Among thin-film solar cells, perovskite solar cells are receiving close review by the researchers due to rapid increase in efficiency during the last decade. Mixed organic–inorganic halide perovskites (CH₃NH₃PbI₃ or MAPbI₃) are easy in fabrication as well as cost-effective but these solar cells are facing challenges of stability under normal environmental conditions. Halide perovskite solar cells (PSCs) are polycrystalline in nature but still lacking the effective optical and electrical properties, and one of the big reasons behind it is the less grain size. In this study, formamidinium lead iodide (FAI) treatment was applied along with deposition of MAPbI₃. The grain size was increased which further decreased defects of film surface and grain boundary between the grains. The treatment also helped improving the crystallinity of films, as well as opto-electronic characteristics of PSCs. The FAI treated PSCs showed more resilience stability than untreated PSCs in terms of aged power conversion efficiency (PCE). The complete investigation was done by comparing the scanning electron microscope (SEM) images, atomic force microscopy (AFM) images, X-ray diffraction peaks, UV-Vis absorbance and mean grain size of the FAI treated and non-FAI treated films.