Vapor-Phase Anion Exchange in CH3NH3PbBr3 Perovskite Films: Continuous Bandgap Tuning and HI-Mediated Corrosion of ITO Substrates

CH3NH3PbBr3 crystalline films were prepared on ITO substrates using the spin-coating method, followed by a vapor-phase anion exchange process in a tube furnace using CH3NH3I to gradually replace the Br anions with I anions. By controlling the reaction time, the structural evolution and changes in optical properties were systematically investigated. X-ray diffraction patterns show that the I anions gradually replace the Br anions in the perovskite lattice as the reaction time increases, leading to lattice expansion and a shift in the diffraction peaks toward lower angles. Scanning electron microscopy reveals that the average grain size increases and the grain boundary reconstructs during the exchange process. Photoluminescence and UV–Vis absorption spectra show that the photoluminescence peak exhibits a continuous redshift, the absorption edge gradually shifts to longer wavelengths, and the optical bandgap decreases steadily toward the value of CH3NH3PbI3. A sharp increase in the resistivity of the ITO substrate was also observed. Control experiments confirm that this change is not due to thermal annealing but to the vapor-phase reaction between CH3NH3I and ITO. In the tube furnace, CH3NH3I is thermally decomposed into HI. HI not only promotes halide substitution but also diffuses to the ITO interface and etches In2O3 into insulating InI3, destroying the original conductive network. Therefore, this process is attributed to a HI-mediated multiphase reaction rather than a simple solid–vapor exchange. Overall, vapor-phase anion exchange provides an effective way to continuously tune the band structure, absorption range, and emission peak of hybrid perovskites, offering a controllable route for multicomponent perovskites and multiband optoelectronic devices. This work also emphasizes the potential chemical corrosion of bottom electrodes during the vapor-phase anion exchange process and suggests that protective measures such as barrier layers or corrosion-resistant electrodes should be considered.