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

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 pi-pi 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.