We first studied in detail of the hybrid perovskite thin film crystal structure by hot-casting method. We found methyl ammonium lead triiodide (MAPbI3) perovskite thin-films grown on Lithium doped nickel oxide surface by hot-casting method leads to a high quality crystalline thin film. The crystallinity is greatly enhanced as revealed by sharp peaks in X-ray diffraction spectrums, which exhibits a characteristic peak splitting observed only in single-crystals and are indicative of a stabilized tetragonal phase. We further study the composition by alloying formamidinium with methyl ammonium in hybrid perovskite structure doped by cesium (FAxMAyCs5PbI3). The obtained thin film by hot-casting method is majorly dominated by the alpha-phase of FAPbI3, with more symmetry structure in trigonal structure than MAPbI3. We fabricate the photovoltaic device with the mixed organic cation thin film and study the device functionality and. We found that there is a direct correlation between the degree of crystallinity and achieving high-efficiency, photostable and reliable photovoltaic devices. Device with methyl ammonium lead triiodide perovskite thin-films grown on Lithium doped nickel oxide leads to high average efficiency of 17%. The devices show reduced trap assisted recombination at the interface and enhanced photo-stability under constant illumination. The FA-MA alloy compound based device reaches peak efficiency over 18% with long term photo-stability because of the symmetry crystal structure.