Thermally Driven Reorganization of Polytypic PbI2 Precursor Films Improves MAPbI3 Formation and Crystallization

Abstract Despite rapid progress in solution-processed MAPbI3, the role of PbI2 polytypism in two-step conversion remains largely unexplored. Here, we show that thermally driven reorganization of polytypic chemical-bath-deposited PbI2 thin films influences the subsequent formation and crystallization of MAPbI3. We thermally treated PbI2 precursor films under vacuum at 175 °C for 0, 24, and 72 h before conversion in methylammonium iodide solution. X-ray diffraction and Rietveld refinement show that annealing does not eliminate PbI2 polytypism but redistributes the 2H/4H/6H/12R stacking landscape toward a more structurally reorganized precursor. This precursor reorganization is associated with improved PbI2-to-MAPbI3 conversion and crystallization. Compared with films derived from untreated precursors, MAPbI3 obtained from the 72 h-treated PbI2 shows less residual PbI2, a higher crystallized MAPbI3 fraction, larger coherent domains from 45 ± 8 to 82 ± 12 nm, and lower microdeformation from ∼0.35 to ∼0.18%. Raman, diffuse-reflectance, and X-ray photoelectron spectroscopy further reveal a sharper vibrational response, a sharper optical edge, and a more uniform Pb−I/I− surface environment after precursor annealing. Devices derived from annealed precursors also show faster and better-defined photoresponse dynamics, despite not reaching the highest absolute photocurrent. In addition, these films retain their structure more effectively after 66 weeks of aging under N2. These results indicate that PbI2 polytypism and thermally driven precursor reorganization are important factors in two-step MAPbI3 formation and suggest precursor reorganization as a useful strategy to improve the structural and functional quality of solution-processed perovskite thin films.