Polytype selection in the antisolvent-free crystallization of formamidinium lead iodide using alkylammonium chlorides†

Understanding the nucleation, crystallization, and phase transition mechanisms in perovskite thin films produced without antisolvent quenching is crucial for improving phase stability and optoelectronic performance at scale. By means of in situ grazing incidence wide-angle X-ray scattering experiments, we show that the nature of alkylammonium chloride (RACl) additives significantly influences the crystallization dynamics and temperature-dependent stability of formamidinium lead iodide (FAPbI₃) perovskite films formed using antisolvent-free processes. The in situ experiments reveal that the effective radius of the RA⁺ cation and the vapor pressure of the RA⁰ conjugate base are critical factors affecting the crystallization pathway and film morphology. C₂–C₃ additives with moderate chain lengths may strike a balance between promoting the early crystallization of the cubic 3C phase while suppressing unwanted phases. This balance promotes the formation of highly crystalline cubic perovskite phase with well-oriented structure and better optoelectronic properties. These findings provide guidance regarding the design of additives to crystallize more stable and efficient perovskite films for optoelectronic applications.