Selenium‐Substitution Asymmetric Acceptor Enables Efficient Binary Organic Solar Cells over 18.3% via Regulating Molecular Stacking and Phase Separation

Abstract Substantial efforts of A–DA′D–A type non‐fullerene acceptors (NFAs) molecular design have impelled power conversion efficiency (PCE) of single junction organic solar cells (OSCs) to exceed 19%. Asymmetric geometry strategy, selenium‐substitution, and end‐group engineering are proven to be effective modification methods. Here, two novel selenium substitution asymmetric NFAs, AsymSSe‐2F, and AsymSSe‐2Cl, are synthesized to investigate the synergistic modification effects on device performance compared with symmetric Y6. When blending AsymSSe‐2F with the wide‐bandgap and high crystallinity polymer D18, a remarkable PCE of 18.31% is yielded, and an excellent fill factor of 79.46% is achieved, which is attributed to the broadened absorption, enhanced π – π stacking, balanced carrier mobilities, and fine phase‐separation morphology. Notably, among the reported selenium‐substituted asymmetric NFAs based OSCs, especially combined with the seldom‐reported D18, this PCE is top‐ranked in binary bulk heterojunction organic solar cells. This work indicates that the combined modification of asymmetric geometry and selenium substitution in NFAs is a promising strategy for fabricating high performance OSCs.