Efficient small-molecule donor with improved structural order and molecular aggregation enabled by side-chain modification

Abstract

Side-chain modification is a proven effective approach for morphology manipulation in organic solar cells (OSCs). However, in-depth analysis and investigation involving side-chain modification towards morphology improvement, including molecular microstructure, orientating packing and aggregation are urgent for all-small-molecule (ASM) systems. Herein, employing a fluorine-modified two-dimension benzodithiophene (BDT) as central unit, we contrastively synthesized two small-molecule donors, namely BDT-F-SR and BDT-F-R, each welding alkylthio side-chains on thienyl of central BDT unit and the other grafted non-sulfuric alkyl side-chains. As predicted, the synergetic side-chain modification of fluorination and alkyl changeover triggers diverse molecular dipole moments and orientations, resulting in different molecular energy levels, thermal stabilities, molecular planarity and order. Eventually, together with the preeminent small-molecule acceptor Y6, BDT-F-R-based ASM OSCs obtain enhanced power conversion efficiency (PCE) of 13.88% compared to BDT-F-SR-based devices (PCE of 12.75%) with more suitable phase-separation and balanced carrier mobilities. The contrast results reveal that alkyl side-chains seem to be a more satisfactory partner for fluorine-modified 2D BDT-based small-molecule donors compared to alkylthio pendants, and highlight the significance of subtle side-chain modification for molecular structural order fun-tuning and morphology control, laying the foundation for efficient ASM OSCs.