Simple and Environmentally Friendly Solution Synthesis of CsPbI₃ Nanoribbons: Investigating the Reversible δ‐ to γ‐Phase Transition

The all‐inorganic lead halide perovskites, including cesium lead iodide (CsPbI 3 ), have attracted significant attention due to their possible applications in photovoltaics, light‐emitting devices, and other optoelectronic technologies, driven by their intrinsic optoelectronic properties. This study introduces a simple, green, and scalable solution synthesis method for the CsPbI 3 perovskite fabrication, utilizing β‐diketonate [Cs(hfa)] n and [Pb(hfa) 2 diglyme] 2 precursors and ethanol as solvent. The as‐synthesized nanoribbons are initially fabricated in the pure yellow δ‐phase, as confirmed by X‐ray diffraction and energy‐dispersive X‐ray analysis. Thermal analysis through differential scanning calorimetry highlights the reversible phase transition from δ‐CsPbI 3 to the black cubic α‐phase. The photoactive and metastable black γ‐CsPbI 3 is obtained via high‐temperature annealing and rapid cooling under inert conditions. Optical characterizations have been performed in order to extrapolate a bandgap of 2.89 eV for the δ‐phase and 1.62 eV for the γ‐phase, while the photoluminescence analysis displays intense emissions at 530 and 700 nm for the δ‐ and γ‐phase, respectively. Ambient photoemission spectroscopy further elucidates the energy levels of the γ‐phase, determining a highest occupied molecular orbital energy of 5.68 eV and a work function of 4.32 eV. These findings demonstrate the possibility to synthesize pure CsPbI 3 and obtain the γ‐CsPbI 3 phase with promising applications.