Revealing the effects of molecular packing on the performances of polymer solar cells based on A-D-C-D-A type non-fullerene acceptors

Although the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) based on non-fullerene acceptors have been increasing rapidly over the last couple of years, little is known about the correlations between molecular structures and blend morphologies. In this work, we design and synthesize three acceptor-donor-core-donor-acceptor (A-D-C-D-A) type non-fullerene acceptors, HF-PCIC, HFO-PCIC and OF-PCIC, which possess the same electron-donating parts (D) and electron-accepting terminals (A), but different benzene-based cores (C). We observe that such minor chemical variations can lead to distinct differences in their photovoltaic properties. The resulting PSCs based on HF-PCIC with a 2,5-difluorobenzene core yield a good PCE of 10.97%, which is higher than those of HFO-PCIC and OF-PCIC based PSCs (8.36% and 9.09%). If the processing solvent is changed from chlorobenzene to chloroform, a further improved PCE of 11.49% is obtained for HF-PCIC-based PSCs due to the formation of finer phase separation domains. This is the highest value among the PSCs with non-fullerene acceptors possessing unfused cores. Through a series of characterization techniques, we disclose that the diverse benzene-based cores influence the molecular geometries of the three non-fullerene acceptors, resulting in varied molecular packing modes and film morphologies. The results suggest that the tuning of non-fullerene acceptors' geometries is an effective method to optimize the film morphology and thus the photovoltaic properties. The unfused-core in A-D-C-D-A small molecules provides a good building block to construct high-performance non-fullerene acceptors.