Modulating Crystallinity and Miscibility via Side-chain Variation Enable High Performance All-Small-Molecule Organic Solar Cells

Side-chain engineering as one of the most important molecular design strategies has been widely used to improve photovoltaic efficiency of active layer materials. Herein, a series of acceptor-π-donor-π-acceptor typed small molecule (SM)-donors (SL1, SL2, SL3, and SL4), on the basis of high-performance SM-donor BTTzR (SL1) with thiazolo[5,4-d]thiazole as the π-bridging units and 3-butylrhodanine as the terminal electron-withdrawing groups, were designed and synthesized to study the effect of the side-chain substitutions of BDT-T on the photovoltaic performance. The investigation shows that the side-chain engineering has no obvious effect on the molecular absorption spectrum and energy levels but significantly influences on the molecular orientation and packing, and the compatibility with the acceptor Y6. Among these SM-donors neat films, SL1 and SL3 with two mixed branched and straight alkyl chains exhibit stronger crystallization in the face-on direction, and SL4 with two shorter alkyl chains achieves more compact packing. In the Y6-based blend films, three SM-donors (SL1, SL3, and SL4) with double alkyl chains on thienyl of BDT-T have much better compatibility compared to SL2 with single long alkyl chain, while their compatibility increases with the increase of alkyl chain length. The stronger crystallization, moderate molecular packing, and better blend compatibility of SL1 offer higher J_sc of 23.2 mA·cm^–2 and fill factor (FF) of 0.68 in the Y6-based all-small-molecule organic solar cells (all-SM-OSCs). Ultimately, the SL1:Y6-based devices achieved a promising power conversion efficiency of 13.9%, which is much higher than that of 11.5% from the SL2:Y6-based devices (J_sc = 21.5 mA·cm^–2 and FF = 0.60). This work indicates that modulating the side chain of SM-donors is a promising strategy to obtain efficient all-SM-OSCs.