TY - JOUR A1 - Al-Sa'di, Mahmoud A1 - Jaiser, Frank A1 - Bagnich, Sergey A. A1 - Unger, Thomas A1 - Blakesley, James C. A1 - Wilke, Andreas A1 - Neher, Dieter T1 - Electrical and optical simulations of a polymer-based phosphorescent organic light-emitting diode with high efficiency JF - Journal of polymer science : B, Polymer physics N2 - A comprehensive numerical device simulation of the electrical and optical characteristics accompanied with experimental measurements of a new highly efficient system for polymer-based light-emitting diodes doped with phosphorescent dyes is presented. The system under investigation comprises an electron transporter attached to a polymer backbone blended with an electronically inert small molecule and an iridium-based green phosphorescent dye which serves as both emitter and hole transporter. The device simulation combines an electrical and an optical model. Based on the known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of all components as well as the measured electrical and optical characteristics of the devices, we model the emissive layer as an effective medium using the dye's HOMO as hole transport level and the polymer LUMO as electron transport level. By fine-tuning the injection barriers at the electron and hole-injecting contact, respectively, in simulated devices, unipolar device characteristics were fitted to the experimental data. Simulations using the so-obtained set of parameters yielded very good agreement to the measured currentvoltage, luminancevoltage characteristics, and the emission profile of entire bipolar light-emitting diodes, without additional fitting parameters. The simulation was used to gain insight into the physical processes and the mechanisms governing the efficiency of the organic light-emitting diode, including the position and extent of the recombination zone, carrier concentration profiles, and field distribution inside the device. The simulations show that the device is severely limited by hole injection, and that a reduction of the hole-injection barrier would improve the device efficiency by almost 50%. KW - conjugated polymers KW - high performance polymers KW - organic electronics KW - organic light-emitting diode KW - simulations KW - TCAD Y1 - 2012 U6 - https://doi.org/10.1002/polb.23158 SN - 0887-6266 VL - 50 IS - 22 SP - 1567 EP - 1576 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Albrecht, Steve A1 - Grootoonk, Bjorn A1 - Neubert, Sebastian A1 - Roland, Steffen A1 - Wordenweber, Jan A1 - Meier, Matthias A1 - Schlatmann, Rutger A1 - Gordijn, Aad A1 - Neher, Dieter T1 - Efficient hybrid inorganic/organic tandem solar cells with tailored recombination contacts JF - Solar energy materials & solar cells : an international journal devoted to photovoltaic, photothermal, and photochemical solar energy conversion N2 - 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. KW - Hybrid solar cells KW - Tandem solar cells KW - Organic solar cells KW - Bulk heterojunction KW - Efficiency optimization Y1 - 2014 U6 - https://doi.org/10.1016/j.solmat.2014.04.020 SN - 0927-0248 SN - 1879-3398 VL - 127 SP - 157 EP - 162 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Albrecht, Steve A1 - Janietz, Silvia A1 - Schindler, Wolfram A1 - Frisch, Johannes A1 - Kurpiers, Jona A1 - Kniepert, Juliane A1 - Inal, Sahika A1 - Pingel, Patrick A1 - Fostiropoulos, Konstantinos A1 - Koch, Norbert A1 - Neher, Dieter T1 - Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells JF - Journal of the American Chemical Society N2 - A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC70BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC70BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells. Y1 - 2012 U6 - https://doi.org/10.1021/ja305039j SN - 0002-7863 VL - 134 IS - 36 SP - 14932 EP - 14944 PB - American Chemical Society CY - Washington 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 - TY - JOUR A1 - Albrecht, Steve A1 - Tumbleston, John R. A1 - Janietz, Silvia A1 - Dumsch, Ines A1 - Allard, Sybille A1 - Scherf, Ullrich A1 - Ade, Harald W. A1 - Neher, Dieter T1 - Quantifying charge extraction in organic solar cells: The case of fluorinated PCPDTBT JF - The journal of physical chemistry letters N2 - We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer. Y1 - 2014 U6 - https://doi.org/10.1021/jz500457b SN - 1948-7185 VL - 5 IS - 7 SP - 1131 EP - 1138 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Albrecht, Steve A1 - Vandewal, Koen A1 - Tumbleston, John R. A1 - Fischer, Florian S. U. A1 - Douglas, Jessica D. A1 - Frechet, Jean M. J. A1 - Ludwigs, Sabine A1 - Ade, Harald W. A1 - Salleo, Alberto A1 - Neher, Dieter T1 - On the efficiency of charge transfer state splitting in polymer: Fullerene solar cells JF - Advanced materials KW - organic solar cells KW - charge generation KW - geminate recombination KW - charge transfer states KW - driving force KW - excess energy KW - morphology KW - spectroelectrochemistry Y1 - 2014 U6 - https://doi.org/10.1002/adma.201305283 SN - 0935-9648 SN - 1521-4095 VL - 26 IS - 16 SP - 2533 EP - 2539 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Alqahtani, Obaid A1 - Babics, Maxime A1 - Gorenflot, Julien A1 - Savikhin, Victoria A1 - Ferron, Thomas A1 - Balawi, Ahmed H. A1 - Paulke, Andreas A1 - Kan, Zhipeng A1 - Pope, Michael A1 - Clulow, Andrew J. A1 - Wolf, Jannic A1 - Burn, Paul L. A1 - Gentle, Ian R. A1 - Neher, Dieter A1 - Toney, Michael F. A1 - Laquai, Frederic A1 - Beaujuge, Pierre M. A1 - Collins, Brian A. T1 - Mixed Domains Enhance Charge Generation and Extraction in Bulk-Heterojunction Solar Cells with Small-Molecule Donors JF - Advanced energy materials N2 - The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)(2), namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes. KW - charge transport KW - domain purity KW - microscopy KW - mixed domains KW - organic solar cells KW - photovoltaic devices KW - resonant X-ray scattering KW - small molecules KW - transient spectroscopy Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201702941 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 19 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Anton, Arthur Markus A1 - Steyrleuthner, Robert A1 - Kossack, Wilhelm A1 - Neher, Dieter A1 - Kremer, Friedrich T1 - Infrared Transition Moment Orientational Analysis on the Structural Organization of the Distinct Molecular Subunits in Thin Layers of a High Mobility n-Type Copolymer JF - Journal of the American Chemical Society N2 - The IR-based method of infrared transition moment orientational analysis (IR-TMOA) is employed to unravel molecular order in thin layers of the semiconducting polymer poly[N,N'-bis(2-octyldodecyl),-1,4,5,8-naphthalene-diimide-2,6-diyl]-alt-5-5'-(2,2'-bithiophene) (P(NDI2OD-T2)). Structure-specific vibrational bands are analyzed in dependence On polarization and inclination of the sample-With respect to the optical axis. By that the molecular Order parameter tensor for the respective molecular moieties with regard to the sample: coordinate system is deduced. Making use of the specificity of the IR spectral range, we are able to determine separately the orientation of atomistic planes defined through the naphthalenediimide (NDI) and bithiophene (T2) units relative to the substrate, and hence, relative to each other. A pronounced solvent effect is observed While chlorobenzene causes the T2 planes to align preferentially parallel to the substrate at an angle of 29 degrees, using a 1:1 chloronaphthalene:xylene mixture results in a reorientation of the T2 units from a face on into an edge on arrangement. In contrast the NDI unit remains unaffected. Additionally, for both solvents evidence is observed for the aggregation of chains in accord With recently published results obtained by UV-vis absorption spectroscopy. Y1 - 2015 U6 - https://doi.org/10.1021/jacs.5b01755 SN - 0002-7863 VL - 137 IS - 18 SP - 6034 EP - 6043 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Asawapirom, Udom A1 - Bulut, F. A1 - Farrell, Tony A1 - Gadermaier, C. A1 - Gamerith, S. A1 - Güntner, Roland A1 - Kietzke, Thomas A1 - Patil, S. A1 - Piok, T. A1 - Montenegro, Rivelino V. D. A1 - Stiller, Burkhard A1 - Tiersch, Brigitte A1 - Landfester, Katharina A1 - List, E. J. W. A1 - Neher, Dieter A1 - Torres, C. S. A1 - Scherf, Ullrich T1 - Materials for polymer electronics applications semiconducting polymer thin films and nanoparticles N2 - The paper presents two different approaches to nanostructured semiconducting polymer materials: (i) the generation of aqueous semiconducting polymer dispersions (semiconducting polymer nanospheres SPNs) and their processing into dense films and layers, and (ii) the synthesis of novel semiconducting polyfluorene-block-polyaniline (PF-b-PANI) block copolymers composed of conjugated blocks of different redox potentials which form nanosized morphologies in the solid state Y1 - 2004 SN - 1022-1360 ER - TY - JOUR A1 - Bagnich, Sergey A. A1 - Bassler, H. A1 - Neher, Dieter T1 - Sensitized phosphorescence of benzil-doped ladder-type methyl-poly(para-phenylene) N2 - The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established. (C) 2004 American Institute of Physics Y1 - 2004 SN - 0021-9606 ER - TY - JOUR A1 - Bagnich, Sergey A. A1 - Bassler, H. A1 - Neher, Dieter T1 - Exciton dynamics in ladder-type methyl-poly(para-phenylene) doped with phosphorescent dyes N2 - The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with platinum-porphyrin dye PtOEP covering the concentration 10(-3)-5% by weight has been measured employing cw and transient techniques. Upon excitation into the range of absorption of the host, strong phosphorescence of the dopant is observed. Possible ways of populating the dopant triplet state are considered. (c) 2004 Elsevier B.V. All rights reserved Y1 - 2005 SN - 0022-2313 ER - TY - JOUR A1 - Bagnich, Sergey A. A1 - Im, C. A1 - Bassler, H. A1 - Neher, Dieter A1 - Scherf, Ullrich T1 - Energy transfer in a ladder-type methyl-poly(para-phenylene) doped by Pt(II)octaethylporphyrin N2 - The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped by platinum-porphyrin dye PtOEP covering the concentration 10(-3) to 5% by weight has been measured employing cw and transient techniques. Upon excitating into the range of absorption of the host strong phosphorescence of the dopant is observed. Possible ways of populating of the dopant triplet state are considered. It is shown that the main channel is singlet-singlet energy transfer among chromophor groups of the polymer followed by Forster-type transfer to the guest and subsequent intersystem crossing. (C) 2003 Elsevier B.V. All rights reserved Y1 - 2004 SN - 0301-0104 ER - TY - JOUR A1 - Bagnich, Sergey A. A1 - Unger, Th. A1 - Jaiser, Frank A1 - Neher, Dieter A1 - Thesen, M. W. A1 - Krüger, H. T1 - Efficient green electrophosphorescence based on ambipolar nonconjugated polymers evaluation of transport and emission properties JF - Journal of applied physics N2 - New materials for polymer organic light-emitting diodes based on a polymer matrix doped with phosphorescent dyes are presented. The matrix system is based on a polystyrene backbone bearing either electron or hole transporting units at the 4-position of each repeat unit. Random copolymers and polymer blend systems of the homopolymers are prepared, both with 62 wt.% electron transporting and 38 wt.