Effects of non-fused and fused substituents in quinoxaline-based central units on conformation, aggregation, and photovoltaic properties of non-fused ring electron acceptors†

Abstract

Non-fused ring electron acceptors (NFREAs) have garnered significant attention in organic photovoltaics (OPVs) due to their promising potential for practical applications. In this study, we prepared two acceptor–donor–acceptor′–donor–acceptor (A–D–A′–D–A) type NFREAs, CRIC and BRIC, featuring a quinoxaline (Qx) derivative with two alkoxy chains at the 6,7-positions as the central A′ unit. Phenanthrene is fused to the Qx ring in BRIC, whereas the 4a–4b bond of the fused phenanthrene is cleaved in CRIC. Theoretical calculations and detailed analyses of ¹H NMR spectroscopy suggest that both CRIC and BRIC most likely adopt an S-shaped conformation, in which one D–A′ linkage forms an intramolecular S⋯O noncovalent bond, while the other forms an S⋯N bond. Notably, BRIC formed aggregates at high concentrations, whereas CRIC did not exhibit such aggregation. When blended with the conjugated polymer donor PBDB-T, CRIC- and BRIC-based OPV devices achieved high power conversion efficiencies (PCEs) of 11.6% and 8.25%, respectively. The relatively lower PCE of the BRIC-based device was attributed to larger domain sizes in the PBDB-T:BRIC blend film, which hindered efficient exciton diffusion to the donor–acceptor interface. These findings offer valuable insights into the molecular design of high-performance Qx-based NFREAs for OPV applications.