Influence of precursor solution temperature on the crystalline nature of mixed halide perovskite thin films grown by one-step deposition method

The influence of the precursor solution temperature (40 °C, 50 °C, 70 °C and 90 °C) on the crystalline nature of the mixed halide methyl ammonium lead iodide perovskite (MAPI) thin films grown using one-step deposition method is investigated. Precursor solution used is a combination of Lead chloride, Lead iodide and Methyl ammonium iodide at a composition of 1:1:4 in Dimethylformamide. Better crystallinity (> 70%) and largest crystallite size (~ 76 nm) are observed in the thin films prepared with solution temperature at 40 °C and 70 °C, respectively. Two different mechanisms that are responsible for better crystallinity and growth of crystallite in the perovskite thin films under study are identified. The observed variation in crystallinity is attributed to the amount of formation of solvent-induced intermediate phase in thin films, as evidenced from the presence of XRD peak at 7.4°. The formation of intermediate phase controls the otherwise fast and uncontrolled crystallization of perovskites to produce better crystalline nature. Variation in the crystallite size is attributed to the nature of the growth of intermediate compound, Methyl ammonium lead chloride, which determines the possibility of template-assisted growth of MAPI. This study emphasizes the critical role of the formation of intermediate phase along with the template-assisted growth in the fabrication of highly crystalline perovskite thin films with large crystallite/grain size that are indispensable for the fabrication of efficient devices.