Synergistic effect of incorporating intra- and inter-molecular charge transfer in nonfullerene acceptor molecules for highly-efficient organic solar cells

Abstract

Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have achieved rapid development benefiting from the near-zero donor/acceptor (D/A) energy offset. However, a specific mechanism underlying the highly-efficient charge generation remains an open question. Up to now, most of the studies only focused on the intra-molecular charge transfer (Intra-CT) character and inter-molecular aggregation structure of nonfullerene acceptor molecules, but ignored their inter-molecular charge transfer (Inter-CT) character. In this study, by emphasizing the intra-molecular A–D–A electronic structure and inter-molecular aggregation mode of typical NFA molecules, we propose a universal quantum model incorporating both Intra- and Inter-CT characters. We confirm that the Inter-CT character is intrinsically present in NFA molecular aggregates. Importantly, owing to the Intra- and Inter-CT synergistic effect, the excited state binding energy in NFA molecular aggregates can be significantly reduced (lowered to ∼0.05 eV). These results clearly demonstrate that the excited state in NFA molecular aggregates is a CT state with both Intra- and Inter-CT characters, by which the charge generation dependence on the D/A energy offset can be eliminated through rational design of NFA molecules.