Simple non-fused small-molecule acceptors with bithiazole core: synthesis, crystallinity and photovoltaic properties

Abstract

It is well-known that most high-efficiency small-molecule acceptors (SMAs) contain a big fused-ring central core. However, their syntheses are multistep and complex. The development of high-efficiency simple non-fused SMA is of great significance for organic solar cells (OSCs). In this study, 4,4′-dimethyl-2,2′-bithiazole was primarily used as a weak electron-accepting (A_w) core to construct a type of non-fused SMAs of BTz-4F-1 and BTz-4Cl-1 with an A_w(π–A)₂ architecture, in which halogenated (fluorinated or chlorinated) cyanoindanone was used as a strong electron-accepting (A) terminal. For comparison, an analogue BTz-4F-0 with 2,2′-bithiazole was synthesized. The effect of methylation in bithiazole and halogenation in cyanoindanone on solubility, crystallinity, morphology and photovoltaic properties was primarily studied. It was found that both SMAs with dimethylbithiazole exhibited a significantly increasing solubility than BTz-4F-0. The fluorinated BTz-4F-1 exhibited an adjustable crystallinity and an increasing solu-bility in comparison with the chlorinated BTz-4Cl-1. Furthermore, BTz-4F-1 exhibited better morphological and photovoltaic properties than BTz-4Cl-1 in the bulk heterojuncation OSCs using polymer J71 as the donor. The obtained power conversion efficiency in the BTz-4F-1 based OSCs is 1.46 times higher than that of the BTz-4Cl-1 based ones. However, the BTz-4F-0 based OSCs were not obtained by a solution process owing to the lower solubility of BTz-4F-0. This study indicates that the crystallinity, morphology and photovoltaic properties of SMAs can be managed by central methylation and terminal fluorination strategies.