@article{BenduhnPiersimoniLondietal.2018,
  author    = {Benduhn, Johannes and Piersimoni, Fortunato and Londi, Giacomo and Kirch, Anton and Widmer, Johannes and Koerner, Christian and Beljonne, David and Neher, Dieter and Spoltore, Donato and Vandewal, Koen},
  title     = {Impact of triplet excited states on the open-circuit voltage of organic solar cells},
  series = {dvanced energy materials},
  volume    = {8},
  journal   = {dvanced energy materials},
  number    = {21},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201800451},
  pages     = {7},
  year      = {2018},
  abstract  = {The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn- and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C-60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T-1) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin-orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T-1 and the CT state does not substantially affect nonradiative voltage losses.},
  language  = {en}
}
@article{NikolisBenduhnHolzmuelleretal.2017,
  author    = {Nikolis, Vasileios C. and Benduhn, Johannes and Holzmueller, Felix and Piersimoni, Fortunato and Lau, Matthias and Zeika, Olaf and Neher, Dieter and Koerner, Christian and Spoltore, Donato and Vandewal, Koen},
  title     = {Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies},
  series = {dvanced energy materials},
  volume    = {7},
  journal   = {dvanced energy materials},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201700855},
  pages     = {122 -- 136},
  year      = {2017},
  abstract  = {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.},
  language  = {en}
}