Bay‐Site Hydrogen‐Bond Engineering of Perylene Diimide Cathode Interlayers for Efficient and Durable Organic Solar Cells

ABSTRACT Cathode interlayers (CILs) are critical to the efficiency and stability of organic solar cells (OSCs), yet conventional small‐molecule CILs often suffer from excessive aggregation, weak interfacial adhesion, and radical‐induced degradation. Here, we demonstrate a bay‐site hydrogen‐bond engineering strategy to address these limitations. Two CILs, PDIN‐B and PDIN‐BOH, were developed by introducing phenyl and phenolic hydroxyl‐functionalized substituents at the perylene diimide (PDI) bay positions. Bay substitution suppresses aggregation and improves energy‐level alignment, while the hydroxyl groups in PDIN‐BOH form a robust hydrogen‐bonding network that locks interfacial morphology, strengthens adhesion with the acceptor L8‐BO, and suppresses radical generation to mitigate photothermal degradation, a behavior distinctly different from conventional side‐chain functionalization, which often promotes radical‐induced damage. Consequently, PDIN‐BOH‐based devices achieve a power conversion efficiency of 20.37% and retain 80.33% of initial performance after 1523 h of continuous one‐sun illumination at 65°C under the International Summit on Organic Photovoltaic Stability (ISOS‐L‐2) protocol, representing a notable demonstration of exceptional durability under coupled photothermal aging. This work establishes bay‐site hydrogen‐bond engineering as a rational design principle for durable organic photovoltaics.