An effective strategy to design high-performance wide bandgap polymer donors for organic solar cells by increasing the backbone curvature

Abstract

Although wide bandgap (WBG) polymer donors are widely used in non-fullerene organic solar cells (OSCs), only a few WBG polymers exhibited high power conversion efficiencies (PCEs). Developing WBG donors with ideal complementary absorption with narrow bandgap non-fullerene acceptors is critical to further improving the photovoltaic performance of OSCs. Here, we provide an effective strategy to widen the bandgap (Eg) of polymers by increasing the curvature of the polymer donor backbone structure and compare it with the fluorination strategy. Both experimental and theoretical results demonstrated that the fluorination strategy reduced the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PBDT-DT2FBT with little effect on the Eg. In contrast, the polymer PBDT-DTfBT, with a large backbone curvature, exhibited a reduced HOMO energy level and a significantly increased LUMO energy level, resulting in a significant increase in the optical Eg of 0.19 eV. The PBDT-DTfBT-based device achieved a high PCE of 15.89%, which far exceeded the PCEs of the PBDT-DTBT (9.84%) and PBDT-DT2FBT-based devices (8.02%). This work could guide the design of high-performance WBG donors.