Suppressing Interlayer Ion Migration With a Tailored Passivator for Highly Air‐Stable Perovskite Solar Cells

ABSTRACT The limited stability of highly efficient perovskite solar cells (PSCs) in the n‐i‐p structure significantly hinders their commercialization, primarily due to severe ion diffusion between the perovskite layer and the hole‐transport layer (HTL). To address the issue, we designed and synthesized two molecularly engineered monophosphonate‐substituted LBCz and bisphosphonate‐substituted 2OBCz as interfacial passivation layers, which were inserted between the perovskite and HTL. Phosphonates that contain the P═O group strongly coordinate with lead ions while forming hydrogen bonds with formamidine (FA + ), thereby inhibiting ion migration. Compared to LBCz, the 2OBCz‐controlled perovskite film showed reduced surface roughness, enhanced crystallinity, and stronger p ‐type character, which improved electron blocking and hole extraction. Meanwhile, 2OBCz exhibited a superior hydrophobicity that can effectively block moisture. As a result, the 2OBCz‐controlled device achieved a champion power conversion efficiency (PCE) of 26.02%, compared to 24.00% for the control device. After 1,000 h storage in ambient air at 65°C with a relative humidity (RH) of 40%–50% (ISOS‐D‐2 protocol), the unencapsulated 2OBCz‐controlled device retained 90.7% of its initial PCE, showing significantly improved air‐stability when compared with the control device.