@article{SchubertSteyrleuthnerBangeetal.2009,
  author    = {Schubert, Marcel and Steyrleuthner, Robert and Bange, Sebastian and Sellinger, Alan and Neher, Dieter},
  title     = {Charge transport and recombination in bulk heterojunction solar cells containing a dicyanoimidazole-based molecular acceptor},
  issn      = {1862-6300},
  doi       = {10.1002/pssa.200925312},
  year      = {2009},
  abstract  = {Carrier transport and recombination have been studied in single component layers and blends of the soluble PPV- derivative poly[2,5-dimethoxy-1,4-phenylenevinylene-2-methoxy-5-(2-ethyl-hexyloxy)- 1,4-phenylenevinylene] (M3EH-PPV) and the small molecule acceptor 4,7-bis(2-(1-hexyl-4,5-dicyanoimidazole-2-yl)vinyl) benzo[c][1,2,5]-thiadiazole (HV-BT). Measurements on single carrier devices show significantly smaller electron mobility in the blend compared to the pure HV- BT layer, which is suggestive of the formation of isolated clusters of the acceptor in a continuous polymer matrix. The significant change in fill factor (FF) with increasing illumination intensity is consistently explained by a model taking into account bimolecular recombination and space charge effects. The decay of the carrier density after photoexcitation has been studied by performing photo-CELIV measurements on pure and blend layers. It is found that the decay at long delay times follows a power-law dependence, which is, however, not consistent with a Langevin-type bimolecular recombination of free charges. A good description of the data is obtained by assuming trimolecular recombination to govern the charge carrier dynamics in these systems.},
  language  = {en}
}
@article{SteyrleuthnerBangeNeher2009,
  author    = {Steyrleuthner, Robert and Bange, Sebastian and Neher, Dieter},
  title     = {Reliable electron-only devices and electron transport in n-type polymers},
  issn      = {0021-8979},
  doi       = {10.1063/1.3086307},
  year      = {2009},
  abstract  = {Current-voltage analysis of single-carrier transport is a popular method for the determination of charge carrier mobilities in organic semiconductors. Although in widespread use for the analysis of hole transport, only a few reports can be found where the method was applied to electron transport. Here, we summarize the experimental difficulties related to the metal electrode leakage currents and nonlinear differential resistance (NDR) effects and explain their origin. We present a modified preparation technique for the metal electrodes and show that it significantly increases the reliability of such measurements. It allows to produce test devices with low leakage currents and without NDR even for thin organic layers. Metal oxides were often discussed as a possible cause of NDR. Our measurements on forcibly oxidized metal electrodes demonstrate that oxide layers are not exclusively responsible for NDR effects. We present electron transport data for two electron-conducting polymers often applied in all-polymer solar cells for a large variety of layer thicknesses and temperatures. The results can be explained by established exponential trapping models.},
  language  = {en}
}
@article{SchubertYinCastellanietal.2009,
  author    = {Schubert, Marcel and Yin, Chunhong and Castellani, Mauro and Bange, Sebastian and Tam, Teck Lip and Sellinger, Alan and Hoerhold, Hans-Heinrich and Kietzke, Thomas and Neher, Dieter},
  title     = {Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells},
  issn      = {0021-9606},
  doi       = {10.1063/1.3077007},
  year      = {2009},
  abstract  = {The authors present organic photovoltaic (OPV) devices comprising a small molecule electron acceptor based on 2- vinyl-4,5-dicyanoimidazole (Vinazene (TM)) and a soluble poly(p-phenylenevinylene) derivative as the electron donor. A strong dependence of the fill factor (FF) and the external quantum efficiency [incident photons converted to electrons (IPCE)] on the heterojunction topology is observed. As-prepared blends provided relatively low FF and IPCE values of 26\% and 4.5\%, respectively, which are attributed to significant recombination of geminate pairs and free carriers in a highly intermixed blend morphology. Going to an all-solution processed bilayer device, the FF and IPCE dramatically increased to 43\% and 27\%, respectively. The FF increases further to 57\% in devices comprising thermally deposited Vinazene layers where there is virtually no interpenetration at the donor/acceptor interface. This very high FF is comparable to values reported for OPV using fullerenes as the electron acceptor. Furthermore, the rather low electron affinity of Vinazene compound near 3.5 eV enabled a technologically important open circuit voltage (V-oc) of 1.0 V.},
  language  = {en}
}
@article{PingelZenNeheretal.2009,
  author    = {Pingel, Patrick and Zen, Achmad and Neher, Dieter and Lieberwirth, Ingo and Wegner, Gerhard and Allard, Sybille and Scherf, Ullrich},
  title     = {Unexpectedly high field-effect mobility of a soluble, low molecular weight oligoquaterthiophene fraction with low polydispersity},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-008-4994-0},
  year      = {2009},
  abstract  = {Layers made from soluble low molecular weight polythiophene PQT-12 with low polydispersity exhibit a highly ordered structure and charge-carrier mobilities of the order of 10(-3) cm(2)/(V s), which we attribute to its proximity to monodispersity. We propose that polydispersity is a decisive factor with regard to structure formation and transport properties of soluble low molecular weight polythiophenes.},
  language  = {en}
}
@article{JoshiPingelGrigorianetal.2009,
  author    = {Joshi, Siddharth and Pingel, Patrick and Grigorian, Souren and Panzner, Tobias and Pietsch, Ullrich and Neher, Dieter and Forster, Michael and Scherf, Ullrich},
  title     = {Bimodal temperature behavior of structure and mobility in high molecular weight p3ht thin films},
  issn      = {0024-9297},
  doi       = {10.1021/Ma900021w},
  year      = {2009},
  abstract  = {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.},
  language  = {en}
}
@article{InalCastellaniSellingeretal.2009,
  author    = {Inal, Sahika and Castellani, Mauro and Sellinger, Alan and Neher, Dieter},
  title     = {Relationship of photophysical properties and the device performance of novel hybrid small-molecular/polymeric solar cells},
  issn      = {1022-1336},
  doi       = {10.1002/marc.200900221},
  year      = {2009},
  abstract  = {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\%.},
  language  = {en}
}