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.