% hole transporting moieties. Adding a green electrophosphorescent dye to the polymer matrix leads to efficient electroluminescence with a maximum current efficiency of 35 cd/A and a maximum external quantum efficiency of up to 10%. The mobilities of electrons and holes in the dye-doped copolymer, as measured by transient electroluminescence, are around 5 x 10(-5) and 5 x 10(-6) cm(2)/Vs, respectively, while the blend of the two homopolymers exhibits slightly lower mobilities of both types of carriers. Despite the pronounced imbalance of charge transport, the device performance is almost entirely limited by the phosphorescence efficiency of the dye, implying balanced flow of holes and electrons into the active region. Also, devices made with either the copolymer or the blend yielded very similar device efficiencies, despite the noticeable difference in electron and hole mobility. It is proposed that electrons are efficiently blocked at the interlayer and that the so-formed space charge assists the balanced injection of holes. Y1 - 2011 U6 - https://doi.org/10.1063/1.3618681 SN - 0021-8979 SN - 1089-7550 VL - 110 IS - 3 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Bange, Sebastian A1 - Schubert, Marcel A1 - Neher, Dieter T1 - Charge mobility determination by current extraction under linear increasing voltages : case of nonequilibrium charges and field-dependent mobilities N2 - 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. Y1 - 2010 UR - http://prb.aps.org/ U6 - https://doi.org/10.1103/Physrevb.81.035209 SN - 1098-0121 ER - TY - JOUR A1 - Bartesaghi, Davide A1 - Perez, Irene del Carmen A1 - Kniepert, Juliane A1 - Roland, Steffen A1 - Turbiez, Mathieu A1 - Neher, Dieter A1 - Koster, L. Jan Anton T1 - Competition between recombination and extraction of free charges determines the fill factor of organic solar cells JF - Nature Communications N2 - Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter theta, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor: acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor: acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination. Y1 - 2015 U6 - https://doi.org/10.1038/ncomms8083 SN - 2041-1723 VL - 6 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Bauer, C. A1 - Böhmer, Roland A1 - Moreno-Flores, S. A1 - Richert, R. A1 - Sillescu, H. A1 - Neher, Dieter T1 - Capacitive scanning dilatometry and frequency dependent thermal expansion of polymer films 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 - Benduhn, Johannes A1 - Piersimoni, Fortunato A1 - Londi, Giacomo A1 - Kirch, Anton A1 - Widmer, Johannes A1 - Koerner, Christian A1 - Beljonne, David A1 - Neher, Dieter A1 - Spoltore, Donato A1 - Vandewal, Koen T1 - Impact of triplet excited states on the open-circuit voltage of organic solar cells JF - dvanced energy materials N2 - The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn- and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C-60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T-1) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin-orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T-1 and the CT state does not substantially affect nonradiative voltage losses. KW - charge-transfer states KW - nonradiative voltage losses KW - organic solar cells KW - triplet excited states Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201800451 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 21 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Benduhn, Johannes A1 - Tvingstedt, Kristofer A1 - Piersimoni, Fortunato A1 - Ullbrich, Sascha A1 - Fan, Yeli A1 - Tropiano, Manuel A1 - McGarry, Kathryn A. A1 - Zeika, Olaf A1 - Riede, Moritz K. A1 - Douglas, Christopher J. A1 - Barlow, Stephen A1 - Marder, Seth R. A1 - Neher, Dieter A1 - Spoltore, Donato A1 - Vandewal, Koen T1 - Intrinsic non-radiative voltage losses in fullerene-based organic solar cells JF - Nature Energy N2 - Organic solar cells demonstrate external quantum efficiencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared with the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant non-radiative recombination. Here, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer-state energies. This observation is explained by considering non-radiative charge-transfer-state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion efficiency of single-junction organic solar cells would be reduced to about 25.5% and the optimal optical gap increases to (1.45-1.65) eV, that is, (0.2-0.3) eV higher than for technologies with minimized non-radiative voltage losses. Y1 - 2017 U6 - https://doi.org/10.1038/nenergy.2017.53 SN - 2058-7546 VL - 2 PB - Nature Publ. Group CY - London 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 - Blakesley, James C. A1 - Neher, Dieter T1 - Relationship between energetic disorder and open-circuit voltage in bulk heterojunction organic solar cells JF - Physical review : B, Condensed matter and materials physics N2 - We simulate organic bulk heterojunction solar cells. The effects of energetic disorder are incorporated through a Gaussian or exponential model of density of states. Analytical models of open-circuit voltage (V(OC)) are derived from the splitting of quasi-Fermi potentials. Their predictions are backed up by more complex numerical device simulations including effects such as carrier-density-dependent charge-carrier mobilities. It is predicted that the V(OC) depends on: (1) the donor-acceptor energy gap; (2) charge-carrier recombination rates; (3) illumination intensity; (4) the contact work functions (if not in the pinning regime); and (5) the amount of energetic disorder. A large degree of energetic disorder, or a high density of traps, is found to cause significant reductions in V(OC). This can explain why V(OC) is often less than expected in real devices. Energetic disorder also explains the nonideal temperature and intensity dependence of V(OC) and the superbimolecular recombination rates observed in many real bulk heterojunction solar cells. Y1 - 2011 U6 - https://doi.org/10.1103/PhysRevB.84.075210 SN - 1098-0121 VL - 84 IS - 7 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Blakesley, James C. A1 - Schubert, Marcel A1 - Steyrleuthner, Robert A1 - Chen, Zhihua A1 - Facchetti, Antonio A1 - Neher, Dieter T1 - Time-of-flight measurements and vertical transport in a high electron-mobility polymer JF - Applied physics letters N2 - 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. Y1 - 2011 U6 - https://doi.org/10.1063/1.3657827 SN - 0003-6951 VL - 99 IS - 18 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Braunger, Steffen A1 - Mundt, Laura E. A1 - Wolff, Christian Michael A1 - Mews, Mathias A1 - Rehermann, Carolin A1 - Jost, Marko A1 - Tejada, Alvaro A1 - Eisenhauer, David A1 - Becker, Christiane A1 - Andres Guerra, Jorge A1 - Unger, Eva A1 - Korte, Lars A1 - Neher, Dieter A1 - Schubert, Martin C. A1 - Rech, Bernd A1 - Albrecht, Steve T1 - Cs(x)FA(1-x)Pb(l(1-y)Br(y))(3) Perovskite Compositions BT - the Appearance of Wrinkled Morphology and its Impact on Solar Cell Performance JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - We report on the formation of wrinkle-patterned surface morphologies in cesium formamidinium-based Cs(x)FA(1-y)Pb(I1-yBry)(3) perovskite compositions with x = 0-0.3 and y = 0-0.3 under various spin-coating conditions. By varying the Cs and Br contents, the perovskite precursor solution concentration and the spin-coating procedure, the occurrence and characteristics of the wrinkle-shaped morphology can be tailored systematically. Cs(0.17)FA(0.83)Pb(I0.83Br0.17)(3) perovskite layers were analyzed regarding their surface roughness, microscopic structure, local and overall composition, and optoelectronic properties. Application of these films in p-i-n perovskite solar cells (PSCs) with indium-doped tin oxide/NiOx/perovskite/C-60/bathocuproine/Cu architecture resulted in up to 15.3 and 17.0% power conversion efficiency for the flat and wrinkled morphology, respectively. Interestingly, we find slightly red-shifted photoluminescence (PL) peaks for wrinkled areas and we are able to directly correlate surface topography with PL peak mapping. This is attributed to differences in the local grain size, whereas there is no indication for compositional demixing in the films. We show that the perovskite composition, crystallization kinetics, and layer thickness strongly influence the formation of wrinkles which is proposed to be related to the release of compressive strain during perovskite crystallization. Our work helps us to better understand film formation and to further improve the efficiency of PSCs with widely used mixed-perovskite compositions. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpcc.8b06459 SN - 1932-7447 SN - 1932-7455 VL - 122 IS - 30 SP - 17123 EP - 17135 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Brinkmann, Kai Oliver A1 - Becker, Tim A1 - Zimmermann, Florian A1 - Kreusel, Cedric A1 - Gahlmann, Tobias A1 - Theisen, Manuel A1 - Haeger, Tobias A1 - Olthof, Selina A1 - Tückmantel, Christian A1 - Günster, M. A1 - Maschwitz, Timo A1 - Göbelsmann, Fabian A1 - Koch, Christine A1 - Hertel, Dirk A1 - Caprioglio, Pietro A1 - Peña-Camargo, Francisco A1 - Perdigón-Toro, Lorena A1 - Al-Ashouri, Amran A1 - Merten, Lena A1 - Hinderhofer, Alexander A1 - Gomell, Leonie A1 - Zhang, Siyuan A1 - Schreiber, Frank A1 - Albrecht, Steve A1 - Meerholz, Klaus A1 - Neher, Dieter A1 - Stolterfoht, Martin A1 - Riedl, Thomas T1 - Perovskite-organic tandem solar cells with indium oxide interconnect JF - Nature N2 - Multijunction solar cells can overcome the fundamental efficiency limits of single-junction devices. The bandgap tunability of metal halide perovskite solar cells renders them attractive for multijunction architectures(1). Combinations with silicon and copper indium gallium selenide (CIGS), as well as all-perovskite tandem cells, have been reported(2-5). Meanwhile, narrow-gap non-fullerene acceptors have unlocked skyrocketing efficiencies for organic solar cells(6,7). Organic and perovskite semiconductors are an attractive combination, sharing similar processing technologies. Currently, perovskite-organic tandems show subpar efficiencies and are limited by the low open-circuit voltage (V-oc) of wide-gap perovskite cells(8) and losses introduced by the interconnect between the subcells(9,10). Here we demonstrate perovskite-organic tandem cells with an efficiency of 24.0 per cent (certified 23.1 per cent) and a high V-oc of 2.15 volts. Optimized charge extraction layers afford perovskite subcells with an outstanding combination of high V-oc and fill factor. The organic subcells provide a high external quantum efficiency in the near-infrared and, in contrast to paradigmatic concerns about limited photostability of non-fullerene cells(11), show an outstanding operational stability if excitons are predominantly generated on the non-fullerene acceptor, which is the case in our tandems. The subcells are connected by an ultrathin (approximately 1.5 nanometres) metal-like indium oxide layer with unprecedented low optical/electrical losses. This work sets a milestone for perovskite-organic tandems, which outperform the best p-i-n perovskite single junctions(12) and are on a par with perovskite-CIGS and all-perovskite multijunctions(13). Y1 - 2022 U6 - https://doi.org/10.1038/s41586-022-04455-0 SN - 0028-0836 SN - 1476-4687 VL - 604 IS - 7905 SP - 280 EP - 286 PB - Nature Research CY - Berlin ER - TY - JOUR A1 - Caprioglio, Pietro A1 - Stolterfoht, Martin A1 - Wolff, Christian Michael A1 - Unold, Thomas A1 - Rech, Bernd A1 - Albrecht, Steve A1 - Neher, Dieter T1 - On the relation between the open-circuit voltage and quasi-fermi level splitting in efficient perovskite solar cells JF - advanced energy materials N2 - Today's perovskite solar cells (PSCs) are limited mainly by their open‐circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity‐dependent measurements of the quasi‐Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin‐type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley‐Queisser theory. This has far‐reaching implications for the applicability of some well‐established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift‐diffusion