Multi-channel exciton dissociation in D18/Y6 complexes for high-efficiency organic photovoltaics

Abstract

Interfacial charge transfer between the donor and acceptor plays a crucial role in determining the photo-induced charge generation mechanisms and efficiencies for organic solar cells. Here, we have theoretically investigated the exciton-dissociation and charge-recombination processes in complexes consisting of a wide-bandgap polymer donor D18 and a narrow-bandgap small-molecule acceptor Y6, which exhibit the best organic photovoltaic performance to date. The results show that besides the lowest charge-transfer (CT₀) state, there are also four higher-lying CT states below the lowest singlet excited state (S₁) of D18, and the excitons on D18 will dissociate into the higher-lying CT states much faster (10¹⁰–10¹² s⁻¹) than into the CT₀ state (10⁷–10⁸ s⁻¹). In contrast, only the CT₀ state is below the S₁ state of Y6 due to the small driving force for hole transfer from Y6 to D18, while the dissociation rates of Y6 excitons into the CT₀ state can be very high (10¹³ s⁻¹). Importantly, the rates of charge recombination are mostly lower than 10⁶ s⁻¹. These results are fully consistent with the highly efficient exciton dissociation and low charge recombination observed by experiments. Our work underlines the importance of multi-channel exciton dissociation for high-efficiency organic photovoltaics.