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.