simulations, the intensity dependence of the QFLS, the QFLS‐VOC offset and the ideality factor are consistently explained by trap‐assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS‐VOC relation is of great importance. KW - electro-optical materials KW - perovskite solar cells KW - photovoltaic devices KW - thin films Y1 - 2019 U6 - https://doi.org/10.1002/aenm.201901631 SN - 1614-6832 SN - 1614-6840 VL - 9 IS - 33 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Caprioglio, Pietro A1 - Zu, Fengshuo A1 - Wolff, Christian Michael A1 - Prieto, Jose A. Marquez A1 - Stolterfoht, Martin A1 - Becker, Pascal A1 - Koch, Norbert A1 - Unold, Thomas A1 - Rech, Bernd A1 - Albrecht, Steve A1 - Neher, Dieter T1 - High open circuit voltages in pin-type perovskite solar cells through strontium addition JF - Sustainable Energy & Fuels N2 - The incorporation of even small amounts of strontium (Sr) into lead-base hybrid quadruple cation perovskite solar cells results in a systematic increase of the open circuit voltage (V-oc) in pin-type perovskite solar cells. We demonstrate via absolute and transient photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C-60 electron-transporting layer. The resulting solar cells exhibited a V-oc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, reducing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high V-oc values and efficiencies in Sr-containing quadruple cation perovskite pin-type solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer. Y1 - 2019 U6 - https://doi.org/10.1039/c8se00509e SN - 2398-4902 VL - 3 IS - 2 SP - 550 EP - 563 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Chen, Zupeng A1 - Savateev, Aleksandr A1 - Pronkin, Sergey A1 - Papaefthimiou, Vasiliki A1 - Wolff, Christian Michael A1 - Willinger, Marc Georg A1 - Willinger, Elena A1 - Neher, Dieter A1 - Antonietti, Markus A1 - Dontsova, Dariya T1 - "The Easier the Better" Preparation of Efficient Photocatalysts-Metastable Poly(heptazine imide) Salts JF - Advanced materials N2 - Cost-efficient, visible-light-driven hydrogen production from water is an attractive potential source of clean, sustainable fuel. Here, it is shown that thermal solid state reactions of traditional carbon nitride precursors (cyanamide, melamine) with NaCl, KCl, or CsCl are a cheap and straightforward way to prepare poly(heptazine imide) alkali metal salts, whose thermodynamic stability decreases upon the increase of the metal atom size. The chemical structure of the prepared salts is confirmed by the results of X-ray photoelectron and infrared spectroscopies, powder X-ray diffraction and electron microscopy studies, and, in the case of sodium poly(heptazine imide), additionally by atomic pair distribution function analysis and 2D powder X-ray diffraction pattern simulations. In contrast, reactions with LiCl yield thermodynamically stable poly(triazine imides). Owing to the metastability and high structural order, the obtained heptazine imide salts are found to be highly active photo-catalysts in Rhodamine B and 4-chlorophenol degradation, and Pt-assisted sacrificial water reduction reactions under visible light irradiation. The measured hydrogen evolution rates are up to four times higher than those provided by a benchmark photocatalyst, mesoporous graphitic carbon nitride. Moreover, the products are able to photocatalytically reduce water with considerable reaction rates, even when glycerol is used as a sacrificial hole scavenger. KW - carbon nitride KW - glycerol oxidation KW - mesocrystals KW - poly(heptazine imide) KW - water reduction reactions Y1 - 2017 U6 - https://doi.org/10.1002/adma.201700555 SN - 0935-9648 SN - 1521-4095 VL - 29 SP - 21800 EP - 21806 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Cimrová, V. A1 - Neher, Dieter A1 - Kostromine, S. A1 - Bieringer, Thomas T1 - Optical anisotropy in films of photoaddressable polymers Y1 - 1999 ER - TY - JOUR A1 - Collado-Fregoso, Elisa A1 - Hood, Samantha N. A1 - Shoaee, Safa A1 - Schröder, Bob C. A1 - McCulloch, Iain A1 - Kassal, Ivan A1 - Neher, Dieter A1 - Durrant, James R. T1 - Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited JF - The journal of physical chemistry letters N2 - In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination. Y1 - 2017 U6 - https://doi.org/10.1021/acs.jpclett.7b01571 SN - 1948-7185 VL - 8 SP - 4061 EP - 4068 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Collado-Fregoso, Elisa A1 - Pugliese, Silvina N. A1 - Wojcik, Mariusz A1 - Benduhn, Johannes A1 - Bar-Or, Eyal A1 - Perdigón-Toro, Lorena A1 - Hörmann, Ulrich A1 - Spoltore, Donato A1 - Vandewal, Koen A1 - Hodgkiss, Justin M. A1 - Neher, Dieter T1 - Energy-gap law for photocurrent generation in fullerene-based organic solar cells BT - the case of low-donor-content blends JF - Journal of the American Chemical Society N2 - The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C-60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state. Y1 - 2019 U6 - https://doi.org/10.1021/jacs.8b09820 SN - 0002-7863 VL - 141 IS - 6 SP - 2329 EP - 2341 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Deschler, Felix A1 - Neher, Dieter A1 - Schmidt-Mende, Lukas T1 - Perovskite semiconductors for next generation optoelectronic applications JF - APL Materials Y1 - 2019 U6 - https://doi.org/10.1063/1.5119744 SN - 2166-532X VL - 7 IS - 8 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Di Pietro, Riccardo A1 - Erdmann, Tim A1 - Carpenter, Joshua H. A1 - Wang, Naixiang A1 - Shivhare, Rishi Ramdas A1 - Formanek, Petr A1 - Heintze, Cornelia A1 - Voit, Brigitte A1 - Neher, Dieter A1 - Ade, Harald W. A1 - Kiriy, Anton T1 - Synthesis of High-Crystallinity DPP Polymers with Balanced Electron and Hole Mobility JF - Chemistry of materials : a publication of the American Chemical Society Y1 - 2017 U6 - https://doi.org/10.1021/acs.chemmater.7b04423 SN - 0897-4756 SN - 1520-5002 VL - 29 SP - 10220 EP - 10232 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Di Pietro, Riccardo A1 - Nasrallah, Iyad A1 - Carpenter, Joshua A1 - Gann, Eliot A1 - Kölln, Lisa Sophie A1 - Thomsen, Lars A1 - Venkateshvaran, Deepak A1 - Sadhanala, Aditya A1 - Chabinyc, Michael A1 - McNeill, Christopher R. A1 - Facchetti, Antonio A1 - Ade, Harald W. A1 - Sirringhaus, Henning A1 - Neher, Dieter T1 - Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors JF - Advanced functional materials Y1 - 2016 U6 - https://doi.org/10.1002/adfm.201602080 SN - 1616-301X SN - 1616-3028 VL - 26 SP - 8011 EP - 8022 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Di Pietro, Riccardo A1 - Venkateshvaran, Deepak A1 - Klug, Andreas A1 - List-Kratochvil, Emil J. W. A1 - Facchetti, Antonio A1 - Sirringhaus, Henning A1 - Neher, Dieter T1 - Simultaneous extraction of charge density dependent mobility and variable contact resistance from thin film transistors JF - Applied physics letters N2 - A model for the extraction of the charge density dependent mobility and variable contact resistance in thin film transistors is proposed by performing a full derivation of the current-voltage characteristics both in the linear and saturation regime of operation. The calculated values are validated against the ones obtained from direct experimental methods. This approach allows unambiguous determination of gate voltage dependent contact and channel resistance from the analysis of a single device. It solves the inconsistencies in the commonly accepted mobility extraction methods and provides additional possibilities for the analysis of the injection and transport processes in semiconducting materials. (C) 2014 AIP Publishing LLC. Y1 - 2014 U6 - https://doi.org/10.1063/1.4876057 SN - 0003-6951 SN - 1077-3118 VL - 104 IS - 19 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Däubler, Thomas Karl A1 - Bittner, Reinhard A1 - Meerholz, Klaus A1 - Neher, Dieter T1 - Charge carrier photogeneration, trapping and space-charge field formation in PVK-based photorefractive materials Y1 - 2000 ER - TY - JOUR A1 - Däubler, Thomas Karl A1 - Cimrova, V. A1 - Pfeiffer, S. A1 - Hörhold, Hans-Heinrich A1 - Neher, Dieter T1 - Field and wavelength dependence of charge carrier photogeneration in soluble PPV derivatives Y1 - 1999 ER - TY - JOUR A1 - Däubler, Thomas Karl A1 - Glowacki, Ireneusz A1 - Scherf, Ullrich A1 - Ulanski, J. A1 - Hörhold, Hans-Heinrich A1 - Neher, Dieter T1 - Photogeneration and transport of charge carriers in hybrid materials of conjugated polymers and dye-sensitized TiO2 Y1 - 1999 ER - TY - JOUR A1 - Däubler, Thomas Karl A1 - Neher, Dieter A1 - Rost, H. A1 - Hörhold, Hans-Heinrich T1 - Efficient bulk photogeneration of charge carriers in arylamino-PPV polymer sandwich cells Y1 - 1999 ER - TY - JOUR A1 - Däubler, Thomas Karl A1 - Pfeiffer, S. A1 - Hörhold, Hans-Heinrich A1 - Neher, Dieter T1 - Photogeneration of charge carriers in segmented arylamino-PPV derivatives Y1 - 1999 ER - TY - JOUR A1 - Egbe, D. A. M. A1 - Carbonnier, B. A1 - Paul, E. L. A1 - Muhlbacher, D. A1 - Kietzke, Thomas A1 - Birckner, Eckhard A1 - Neher, Dieter A1 - Grummt, U. W. A1 - Pakula, T. T1 - Diyne-containing PPVs : Solid-state properties and comparison of their photophysical and electrochemical properties with those of their Yne-containing counterparts N2 - Diyne-containing poly(p-phenylene-vinylene)s, 4a-d, of general chemical structure-(Ph-C&3bond; C-C&3bond; C-Ph- CH&3bond; CH-Ph-CH&3bond; CH-)(n), obtained through polycondensation reactions of 1,4-bis(4-formyl-2,5-dioctyloxyphenyl)- buta-1,3-diyne (2) with various 2,5-dialkoxy-p-xylylenebis(diethylphosphonates), 3a-d, are the subject of this report. The polymers exhibit great disparity in their degree of polymerization, n, which might be ascribed to side-chain-related differences in reactivity of the reactive species during the polycondensation process and which led to n-dependent absorption (solution and solid state) and emission (solution) behaviors of the polymers. Polarizing optical microscopy and differential scanning calorimetry are employed to probe their thermal behavior. The structure is investigated by means of wide-angle X-ray diffraction for both isotropic and macroscopically oriented samples. Comparison of photophysical (experimental and theoretical) and electrochemical properties of the polymers with those of their yne- containing counterparts 6a-d [-(Ph-C&3bond; C-Ph-CH&3bond; CH-Ph-CH&3bond; CH-)(n)] has been carried out. Similar photophysical behavior was observed for both types of polymers despite the difference in backbone conjugation pattern. The introduction of a second yne unit in 4 lowers the HOMO and LUMO levels, thereby enhancing the electron affinity of polymers 4 compared to polymers 6. The "wider opening" introduced by the second yne unit facilitates moreover the movement of charges during the electrochemical processes leading to minimal discrepancy, Delta E-g between the optical and electrochemical band gap energies. Polymers 6, in contrast, show significant side-chain-dependent Delta E-g values. Low turn-on voltages between 2 and 3 V and maximal luminous efficiencies between 0.32 and 1.25 cd/A were obtained from LED devices of configuration ITO/PEDOT:PSS/polymer 4/Ca/Al Y1 - 2005 SN - 0024-9297 ER - TY - JOUR A1 - Egbe, D. A. M. A1 - Kietzke, Thomas A1 - Carbonnier, B. A1 - Muhlbacher, D. A1 - Horhold, H. H. A1 - Neher, Dieter A1 - Pakula, T. T1 - Synthesis, characterization, and photophysical, electrochemical, electroluminescent, and photovoltaic properties of yne-containing CN-PPVs N2 - Alkoxy-substituted CN-containing phenylene-vinylene-alt-phenylene-ethynylene hybrid polymers (CN-PPV-PPE), 3a, 3b, and 7a, were obtained from luminophoric dialdehydes 1 by step growth polymerization via Knoevenagel reaction as high molecular-weight materials. Corresponding CN-free polymers 3c and 7b and an