Enhanced Near‐Infrared Amplified Spontaneous Emission and Stability Improvement of Air‐Processed Pure Black‐Phase Formamidinium Lead Iodide Perovskite Films

Abstract Formamidine lead halide perovskite material is regarded as a competitive candidate for light amplification and lasers due to its ideal bandgap range and excellent optical properties. The formamidinium perovskites undergo a cubic‐to‐hexagonal transition in humid conditions, leading to the formation of yellow δ‐phase FAPbI 3 , which degrades laser device performance and stability. To address this, we propose using methylamine formate (MAFa) as an ionic liquid solvent for stable black‐phase α‐FAPbI 3 perovskite films. Compared to traditional N, N‐dimethylformamide (DMF) solvent (FAPI‐DMF), FAPI‐MF films show improved ASE performance with lower threshold and higher gain under nanosecond laser excitation. Temperature‐dependent photoluminescence (PL) and transient absorption (TA) spectra reveal higher exciton binding energy (72.9 meV) and longer optical gain lifetime for FAPI‐MF films, attributed to the vertical arrangement of lead iodide in nano‐sized ion channels that promote α‐FAPbI 3 formation. The FAPI‐MF films also exhibit excellent photostability, thermal stability, and resistance to humidity, ensuring continuous laser and long‐term stability. This work extends the perovskite family’s spectral range from visible to near‐infrared light, offering a color‐tunable emitter with broad applications, including chemical analysis, biomedical research, and food safety testing.