C-Shape or S-Shape? The Molecular Geometry Control of Fused-Ring Nonfullerene Acceptors for Lower Energy Loss in Organic Solar Cells

To elucidate the pivotal influence of molecular geometry in fused-ring nonfullerene acceptors (FRNFAs) on material properties and device performance of organic solar cells (OSCs), we designed and synthesized two isomeric molecules C-F and S-F, featuring C-shaped and S-shaped geometries with the acceptor-donor-acceptor conjugated framework. The alteration in geometries demonstrated negligible effects on the optical and electrochemical properties. Significantly, single crystal X-ray crystallography analyses uncovered that C-F exhibited a wave network packing, while S-F favored a linear brick packing with the intermolecular packing of end groups, different from the previously reported three-dimensional (3D) stacking network of C-shaped Y series FRNFAs. Despite the absence of 3D network packing, OSCs utilizing C-F demonstrated a remarkable power conversion efficiency of 17.0%, with lower voltage loss compared to the devices based on S-F. This study further underscores the essential role of the C-shaped geometry in FRNFAs, providing valuable insights for future molecular design for high-performance OSCs.