ethynylene-free polymer 5 with similar side chains were synthesized for the purpose of comparison. The chemical structures of the polymers were confirmed by IR, H-1 and C-13 NMR, and elemental analysis. Thermal characterization was conducted by means of thermogravimetric analysis and differential scanning calorimetry. Morphology was investigated by means of optical microscopy and small-angle light scattering. The final morphologies are determined by the molecular characteristics (side chains volume fraction, backbone stiffness) of the studied polymers. All the CN-containing polymers 3b, 5, and 7a exhibit higher fluorescence quantum yield in solid state (50 to 60%), but lower quantum yields (12-40%) in dilute chloroform solution, in total contrast to CN-free polymers 3c, 3d, and 7b. Identical optical, E-g(opt), and electrochemical band gap energies, E- g(ec), were obtained for 3b, 3c and 3d with intrinsic self-assembly ability, whereas a discrepancy, DeltaE(g), was observed in the cases of the fully substituted polymers 5, 7a, and 7b, whose values are dependent on the level of backbone stiffness and length of the side groups combined with the presence or absence of CN units. The incorporation of CN units in 3b and 7a lowers their respective LUMO level by 220 and 350 meV compared to their corresponding CN-free counterparts 3c and 7b, suggesting an improvement of the electron-accepting strength. Polymers 3b and 7a are efficient electron acceptors suitable for photovoltaic application. The experiments indicate that 3b is a better electron acceptor when used together with M3EH-PPV, but transport properties seem to be better for 7a. With 3b, high external quantum efficiencies of up to 23%, an open circuit voltage of up to 1.52 V, and a white light energy efficiency of 0.65% could be realized in bilayer solar cell devices. LED-devices of configuration ITO/PEDOT:PSS/polymer/Ca/Al from 3b, 3c, 7a, and 7b showed low turn-on voltages between 2 and 2.5 V. The CN-free polymers 3c and 7b exhibit far better EL parameters than their corresponding CN containing counterparts 3b and 7a Y1 - 2004 ER - TY - JOUR A1 - Egbe, D. A. M. A1 - Ulbricht, C. A1 - Orgis, Thomas A1 - Carbonnier, B. A1 - Kietzke, Thomas A1 - Peip, M. A1 - Metzner, M. A1 - Gericke, M. A1 - Birckner, Eckhard A1 - Pakula, T. A1 - Neher, Dieter A1 - Grumm, U. W. T1 - Odd-even effects and the influence of length and specific positioning of alkoxy side chains on the optical properties of PPE-PPV polymers N2 - This contribution reports the combined influences of odd-even effects and the specific positioning of alkoxy side chains OR1 = (OCn+H-10(2(n+10)+1)) and OR2 = (OCnH2n+1) (with n = 6, 7, 8, 9) on the phenylene-ethynylene and phenylene- vinylene segments, respectively, on the optical properties of hybrid polymers P(n+10)/n of general repeating unit: -Ph-C equivalent to C-Ph-C equivalent to C-Ph-CH=CH-Ph-CH=CH-. For the polymeric materials, visual color impression varies alternatively between orange red (P16/6 and P18/8) and yellow (P17/7 and P19/9) according to the odd and even features of the alkoxy side chains, where odd or even relates to the total number of sp(3)-hybridized atoms within the side chains. This side chain related effect is ascribed to both absorptive and emissive behaviors of the polymers on the basis of photophysical investigations in the bulk. Almost identical thin film absorption spectra were obtained for all four materials; however, the photoluminescence of the odd polymers, P16/6 (lambda(f) = 556 nm) and P18/ 8 (lambda(f) = 614 nm), was red-shifted relative to that of their even counterparts (lambda(f) = 535 nm). Further, the P18/8 maximum at 614 nm can be readily assigned to excimer emission, as evidenced by the largest Stokes shift (5600 cm(- 1)), largest fwhmf-value (3700 cm(-1))(,) and the lowest Phi(f)-value of 24%. The strong pi-pi interchain interaction in P18/8, due to loose alkoxy side chains packing, does not only favor fluorescence quenching but also enable an effective inter- as well as intra-molecular recombination of the generated positive and negative polarons in electrolurninescence, which explains the good EL properties of this polymer irrespective of the solvent used. A voltage-dependent blue shift of the EL spectra of up to 100 nm was observed for P18/8 devices prepared from aromatic solvents. This red to green EL shift as observed with increasing voltage is assigned to conformational changes of the polymer chains with increasing temperature Y1 - 2005 SN - 0897-4756 ER - TY - JOUR A1 - Fang, Lijia A1 - Holzmueller, Felix A1 - Matulaitis, Tomas A1 - Baasner, Anne A1 - Hauenstein, Christoph A1 - Benduhn, Johannes A1 - Schwarze, Martin A1 - Petrich, Annett A1 - Piersimoni, Fortunato A1 - Scholz, Reinhard A1 - Zeika, Olaf A1 - Koerner, Christian A1 - Neher, Dieter A1 - Vandewal, Koen A1 - Leo, Karl T1 - Fluorine-containing low-energy-gap organic dyes with low voltage losses for organic solar cells JF - Synthetic metals : the journal of electronic polymers and electronic molecular materials N2 - Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum-deposited organic solar cells. These molecules comprise a donor acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-donating and electron-withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with, similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C-60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8%. Moreover, the voltage loss for the PRTF device due to the non radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (>0.34V). (C) 2016 Published by Elsevier B.V. KW - (Z)-isomer KW - Donor materials KW - CH center dot center dot center dot F hydrogen bonds KW - Sublimation with good yield KW - Low voltage losses Y1 - 2016 U6 - https://doi.org/10.1016/j.synthmet.2016.10.025 SN - 0379-6779 VL - 222 SP - 232 EP - 239 PB - Elsevier CY - Lausanne ER - TY - JOUR A1 - Fiesel, R. A1 - Neher, Dieter A1 - Scherf, Ullrich T1 - On the solid state aggregation of chiral substituted poly(para-phenylene)s (PPPs) Y1 - 1999 ER - TY - JOUR A1 - Fischer, Florian S. U. A1 - Trefz, Daniel A1 - Back, Justus A1 - Kayunkid, Navaphun A1 - Tornow, Benjamin A1 - Albrecht, Steve A1 - Yager, Kevin G. A1 - Singh, Gurpreet A1 - Karim, Alamgir A1 - Neher, Dieter A1 - Brinkmann, Martin A1 - Ludwigs, Sabine T1 - Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto-Electronic Properties JF - Advanced materials N2 - PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance. Y1 - 2015 U6 - https://doi.org/10.1002/adma.201403475 SN - 0935-9648 SN - 1521-4095 VL - 27 IS - 7 SP - 1223 EP - 1228 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Foertig, Alexander A1 - Kniepert, Juliane A1 - Gluecker, Markus A1 - Brenner, Thomas J. K. A1 - Dyakonov, Vladimir A1 - Neher, Dieter A1 - Deibel, Carsten T1 - Nongeminate and geminate recombination in PTB7: PCBM solar cells JF - Advanced functional materials KW - organic semiconductors KW - organic solar cells KW - conjugated polymers KW - charge carrier recombination Y1 - 2014 U6 - https://doi.org/10.1002/adfm.201302134 SN - 1616-301X SN - 1616-3028 VL - 24 IS - 9 SP - 1306 EP - 1311 PB - Wiley-VCH CY - Weinheim 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 - 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 - Fritsch, Tobias A1 - Kurpiers, Jona A1 - Roland, Steffen A1 - Tokmoldin, Nurlan A1 - Shoaee, Safa A1 - Ferron, Thomas A1 - Collins, Brian A. A1 - Janietz, Silvia A1 - Vandewal, Koen A1 - Neher, Dieter T1 - On the interplay between CT and singlet exciton emission in organic solar cells with small driving force and its impact on voltage loss JF - Advanced energy materials N2 - The interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S-1) determines the recombination pathway and the resulting open circuit voltage (V-OC) of organic solar cells. By combining a well-aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state (E-CT) is varied by 130 meV while leaving the S-1 energy of the polymer (ES1\[{E_{{{\rm{S}}_1}}}\]) unaffected. It is found that the polymer exciton dominates the radiative properties of the blend when ECT\[{E_{{\rm{CT}}}}\] approaches ES1\[{E_{{{\rm{S}}_1}}}\], while the V-OC remains limited by the non-radiative decay of the CT state. It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non-radiative voltage loss because it lowers the radiative V-OC limit (V-OC,V-rad), but not because it is more emissive. The analysis further suggests that electronic coupling between the CT state and the S-1 will not improve the V-OC, but rather reduce the V-OC,V-rad. It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons. KW - external quantum efficiency KW - organic photovoltaics KW - ternary blends KW - voltage losses Y1 - 2022 U6 - https://doi.org/10.1002/aenm.202200641 SN - 1614-6832 SN - 1614-6840 VL - 12 IS - 31 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Galbrecht, Frank A1 - Yang, X. H. A1 - Nehls, B. S. A1 - Neher, Dieter A1 - Farrell, Tony A1 - Scherf, Ullrich T1 - Semiconducting polyfluorenes with electrophosphorescent on-chain platinum-salen chromophores N2 - The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported Y1 - 2005 SN - 1359-7345 ER - TY - JOUR A1 - García-Benito, Inés A1 - Quarti, Claudio A1 - Queloz, Valentin I. E. A1 - Hofstetter, Yvonne J. A1 - Becker-Koch, David A1 - Caprioglio, Pietro A1 - Neher, Dieter A1 - Orlandi, Simonetta A1 - Cavazzini, Marco A1 - Pozzi, Gianluca A1 - Even, Jacky A1 - Nazeeruddin, Mohammad Khaja A1 - Vaynzof, Yana A1 - Grancini, Giulia T1 - Fluorination of organic spacer impacts on the structural and optical response of 2D perovskites JF - Frontiers in Chemistry N2 - Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)(2)PbI4 and (Lf)(2)PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors. KW - fluorinated organic spacer KW - 2D perovskites KW - phase transition KW - temperature dependence KW - excitonic materials Y1 - 2020 U6 - https://doi.org/10.3389/fchem.2019.00946 SN - 2296-2646 VL - 7 SP - 1 EP - 11 PB - Frontiers Media CY - Lausanne ER - TY - JOUR A1 - Gattinger, P. A1 - Gurka, M. A1 - Craats, A. M. van de A1 - Rengel, Heiko A1 - Warman, J. M. A1 - Buck, M. A1 - Neher, Dieter T1 - Mechanism of charge transport in anisotropic layers of a phthalocyanine polymer Y1 - 1999 ER - TY - JOUR A1 - Gehrig, Dominik W. A1 - Roland, Steffen A1 - Howard, Ian A. A1 - Kamm, Valentin A1 - Mangold, Hannah A1 - Neher, Dieter A1 - Laquai, Frederic T1 - Efficiency-limiting processes in low-bandgap polymer:Perylene diimide photovoltaic blends JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - 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. Y1 - 2014 U6 - https://doi.org/10.1021/jp503366m SN - 1932-7447 VL - 118 IS - 35 SP - 20077 EP - 20085 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Gerhard, Reimund A1 - Wegener, Michael A1 - Künstler, Wolfgang A1 - Wirges, Werner A1 - Görne, Thomas A1 - Urayama, D. A1 - Neher, Dieter T1 - Inverse piezoelectricity of porous PTFE films with bipolar space charge Y1 - 2000 ER - TY - JOUR A1 - Ghani, Fatemeh A1 - Opitz, Andreas A1 - Pingel, Patrick A1 - Heimel, Georg A1 - Salzmann, Ingo A1 - Frisch, Johannes A1 - Neher, Dieter A1 - Tsami, Argiri A1 - Scherf, Ullrich A1 - Koch, Norbert T1 - Charge Transfer in and Conductivity of Molecularly Doped Thiophene-Based Copolymers JF - Journal of polymer science : B, Polymer physics N2 - The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground-state integer charge transfer and charge-transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge-transfer interactions between the common molecular p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and a systematic series of thiophene-based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge-transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design. KW - charge transfer KW - conducting polymers KW - doping KW - thiophene Y1 - 2015 U6 - https://doi.org/10.1002/polb.23631 SN - 0887-6266 SN - 1099-0488 VL - 53 IS - 1 SP - 58 EP - 63 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Gorenflot, Julien A1 - Paulke, Andreas A1 - Piersimoni, Fortunato A1 - Wolf, Jannic A1 - Kan, Zhipeng A1 - Cruciani, Federico A1 - El Labban, Abdulrahman A1 - Neher, Dieter A1 - Beaujuge, Pierre M. A1 - Laquai, Frederic T1 - From recombination dynamics to device performance BT - quantifying the efficiency of exciton dissociation, charge separation, and extraction in bulk heterojunction solar cells with Fluorine-Substituted polymer donors JF - dvanced energy materials N2 - An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time-delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two-pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device-relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide-bandgap donor polymers: poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-thiophene) (PBDT[2H]T) combined with PC71BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)-state energy is shown to lead to significantly lower energy losses (resulting in higher VOC) during charge generation compared to P3HT:PCBM. KW - bulk heterojunction KW - charge generation yield KW - charge recombination yield KW - polymer solar cells KW - transient absorption spectroscopy Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201701678 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 4 PB - Wiley-VCH CY - Weinheim 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 - Grischek, Max A1 - Caprioglio, Pietro A1 - Zhang, Jiahuan A1 - Pena-Camargo, Francisco A1 - Sveinbjornsson, Kari A1 - Zu, Fengshuo A1 - Menzel, Dorothee A1 - Warby, Jonathan A1 - Li, Jinzhao A1 - Koch, Norbert A1 - Unger, Eva A1 - Korte, Lars A1 - Neher, Dieter A1 - Stolterfoht, Martin A1 - Albrecht, Steve T1 - Efficiency Potential and Voltage Loss of Inorganic CsPbI2Br Perovskite Solar Cells JF - Solar RRL N2 - Inorganic perovskite solar cells show excellent thermal stability, but the reported power conversion efficiencies are still lower than for organic-inorganic perovskites. This is mainly caused by lower open-circuit voltages (V(OC)s). Herein, the reasons for the low V-OC in inorganic CsPbI2Br perovskite solar cells are investigated. Intensity-dependent photoluminescence measurements for different layer stacks reveal that n-i-p and p-i-n CsPbI2Br solar cells exhibit a strong mismatch between quasi-Fermi level splitting (QFLS) and V-OC. Specifically, the CsPbI2Br p-i-n perovskite solar cell has a QFLS-e center dot V-OC mismatch of 179 meV, compared with 11 meV for a reference cell with an organic-inorganic perovskite of similar bandgap. On the other hand, this study shows that the CsPbI2Br films with a bandgap of 1.9 eV have a very low defect density, resulting in an efficiency potential of 20.3% with a MeO-2PACz hole-transporting layer and 20.8% on compact TiO2. Using ultraviolet photoelectron spectroscopy measurements, energy level misalignment is identified as a possible reason for the QFLS-e center dot V-OC mismatch and strategies for overcoming this V-OC limitation are discussed. This work highlights the need to control the interfacial energetics in inorganic perovskite solar cells, but also gives promise for high efficiencies once this issue is resolved. KW - CsPbI2Br KW - efficiency potentials KW - inorganic perovskites KW - photoluminescence KW - solar cells KW - voltage losses Y1 - 2022 U6 - https://doi.org/10.1002/solr.202200690 SN - 2367-198X VL - 6 IS - 11 PB - Wiley-VCH CY - Weinheim ER - 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 - Hahn, Tobias A1 - Tscheuschner, Steffen A1 - Saller, Christina A1 - Strohriegl, Peter A1 - Boregowda, Puttaraju A1 - Mukhopadhyay, Tushita A1 - Patil, Satish A1 - Neher, Dieter A1 - Bässler, Heinz A1 - Köhler, Anna T1 - Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C-60 and PCBM JF - The journal of physical chemistry : C, Nanomaterials and interfaces Y1 - 2016 U6 - https://doi.org/10.1021/acs.jpcc.6b08471 SN - 1932-7447 VL - 120 SP - 25083 EP - 25091 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Hofacker, Andreas A1 - Neher, Dieter T1 - Dispersive and steady-state recombination in organic disordered semiconductors JF - Physical review : B, Condensed matter and materials physics N2 - Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed. Y1 - 2017 U6 - https://doi.org/10.1103/PhysRevB.96.245204 SN - 2469-9950 SN - 2469-9969 VL - 96 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Hofacker, Andreas A1 - Neher, Dieter T1 - Dispersive and steady-state recombination in organic disordered semiconductors JF - Physical review : B, Condensed matter and materials physics N2 - Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed. Y1 - 2017 U6 - https://doi.org/10.1103/PhysRevB.96.245204 SN - 2469-9950 SN - 2469-9969 VL - 96 SP - 5640 EP - 5649 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Hoffmann, Sebastian T. A1 - Jaiser, Frank A1 - Hayer, Anna A1 - Baessler, Heinz A1 - Unger, Thomas A1 - Athanasopoulos, Stavros A1 - Neher, Dieter A1 - Koehler, Anna T1 - How Do Disorder, Reorganization, and Localization Influence the Hole Mobility in Conjugated Copolymers? JF - JOURNAL OF THE AMERICAN CHEMICAL SOCIETY N2 - In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties. Y1 - 2013 U6 - https://doi.org/10.1021/ja308820j SN - 0002-7863 VL - 135 IS - 5 SP - 1772 EP - 1782 PB - AMER CHEMICAL SOC CY - WASHINGTON ER - TY - JOUR A1 - Hofmann, Alexander J. L. A1 - Züfle, Simon A1 - Shimizu, Kohei A1 - Schmid, Markus A1 - Wessels, Vivien A1 - Jäger, Lars A1 - Altazin, Stephane A1 - Ikegami, Keitaro A1 - Khan, Motiur Rahman A1 - Neher, Dieter A1 - Ishii, Hisao A1 - Ruhstaller, Beat A1 - Brütting, Wolfgang T1 - Dipolar Doping of Organic Semiconductors to Enhance Carrier Injection JF - Physical review applied N2 - If not oriented perfectly isotropically, the strong dipole moment of polar organic semiconductor materials such as tris-(8-hydroxyquinolate)aluminum (Alq3) will lead to the buildup of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this having been a known fact for years, the implications of such high potentials within an organic layer stack have only been studied recently. In this work, the influence of the GSP on hole injection into organic layers is investigated. Therefore, we apply a concept called dipolar doping to devices consisting of the prototypical organic materials N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as nonpolar host and Alq3 as dipolar dopant with different mixing ratios to tune the GSP. The mixtures are investigated in single-layer monopolar devices as well as bilayer metal/insulator/semiconductor structures. Characterization is done electrically using current-voltage (I-V) characteristics, impedance spectroscopy, and charge extraction by linearly increasing voltage and time of flight, as well as with ultraviolet photoelectron spectroscopy. We find a maximum in device performance for moderate to low doping concentrations of the polar species in the host. The observed behavior can be described on the basis of the Schottky effect for image-force barrier lowering, if the changes in the interface dipole, the carrier mobility, and the GSP induced by dipolar doping are taken into account. KW - Carrier dynamics KW - Electric polarization KW - Optoelectronics KW - Organic electronics KW - Doped semiconductors KW - Interfaces KW - Organic LEDs KW - Organic semiconductors Y1 - 2019 U6 - https://doi.org/10.1103/PhysRevApplied.12.064052 SN - 2331-7019 VL - 12 IS - 6 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Hosseini, Seyed Mehrdad A1 - Roland, Steffen A1 - Kurpiers, Jona A1 - Chen, Zhiming A1 - Zhang, Kai A1 - Huang, Fei A1 - Armin, Ardalan A1 - Neher, Dieter A1 - Shoaee, Safa T1 - Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells BT - A Comparative Study of Charge Generation and Extraction JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fundamental questions regarding nanomorphology, interfacial states, charge generation and extraction, and losses in these systems. Herein, we present a comparative study of bulk heterojunction solar cells composed of a recently introduced naphthothiadiazole-based polymer (NT812) as the electron donor and two different acceptor molecules, namely, [6,6]-phenyl-C71-butyric acid methyl ester (PCBM)[70] and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). A comparison between the photovoltaic performance of these two types of solar cells reveals that the open-circuit voltage (Voc) of the NT812:ITIC-based solar cell is larger, but the fill factor (FF) is lower than that of the NT812:PCBM[70] device. We find the key reason behind this reduced FF in the ITIC-based device to be faster nongeminate recombination relative to the NT812:PCBM[70] system. Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpcc.8b11669 SN - 1932-7447 VL - 123 IS - 11 SP - 6823 EP - 6830 PB - American Chemical Society CY - Washington 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 - Hörmann, Ulrich A1 - Zeiske, Stefan A1 - Park, Soohyung A1 - Schultz, Thorsten A1 - Kickhoefel, Sebastian A1 - Scherf, Ullrich A1 - Blumstengel, Sylke A1 - Koch, Norbert A1 - Neher, Dieter T1 - Direct observation of state-filling at hybrid tin oxide/organic interfaces JF - Applied physics letters N2 - Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. Here, we combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer [ladder type poly-(para-phenylene)], we find a one-to-one correspondence between the blue-shift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which we unambiguously assign to state filling. Our data are resembled best by a model considering the combination of an exponential density of states with a doped semiconductor. Published under license by AIP Publishing. Y1 - 2019 U6 - https://doi.org/10.1063/1.5082704 SN - 0003-6951 SN - 1077-3118 VL - 114 IS - 18 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Hörmann, Ulrich A1 - Zeiske, Stefan A1 - Piersimoni, Fortunato A1 - Hoffmann, Lukas A1 - Schlesinger, Raphael A1 - Koch, Norbert A1 - Riedl, Thomas A1 - Andrienko, Denis A1 - Neher, Dieter T1 - Stark effect of hybrid charge transfer states at planar ZnO/organic interfaces JF - Physical review : B, Condensed matter and materials physics N2 - We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevB.98.155312 SN - 2469-9950 SN - 2469-9969 VL - 98 IS - 15 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Ilnytskyi, Jaroslav M. A1 - Neher, Dieter A1 - Saphiannikova, Marina T1 - Opposite photo-induced deformations in azobenzene-containing polymers with different molecular architecture molecular dynamics study JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Photo-induced deformations in azobenzene-containing polymers (azo-polymers) are central to a number of applications, such as optical storage and fabrication of diffractive elements. The microscopic nature of the underlying opto-mechanical coupling is yet not clear. In this study, we address the experimental finding that the scenario of the effects depends on molecular architecture of the used azo-polymer. Typically, opposite deformations in respect to the direction of light polarization are observed for liquid crystalline and amorphous azo-polymers. In this study, we undertake molecular dynamics simulations of two different models that mimic these two types of azo-polymers. We employ hybrid force field modeling and consider only trans-isomers of azobenzene, represented as Gay-Berne sites. The effect of illumination on the orientation of the chromophores is considered on the level of orientational hole burning and emphasis is given to the resulting