Defect-Induced Bandgap Widening and Abnormal Photoluminescence in Formamidinium Tin Iodide Perovskite Microcrystals

Formamidinium tin iodide (FASnI₃) has emerged as a promising semiconductor material for various optoelectronic applications. However, the structure–photophysical property relationship remains ambiguous, because of the ready occurrence of structural defects in the fragile lattice. Here, using FASnI₃ microcrystals synthesized with tailored reaction conditions, we unveil that structural defects can induce bandgap widening and abnormal photoluminescence. Based on combined analysis of X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and optical spectroscopy, we propose that bandgap widening could stem from defect-mediated lattice distortion. Temperature-dependent photoluminescence measurements lead us to the discovery of a new near-infrared photoluminescence band between 185 and 10 K and negative thermal quenching in a broad range of 110–200 K. We believe that the knowledge gained here may not only offer a plausible roadmap to prepare high-quality organic–inorganic tin halide perovskite crystals but can also deepen our understanding on the relationship between defects and photophysical properties of tin-based perovskites.