@article{GhaniOpitzPingeletal.2015,
  author    = {Ghani, Fatemeh and Opitz, Andreas and Pingel, Patrick and Heimel, Georg and Salzmann, Ingo and Frisch, Johannes and Neher, Dieter and Tsami, Argiri and Scherf, Ullrich and Koch, Norbert},
  title     = {Charge Transfer in and Conductivity of Molecularly Doped Thiophene-Based Copolymers},
  series = {Journal of polymer science : B, Polymer physics},
  volume    = {53},
  journal   = {Journal of polymer science : B, Polymer physics},
  number    = {1},
  publisher = {Wiley-Blackwell},
  address   = {Hoboken},
  issn      = {0887-6266},
  doi       = {10.1002/polb.23631},
  pages     = {58 -- 63},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}
@article{PiersimoniSchlesingerBenduhnetal.2015,
  author    = {Piersimoni, Fortunato and Schlesinger, Raphael and Benduhn, Johannes and Spoltore, Donato and Reiter, Sina and Lange, Ilja and Koch, Norbert and Vandewal, Koen and Neher, Dieter},
  title     = {Charge Transfer Absorption and Emission at ZnO/Organic Interfaces},
  series = {The journal of physical chemistry letters},
  volume    = {6},
  journal   = {The journal of physical chemistry letters},
  number    = {3},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1948-7185},
  doi       = {10.1021/jz502657z},
  pages     = {500 -- 504},
  year      = {2015},
  abstract  = {We investigate hybrid charge transfer states (HCTS) at the planar interface between a-NPD and ZnO by spectrally resolved electroluminescence (EL) and external quantum efficiency (EQE) measurements. Radiative decay of HCTSs is proven by distinct emission peaks in the EL spectra of such bilayer devices in the NIR at energies well below the bulk a-NPD or ZnO emission. The EQE spectra display low energy contributions clearly red-shifted with respect to the a-NPD photocurrent and partially overlapping with the EL emission. Tuning of the energy gap between the ZnO conduction band and a-NPD HOMO level (E-int) was achieved by modifying the ZnO surface with self-assembled monolayers based on phosphonic acids. We find a linear dependence of the peak position of the NIR EL on E-int, which unambiguously attributes the origin of this emission to radiative recombination between an electron on the ZnO and a hole on a-NPD. In accordance with this interpretation, we find a strictly linear relation between the open-circuit voltage and the energy of the charge state for such hybrid organicinorganic interfaces.},
  language  = {en}
}
@article{RolandNeubertAlbrechtetal.2015,
  author    = {Roland, Steffen and Neubert, Sebastian and Albrecht, Steve and Stannowski, Bernd and Seger, Mark and Facchetti, Antonio and Schlatmann, Rutger and Rech, Bernd and Neher, Dieter},
  title     = {Hybrid Organic/Inorganic Thin-Film Multijunction Solar Cells Exceeding 11\% Power Conversion Efficiency},
  series = {Advanced materials},
  volume    = {27},
  journal   = {Advanced materials},
  number    = {7},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0935-9648},
  doi       = {10.1002/adma.201404698},
  pages     = {1262 -- 1267},
  year      = {2015},
  abstract  = {Hybrid multijunction solar cells comprising hydrogenated amorphous silicon and an organic bulk heterojunction are presented, reaching 11.7\% power conversion efficiency. The benefits of merging inorganic and organic subcells are pointed out, the optimization of the cells, including optical modeling predictions and tuning of the recombination contact are described, and an outlook of this technique is given.},
  language  = {en}
}
@article{FischerTrefzBacketal.2015,
  author    = {Fischer, Florian S. U. and Trefz, Daniel and Back, Justus and Kayunkid, Navaphun and Tornow, Benjamin and Albrecht, Steve and Yager, Kevin G. and Singh, Gurpreet and Karim, Alamgir and Neher, Dieter and Brinkmann, Martin and Ludwigs, Sabine},
  title     = {Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto-Electronic Properties},
  series = {Advanced materials},
  volume    = {27},
  journal   = {Advanced materials},
  number    = {7},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0935-9648},
  doi       = {10.1002/adma.201403475},
  pages     = {1223 -- 1228},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}
@article{LysyakovaLomadzeNeheretal.2015,
  author    = {Lysyakova, Liudmila and Lomadze, Nino and Neher, Dieter and Maximova, Ksenia and Kabashin, Andrei V. and Santer, Svetlana},
  title     = {Light-Tunable Plasmonic Nanoarchitectures Using Gold Nanoparticle-Azobenzene-Containing Cationic Surfactant Complexes},