deformation of the polymer matrix. We reproduce deformations of opposite sign for the two models being considered here and discuss the relevant microscopic mechanisms in both cases. KW - amorphous state KW - light polarisation KW - liquid crystal polymers KW - molecular dynamics method KW - optical hole burning KW - photochemistry Y1 - 2011 U6 - https://doi.org/10.1063/1.3614499 SN - 0021-9606 VL - 135 IS - 4 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Ilnytskyi, Jaroslav M. A1 - Saphiannikova, Marina A1 - Neher, Dieter A1 - Allen, Michael P. T1 - Modelling elasticity and memory effects in liquid crystalline elastomers by molecular dynamics simulations JF - Soft matter N2 - We performed molecular dynamics simulations of a liquid crystal elastomer of side-chain architecture. The network is formed from a melt of 28 molecules each having a backbone of 100 hydrocarbon monomers, to which 50 side chains are attached in a syndiotactic way. Crosslinking is performed in the smectic A phase. We observe an increase of the smectic-isotropic phase transition temperature of about 5 degrees as compared to the uncrosslinked melt. Memory effects in liquid crystalline order and in sample shape are well reproduced when the elastomer is driven through the smectic-isotropic transition. Above this transition, in the isotropic phase, the polydomain smectic phase is induced by a uniaxial load. Below the transition, in a monodomain smectic A phase, both experimentally observed effects of homogeneous director reorientation and stripe formation are reproduced when the sample is stretched along the director. When the load is applied perpendicularly to the director, the sample demonstrates reversible deformation with no change of liquid crystalline order, indicating elasticity of the two-dimensional network of polymer layers. Y1 - 2012 U6 - https://doi.org/10.1039/c2sm26499d SN - 1744-683X VL - 8 IS - 43 SP - 11123 EP - 11134 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Ilnytskyi, Jaroslav A1 - Saphiannikova, Marina A1 - Neher, Dieter T1 - Photo-induced deformations in azobenzene-containing side-chain polymers : molecular dynamics study N2 - We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director, forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation of the sample is found to depend on the field strength. In a weak field the original smectic order is melted gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction of the polymer along the field direction (the effect similar to the one observed experimentally in azopenzene containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the preferential direction of chromophores perperdicular to the field. Y1 - 2006 UR - http://www.icmp.lviv.ua/journal/Contents.html SN - 1607-324X 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 - Inal, Sahika A1 - Chiappisi, Leonardo A1 - Kölsch, Jonas D. A1 - Kraft, Mario A1 - Appavou, Marie-Sousai A1 - Scherf, Ullrich A1 - Wagner, Manfred A1 - Hansen, Michael Ryan A1 - Gradzielski, Michael A1 - Laschewsky, André A1 - Neher, Dieter T1 - Temperature-regulated fluorescence and association of an Oligo(ethyleneglycol)methacrylate-based copolymer with a conjugated Polyelectrolyte-the effect of solution ionic strength JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - Aqueous mixtures of a dye-labeled non-ionic thermoresponsive copolymer and a conjugated cationic polyelectrolyte are shown to exhibit characteristic changes in fluorescence properties in response to temperature and to the presence of salts, enabling a double-stimuli responsiveness. In such mixtures at room temperature, i.e., well below the lower critical solution temperature (LCST), the emission of the dye is strongly quenched due to energy transfer to the polycation, pointing to supramolecular interactions between the two macromolecules. Increasing the concentration of salts weakens the interpolymer interactions, the extent of which is simultaneously monitored from the change in the relative emission intensity of the components. When the mixture is heated above its LCST, the transfer efficiency is significantly reduced, signaling a structural reorganization process, however, surprisingly only if the mixture contains salt ions. To elucidate the reasons behind such thermo- and ion-sensitive fluorescence characteristics, we investigate the effect of salts of alkali chlorides, in particular of NaCl, on the association behavior of these macromolecules before and after the polymer phase transition by a combination of UV-vis, fluorescence, and H-1 NMR spectroscopy with light scattering and small-angle neutron scattering measurements. Y1 - 2013 U6 - https://doi.org/10.1021/jp408864s SN - 1520-6106 VL - 117 IS - 46 SP - 14576 EP - 14587 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Inal, Sahika A1 - Koelsch, Jonas D. A1 - Chiappisi, Leonardo A1 - Janietz, Dietmar A1 - Gradzielski, Michael A1 - Laschewsky, André A1 - Neher, Dieter T1 - Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers JF - Journal of materials chemistry : C, Materials for optical and electronic devices N2 - We demonstrate new fluorophore-labelled materials based on acrylamide and on oligo(ethylene glycol) (OEG) bearing thermoresponsive polymers for sensing purposes and investigate their thermally induced solubility transitions. It is found that the emission properties of the polarity-sensitive (solvatochromic) naphthalimide derivative attached to three different thermoresponsive polymers are highly specific to the exact chemical structure of the macromolecule. While the dye emits very weakly below the LCST when incorporated into poly(N-isopropylacrylamide) (pNIPAm) or into a polyacrylate backbone bearing only short OEG side chains, it is strongly emissive in polymethacrylates with longer OEG side chains. Heating of the aqueous solutions above their cloud point provokes an abrupt increase of the fluorescence intensity of the labelled pNIPAm, whereas the emission properties of the dye are rather unaffected as OEG-based polyacrylates and methacrylates undergo phase transition. Correlated with laser light scattering studies, these findings are ascribed to the different degrees of pre-aggregation of the chains at low temperatures and to the extent of dehydration that the phase transition evokes. It is concluded that although the temperature-triggered changes in the macroscopic absorption characteristics, related to large-scale alterations of the polymer chain conformation and aggregation, are well detectable and similar for these LCST-type polymers, the micro-environment provided to the dye within each polymer network differs substantially. Considering sensing applications, this finding is of great importance since the temperature-regulated fluorescence response of the polymer depends more on the macromolecular architecture than the type of reporter fluorophore. Y1 - 2013 U6 - https://doi.org/10.1039/c3tc31304b SN - 2050-7526 VL - 1 IS - 40 SP - 6603 EP - 6612 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Inal, Sahika A1 - Koelsch, Jonas D. A1 - Chiappisi, Leonardo A1 - Kraft, Mario A1 - Gutacker, Andrea A1 - Janietz, Dietmar A1 - Scherf, Ullrich A1 - Gradzielski, Michael A1 - Laschewsky, André A1 - Neher, Dieter T1 - Temperature-Regulated Fluorescence Characteristics of Supramolecular Assemblies Formed By a Smart Polymer and a Conjugated Polyelectrolyte JF - MACROMOLECULAR CHEMISTRY AND PHYSICS N2 - Aqueous mixtures of a coumarin-labeled non-ionic thermoresponsive copolymer and a cationic polythiophene exhibit marked changes in their fluorescence properties upon heating. At room temperature, emission from the label is significantly quenched due to energy transfer to the conjugated polyelectrolyte. Heating the mixture reduces the energy-transfer efficiency markedly, resulting in a clearly visible change of the emission color. Although the two macromolecules associate strongly at room temperature, the number of interacting sites is largely reduced upon the phase transition. Crucially, the intermolecular association does not suppress the responsiveness of the smart polymer, meaning that this concept should be applicable to chemo- or bioresponsive polymers with optical read-out, for example, as a sensor device. KW - aqueous solutions KW - conjugated polyelectrolytes KW - fluorescence (or Forster) KW - resonance energy transfer KW - phase transitions KW - thermoresponsive polymers Y1 - 2013 U6 - https://doi.org/10.1002/macp.201200493 SN - 1022-1352 VL - 214 IS - 4 SP - 435 EP - 445 PB - WILEY-V C H VERLAG GMBH CY - WEINHEIM ER - TY - JOUR A1 - Inal, Sahika A1 - Kölsch, Jonas D. A1 - Chiappisi, Leonardo A1 - Janietz, Dietmar A1 - Gradzielski, Michael A1 - Laschewsky, André A1 - Neher, Dieter T1 - Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers N2 - We demonstrate new fluorophore-labelled materials based on acrylamide and on oligo(ethylene glycol) (OEG) bearing thermoresponsive polymers for sensing purposes and investigate their thermally induced solubility transitions. It is found that the emission properties of the polarity-sensitive (solvatochromic) naphthalimide derivative attached to three different thermoresponsive polymers are highly specific to the exact chemical structure of the macromolecule. While the dye emits very weakly below the LCST when incorporated into poly(N-isopropylacrylamide) (pNIPAm) or into a polyacrylate backbone bearing only short OEG side chains, it is strongly emissive in polymethacrylates with longer OEG side chains. Heating of the aqueous solutions above their cloud point provokes an abrupt increase of the fluorescence intensity of the labelled pNIPAm, whereas the emission properties of the dye are rather unaffected as OEG-based polyacrylates and methacrylates undergo phase transition. Correlated with laser light scattering studies, these findings are ascribed to the different degrees of pre-aggregation of the chains at low temperatures and to the extent of dehydration that the phase transition evokes. It is concluded that although the temperature-triggered changes in the macroscopic absorption characteristics, related to large-scale alterations of the polymer chain conformation and aggregation, are well detectable and similar for these LCST-type polymers, the micro-environment provided to the dye within each polymer network differs substantially. Considering sensing applications, this finding is of great importance since the temperature-regulated fluorescence response of the polymer depends more on the macromolecular architecture than the type of reporter fluorophore. Y1 - 2013 UR - http://pubs.rsc.org/en/content/articlepdf/2013/tc/c3tc31304b U6 - https://doi.org/10.1039/C3TC31304B ER - TY - JOUR A1 - Inal, Sahika A1 - Kölsch, Jonas D. A1 - Sellrie, Frank A1 - Schenk, Jörg A. A1 - Wischerhoff, Erik A1 - Laschewsky, André A1 - Neher, Dieter T1 - A water soluble fluorescent polymer as a dual colour sensor for temperature and a specific protein JF - Journal of materials chemistry : B, Materials for biology and medicine N2 - We present two thermoresponsive water soluble copolymers prepared via free radical statistical copolymerization of N-isopropylacrylamide (NIPAm) and of oligo(ethylene glycol) methacrylates (OEGMAs), respectively, with a solvatochromic 7-(diethylamino)-3-carboxy-coumarin (DEAC)-functionalized monomer. In aqueous solutions, the NIPAm-based copolymer exhibits characteristic changes in its fluorescence profile in response to a change in solution temperature as well as to the presence of a specific protein, namely an anti-DEAC antibody. This polymer emits only weakly at low temperatures, but exhibits a marked fluorescence enhancement accompanied by a change in its emission colour when heated above its cloud point. Such drastic changes in the fluorescence and absorbance