Effects of Halogenation of Small-Molecule and Polymeric Acceptors for Efficient Organic Solar Cells

Tuning the properties of non-fullerene acceptors (NFAs) through halogenation, including fluorination and chlorination, represents one of the most promising strategies to boost the performance of organic solar cells (OSCs). However, it remains unclear how the F and Cl choice influences the molecular packing and performance between small-molecule and polymeric acceptors. Here, a series of small-molecule and polymeric acceptors with different amounts and types of halogenation is synthesized, and the effects of fluorination and chlorination between small-molecule and polymeric acceptors are investigated. It is found that chlorinated small-molecule acceptors lead to longer exciton diffusion length and better performance compared to the corresponding fluorinated ones, which attributes to their stronger intermolecular packing mode. For polymer acceptors, in contrast, the fluorinated polymers achieve a denser packing mode and better performance, because chlorinated polymers exhibit reduced intrachain conjugation between end group moieties and linker units. This study demonstrates different halogenation effects on the packing modes and performances for small-molecule and polymeric acceptors, which provides important guidance for the molecule design of high-performance acceptors for OSCs.