19.5%‐Efficiency Organic Solar Cells with High Thickness Tolerance and Exceptional Device Stability Enabled by TEMPO‐Functionalized Perylene Diimide as a Cathode Interface Layer

Abstract The cathode interface material is pivotal for achieving excellent photovoltaic performance and device stability in organic solar cells (OSCs). However, currently, few interface materials can simultaneously satisfy multiple criteria such as low cost, high efficiency, insensitivity to film thickness, and good stability. To address this challenge, a novel perylene diimide (PDI) derivative, P‐C3T is designed and synthesized, by incorporating stable free radical unit TEMPO and polar quaternary ammonium salt as side chains. P‐C3T is facile to synthesize and cost‐effective, while also exhibiting good conductivity and excellent ohmic contact characteristics in OSCs. When utilized as the cathode interface material, the device based on D18:L8‐BO achieves a remarkable power conversion efficiency (PCE) of 19.52%, which is one of the highest levels reported for binary single‐junction OSCs. Notably, devices based on P‐C3T exhibit remarkable insensitivity to film thickness, with the PCE remaining at 95% of the initial efficiency even at a thickness of 40 nm. Furthermore, P‐C3T ‐based OSCs demonstrate exceptional storage stability and improved light stability. In summary, with its high PCE, excellent thickness insensitivity, and good stability, P‐C3T is anticipated to be a promising candidate material for large‐area roll‐to‐roll processing of OSCs.