@article{VandewalBenduhnSchellhammeretal.2017,
  author    = {Vandewal, Koen and Benduhn, Johannes and Schellhammer, Karl Sebastian and Vangerven, Tim and R{\"u}ckert, Janna E. and Piersimoni, Fortunato and Scholz, Reinhard and Zeika, Olaf and Fan, Yeli and Barlow, Stephen and Neher, Dieter and Marder, Seth R. and Manca, Jean and Spoltore, Donato and Cuniberti, Gianaurelio and Ortmann, Frank},
  title     = {Absorption Tails of Donor},
  series = {Journal of the American Chemical Society},
  volume    = {139},
  journal   = {Journal of the American Chemical Society},
  number    = {4},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0002-7863},
  doi       = {10.1021/jacs.6b12857},
  pages     = {1699 -- 1704},
  year      = {2017},
  abstract  = {In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D A complexes occur at photon energies below the optical gaps of both the donors and the C-60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C-60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.},
  language  = {en}
}
@article{BenduhnTvingstedtPiersimonietal.2017,
  author    = {Benduhn, Johannes and Tvingstedt, Kristofer and Piersimoni, Fortunato and Ullbrich, Sascha and Fan, Yeli and Tropiano, Manuel and McGarry, Kathryn A. and Zeika, Olaf and Riede, Moritz K. and Douglas, Christopher J. and Barlow, Stephen and Marder, Seth R. and Neher, Dieter and Spoltore, Donato and Vandewal, Koen},
  title     = {Intrinsic non-radiative voltage losses in fullerene-based organic solar cells},
  series = {Nature Energy},
  volume    = {2},
  journal   = {Nature Energy},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2058-7546},
  doi       = {10.1038/nenergy.2017.53},
  pages     = {6},
  year      = {2017},
  abstract  = {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.},
  language  = {en}
}