Persistent PbI2-Passivated Microdomains in As-Prepared MAPbI3 Perovskite Thin Film Revealed by Spatially Resolved Photoluminescence and Raman Maps

Methylammonium lead iodide (MAPbI3) perovskite has been widely studied for its optoelectronic properties, but its polycrystalline thin films inevitably contain grain boundaries and defects that degrade performance and stability. PbI2 is often considered a destabilizing agent in perovskites, yet it has also been reported to act as a passivation agent, making its role a subject of debate. Here, we performed integrated optical, photoluminescence (PL), and ultra-low-frequency Raman mapping on a 100 μm2 region of MAPbI3 thin films to study the roles of PbI2. The analyses resolved α-phase MAPbI3-rich, PbI2-rich, and mixed-phase domains, revealing heterogeneous PbI2 distribution with PL quenching at defect sites. In addition, after light-induced degradation, PL changes were governed by both the local microstructure and PbI2 distribution. Notably, PbI2-rich regions retained PL, evidencing a protective passivation effect. These findings demonstrate that the beneficial role of PbI2 is key to designing more stable perovskite devices.