The influence of intramolecular noncovalent interactions in unsymmetrical squaraines on material properties, film morphology and photovoltaic performance

Abstract Three unsymmetrical squaraines (USQs) with different number of hydroxyl groups ( BIISQ bearing two hydroxyl substituents, BIISQ-MOH bearing mono-one, and BIISQ-NOH bearing none) have been synthesized to understand of the insightful molecular structure-property relationship of photovoltaic materials. Our research shows that the introduction of strong intramolecular hydrogen bonding (HB) interactions between donor (D) and acceptor (A) units of squaraines can affect the whole π-conjugated properties, such as deepening the highest occupied molecular orbital (HOMO) energy level and thus enhancing the open-circuit voltage ( V oc ), increasing the dipole moment and improving hole mobility, weakening the electron-withdrawing ability of centric four-membered rings (as A unit in squaraines) and then improving the compatibility between USQ and [6,6]-phenyl-C 71 butyric acid methyl ester (PC 71 BM). Eventually, the bulk-heterojunction organic photovoltaic device (BHJ-OPV) based on BIISQ has obtained the best performance, which is approximately three times higher than that of BIISQ-NOH -based one. Interestingly, our research also finds that thin layer chromatography (TLC) analysis might be a simple, cheap, and rapid way to preliminarily forecast the compatibility between donor materials and PC 71 BM when the molecular skeletons of donor materials are relatively similar.