Effects of Mixed Halogenation on the Performance of Nonfullerene Acceptors in All-Small Molecule Organic Solar Cells

Mixed halogen substitution on the end groups of organic photovoltaic materials has been proven to be an effective way to generate materials with enhanced performance. A systematic comparison of the mixed halogen atom types can provide comprehensive information on this strategy; however, this is usually limited by the lack of a facile synthesis route. Herein, a new class of end groups featuring mixed halogenation, easily obtained via two steps with excellent yields, is reported. The new end groups are applied in synthesizing acceptor materials, and their performance is compared in organic solar cell devices. Combined with the small molecule donor BTR-Cl, the best-performing acceptor BTP-FCl-FCl achieves a power conversion efficiency of 15.27% and a high fill factor (FF) of 75.94%. Systematic studies of the mixed halogenation analogues in devices reveal that the higher FF is consistent with the longer carrier lifetime, faster charge extraction, weaker bimolecular recombination, and is associated with the favorable BTR-Cl:BTP-FC1-FC1 domain size and molecular packing, all of which are derived from the different halogen atoms. The results demonstrate that selecting suitable halogen atoms is vital in achieving high-performance organic solar cells.