  series = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  volume    = {119},
  journal   = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  number    = {7},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1932-7447},
  doi       = {10.1021/jp511232g},
  pages     = {3762 -- 3770},
  year      = {2015},
  abstract  = {When arranged in a proper nanoaggregate architecture, gold nanoparticles can offer controllable plasmon-related absorption/scattering, yielding distinct color effects that depend critically on the relative orientation and distance between nanoparticle constituents. Herein, we report on the implementation of novel plasmonic nanoarchitectures based on complexes between gold nanoparticles and an azobenzene-modified cationic surfactant that can exhibit a light-tunable plasmonic response. The formation of such complexes becomes possible through the use of strongly negatively charged bare gold nanoparticles (similar to 10-nm diameter) prepared by the method of laser ablation in deionized water. Driven by electrostatic interactions, the cationic surfactant molecules attach and form a shell around the negatively charged nanoparticles, resulting in neutralization of the particle charge or even overcompensation beyond which the nanoparticles become positively charged. At low and high surfactant concentrations, Au nanoparticles are negatively and positively charged, respectively, and are represented by single species due to electric repulsion effects having absorption peaks around 523-527 nm, whereas at intermediate concentrations, the Au nanoparticles become neutral, forming nanoscale 100-nm clusterlike aggregates and exhibiting an additional absorption peak at gimel > 600 nm and a visible change in the color of the solution from red to blue. Because of the presence of the photosensitive azobenzene unit in the surfactant tail that undergoes trans-to-cis isomerization under irradiation with UV light, we then demonstrate a light-controlled nanoclustering of nanoparticles, yielding a switch in the plasmonic absorption band and a related change in the solution color. The formed hybrid architectures with a light-controlled plasmonic response could be important for a variety of tasks, including biomedical, surface-enhanced Raman spectroscopy (SERS), data transmission, and storage applications.},
  language  = {en}
}
@article{LangeReiterKniepertetal.2015,
  author    = {Lange, Ilja and Reiter, Sina and Kniepert, Juliane and Piersimoni, Fortunato and Paetzel, Michael and Hildebrandt, Jana and Brenner, Thomas J. K. and Hecht, Stefan and Neher, Dieter},
  title     = {Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures},
  series = {Applied physics letters},
  volume    = {106},
  journal   = {Applied physics letters},
  number    = {11},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.4916182},
  pages     = {5},
  year      = {2015},
  abstract  = {An approach is presented to modify the work function of solution-processed sol-gel derived zinc oxide (ZnO) over an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using poly(3-hexylthiophene): phenyl-C71-butyric acid methyl ester as the active layer. These devices compete with or even surpass the performance of the reference on indium tin oxide/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Our findings highlight the potential of properly modified ZnO as electron or hole extracting electrodes in hybrid optoelectronic devices. (C) 2015 AIP Publishing LLC.},
  language  = {en}
}
@article{WuerfelNeherSpiesetal.2015,
  author    = {W{\"u}rfel, Uli and Neher, Dieter and Spies, Annika and Albrecht, Steve},
  title     = {Impact of charge transport on current-voltage characteristics and power-conversion efficiency of organic solar cells},
  series = {Nature Communications},
  volume    = {6},
  journal   = {Nature Communications},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2041-1723},
  doi       = {10.1038/ncomms7951},
  pages     = {9},
  year      = {2015},
