TY - JOUR A1 - Shivhare, Rishi A1 - Erdmann, Tim A1 - Hoermann, Ulrich A1 - Collado-Fregoso, Elisa A1 - Zeiske, Stefan A1 - Benduhn, Johannes A1 - Ullbrich, Sascha A1 - Huebner, Rene A1 - Hambsch, Mike A1 - Kiriy, Anton A1 - Voit, Brigitte A1 - Neher, Dieter A1 - Vandewal, Koen A1 - Mannsfeld, Stefan C. B. T1 - Alkyl Branching Position in Diketopyrrolopyrrole Polymers BT - Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells JF - Chemistry of materials : a publication of the American Chemical Society N2 - Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency. Y1 - 2018 U6 - https://doi.org/10.1021/acs.chemmater.8b02739 SN - 0897-4756 SN - 1520-5002 VL - 30 IS - 19 SP - 6801 EP - 6809 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Li, Hongguang A1 - Babu, Sukumaran Santhosh A1 - Turner, Sarah T. A1 - Neher, Dieter A1 - Hollamby, Martin J. A1 - Seki, Tomohiro A1 - Yagai, Shiki A1 - Deguchi, Yonekazu A1 - Möhwald, Helmuth A1 - Nakanishi, Takashi T1 - Alkylated-C-60 based soft materials regulation of self-assembly and optoelectronic properties by chain branching JF - Journal of materials chemistry : C, Materials for optical and electronic devices N2 - Derivatization of fullerene (C-60) with branched aliphatic chains softens C-60-based materials and enables the formation of thermotropic liquid crystals and room temperature nonvolatile liquids. This work demonstrates that by carefully tuning parameters such as type, number and substituent position of the branched chains, liquid crystalline C-60 materials with mesophase temperatures suited for photovoltaic cell fabrication and room temperature nonvolatile liquid fullerenes with tunable viscosity can be obtained. In particular, compound 1, with branched chains, exhibits a smectic liquid crystalline phase extending from 84 degrees C to room temperature. Analysis of bulk heterojunction (BHJ) organic solar cells with a ca. 100 nm active layer of compound 1 and poly(3-hexylthiophene) (P3HT) as an electron acceptor and an electron donor, respectively, reveals an improved performance (power conversion efficiency, PCE: 1.6 + 0.1%) in comparison with another compound, 10 (PCE: 0.5 + 0.1%). The latter, in contrast to 1, carries linear aliphatic chains and thus forms a highly ordered solid lamellar phase at room temperature. The solar cell performance of 1 blended with P3HT approaches that of PCBM/P3HT for the same active layer thickness. This indicates that C-60 derivatives bearing branched tails are a promising class of electron acceptors in soft (flexible) photovoltaic devices. Y1 - 2013 U6 - https://doi.org/10.1039/c3tc00066d SN - 2050-7526 VL - 1 IS - 10 SP - 1943 EP - 1951 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Li, Hongguang A1 - Babu, Sukumaran Santhosh A1 - Turner, Sarah T. A1 - Neher, Dieter A1 - Hollamby, Martin J. A1 - Tomohito, Seki A1 - Yagai, Shiki A1 - deguchi, Yonekazu A1 - Möhwald, Helmuth A1 - Nakanishi, Takashi T1 - Alkylated-C60 based soft materials: regulation of self-assembly and optoelectronic properties by chain branching N2 - Derivatization of fullerene (C60) with branched aliphatic chains softens C60-based materials and enables the formation of thermotropic liquid crystals and room temperature nonvolatile liquids. This work demonstrates that by carefully tuning parameters such as type, number and substituent position of the branched chains, liquid crystalline C60 materials with mesophase temperatures suited for photovoltaic cell fabrication and room temperature nonvolatile liquid fullerenes with tunable viscosity can be obtained. In particular, compound 1, with branched chains, exhibits a smectic liquid crystalline phase extending from 84°C to room temperature. Analysis of bulk heterojunction (BHJ) organic solar cells with a ca. 100 nm active layer of compound 1 and poly(3-hexylthiophene) (P3HT) as an electron acceptor and an electron donor, respectively, reveals an improved performance (power conversion efficiency, PCE: 1.6 ñ 0.1%) in comparison with another compound, 10 (PCE: 0.5 ñ 0.1%). The latter, in contrast to 1, carries linear aliphatic chains and thus forms a highly ordered solid lamellar phase at room temperature. The solar cell performance of 1 blended with P3HT approaches that of PCBM/P3HT for the same active layer thickness. This indicates that C60 derivatives bearing branched tails are a promising class of electron acceptors in soft (flexible) photovoltaic devices. Y1 - 2013 UR - http://pubs.rsc.org/en/content/articlepdf/2013/tc/c3tc00066d U6 - https://doi.org/10.1039/C3TC00066D ER - TY - JOUR A1 - Srikhirin, T. A1 - Cimrova, V. A1 - Schiewe, B. A1 - Tzolov, M. A1 - Hagen, R. A1 - Kostromine, S. A1 - Bieringer, Thomas A1 - Neher, Dieter T1 - An Investigation of the photoinduced changes of absoprtion of high-performance photoaddressable Polymers Y1 - 2002 ER - TY - JOUR A1 - Pranav, Manasi A1 - Hultzsch, Thomas A1 - Musiienko, Artem A1 - Sun, Bowen A1 - Shukla, Atul A1 - Jaiser, Frank A1 - Shoaee, Safa A1 - Neher, Dieter T1 - Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells JF - APL materials : high impact open access journal in functional materials science N2 - Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high-performance donor-acceptor systems. Here, we probe the origin of field-dependent free-charge generation and photoluminescence in wnon-fullereneacceptor (NFA)-based organic solar cells using the polymer PM6 and the NFA Y5-a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artifacts from non-geminate recombination of photogenerated and dark charge carriers near the electrodes. We then correlate free charge generation with steady-state photoluminescence intensity and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low-offset system is entirely controlled by the field-dependent dissociation of local excitons into charge-transfer states. (c) 2023 Author(s). Y1 - 2023 U6 - https://doi.org/10.1063/5.0151580 SN - 2166-532X VL - 11 IS - 6 PB - AIP Publishing CY - Melville ER - TY - JOUR A1 - Stolterfoht, Martin A1 - Wolff, Christian Michael A1 - Amir, Yohai A1 - Paulke, Andreas A1 - Perdigon-Toro, Lorena A1 - Caprioglio, Pietro A1 - Neher, Dieter T1 - Approaching the fill factor Shockley-Queisser limit in stable, dopant-free triple cation perovskite solar cells JF - Energy & Environmental Science N2 - Perovskite solar cells now compete with their inorganic counterparts in terms of power conversion efficiency, not least because of their small open-circuit voltage (V-OC) losses. A key to surpass traditional thin-film solar cells is the fill factor (FF). Therefore, more insights into the physical mechanisms that define the bias dependence of the photocurrent are urgently required. In this work, we studied charge extraction and recombination in efficient triple cation perovskite solar cells with undoped organic electron/hole transport layers (ETL/HTL). Using integral time of flight we identify the transit time through the HTL as the key figure of merit for maximizing the fill factor (FF) and efficiency. Complementarily, intensity dependent photocurrent and V-OC measurements elucidate the role of the HTL on the bias dependence of non-radiative and transport-related loss channels. We show that charge transport losses can be completely avoided under certain conditions, yielding devices with FFs of up to 84%. Optimized cells exhibit power conversion efficiencies of above 20% for 6 mm(2) sized pixels and 18.9% for a device area of 1 cm(2). These are record efficiencies for hybrid perovskite devices with dopant-free transport layers, highlighting the potential of this device technology to avoid charge-transport limitations and to approach the Shockley-Queisser limit. Y1 - 2017 U6 - https://doi.org/10.1039/c7ee00899f SN - 1754-5692 SN - 1754-5706 VL - 10 SP - 1530 EP - 1539 PB - Royal Society of Chemistry CY - Cambridge 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 - Lange, Ilja A1 - Blakesley, James C. A1 - Frisch, Johannes A1 - Vollmer, Antje A1 - Koch, Norbert A1 - Neher, Dieter T1 - Band bending in conjugated polymer layers JF - Physical review letters N2 - We use the Kelvin probe method to study the energy-level alignment of four conjugated polymers deposited on various electrodes. Band bending is observed in all polymers when the substrate work function exceeds critical values. Through modeling, we show that the band bending is explained by charge transfer from the electrodes into a small density of states that extends several hundred meV into the band gap. The energetic spread of these states is correlated with charge-carrier mobilities, suggesting that the same states also govern charge transport in the bulk of these polymers. Y1 - 2011 U6 - https://doi.org/10.1103/PhysRevLett.106.216402 SN - 0031-9007 VL - 106 IS - 21 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Perdigon-Toro, Lorena A1 - Zhang, Huotian A1 - Markina, Anastaa si A1 - Yuan, Jun A1 - Hosseini, Seyed Mehrdad A1 - Wolff, Christian Michael A1 - Zuo, Guangzheng A1 - Stolterfoht, Martin A1 - Zou, Yingping A1 - Gao, Feng A1 - Andrienko, Denis A1 - Shoaee, Safa A1 - Neher, Dieter T1 - Barrierless free charge generation in the high-performance PM6:Y6 bulk heterojunction non-fullerene solar cell JF - Advanced materials N2 - Organic solar cells are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high-performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near-unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier. KW - driving force KW - non-fullerene acceptors KW - organic solar cells KW - photocurrent generation Y1 - 2020 U6 - https://doi.org/10.1002/adma.201906763 SN - 0935-9648 SN - 1521-4095 VL - 32 IS - 9 PB - Wiley-VCH CY - Weinheim 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 -