@article{AlbrechtJanietzSchindleretal.2012,
  author    = {Albrecht, Steve and Janietz, Silvia and Schindler, Wolfram and Frisch, Johannes and Kurpiers, Jona and Kniepert, Juliane and Inal, Sahika and Pingel, Patrick and Fostiropoulos, Konstantinos and Koch, Norbert and Neher, Dieter},
  title     = {Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells},
  series = {Journal of the American Chemical Society},
  volume    = {134},
  journal   = {Journal of the American Chemical Society},
  number    = {36},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0002-7863},
  doi       = {10.1021/ja305039j},
  pages     = {14932 -- 14944},
  year      = {2012},
  abstract  = {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.},
  language  = {en}
}
@article{SalzmannHeimelDuhmetal.2012,
  author    = {Salzmann, Ingo and Heimel, Georg and Duhm, Steffen and Oehzelt, Martin and Pingel, Patrick and George, Benjamin M. and Schnegg, Alexander and Lips, Klaus and Blum, Ralf-Peter and Vollmer, Antje and Koch, Norbert},
  title     = {Intermolecular hybridization governs molecular electrical doping},
  series = {Physical review letters},
  volume    = {108},
  journal   = {Physical review letters},
  number    = {3},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {0031-9007},
  doi       = {10.1103/PhysRevLett.108.035502},
  pages     = {5},
  year      = {2012},
  abstract  = {Current models for molecular electrical doping of organic semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these inconsistencies for prototypical systems, we present experimental and theoretical evidence for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.},
  language  = {en}
}