20.5% Efficiency Organic Photovoltaics Enabled by Strong Photo‐Responsive Aromatic‐Diimide Cathode Interlayer

ABSTRACT Organic photovoltaic (OPV) devices are operating under illumination, therefore, the material properties under light must be prioritized in designing high‐efficiency devices. Compared to the active layer, the impact of photo‐responsive behaviors of the cathode interlayer material (CIM) on device performance has received limited attention. In this study, a coronene‐diimide‐based CIM (CDI‐4N) is developed, exhibiting a significantly stronger intramolecular charge transfer (ICT) effect upon illumination compared to its benzo[ghi]perylene‐imide counterpart (BPI‐2N). The distinct excited‐state charge separation in CDI‐4N leads to stronger light‐induced self‐doping behavior and thus enhanced conductivity. Notably, 2.8 times increase in polarized electrostatic potential (ESP) surface area ratio is detected in CDI‐4N upon photoexcitation. Furthermore, CDI‐4N also displays stronger work function adjustability, more appropriate energy level alignment and improved crystallinity. These properties enable optimized interfacial electron extraction and suppressed recombination. Consequently, the CDI‐4N‐based OPV devices achieve a power conversion efficiency (PCE) of 20.48% with an impressive fill factor (FF) of 82.53% in the D18:L8‐BO binary system. This comprehensive investigation establishes photo‐response engineering as a paradigm for high‐efficiency CIMs.