Low band-gap modulation of isoindigo-based copolymers toward high open-circuit voltage of polymer solar cells

Designing low band-gap-conjugated polymers coupled with low HOMO levels attracts great attention in the field of polymer solar cells (PSCs). By using donor–acceptor (D-A) copolymerization strategy, we designed and synthesized a series of low band-gap copolymers with deep HOMO levels via introducing an isoindigo (IID) acceptor unit in the copolymers with the donor unit of fluorene (F) (PIID-F), carbazole (Cz) (PIID-Cz), thiophene (Th) (PIID-Th), dithiophene (DTh) (PIID-DTh), or dithienosilole (DTS) (PIID-DTS). The HOMO level of the copolymers, measured by electrochemical cyclic voltammetry, varies from −5.3 eV to −5.8 eV, depending on different donor units in the copolymers. However, the LUMO levels of all the copolymers are fixed at about −3.6 eV, which is mainly determined by IID acceptor unit due to its strong electron-withdrawing ability. The new results will provide an effect help in designing IID based molecular structures. Among the copolymers, PIID-DTS has a low band gap of 1.58 eV and possesses a low-lying HOMO energy level of −5.33 eV. The PSCs based on PIID-DTS as donor and PC70BM as acceptor exhibited a high open-circuit voltage (Voc) of 0.93 V and a primary power conversion efficiency of 2.45%.