Metal halide perovskites have attracted attention for their exceptional optoelectronic properties, such as defect tolerance, long charge-carrier lifetimes, and extended diffusion lengths. Despite their promise for next-generation optoelectronic devices, reproducible fabrication of high-quality thin films remains challenging due to the sensitive crystallization process, especially during thermal annealing.

In situ photoluminescence (PL) has emerged as a powerful technique for real-time monitoring of perovskite film formation, providing dynamic insight into crystallization processes. Rather than relying on extensive trial-and-error parameter optimization, this approach enables a targeted investigation of the fundamental mechanisms governing annealing-induced phase evolution and grain growth. It also provides insight into defect formation, thereby informing optimal processing strategies and defect passivation. A critical challenge in this field is the phase instability of formamidinium lead iodide (FAPbI₃). While the black α-FAPbI₃ phase exhibits excellent optoelectronic properties, it is metastable at room temperature and tends to transform into the thermodynamically favoured yellow δ-FAPbI₃ phase, leading to severe performance degradation. Although high-temperature annealing is commonly employed to promote the formation of the α phase, a comprehensive understanding of the crystallization pathways is still lacking.

In this work, we systematically investigate the influence of annealing temperature on the crystallization behaviour of FAPbI₃ thin films. Using in situ PL, complemented by X-ray diffraction and photoluminescence quantum yield measurements, we identify a threshold annealing temperature that delineates two distinct material states. Films annealed at lower temperatures exhibit poor optoelectronic quality and limited stability, and certain films switch to yellow δ-FAPbI₃, detected by ex situ XRD measurements, whereas higher-temperature annealing leads to the formation of a qualitatively different material with markedly improved structural and optical properties. Even after ambient exposure, such annealed samples offer almost pristine α-FAPbI₃ (above 96%). Real-time PL measurements reveal additional features during annealing and serve as reliable indicators of the final film quality. One of the most interesting features is an additional dip in photoluminescence and a simultaneous maximum in the obtained Urbach energy; this occurs only above 140 °C. Films that don’t show such real-time PL development undergo quick degradation and have very poor optoelectronic quality. The origin of this behaviour and its implications for the reproducible fabrication of high-quality α-FAPbI₃ thin films are discussed.

These results reveal a distinct signature photoluminescence evolution, demonstrating the formation of a stable FAPbI₃ thin film. In situ photoluminescence reveals how annealing conditions govern perovskite crystallization and final film quality, providing guidance for reproducible fabrication of high-performance materials.