TY - JOUR A1 - Ligorio, G. A1 - Nardi, M. V. A1 - Steyrleuthner, Robert A1 - Ihiawakrim, D. A1 - Crespo-Monteiro, N. A1 - Brinkmann, Martin A1 - Neher, Dieter A1 - Koch, N. T1 - Metal nanoparticle mediated space charge and its optical control in an organic hole-only device JF - Applied physics letters N2 - We reveal the role of localized space charges in hole-only devices based on an organic semiconductor with embedded metal nanoparticles (MNPs). MNPs act as deep traps for holes and reduce the current density compared to a device without MNPs by a factor of 10(4) due to the build-up of localized space charge. Dynamic MNPs charged neutrality can be realized during operation by electron transfer from excitons created in the organic matrix, enabling light sensing independent of device bias. In contrast to the previous speculations, electrical bistability in such devices was not observed. (C) 2016 AIP Publishing LLC. Y1 - 2016 U6 - https://doi.org/10.1063/1.4945710 SN - 0003-6951 SN - 1077-3118 VL - 108 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Zerson, Mario A1 - Neumann, Martin A1 - Steyrleuthner, Robert A1 - Neher, Dieter A1 - Magerle, Robert T1 - Surface Structure of Semicrystalline Naphthalene Diimide-Bithiophene Copolymer Films Studied with Atomic Force Microscopy JF - Macromolecules : a publication of the American Chemical Society Y1 - 2016 U6 - https://doi.org/10.1021/acs.macromol.6b00988 SN - 0024-9297 SN - 1520-5835 VL - 49 SP - 6549 EP - 6557 PB - American Chemical Society CY - Washington 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 - Nikolis, Vasileios C. A1 - Benduhn, Johannes A1 - Holzmueller, Felix A1 - Piersimoni, Fortunato A1 - Lau, Matthias A1 - Zeika, Olaf A1 - Neher, Dieter A1 - Koerner, Christian A1 - Spoltore, Donato A1 - Vandewal, Koen T1 - Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies JF - dvanced energy materials N2 - High photon energy losses limit the open-circuit voltage (V-OC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the V-OC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha-sexithiophene (alpha-6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the V-OC of an alpha-6T/SubNc/SubPc fullerene-free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D-A interface. By accurately measuring the optical gap (E-opt) and the energy of the charge-transfer state (E-CT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. E-opt - qV(OC) losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the V-OC-optimized devices, the low-energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low-voltage losses can be combined with a high EQE in organic photovoltaic devices. KW - energy losses KW - nonradiative recombination KW - open-circuit voltage KW - organic solar cells KW - voltage losses Y1 - 2017 U6 - https://doi.org/10.1002/aenm.201700855 SN - 1614-6832 SN - 1614-6840 VL - 7 SP - 122 EP - 136 PB - Wiley-VCH CY - Weinheim 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 - Mechau, Norman A1 - Saphiannikova, Marina A1 - Neher, Dieter T1 - Molecular tracer diffusion in thin azobenzene polymer layers JF - Applied physics letters N2 - Translational diffusion of fluorescent tracer molecules in azobenzene polymer layers is studied at different temperatures and under illumination using the method of fluorescence recovery after photobleaching. Diffusion is clearly observed in the dark above the glass transition temperature, while homogeneous illumination at 488 nm and 100 mW/cm(2) does not cause any detectable diffusion of the dye molecules within azobenzene layers. This implies that the viscosity of azobenzene layers remains nearly unchanged under illumination with visible light in the absence of internal or external forces. (c) 2006 American Institute of Physics. Y1 - 2006 U6 - https://doi.org/10.1063/1.2405853 SN - 0003-6951 VL - 89 IS - 25 PB - Elsevier CY - Melville 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 - Li, Tian-yi A1 - Benduhn, Johannes A1 - Li, Yue A1 - Jaiser, Frank A1 - Spoltore, Donato A1 - Zeika, Olaf A1 - Ma, Zaifei A1 - Neher, Dieter A1 - Vandewal, Koen A1 - Leo, Karl T1 - Boron dipyrromethene (BODIPY) with meso-perfluorinated alkyl substituents as near infrared donors in organic solar cells JF - Journal of materials chemistry : A, Materials for energy and sustainability N2 - Three furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at similar to 800 nm with absorption coefficient of over 280 000 cm(-1). Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at similar to 790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm(-2). Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor-acceptor interface were comparable in the range of 0.14-0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate V-oc of 0.76 V. Y1 - 2018 U6 - https://doi.org/10.1039/c8ta06261g SN - 2050-7488 SN - 2050-7496 VL - 6 IS - 38 SP - 18583 EP - 18591 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Salert, Beatrice Ch. D. A1 - Krueger, Hartmut A1 - Bagnich, Sergey A. A1 - Unger, Thomas A1 - Jaiser, Frank A1 - Al-Sa'di, Mahmoud A1 - Neher, Dieter A1 - Hayer, Anna A1 - Eberle, Thomas T1 - New polymer matrix system for phosphorescent organic light-emitting diodes and the role of the small molecular co-host JF - Journal of polymer science : A, Polymer chemistry N2 - A new matrix system for phosphorescent organic light-emitting diodes (OLEDs) based on an electron transporting component attached to an inert polymer backbone, an electronically neutral co-host, and a phosphorescent dye that serves as both emitter and hole conductor are presented. The inert co-host is used either as small molecules or covalently connected to the same chain as the electron-transporting host. The use of a small molecular inert co-host in the active layer is shown to be highly advantageous in comparison to a purely polymeric matrix bearing the same functionalities. Analysis of the dye phosphorescence decay in pure polymer, small molecular co-host film, and their blend lets to conclude that dye molecules distribute mostly in the small molecular co-host phase, where the co-host prevents agglomeration and self-quenching of the phosphorescence as well as energy transfer to the electron transporting units. In addition, the co-host accumulates at the anode interface where it acts as electron blocking layer and improves hole injection. This favorable phase separation between polymeric and small molecular components results in devices with efficiencies of about 47 cd/A at a luminance of 1000 cd/m(2). Investigation of OLED degradation demonstrates the presence of two time regimes: one fast component that leads to a strong decrease at short times followed by a slower decrease at longer times. Unlike the long time degradation, the efficiency loss that occurs at short times is reversible and can be recovered by annealing of the device at 180 degrees C. We also show that the long-time degradation must be related to a change of the optical and electrical bulk properties. KW - charge transport KW - conducting polymer KW - degradation KW - host-guest systems KW - light-emitting diodes KW - random copolymer KW - synthesis KW - UV-vis spectroscopy Y1 - 2013 U6 - https://doi.org/10.1002/pola.26409 SN - 0887-624X VL - 51 IS - 3 SP - 601 EP - 613 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Zen, Achmad A1 - Pflaum, J. A1 - Hirschmann, S. A1 - Zhuang, W. A1 - Jaiser, Frank A1 - Asawapirom, Udom A1 - Rabe, J. P. A1 - Scherf, Ullrich A1 - Neher, Dieter T1 - Effect of molecular weight and annealing of poly (3-hexylthiophene)s on the performance of organic field-effect transistors N2 - The optical, structural, and electrical properties of thin layers made from poly(3-hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater 2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X-ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 1.50 degreesC . However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue-shift of the solid-state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low-molecular weight P3HT layers Y1 - 2004 ER -