Fused Benzothiadiazole: A Building Block for n‐Type Organic Acceptor to Achieve High‐Performance Organic Solar Cells

Narrow bandgap n‐type organic semiconductors (n‐OS) have attracted great attention in recent years as acceptors in organic solar cells (OSCs), due to their easily tuned absorption and electronic energy levels in comparison with fullerene acceptors. Herein, a new n‐OS acceptor, Y5, with an electron‐deficient‐core‐based fused structure is designed and synthesized, which exhibits a strong absorption in the 600–900 nm region with an extinction coefficient of 1.24 × 105 cm−1, and an electron mobility of 2.11 × 10−4 cm2 V−1 s−1. By blending Y5 with three types of common medium‐bandgap polymers (J61, PBDB‐T, and TTFQx‐T1) as donors, all devices exhibit high short‐circuit current densities over 20 mA cm−2. As a result, the power conversion efficiency of the Y5‐based OSCs with J61, TTFQx‐T1, and PBDB‐T reaches 11.0%, 13.1%, and 14.1%, respectively. This indicates that Y5 is a universal and highly efficient n‐OS acceptor for applications in organic solar cells. Employing an electron‐deficient‐core‐based fused structure instead of a fused donor unit represents a new strategy to adjust the optoelectronic properties of acceptor–donor–acceptor‐type n‐type organic semiconductors. The electron‐deficient‐core based on benzothiadiazole together with dicyanomethylene derivative realizes a low bandgap, high electron mobility, and suitable energy level simultaneously, affording universal and high performances when blending with different donor polymers.