Local Excitation/Charge-Transfer Hybridization Simultaneously Promotes Charge Generation and Reduces Nonradiative Voltage Loss in Nonfullerene Organic Solar Cells

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Abstract
High power conversion efficiencies in state-of-the-art non-fullerene organic solar cells (NF OSCs) call for elucidation of the underlying working mechanisms of both high photocurrent densities and low non-radiative voltage losses under small energy offsets. Here, to address this fundamental issue, we have assessed the nature of interfacial charge-transfer (CT) states in a representative small-molecule NF OSC (DRTB-T:IT-4F) by time-dependent density functional theory calculations. The calculated results point to that the CT states can borrow considerable oscillator strengths from the energy-close local-excitation (LE) states or be fully hybridized with these LE states by molecular aggregation at the donor-acceptor interfaces. The LE/CT hybridization can promote charge generation by direct population of thermalized CT or LE/CT states under illumination. At the same time, the increased oscillator strengths of the lowest CT state will improve the luminescence quantum efficiencies and thus reduce non-radiative voltage losses. Our work suggests that it is crucial to tune the LE/CT hybridization by optimization of the donor and acceptor molecular and interfacial structures for further improving the NF OSC performance.
Funding Information
  • Ministry of Science and Technology of the People's Republic of China (2017YFA0204502)
  • National Natural Science Foundation of China (51773208, 91833305, 51803216)
  • Chinese Academy of Sciences (XDB12020200)

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