ABSTRACT Surface defects such as iodine vacancies initiate lattice degradation via I − migration and proton transfer, resulting in structural collapse. This degradation propagates into the bulk via coupled ion and vacancy diffusion, thereby accelerating irreversible performance decay. Conventional surface‐passivation methods are often limited by weak intermolecular interactions and suboptimal stability. In this study, we developed an in situ interfacial polymerization strategy that leverages the