Off-Stoichiometric Methylammonium Iodide Passivated Large-Grain Perovskite Film in Ambient Air for Efficient Inverted Solar Cells

Hot-casting is a promising technique of depositing high-quality organic–inorganic hybrid perovskite thin films with large crystal grain size. Here, we reported the that the crystallinity and grain size of perovskite films could be systematically tailored by modulating the stoichiometry of the precursor solution in the hot-casting process under ambient condition with a relative humidity of 40%. It was found that a slight excess of methylammonium iodide (MAI) in the precursor solution could effectively compensate the MAI loss due to the high substrate temperature. A significant increase in the grain size and crystallinity of the perovskite film was observed together with a decrease in defect density and a carrier concentration enhancement in MAI-rich samples. The corresponding devices exhibited a notable increase in fill factor (up to 80.70%) and short-circuit current density. In addition, in MAI-deficient samples, an enrichment of PbI₂ at the grain boundaries was directly observed by optical microscopy and laser confocal microscopy. Time-resolved photoluminescence spectroscopy revealed an increase in the charge carrier lifetime in the MAI-deficient samples, which was in line with the previous results with a small amount of excess PbI₂ in the perovskite film. This work highlights a new strategy to prepare high-quality perovskite thin films with excellent crystal quality under ambient condition.