spectra are observed also upon injection of the anti-DEAC antibody, attributed to the specific binding of the antibody to DEAC moieties. Importantly, protein binding occurs exclusively when the polymer is in the well hydrated state below the cloud point, enabling a temperature control on the molecular recognition event. On the other hand, heating of the polymer-antibody complexes releases a fraction of the bound antibody. In the presence of the DEAC-functionalized monomer in this mixture, the released antibody competitively binds to the monomer and the antibody-free chains of the polymer undergo a more effective collapse and inter-aggregation. In contrast, the emission properties of the OEGMA-based analogous copolymer are rather insensitive to the thermally induced phase transition or to antibody binding. These opposite behaviours underline the need for a carefully tailored molecular design of responsive polymers aimed at specific applications, such as biosensing. Y1 - 2013 U6 - https://doi.org/10.1039/c3tb21245a SN - 2050-750X SN - 2050-7518 VL - 1 IS - 46 SP - 6373 EP - 6381 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Inal, Sahika A1 - Kölsch, Jonas D. A1 - Selrie, Frank A1 - Schenk, Jörg A. A1 - Wischerhoff, Erik A1 - Laschewsky, André A1 - Neher, Dieter T1 - A water soluble fluorescent polymer as a dual colour sensor for temperature and a specific protein N2 - We present two thermoresponsive water soluble copolymers prepared via free radical statistical copolymerization of N-isopropylacrylamide (NIPAm) and of oligo(ethylene glycol) methacrylates (OEGMAs), respectively, with a solvatochromic 7-(diethylamino)-3-carboxy-coumarin (DEAC)-functionalized monomer. In aqueous solutions, the NIPAm-based copolymer exhibits characteristic changes in its fluorescence profile in response to a change in solution temperature as well as to the presence of a specific protein, namely an anti-DEAC antibody. This polymer emits only weakly at low temperatures, but exhibits a marked fluorescence enhancement accompanied by a change in its emission colour when heated above its cloud point. Such drastic changes in the fluorescence and absorbance spectra are observed also upon injection of the anti-DEAC antibody, attributed to the specific binding of the antibody to DEAC moieties. Importantly, protein binding occurs exclusively when the polymer is in the well hydrated state below the cloud point, enabling a temperature control on the molecular recognition event. On the other hand, heating of the polymer-antibody complexes releases a fraction of the bound antibody. In the presence of the DEAC-functionalized monomer in this mixture, the released antibody competitively binds to the monomer and the antibody-free chains of the polymer undergo a more effective collapse and inter-aggregation. In contrast, the emission properties of the OEGMA-based analogous copolymer are rather insensitive to the thermally induced phase transition or to antibody binding. These opposite behaviours underline the need for a carefully tailored molecular design of responsive polymers aimed at specific applications, such as biosensing. Y1 - 2013 UR - http://pubs.rsc.org/en/content/articlepdf/2013/tb/c3tb21245a U6 - https://doi.org/10.1039/c3tb21245a 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 - Jošt, Marko A1 - Albrecht, Steve A1 - Kegelmann, Lukas A1 - Wolff, Christian Michael A1 - Lang, Felix A1 - Lipovšek, Benjamin A1 - Krč, Janez A1 - Korte, Lars A1 - Neher, Dieter A1 - Rech, Bernd A1 - Topič, Marko T1 - Efficient light management by textured nanoimprinted layers for perovskite solar cells JF - ACS photonics N2 - Inorganic-organic perovskites like methylammonium-lead-iodide have proven to be an effective class of 17 materials for fabricating efficient solar cells. To improve their performance, light management techniques using textured surfaces, similar to those used in established solar cell technologies, should be considered. Here, we apply a light management foil created by UV nanoimprint lithography on the glass side of an inverted (p-i-n) perovskite solar cell with 16.3% efficiency. The obtained 1 mA cm(-2) increase in the short-circuit current density translates to a relative improvement in cell performance of 5%, which results in a power conversion efficiency of 17.1%. Optical 3D simulations based on experimentally obtained parameters were used to support the experimental findings. A good match between the simulated and experimental data was obtained, validating the model. Optical simulations reveal that the main improvement in device performance is due to a reduction in total reflection and that relative improvement in the short-circuit current density of up to 10% is possible for large-area devices. Therefore, our results present the potential of light management foils for improving the device performance of perovskite solar cells and pave the way for further use of optical simulations in the field of perovskite solar cells. KW - perovskite solar cells KW - antireflection KW - light management KW - UV nanoimprint lithography KW - optical simulations Y1 - 2017 U6 - https://doi.org/10.1021/acsphotonics.7b00138 SN - 2330-4022 VL - 4 SP - 1232 EP - 1239 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Karageorgiev, Peter A1 - Neher, Dieter A1 - Schulz, Burkhard A1 - Stiller, Burkhard A1 - Pietsch, Ullrich A1 - Giersig, Michael A1 - Brehmer, Ludwig T1 - From anisotropic photo-fluidity towards nanomanipulation in the optical near-field N2 - An increase in random molecular vibrations of a solid owing to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. Here we report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it shows considerable fluidity only in the direction parallel to the light field vector under linear polarized light. The fluidization phenomenon is related to photoinduced motion of azobenzene-functionalized molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also describe here how the photofluidization allows nanoscopic elements of matter to be precisely manipulated Y1 - 2005 SN - 1476-1122 ER - TY - JOUR A1 - Kegelmann, Lukas A1 - Tockhorn, Philipp A1 - Wolff, Christian Michael A1 - Márquez, José A. A1 - Caicedo Dávila, Sebastián A1 - Korte, Lars A1 - Unold, Thomas A1 - Loevenich, Wilfried A1 - Neher, Dieter A1 - Rech, Bernd A1 - Albrecht, Steve T1 - Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in Perovskite Solar Cells JF - ACS applied materials & interfaces N2 - Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages (V-OC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the V-OC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM. KW - perovskite solar cell KW - selective contact KW - spiro-OMeTAD KW - PEDOT KW - recombination KW - passivation KW - quasi-Fermi level splitting Y1 - 2019 U6 - https://doi.org/10.1021/acsami.9b01332 SN - 1944-8244 SN - 1944-8252 VL - 11 IS - 9 SP - 9172 EP - 9181 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kegelmann, Lukas A1 - Wolff, Christian Michael A1 - Awino, Celline A1 - Lang, Felix A1 - Unger, Eva L. A1 - Korte, Lars A1 - Dittrich, Thomas A1 - Neher, Dieter A1 - Rech, Bernd A1 - Albrecht, Steve T1 - It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis JF - ACS applied materials & interfaces N2 - Solar cells made from inorganic organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 degrees C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C-60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop, the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells. KW - perovskite solar cell KW - electron contact KW - double-layer KW - regular planar architecture KW - hysteresis KW - fullerene KW - metal oxide Y1 - 2017 U6 - https://doi.org/10.1021/acsami.7b00900 SN - 1944-8244 VL - 9 SP - 17246 EP - 17256 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kelly, Mary Allison A1 - Roland, Steffen A1 - Zhang, Qianqian A1 - Lee, Youngmin A1 - Kabius, Bernd A1 - Wang, Qing A1 - Gomez, Enrique D. A1 - Neher, Dieter A1 - You, Wei T1 - Incorporating Fluorine Substitution into Conjugated Polymers for Solar Cells BT - three Different Means, Same Results JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Fluorinating conjugated polymers is a proven strategy for creating high performance materials in polymer solar cells, yet few studies have investigated the importance of the fluorination method. We compare the performance of three fluorinated systems: a poly(benzodithieno-dithienyltriazole) (PBnDT-XTAZ) random copolymer where 50% of the acceptor units are difluorinated, PBnDT-mFTAZ where every acceptor unit is monofluorinated, and a 1:1 physical blend of the difluorinated and nonfluorinated polymer. All systems have the same degree of fluorination (50%) yet via different methods (chemically vs physically, random vs regular). We show that these three systems have equivalent photovoltaic behavior:,similar to 5.2% efficiency with a short-circuit current (J(sc)) at,similar to 11 mA cm(-2), an open-circuit voltage (v(oc)) at 0.77 V, and a fill factor (FF) of similar to 60%. Further investigation of these three systems demonstrates that the charge generation, charge extraction, and charge transfer state are essentially identical for the three studied systems. Transmission electron microscopy shows no significant differences in the morphologies. All these data illustrate that it is possible to improve performance not only via regular or random fluorination but also by physical addition via a ternary blend. Thus, our results demonstrate the versatility of incorporating fluorine in the active layer of polymer solar cells to enhance device performance. Y1 - 2017 U6 - https://doi.org/10.1021/acs.jpcc.6b10993 SN - 1932-7447 VL - 121 IS - 4 SP - 2059 EP - 2068 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kietzke, Thomas A1 - Egbe, Daniel A. M. A1 - Hörhold, Hans-Heinrich A1 - Neher, Dieter T1 - Comparative study of M3EH-PPV-based bilayer photovoltaic devices N2 - We have recently shown that efficient polymer solar cells can be fabricated by using a weakly soluble derivative of poly-p-vinylene (M3EH-PPV) as the electron donor. Here we present studies on bilayer devices using organic electron acceptors with varying LUMO levels and M3EH-PPV. It is found that the open-circuit voltage scales linearly with the LUMO level of the acceptor, reaching values as high as 1.5 V when cyano-substituted poly(p-phenyleneethynylene)-alt- poly(p-phenylenevinylene) copolymers are used. Further, we discovered that for an increasing number of triple bonds in the repeat unit of the acceptor polymer the device performance decreases with increasing thickness of the acceptor layer. Also, the quantum efficiency was smaller when using polymers with higher LUMO levels. Thus, further effort is needed to design optimum acceptor polymers for devices exhibiting large open-circuit voltage and high quantum efficiency Y1 - 2006 UR - http://pubs.acs.org/journal/mamobx U6 - https://doi.org/10.1021/Ma0601991 SN - 4018-4022 ER - TY - JOUR A1 - Kietzke, Thomas A1 - Horhold, H. H. A1 - Neher, Dieter T1 - Efficient polymer solar cells based on M3EH-PPV N2 - We report on polymer blend solar cells with an external quantum efficiency of more than 30% and a hi-h overall energy conversion efficiency (ECE) under white light illumination (100 mW/cm(2)) Of Lip to 1.7% using a blend of M3EH- PPV (poly [2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5(2-ethylhexyloxy)-(1,4-pheiiylene-1,2-ethenylene)]) and CN-ether-PPV (poly[oxa-1,4-phenylene-1,2(1-cyano)ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)ethellyiene-1,4- phenylene]). We attribute these high efficiencies to the formation of a vertically composition graded structure during spin coating Photoluminescence measurements performed on the blend layers indicated the