Highly Planar Dimeric M‐Series Acceptors via π‐Bridge Engineering for Efficient and Stable Organic Solar Cells ^†

Comprehensive Summary Dimerization of small‐molecule acceptors (SMAs) is an effective strategy to suppress molecular diffusion and enhance the stability of organic solar cells (OSCs), yet many dimerized SMAs (DSMAs) suffer from twisted backbones due to nonplanar SMA units and rotatable σ‐bonds, limiting molecular packing and charge transport. Here, two dimerized M‐series SMAs, DMS and DMSe, were designed by combining fluorinated central indanone units with thiophene or selenophene π‐bridges to modulate backbone planarity and intermolecular packing. The selenophene‐linked DMSe exhibits stronger intramolecular noncovalent interactions, resulting in enhanced backbone planarity, tighter π‐π stacking, improved charge transport, and favorable phase‐separated morphology. When blended with PM6, DMSe‐based OSCs achieve a power conversion efficiency of 18.01%, surpassing 17.12% for DMS‐based counterparts, while also demonstrating superior thermal and photostability. The improved performance is attributed to higher exciton dissociation, more balanced charge carrier mobilities, and increased face‐on molecular orientation. These results highlight the critical role of synergistic π‐bridge and central end‐group engineering in modulating dimer geometry, optimizing blend morphology, and enhancing device performance and stability.