Relationship between cis-trans isomerism and optical and electrical properties based on benzidiimidazole-thiophene copolymer

The absorption gap and charge mobility of photovoltaic (PV) materials are usually important parameters to be optimized for enhancement of power conversion efficiency (PCE) of organic PV cells. Herein, a series of novel cis/trans-isomers of benzidiimidazole-thiophene copolymers (PBDIT) with different alkyl chain were conveniently synthesized. The relationship between the configurational isomerism and the optical and electrical properties of conjugated polymer was firstly investigated. Compared with their cis-isomers, only the trans-PBDITs can form quinoid structures, causing the narrower band gap and corresponding red-shifted and broader optical absorption. Theoretical calculation also illustrated that trans-PBDITs can form much smaller dihedral angles between imidazole and thiophene rings than their cis-counterparts, which facilitated the better coplanar structure and conjugated system, inducing the absorption red-shifted and higher charge mobility. The illumination of relationship between the configurational isomerism and the optical and electrical properties paves a way to design benzidiimidazole-thiophene copolymers with tunable absorption bands through regulation of side chains, which renders them promising candidates for various applications in organic PV devices.