Exciton Binding Energy of Non-Fullerene Electron Acceptors

The past three years have witnessed the power conversion efficiency (PCE) of organic solar cells (OSCs) rocketing to over 18%, due to outstanding advantages of non-fullerene acceptors (NFAs). However, large exciton binding energy (E _b) caused by strong Coulombic force is still one of the main limiting factors for high-performance OSCs. Thus, it is critical to reduce the E _b for further enhancement of device performance. Many strategies have been developed to reduce the E _b of organic materials previously. In this perspective, the calculation methods for E _b and the relationship between E _b and voltage loss (V _loss) are discussed. Then, the effects of the properties of small-molecule acceptors on E _b from the perspectives of fused-ring donor cores, end groups, side chains, and molecular packing are discussed. Finally, the potential directions for reducing E _b and pointing out the trade-off between E _b and bandgaps/miscibility are put forward. It is hoped that this perspective could provide a new thinking of a molecular design for the breakthrough of OSCs.