Fluorination of a polymer donor through the trifluoromethyl group for high-performance polymer solar cells

The fluorination of polymer donors through substituting hydrogen atoms by fluorine atoms on aromatic rings have been widely used in highly efficient polymer solar cells (PSCs). However, this fluorination process suffers from complicated synthesis and low yields. Herein, through replacing the methyl group with a trifluoromethyl group, the feasibility of synthetically-simple fluorination towards high-performance polymer donors is demonstrated. Two structurally similar polymer donors, one having a trifluoromethyl (-CF3) group and the other with a methyl (-CH3) pendant group for comparison, were designed and synthesized. By comparing these two donors, we found that the -CF(3)group addition lowered the HOMO energy level and increased absorption through improved intermolecular interactions. Single-junction solar cells based on the trifluoromethylated donor yield a maximum power conversion efficiency of 13.5%, representing a nearly two-fold increase compared with that of devices using the methylated counterpart. These findings reveal great potential of fluorinating polymer donors by the trifluoromethyl group to improve solar cell performance.