formation of exciplexes between both types of polymers, which we propose to be one factor preventing even higher efficiencies Y1 - 2005 ER - TY - JOUR A1 - Kietzke, Thomas A1 - Neher, Dieter A1 - Kumke, Michael Uwe A1 - Montenegro, Rivelino V. D. A1 - Landfester, Katharina A1 - Scherf, Ullrich T1 - A nanoparticle approach to control the phase separation in polyfluorene photovoltaic devices N2 - Polymer solar cell devices with nanostructured blend layers have been fabricated using single- and dual- component polymer nanospheres. Starting from an electron-donating and an electron-accepting polyfluorene derivative, PFB and F8BT, dissolved in suitable organic solvents, dispersions of solid particles with mean diameters of ca. 50 nm, containing either the pure polymer components or a mixture of PFB and F8BT in each particle, were prepared with the miniemulsion process. Photovoltaic devices based on these particles have been studied with respect to the correlation between external quantum efficiency and layer composition. It is shown that the properties of devices containing a blend of single-component PFB and F8BT particles differ significantly from those of solar cells based on blend particles, even for the same layer composition. Various factors determining the quantum efficiency in both kinds of devices are identified and discussed, taking into account the spectroscopic properties of the particles. An external quantum efficiency of ca. 4% is measured for a device made from polymer blend nanoparticles containing PFB:F8BT at a weight ratio of 1:2 in each individual nanosphere. This is among the highest values reported so far for photovoltaic cells using this material combination Y1 - 2004 ER - TY - JOUR A1 - Kietzke, Thomas A1 - Stiller, Burkhard A1 - Landfester, Katharina A1 - Montenegro, Rivelino V. D. A1 - Neher, Dieter T1 - Probing the local optical properties of layers prepared from polymer nanoparticles N2 - It is well known that the performance of solar cells based on a blend of hole-accepting and electron-accepting conjugated polymers as the active material depend crucially on the length scale of the resulting phase separated morphology. However, a direct control of this morphology is difficult if the layer is prepared from an organic solvent. To circumvent this difficulty, recently a universal method to fabricate defined nano-structured blend layer using nanoparticles dispersed in water was demonstrated. These nanoparticles were prepared with the miniemulsion method, which allows for the preparation of semiconducting polymer nanospheres (SPNs) with diameters in the range of 30 to 300 nanometres. Since the process starts from the active material dissolved in a common solvent, it can be applied to the fabrication of nanoparticles of blends of polymers with oligomers or even with inorganic materials. We present here for the first time scanning near field optical microscopy (SNOM) investigations on these novel nanostructured polymer layers. We show that by spin-coating a mixture of two different dispersions a nanoparticle monolayer with a statistically distribution of the nanoparticles can be obtained. Mixing conjugated polymer nanoparticles with some inert particles like polystyrene beads may allow for the preparation of nano-sized light emitters Y1 - 2005 SN - 0379-6779 ER - TY - JOUR A1 - Kniepert, Juliane A1 - Lange, Ilja A1 - Heidbrink, Jan A1 - Kurpiers, Jona A1 - Brenner, Thomas J. K. A1 - Koster, L. Jan Anton A1 - Neher, Dieter T1 - Effect of Solvent Additive on Generation, Recombination, and Extraction in PTB7:PCBM Solar Cells: A Conclusive Experimental and Numerical Simulation Study JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Time-delayed collection field (TDCF), bias-assisted charge extraction (BACE), and space charge-limited current (SCLC) measurements are combined with complete numerical device simulations to unveil the effect of the solvent additive 1,8-diiodooctane (DIO) on the performance of PTB7:PCBM bulk heterojunction solar cells. DIO is shown to increase the charge generation rate, reduce geminate and bimolecular recombination, and increase the electron mobility. In total, the reduction of loss currents by processing with the additive raises the power conversion efficiency of the PTB7:PCBM blend by a factor of almost three. The lower generation rates and higher geminate recombination losses in devices without DIO are consistent with a blend morphology comprising large fullerene clusters embedded within a PTB7-rich matrix, while the low electron mobility suggests that these fullerene clusters are themselves composed of smaller pure fullerene aggregates separated by disordered areas. Our device simulations show unambiguously that the effect of the additive on the shape of the currentvoltage curve (J-V) cannot be ascribed to the variation of only the mobility, the recombination, or the field dependence of generation. It is only when the changes of all three parameters are taken into account that the simulation matches the experimental J-V characteristics under all illumination conditions and for a wide range of voltages. Y1 - 2015 U6 - https://doi.org/10.1021/jp512721e SN - 1932-7447 VL - 119 IS - 15 SP - 8310 EP - 8320 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kniepert, Juliane A1 - Lange, Ilja A1 - van der Kaap, Niels J. A1 - Koster, L. Jan Anton A1 - Neher, Dieter T1 - A conclusive view on charge generation, recombination, and extraction in As-prepared and annealed P3HT:PCBM blends: combined experimental and simulation work JF - dvanced energy materials N2 - 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. Y1 - 2014 U6 - https://doi.org/10.1002/aenm.201301401 SN - 1614-6832 SN - 1614-6840 VL - 4 IS - 7 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kniepert, Juliane A1 - Paulke, Andreas A1 - Perdigón-Toro, Lorena A1 - Kurpiers, Jona A1 - Zhang, Huotian A1 - Gao, Feng A1 - Yuan, Jun A1 - Zou, Yingping A1 - Le Corre, Vincent M. A1 - Koster, Lambert Jan Anton A1 - Neher, Dieter T1 - Reliability of charge carrier recombination data determined with charge extraction methods JF - Journal of applied physics N2 - Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k(2) = 1.7 x 10(-17)m(3) s(-1). BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that surface recombination is of minor importance despite the small active layer thickness. Published under license by AIP Publishing. Y1 - 2019 U6 - https://doi.org/10.1063/1.5129037 SN - 0021-8979 SN - 1089-7550 VL - 126 IS - 20 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Kniepert, Juliane A1 - Schubert, Marcel A1 - Blakesley, James C. A1 - Neher, Dieter T1 - Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments JF - The journal of physical chemistry letters N2 - 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. Y1 - 2011 U6 - https://doi.org/10.1021/jz200155b SN - 1948-7185 VL - 2 IS - 7 SP - 700 EP - 705 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kraffert, Felix A1 - Steyrleuthner, Robert A1 - Albrecht, Steve A1 - Neher, Dieter A1 - Scharber, Markus C. A1 - Bittl, Robert A1 - Behrends, Jan T1 - Charge Separation in PCPDTBT : PCBM Blends from an EPR Perspective JF - The journal of physical chemistry Y1 - 2014 U6 - https://doi.org/10.1021/jp509650v SN - 1932-7447 VL - 118 IS - 49 SP - 28482 EP - 28493 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kroh, Daniel A1 - Eller, Fabian A1 - Schötz, Konstantin A1 - Wedler, Stefan A1 - Perdigón-Toro, Lorena A1 - Freychet, Guillaume A1 - Wei, Qingya A1 - Dörr, Maximilian A1 - Jones, David A1 - Zou, Yingping A1 - Herzig, Eva M. A1 - Neher, Dieter A1 - Köhler, Anna T1 - Identifying the signatures of intermolecular interactions in blends of PM6 with Y6 and N4 using absorption spectroscopy JF - Advanced functional materials N2 - In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the-art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in the blend films and accounts for the different solar cell efficiencies reported with the two blends. KW - charge-transfer states KW - Frank-Condon analysis KW - morphology KW - organic solar cells Y1 - 2022 U6 - https://doi.org/10.1002/adfm.202205711 SN - 1616-301X SN - 1616-3028 VL - 32 IS - 44 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kuehn, Sergei A1 - Pingel, Patrick A1 - Breusing, Markus A1 - Fischer, Thomas A1 - Stumpe, Joachim A1 - Neher, Dieter A1 - Elsaesser, Thomas T1 - High-Resolution Near-Field Optical Investigation of Crystalline Domains in Oligomeric PQT-12 Thin Films JF - Advanced functional materials N2 - The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin films. Using a combination of spectroscopic methods, including scanning near-field optical microscopy, we study the domain structure and packing quality of highly crystalline thin films of oligomeric PQT-12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identification of regions with uniform molecular orientation. We find that a hierarchical order on three different length scales exists in these layers, made up of distinct well-ordered dichroic areas at the ten-micrometer-scale, which are sub-divided into domains with different molecular in-plane orientation. These serve as a template for the formation of smaller needle-like crystallites at the layer surface. A high degree of crystalline order is believed to be the cause of the rather high field-effect mobility of these layers of 10(-3) cm 2 V(-1) s(-1), whereas it is limited by the presence of domain boundaries at macroscopic distances. Y1 - 2011 U6 - https://doi.org/10.1002/adfm.201001978 SN - 1616-301X VL - 21 IS - 5 SP - 860 EP - 868 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Kulikovsky, Lazar A1 - Neher, Dieter A1 - Mecher, E. A1 - Meerholz, Klaus A1 - Horhold, H. H. A1 - Ostroverkhova, O. T1 - Photocurrent dynamics in a poly(phenylene vinylene)-based photorefractive composite N2 - All parameters describing the charge carrier dynamics in a poly(phenylene vinylene)-based photorefractive (PR) composite relevant to PR grating dynamics were determined using photoconductivity studies under various illumination conditions. In particular, the values of the coefficients for trap filling and recombination of charges with ionized sensitizer molecules could be extracted independently. It is concluded that the PR growth time without preillumination is mostly determined by the competition between deep trap filling and recombination with ionized sensitizer molecules. Further, the pronounced increase in PR speed upon homogeneous preillumination (gating) as reported recently is quantitatively explained by deep trap filling Y1 - 2004 SN - 1098-0121 ER - TY - JOUR A1 - Kurpiers, Jona A1 - Ferron, Thomas A1 - Roland, Steffen A1 - Jakoby, Marius A1 - Thiede, Tobias A1 - Jaiser, Frank A1 - Albrecht, Steve A1 - Janietz, Silvia A1 - Collins, Brian A. A1 - Howard, Ian A. A1 - Neher, Dieter T1 - Probing the pathways of free charge generation in organic bulk heterojunction solar cells JF - Nature Communications N2 - The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-04386-3 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Kurpiers, Jona A1 - Neher, Dieter T1 - Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend JF - Scientific reports N2 - Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions. Y1 - 2016 U6 - https://doi.org/10.1038/srep26832 SN - 2045-2322 VL - 6 PB - Nature Publishing Group CY - London ER - TY - JOUR A1 - Kurpiers, Jona A1 - Neher, Dieter T1 - Dispersive Non-Geminate Recombination in an Amorphous Polymer: Fullerene Blend JF - Scientific reports N2 - Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer: fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions. Y1 - 2016 U6 - https://doi.org/10.1038/srep26832 SN - 2045-2322 VL - 6 PB - Nature Publ. Group CY - London ER -