Exploring the formation of formamidinium-based hybrid perovskites by antisolvent methods: in situ GIWAXS measurements during spin coating

Antisolvent methods - solvent engineering and Lewis base adduct - are the most used to prepare high efficient perovskite solar cells (PSCs). These two methods differ only by the ratio between perovskite (PVSK) and dimethyl sulfoxide (DMSO). In this study, the difference among these two methods and the effect of the relative humidity (rH) on the crystallization process were evaluated by in situ Grazing-Incidence Wide Angle X-ray Scattering (in situ GIWAXS) using synchrotron radiation. The technique was applied to the first stages of formation of formamidinium-based perovskites films, under real preparation conditions of spin coating, at different humidity conditions and time window to inject the antisolvent. The higher amount of DMSO in the solvent engineering method prolongs the duration of the colloidal gel, which extends the time window for the antisolvent injection and, as consequence, facilitates obtaining films with homogeneous morphology. Our results confirm that the formation of cesium/formamidinium-based perovskite takes place through the conversion of 2H-4H hexagonal polytypes directly to black perovskite without thermal annealing, independent on the rH or method employed. In contrast, cesium-free, methylammonium/formamidinium-based perovskite follows the 2H-4H-6H polytype sequence. Interestingly, at higher amounts of DMSO and rH (40%), pure iodide intermediate (MA2Pb3I8.2DMSO) is formed in the cesium-free perovskite, which is undesirable in mixed halide perovskite. Our findings shed new light on the complexity of PVSK film formation, by identifying all crystalline phases in the process, and give clues to manage composition and enviroment during film processing, thus having an impact on the efficiency optimization and solar cell manufacturing.