Trifluoromethylation Enables a 3D Interpenetrated Low-Band-Gap Acceptor for Efficient Organic Solar Cells

Trifluoromethylation has been used for nonfullerene acceptors, resulting in an ultra-narrow band-gap molecule named BTIC-CF3-gamma. Compared to its fluorinated and chlorinated analogs, BTIC-CF3-gamma possesses the most red-shifted absorptions. The single-crystal structure of BTIC-CF3-gamma reveals that the cooperated pi-pi interactions from H aggregations of central fused cores and J aggregations of end groups lead to a three-dimensional (3D) interpenetrating network, which provides more electron-hopping junctions between different acceptor molecules. After blending with PBDB-TF, a champion power conversion efficiency (PCE) of 15.59% is realized, which is the highest value in reported ultra-narrow band-gap acceptors. Notably, a PCE of 16.50% is also achieved in BTIC-CF3-gamma-based ternary devices due to their red-shifted absorptions. Overall, our results provide a visualized understanding of molecule packings of this family of acceptors and also demonstrate that trifluoromethylation is a useful strategy to design highly efficient ultra-low band-gap acceptors for the next nonfullerene solar devices.