Effect of Fluorinated End-Groups on the Exciton Dynamics and Charge Transfer of Non-fused Ring Acceptors

In recent years, the optimization of non-fused ring electron acceptors (NFREAs) has become an important topic, which includes pi extension, side-chain engineering, and end-group halogenation. While the performance characterization of different NFREAs, including their morphology and power conversion efficiencies, has been extensively studied, the influence of different optimization methods on their photophysical processes is not clear. Here, we analyze the effect of end-group fluoridation on exciton dynamics and charge transfer (CT) in NFREA-based organic photovoltaics in detail using a novel NFREA Isopropyl-0F and its end-group fluorinated homologue Isopropyl-2F by means of spectroscopy, in order to understand the physical mechanism for the differences in their performance. The transient absorption spectra show that the more ordered molecular arrangement in the Isopropyl-2F-based heterojunction resulted in faster exciton diffusion and higher hole transfer efficiency, which were conducive to CT and reduced recombination loss. These results well explain the improved properties after end-group fluoridation.