Alkylthio side-chain engineering on the quinoxaline-based electron acceptors for efficient organic solar cells

Alkylthio side-chain engineering plays an important role in constructing high-performance small-molecule acceptors (SMAs) for organic solar cells (OSCs). In this work, two ethylthio-substituted quinoxaline core-based SMAs (BQS-F and BQDS-F) are developed to elucidate the impact of alkylthio substituents on their physicochemical and photoelectric properties. Comparative analysis reveals that the dialkylthio-substituted BQDS-F exhibited a more planar molecular backbone, red-shifted absorption spectrum, upshifted molecular orbital energy levels, and enhanced crystallinity compared to the monoalkylthio-substituted BQS-F. However, the poor thermodynamic compatibility of BQDS-F with the polymer donor D18 leads to excessively phase-separated heterojunction textures. By contrast, more homogeneous and uniform phase separation is realized in D18:BQS-F blend film, resulting in enhanced exciton dissociation efficiency, improved charge carrier mobility, and suppressed charge recombination. As a result, the BQS-F-based device affords a much higher efficiency of 15.52 % compared to the BQDS-F-based device (12.21 %). The results clarify the effect of ethylthio substituents on quinoxaline core-based SMAs, which are instructive to further advance the development of organic photovoltaics.