Benzothiadiazole‐Fused Cyanoindone: A Superior Building Block for Designing Ultra‐Narrow Bandgap Electron Acceptor with Long‐Range Ordered Stacking

There are great demands of developing ultra‐narrow bandgap electron acceptors for multifunctional electronic devices, particularly semi‐transparent organic photovoltaics (OPVs) for building‐integrated applications. However, current ultra‐narrow bandgap materials applied in OPVs, primarily based on electron‐rich cores, exhibit defects of high‐lying energy levels and inferior performance. We herein proposed a novel strategy by designing the benzothiazole‐fused cyanoindone (BTC) unit with ultra‐strong electron‐withdrawing ability as the terminal to synthesize the acceptor BTC‐2. The BTC unit imparts red‐shifted absorption up to 1000 nm to BTC‐2, attributed to enhanced intramolecular charge transfer and the quinoid resonance effect. Additionally, BTC‐2 features deep‐lying energy levels with the highest occupied molecular orbital level of –5.81 eV, due to the ultra‐strong electron‐withdrawing ability of BTC. Furthermore, BTC‐2 exhibits long‐range ordering in both molecular packing and macroscopic blend morphology, resulting from shoulder‐to‐shoulder packing of two BTC units, leading to an ultra‐long exciton lifetime over 1.1 ns. These superiorities facilitated a 17.17% efficiency in the binary OPV device with an extremely high photocurrent of 30.34 mA cm−2, representing the best performance for ultra‐narrow bandgap electron acceptors, and a record light utilization efficiency of 4.88% in binary semi‐transparent systems. Overall, BTC is a superior building block for designing ultra‐narrow bandgap electron acceptors.