@article{UllbrichBenduhnJiaetal.2019,
  author    = {Ullbrich, Sascha and Benduhn, Johannes and Jia, Xiangkun and Nikolis, Vasileios C. and Tvingstedt, Kristofer and Piersimoni, Fortunato and Roland, Steffen and Liu, Yuan and Wu, Jinhan and Fischer, Axel and Neher, Dieter and Reineke, Sebastian and Spoltore, Donato and Vandewal, Koen},
  title     = {Emissive and charge-generating donor-acceptor interfaces for organic optoelectronics with low voltage losses},
  series = {Nature materials},
  volume    = {18},
  journal   = {Nature materials},
  number    = {5},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {1476-1122},
  doi       = {10.1038/s41563-019-0324-5},
  pages     = {459 -- 464},
  year      = {2019},
  abstract  = {Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes(1,2). Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001\% range(3,4). In contrast, the electroluminescence external quantum yield reaches up to 16\% in D-A-based organic light-emitting diodes(5-7). Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons(8-11).},
  language  = {en}
}
@article{ShivhareErdmannHoermannetal.2018,
  author    = {Shivhare, Rishi and Erdmann, Tim and Hoermann, Ulrich and Collado-Fregoso, Elisa and Zeiske, Stefan and Benduhn, Johannes and Ullbrich, Sascha and Huebner, Rene and Hambsch, Mike and Kiriy, Anton and Voit, Brigitte and Neher, Dieter and Vandewal, Koen and Mannsfeld, Stefan C. B.},
  title     = {Alkyl Branching Position in Diketopyrrolopyrrole Polymers},
  series = {Chemistry of materials : a publication of the American Chemical Society},
  volume    = {30},
  journal   = {Chemistry of materials : a publication of the American Chemical Society},
  number    = {19},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0897-4756},
  doi       = {10.1021/acs.chemmater.8b02739},
  pages     = {6801 -- 6809},
  year      = {2018},
  abstract  = {Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency.},
  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}
}