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

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 (S-1) of D18, and the excitons on D18 will dissociate into the higher-lying CT states much faster (10(10)-10(12)s(-1)) than into the CT(0)state (10(7)-10(8)s(-1)). In contrast, only the CT(0)state is below the S(1)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 CT(0)state can be very high (10(13)s(-1)). Importantly, the rates of charge recombination are mostly lower than 10(6)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.