Side-chain engineering of bifunctional linkage unit-linked giant molecular acceptors for organic solar cells†

Side-chain engineering serves as a simple yet effective strategy to regulate the molecular packing and optimize the active-layer morphology in organic solar cells (OSCs). However, in the emerging class of giant molecular acceptors (GMAs) constructed using bifunctional linkage units, current optimization efforts have predominantly focused on modifying the linkers, with limited exploration of side-chain modulation. To address this gap, GMAs DY-C11 and DY-BO, featuring sterically differentiated side chains, were designed and synthesized in this study. DY-C11, with an optimized geometric configuration, exhibits enhanced molecular stacking and facilitates fibrillation when blended with the polymer donor PM6. Consequently, the DY-C11-based device exhibits a remarkable fill factor (FF) of 73.31%, a short-circuit current density (J_sc) of 22.05 mA cm⁻², and a power conversion efficiency (PCE) of 14.80%, significantly outperforming the DY-BO-based counterpart. This study elucidates the critical role of side-chain steric effects in governing the optoelectronic properties of bifunctionally linked GMAs, providing essential guidelines for designing high-performance OSC materials through rational side-chain engineering.