  abstract  = {This work elucidates the impact of charge transport on the photovoltaic properties of organic solar cells. Here we show that the analysis of current-voltage curves of organic solar cells under illumination with the Shockley equation results in values for ideality factor, photo-current and parallel resistance, which lack physical meaning. Drift-diffusion simulations for a wide range of charge-carrier mobilities and illumination intensities reveal significant carrier accumulation caused by poor transport properties, which is not included in the Shockley equation. As a consequence, the separation of the quasi Fermi levels in the organic photoactive layer (internal voltage) differs substantially from the external voltage for almost all conditions. We present a new analytical model, which considers carrier transport explicitly. The model shows excellent agreement with full drift-diffusion simulations over a wide range of mobilities and illumination intensities, making it suitable for realistic efficiency predictions for organic solar cells.},
  language  = {en}
}
@article{KniepertLangeHeidbrinketal.2015,
  author    = {Kniepert, Juliane and Lange, Ilja and Heidbrink, Jan and Kurpiers, Jona and Brenner, Thomas J. K. and Koster, L. Jan Anton and Neher, Dieter},
  title     = {Effect of Solvent Additive on Generation, Recombination, and Extraction in PTB7:PCBM Solar Cells: A Conclusive Experimental and Numerical Simulation Study},
  series = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  volume    = {119},
  journal   = {The journal of physical chemistry : C, Nanomaterials and interfaces},
  number    = {15},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1932-7447},
  doi       = {10.1021/jp512721e},
  pages     = {8310 -- 8320},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}
@article{BartesaghiPerezKniepertetal.2015,
  author    = {Bartesaghi, Davide and Perez, Irene del Carmen and Kniepert, Juliane and Roland, Steffen and Turbiez, Mathieu and Neher, Dieter and Koster, L. Jan Anton},
  title     = {Competition between recombination and extraction of free charges determines the fill factor of organic solar cells},
  series = {Nature Communications},
  volume    = {6},
  journal   = {Nature Communications},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2041-1723},
  doi       = {10.1038/ncomms8083},
  pages     = {10},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}
@article{AntonSteyrleuthnerKossacketal.2015,
  author    = {Anton, Arthur Markus and Steyrleuthner, Robert and Kossack, Wilhelm and Neher, Dieter and Kremer, Friedrich},
  title     = {Infrared Transition Moment Orientational Analysis on the Structural Organization of the Distinct Molecular Subunits in Thin Layers of a High Mobility n-Type Copolymer},
  series = {Journal of the American Chemical Society},
  volume    = {137},
  journal   = {Journal of the American Chemical Society},
  number    = {18},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0002-7863},
  doi       = {10.1021/jacs.5b01755},
  pages     = {6034 -- 6043},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}
@article{XuShalomPiersimonietal.2015,
  author    = {Xu, Jingsan and Shalom, Menny and Piersimoni, Fortunato and Antonietti, Markus and Neher, Dieter and Brenner, Thomas J. K.},
  title     = {Color-Tunable Photoluminescence and NIR Electroluminescence in Carbon Nitride Thin Films and Light-Emitting Diodes},
  series = {Advanced optical materials},
  volume    = {3},
  journal   = {Advanced optical materials},
  number    = {7},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2195-1071},
  doi       = {10.1002/adom.201500019},
  pages     = {913 -- 917},
  year      = {2015},
  language  = {en}
}
@article{LuKochNeher2015,
  author    = {Lu, Guanghao and Koch, Norbert and Neher, Dieter},
  title     = {In-situ tuning threshold voltage of field-effect transistors based on blends of poly(3-hexylthiophene) with an insulator electret},
  series = {Applied physics letters},
  volume    = {107},
  journal   = {Applied physics letters},
  number    = {6},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.4928554},
  pages     = {5},
  year      = {2015},
  abstract  = {Blending the conjugated polymer poly(3-hexylthiophene) (P3HT) with the insulating electret polystyrene (PS), we show that the threshold voltage V-t of organic field-effect transistors (OFETs) can be easily and reversely tuned by applying a gate bias stress at 130 degrees C. It is proposed that this phenomenon is caused by thermally activated charge injection from P3HT into PS matrix, and that this charge is immobilized within the PS matrix after cooling down to room temperature. Therefore, room-temperature hysteresis-free FETs with desired V-t can be easily achieved. The approach is applied to reversely tune the OFET mode of operation from accumulation to depletion, and to build inverters. (C) 2015 AIP Publishing LLC.},
  language  = {en}
}