A comprehensive review on heteroatom engineering in fused-ring electron acceptors: from molecular design to device efficiency.

Organic solar cells (OSCs) using bulk-heterojunction (BHJ) blends of polymer donors and non-fullerene acceptors (NFAs) have garnered considerable attention in recent years. NFAs, especially fused-ring electron acceptors (FREAs) adopting acceptor-donor-acceptor (A-D-A) and A-DA'D-A structures, have contributed most to high-efficiency OSCs, pushing the power conversion efficiency (PCE) to over 20%. The rapid advancement of novel FREAs is primarily driven by their tunable molecular architectures, broad absorption characteristics, and the ability to finely adjust energy levels and molecular packing through chemical modification. Typically, FREAs consist of coplanar fused aromatic ring systems serving as donor (D) cores, flanked by two highly electron-deficient acceptor (A) end groups. In this review, we summarize recent advances in heteroatom engineering on central fused-ring cores in FREAs for high-efficiency OSCs. The influence of heteroatom incorporation on light absorption, energy levels, electron mobility, and overall photovoltaic performance is systematically discussed. Furthermore, some new guidelines are proposed for future molecular design in the aspect of light absorption, fill factor, driving force, and voltage loss. Finally, we present an outlook on the remaining challenges and highlight promising directions toward the commercialization of OSCs.