Non-fullerene electron acceptors constructed by simple electron-withdrawing core: Distinct effect of bithiazole vs thiazolothiazole core on photovoltaic properties

Developing simple-structure and high-efficiency non-fullerene small molecule acceptors (NF-SMAs) are crucial for application of organic solar cells (OSCs). To deeply understand the effect of electron-withdrawing central building block on photo-electronic properties and better design simple-structure NF-SMAs, in this work, a novel simple SMA of BTz1 with an A (pi-A'-A '')(2) framework was synthesized and characterized, in which a unfused-ring weak acceptor (A) of bithiazole and two adjacent strong acceptors of A'-A '' are used as molecular central and terminal units, respectively. And the UV absorption, energy level, crystallinity, mobility, morphology and photovoltaic per-formance of BTz1 were studied systematically. It is found that BTz1 exhibits similar molecular conformations and photophysical properties, but a low HOMO and high LUMO energy level in comparison with its analogue of TTz1 based on thiazolothiazole center. Furthermore, significant different photovoltaic properties are observed in the BTz1 and ITz1 based PSCs using polymer J71 as donor in an inverted device structure. The BTz1-based OSCs exhibit an ascending open-circuit voltage (V-oc) of 0.96 V, but TTz1-based devices display a significantly increasing power conversion efficiency (PCE) of 8.77%. The better film morphology of photoactive layer makes the TTz1-based OSCs exhibit higher PCE than the BTz1-based OSCs. This work indicates that tuning electron-withdrawing central structure is crucial to construct simple-structure and high-efficiency NF-SMAs.