Unlocking the Ambient Temperature Effect on FA‐Based Perovskites Crystallization by In Situ Optical Method

Abstract Multiple cation‐composited perovskites are demonstrated as a promising approach to improving the performance and stability of perovskite solar cells (PSCs). However, recipes developed for fabricating high‐performance perovskites in laboratories are always not transferable in large‐scale production, as perovskite crystallization is highly sensitive to processing conditions. Here, using an in situ optical method, the ambient temperature effect on the crystallization process in multiple cation‐composited perovskites is investigated. It is found that the typical solvent‐coordinated intermediate phase in methylammonium lead iodide (MAPbI 3 ) is absent in formamidinium lead iodide (FAPbI 3 ), and nucleation is almost completed in FAPbI 3 right after spin‐coating. Interestingly, it is found that there is noticeable nuclei aggregation in Formamidinium (FA)‐based perovskites even during the spin‐coating process, which is usually only observed during the annealing in MAPbI 3 . Such aggregation is further promoted at a higher ambient temperature or in higher FA content. Instead of the general belief of stress release‐induced crack formation, it is proposed that the origin of the cracks in FA‐based perovskites is due to the aggregation‐induced solute depletion effect. This work reveals the limiting factors for achieving high‐quality FA‐based perovskite films and helps to unlock the existing narrow processing window for future large‐scale production.