1 - Yin, Chunhong A1 - Castellani, Mauro A1 - Bange, Sebastian A1 - Tam, Teck Lip A1 - Sellinger, Alan A1 - Hoerhold, Hans-Heinrich A1 - Kietzke, Thomas A1 - Neher, Dieter T1 - Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells N2 - 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. Y1 - 2009 UR - http://jcp.aip.org/ U6 - https://doi.org/10.1063/1.3077007 SN - 0021-9606 ER - TY - JOUR A1 - Inal, Sahika A1 - Castellani, Mauro A1 - Sellinger, Alan A1 - Neher, Dieter T1 - Relationship of photophysical properties and the device performance of novel hybrid small-molecular/polymeric solar cells N2 - We investigate solar cells comprised of a vinazene derivative (HV-BT) as the electron acceptor and the well- known polymer poly(3-hexylthiophene) as the electron donor. In the as-prepared blend, most of the excited state species, including the excimers on HV-BT, are quenched at the heterojunction. Although the photophysical properties of the blends change upon annealing, the blend solar cells largely remain uninfluenced by such treatments. A significant improvement is, however, observed when inducing phase separation at a longer length scale, for example, in solution-processed bilayer devices. Hereby, both the fill factor (FF) and the open circuit voltage are considerably increased, pointing to the importance of the heterojunction topology and the layer composition at the charge extracting contacts. An optimized device exhibits a power conversion efficiency of close to 1%. Y1 - 2009 UR - http://www3.interscience.wiley.com/cgi-bin/jhome/10003270 U6 - https://doi.org/10.1002/marc.200900221 SN - 1022-1336 ER - TY - JOUR A1 - Steyrleuthner, Robert A1 - Bange, Sebastian A1 - Neher, Dieter T1 - Reliable electron-only devices and electron transport in n-type polymers N2 - Current-voltage analysis of single-carrier transport is a popular method for the determination of charge carrier mobilities in organic semiconductors. Although in widespread use for the analysis of hole transport, only a few reports can be found where the method was applied to electron transport. Here, we summarize the experimental difficulties related to the metal electrode leakage currents and nonlinear differential resistance (NDR) effects and explain their origin. We present a modified preparation technique for the metal electrodes and show that it significantly increases the reliability of such measurements. It allows to produce test devices with low leakage currents and without NDR even for thin organic layers. Metal oxides were often discussed as a possible cause of NDR. Our measurements on forcibly oxidized metal electrodes demonstrate that oxide layers are not exclusively responsible for NDR effects. We present electron transport data for two electron-conducting polymers often applied in all-polymer solar cells for a large variety of layer thicknesses and temperatures. The results can be explained by established exponential trapping models. Y1 - 2009 UR - http://jap.aip.org/ U6 - https://doi.org/10.1063/1.3086307 SN - 0021-8979 ER - TY - JOUR A1 - Rengel, Heiko A1 - Altmann, Markus A1 - Neher, Dieter A1 - Harrison, Craig B. A1 - Myrick, Michael L. A1 - Bunz, Uwe H. F. T1 - Assignment of the optical transitions in 1,3- diethynylcyclobutadiene (cyclopentadienyl)cobalt oligomers Y1 - 1999 ER - TY - JOUR A1 - Miteva, T. A1 - Meisel, A. A1 - Nothofer, Heinz-Georg A1 - Scherf, Ullrich A1 - Knoll, W. A1 - Neher, Dieter A1 - Grell, M. A1 - Lupo, D. A1 - Yasuda, A. T1 - Polarized electroluminescence from highly aligned liquid-crystalline polymers Y1 - 2000 ER - TY - JOUR A1 - Miteva, T. A1 - Kloppenburg, L. A1 - Neher, Dieter A1 - Bunz, Uwe H. F. T1 - Interplay of Thermochromicity and Liquid Crystalline Behavior in Poly(p-phenyleneethynylen)s:p-p-Interactions or Planarization of the Conjugated Backbone? Y1 - 2000 ER - TY - JOUR A1 - Bauer, C. A1 - Umbasch, G. A1 - Giessen, H. A1 - Meisel, A. A1 - Nothofer, Heinz-Georg A1 - Neher, Dieter A1 - Scherf, Ullrich A1 - Marth, R. T1 - Polarized Photoluminescence and Spectral Narrowing in an oriented Polyfluorene Thin Film Y1 - 2000 ER - TY - JOUR A1 - Grell, M. A1 - Knoll, W. A1 - Lupo, D. A1 - Meisel, A. A1 - Miteva, T. A1 - Neher, Dieter A1 - Nothofer, Heinz-Georg A1 - Scherf, Ullrich A1 - Yasuda, H. T1 - Blue polarized electroluminescence from a liquid crystalline polyfluorene Y1 - 1999 ER - TY - JOUR A1 - Urayama, Kenji A1 - Kircher, Oliver A1 - Böhmer, Roland A1 - Neher, Dieter T1 - Investigations of ferroelectric-to-paraelectric phase transition of vinylidenefluoride trifluoroethylene copolymer thin films by electromechanical interferometry Y1 - 1999 ER - TY - JOUR A1 - Bittner, Reinhard A1 - Däubler, Thomas Karl A1 - Neher, Dieter A1 - Meerholz, Klaus T1 - Influence of the glass-transition and the chromophore content on the steady-state performance of PVK-based photorefractive polymers Y1 - 1999 ER - TY - JOUR A1 - Former, C. A1 - Wagner, H. A1 - Richert, R. A1 - Neher, Dieter A1 - Müllen, K. T1 - Orientation and dynamics of chainlike dipole arrays: Donor-acceptor-substituted oligophenylenevinylenes in a polymer matrix Y1 - 1999 ER - TY - JOUR A1 - Hosseini, Seyed Mehrdad A1 - Tokmoldin, Nurlan A1 - Lee, Young Woong A1 - Zou, Yingping A1 - Woo, Han Young A1 - Neher, Dieter A1 - Shoaee, Safa T1 - Putting order into PM6:Y6 solar cells to reduce the langevin recombination in 400 nm thick junction JF - Solar RRL N2 - Increasing the active layer thickness without sacrificing the power conversion efficiency (PCE) is one of the great challenges faced by organic solar cells (OSCs) for commercialization. Recently, PM6:Y6 as an OSC based on a non-fullerene acceptor (NFA) has excited the community because of its PCE reaching as high as 15.9%; however, by increasing the thickness, the PCE drops due to the reduction of the fill factor (FF). This drop is attributed to change in mobility ratio with increasing thickness. Furthermore, this work demonstrates that by regulating the packing and the crystallinity of the donor and the acceptor, through volumetric content of chloronaphthalene (CN) as a solvent additive, one can improve the FF of a thick PM6:Y6 device (approximate to 400 nm) from 58% to 68% (PCE enhances from 12.2% to 14.4%). The data indicate that the origin of this enhancement is the reduction of the structural and energetic disorders in the thick device with 1.5% CN compared with 0.5% CN. This correlates with improved electron and hole mobilities and a 50% suppressed bimolecular recombination, such that the non-Langevin reduction factor is 180 times. This work reveals the role of disorder on the charge extraction and bimolecular recombination of NFA-based OSCs. KW - charge carrier extraction KW - energetic disorders KW - non-fullerene acceptors KW - non-Langevin reduction factors KW - thick junctions Y1 - 2020 U6 - https://doi.org/10.1002/solr.202000498 SN - 2367-198X VL - 4 IS - 11 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Vandewal, Koen A1 - Albrecht, Steve A1 - Hoke, Eric T. A1 - Graham, Kenneth R. A1 - Widmer, Johannes A1 - Douglas, Jessica D. A1 - Schubert, Marcel A1 - Mateker, William R. A1 - Bloking, Jason T. A1 - Burkhard, George F. A1 - Sellinger, Alan A1 - Frechet, Jean M. J. A1 - Amassian, Aram A1 - Riede, Moritz K. A1 - McGehee, Michael D. A1 - Neher, Dieter A1 - Salleo, Alberto T1 - Efficient charge generation by relaxed charge-transfer states at organic interfaces JF - Nature materials N2 - 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. Y1 - 2014 U6 - https://doi.org/10.1038/NMAT3807 SN - 1476-1122 SN - 1476-4660 VL - 13 IS - 1 SP - 63 EP - 68 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Schubert, Marcel A1 - Dolfen, Daniel A1 - Frisch, Johannes A1 - Roland, Steffen A1 - Steyrleuthner, Robert A1 - Stiller, Burkhard A1 - Chen, Zhihua A1 - Scherf, Ullrich A1 - Koch, Norbert A1 - Facchetti, Antonio A1 - Neher, Dieter T1 - Influence of aggregation on the performance of All-Polymer Solar Cells containing Low-Bandgap Naphthalenediimide Copolymers JF - dvanced energy materials N2 - 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. KW - aggregation KW - morphology KW - naphthalenediimide KW - organic semiconductors KW - organic photovoltaics Y1 - 2012 U6 - https://doi.org/10.1002/aenm.201100601 SN - 1614-6832 VL - 2 IS - 3 SP - 369 EP - 380 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Frisch, Johannes A1 - Schubert, Marcel A1 - Preis, Eduard A1 - Rabe, Jürgen P. A1 - Neher, Dieter A1 - Scherf, Ullrich A1 - Koch, Norbert T1 - Full electronic structure across a polymer heterojunction solar cell JF - Journal of materials chemistry N2 - 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. Y1 - 2012 U6 - https://doi.org/10.1039/c1jm14968g SN - 0959-9428 VL - 22 IS - 10 SP - 4418 EP - 4424 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Albrecht, Steve A1 - Schaefer, Sebastian A1 - Lange, Ilja A1 - Yilmaz, Seyfullah A1 - Dumsch, Ines A1 - Allard, Sybille A1 - Scherf, Ullrich A1 - Hertwig, Andreas A1 - Neher, Dieter T1 - Light management in PCPDTBT:PC70BM solar cells: A comparison of standard and inverted device structures JF - Organic electronics : physics, materials and applications N2 - We compare standard and inverted bulk heterojunction solar cells composed of PCPDTBT:PC70BM blends. Inverted devices comprising 100 nm thick active layers exhibited short circuit currents of 15 mA/cm(2), 10% larger than in corresponding standard devices. Modeling of the optical field distribution in the different device stacks proved that this enhancement originates from an increased absorption of incident light in the active layer. Internal quantum efficiencies (IQEs) were obtained from the direct comparison of experimentally derived and modeled currents for different layer thicknesses, yielding IQEs of similar to 70% for a layer thickness of 100 nm. Simulations predict a significant increase of the light harvesting efficiency upon increasing the layer thickness to 270 nm. However, a continuous deterioration of the photovoltaic properties with layer thickness was measured for both device architectures, attributed to incomplete charge extraction. On the other hand, our optical modeling suggests that inverted devices based on PCPDTBT should be able to deliver high power conversion efficiencies (PCEs) of more than 7% provided that recombination losses can be reduced. KW - Organic solar cells KW - Inverted solar cells KW - PCPDTBT KW - Low band-gap KW - Optical modeling Y1 - 2012 U6 - https://doi.org/10.1016/j.orgel.2011.12.019 SN - 1566-1199 VL - 13 IS - 4 SP - 615 EP - 622 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Albrecht, Steve A1 - Schindler, Wolfram A1 - Kurpiers, Jona A1 - Kniepert, Juliane A1 - Blakesley, James C. A1 - Dumsch, Ines A1 - Allard, Sybille A1 - Fostiropoulos, Konstantinos A1 - Scherf, Ullrich A1 - Neher, Dieter T1 - On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM influence of solvent additives JF - The journal of physical chemistry letters N2 - We have applied time-delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) to investigate the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. The results suggest that the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimolecular recombination coefficients, which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a negative field dependence of mobility, which we suggest to cause bias-dependent recombination coefficients. Y1 - 2012 U6 - https://doi.org/10.1021/jz3000849 SN - 1948-7185 VL - 3 IS - 5 SP - 640 EP - 645 PB - American Chemical Society CY - Washington ER -