Effects of Electron-Donating and Electron-Accepting Substitution on Photovoltaic Performance in Benzothiadiazole-Based A–D–A′–D–A-Type Small-Molecule Acceptor Solar Cells

A–D–A′–D–A-type nonfused-ring acceptors (NFRAs) have recently received extensive attention because of their suitable tuning of absorption spectra, frontier energy levels, and promising sunlight harvesting capability. However, no attention has yet been paid to the effects of A′ core substitutions on optoelectronic, morphological, and photovoltaic properties in high-performance organic solar cells (OSCs). In this work, to deeply understand the effects of electron-donating and electron accepting substitutions on the A′ core, we designed and synthesized three A–D–A′–D–A-type NFRAs, BTCPDT, BTCPDTO4, and BTCPDTF, with different substitutions on the benzo[c][1,2,5]thiadiazole core, such as hydrogen, alkoxy, and fluorine groups, respectively. Based on the characterization results, BTCPDT, BTCPDTO4, and BTCPDTF showed considerable variations in optical, electrochemical, and morphological properties because of their different electronegativity, steric hindrance, and intramolecular charge transfer effects. Among the NFRAs in this study, BTCPDTO4 showed the highest absorption coefficient, a high-lying frontier energy level, well-mixed and interpenetrating network morphology with enhanced charge transfer/transport, and suppressed charge recombination, which result in an impressive efficiency of 11.85% in an NFRA-OSC.