Hybrid perovskite are on a trajectory towards realizing the most efficient single-junction devices using low-temperature processing schemes. However, a critical issue is the limited understanding of how the structure and crystalline quality of the perovskite thin-films affects its opto-electronic properties and the stability. In my talk, I will focus on two aspect: crystal structure of hybrid perovskite thin film and the impact towards device performance and stability. 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 crystal structure transitioned from tetragonal phase (MA) to cubic phase (FA). We fabricate planar photovoltaic device using the alloy film for characterization and found a direct correlation between the degree of crystallinity and achieving high-efficiency, photo-stable and reliable photovoltaic devices. The FA-MA alloy compound based device reaches peak efficiency over 19% with long term photo-stability (>800 hours constant 1-Sun illumination) because of the replacement of MA molecules. This high efficiency is mainly from high short circuit current due to reduced optical band gap and high open circuit voltage (~1.08V). The alloy film yields a high photoluminescence quantum efficiency as well as high electro-luminescence quantum efficiency, which explains the high open circuit voltage value.