Unlocking the Ambient Temperature Effect on FA-based Perovskites Crystallization by In-situ Optical Method.

Multiple cation-composited perovskites have been demonstrated as a promising approach to improving the performance and stability of perovskite solar cells (PSCs). [ 1 ] 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. [ 2 ] Here, using an in-situ optical method, we investigate the ambient temperature effect on the crystallization process in multiple cation-composited perovskites. 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 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, [ 3 ] we propose that the origin of the cracks in FA-based perovskites is due to the aggregation-induced solute depletion effect. The crystallization process deduced from the in-situ results is consistent with the consideration of formation energies of different compositions calculated by the density functional theory (DFT). 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. This article is protected by copyright. All rights reserved.