Engineered Biographical Defects in Two-Dimensional Nonanediaminium Lead Iodide Perovskites: From Synthesis to Reversible Photodegradation

Abstract The choice of lead precursor (PbO versus PbI2), the Pb:diamine stoichiometric ratio, and the crystallization regime influence the defect landscape and optical stability of the layered hybrid perovskite (NonDA)PbI4. Diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, and low-temperature photoluminescence reveal that structural defects, specifically iodine vacancies and organic-cation vacancies, are introduced during the synthesis, with their concentrations determined by the thermal history and the lead source. Aging results in the emergence of three sub-bandgap absorption bands attributed to surface V-centers, interstitial iodine, and bulk V-centers, along with anisotropic lattice contraction caused by the loss of the organic spacer. The 542 nm photoluminescence band is assigned to excitons localized on empty iodine vacancies, VI+, and the 531/542 intensity ratio is established as a quantitative indicator of the vacancy filling state. This ratio can be reversibly modulated by thermal annealing and UV irradiation, providing direct evidence for the redox cycle “F-center + I2 ↔ VI+ + 2I–”. An excess of PbI2 in the synthesis of (NonDA)PbI4 results in the irreversible decomposition of the inorganic framework, in contrast to the reversible vacancy filling observed in stoichiometrically synthesized samples. This approach enables identification of the dominant defect type and prediction of long-term stability at the synthesis stage.