Crystallinity and Compatibility Regulated by Donor End‐Capped Fluorination Enabled Efficiency Over 16% in Additive‐Free All‐Small‐Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs) with completely definite chemical structure are an ideal model to establish the relationship between molecular structure and device performance via aggregates. The end-capped acceptor unit is of great significance in the regulation of aggregates by essential molecular interactions. However, the successful end-capped acceptor units for small-molecule donors have been rather poorly studied and only focused on the alkyl substituted rhodamine, limiting further development for ASM-OSCs. In this study, three novel end-capped acceptors are designed by fluorinating the benzyl-substituted rhodamine, and results show that both the fluorinated position and quantity make significant effect on the dynamic film formation, aggregation and photoelectric properties in both the pure and blend films. Pairing with BTP-eC9, the MPhS-m-1F with single meta-fluorinated end groups performs a satisfactory PCE of 16.10% with the most efficient charge carrier management, which is one of the highest efficiencies in additive-free ASM-OSCs. These results open a window for the diverse of efficient end-capped units and provide design guidance for their further improvements.