Photogeneration in Organic Solar Cells (Photogen)

• Applicants: Professor Dr. Carsten Deibel; Professor Dr. Karl Leo, since 1/2018
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 362992821

• Final Report Year: 2021

Final Report Abstract

Within our joint research project, we gained deep insights on the nature of intermolecular charge-transfer states at the donor-acceptor interface which are typically employed in organic solar cells but are also used in organic photodetectors and so-called exciplex organic-light-emitting diodes. Studying intermolecular charge-transfer states is of key importance since they are responsible for charge carrier separation and recombination in all the mentioned optoelectronic devices and are therefore directly ruling their device performance. Here, our research focused on two main aspects: understanding and quantifying the chargetransfer state binding energy being relevant for the charge carrier generation efficiency and unveiling the nature of the charge-transfer state lineshape broadening. The main findings, we gained within this research project, are summarised below: • Quantum chemical calculations revealed that the structural environment at the donor-acceptor interface plays a significant role in the photogeneration process. Quadrupole moments as well as charge delocalisation along the backbone are reducing the Coulomb interaction and thereby improve the photogeneration yield. • Employing high-gap donor-acceptor phases, we were able to show for the very first time that it is possible to realise an efficiently emitting and charge generating organic optoelectronic device at the same time. Reducing the binding energy of the charge-transfer state leads to favourable charge carrier generation but also to extremely low turn-on voltages for the electroluminescence which is important for energy efficient organic-light-emitting diodes. • By comprehensive studies on the linewidth employing temperature-dependent very sensitive photoluminescence, electroluminescence and external quantum efficiency measurements along with quantum chemical simulations and an in-depth modelling of the charge-transfer state density of states, we found that static energetic disorder does not play a dominant role in the formation of charge-transfer states. • We proposed design rules for future organic semiconductors to reduce the subgap absorption in organic solar cells and thereby reducing voltage losses in these system which will lead to higher power conversion efficiencies. • The energy of the charge-transfer state (ECT ) is necessary to quantify the photogeneration driving force ∆Eet . Compared to models involving static energetic disorder to explain spectral chargetransfer state properties, the purely dynamic approach predicts a by more than 100 meV lower ECT in line with temperature-dependent open-circuit voltage measurements.

Publications

• “Emissive and charge-generating donor–acceptor interfaces for organic optoelectronics with low voltage losses”, Nature Materials 18, 459–464 (2019)
  S. Ullbrich et al.
  
• “Molecular vibrations reduce the maximum achievable photovoltage in organic solar cells”, Nature Communications 11, 1488 (2020)
  M. Panhans et al.
  
• “Temperature dependence of the spectral line-width of charge-transfer state emission in organic solar cells; static vs. dynamic disorder”, Materials Horizons 7, 1888–1900 (2020)
  K. Tvingstedt et al.
  
• “Band gap engineering in blended organic semiconductor films based on dielectric interactions”, Nature Materials (2021)
  K. Ortstein et al.
  
• “Charge photogeneration in non-fullerene organic solar cells: influence of excess energy and electrostatic interactions”, Advanced Functional Materials 31, 2007479 (2021)
  M. Saladina et al.
  
• “Enhancing sub-bandgap external quantum efficiency by photomultiplication for narrowband organic near-infrared photodetectors”, Nature Communications 12, 4259 (2021)
  J. Kublitski et al.
  
• “Reducing non-radiative voltage losses by methylation of push-pull molecular donors in organic solar cells”, ChemSusChem 14 (2021)
  L. Baisinger et al.
  
• “Reverse dark current in organic photodetectors and the major role of traps as source of noise”, Nature Communications 12, 551 (2021)
  J. Kublitski et al.
  
• “Stacked Dual-Wavelength Near-Infrared Organic Photodetectors”, Advanced Optical Materials 9, 2001784 (2021)
  Y. Wang et al.
  
• “Temperature-dependent charge-transfer-state absorption and emission reveal the dominant role of dynamic disorder in organic solar cells”, Physical Review Applied 15, 064009 (2021)
  C. Göhler et al.