Perovskite is one of the most popular materials for solar cell application because of its high absorption coefficient, defect tolerance and tunable bandgap. The power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have exceeded 25% to date. To put this technology into the market, not only efficiency but the stability of the PSCs should catch up with Si-based solar cells. Perovskite has been reported to be sensitive to environmental stresses, such as moisture, oxygen, heat, light and their combination. In this talk, we will discuss the role of defects in oxygen-induced photostability in perovskite, and employing amino acid additives to passivate the defects. The Low energy ion scattering (LEIS) was used to detect the termination of perovskite lattice. We found out that the amino acids are terminated at the outermost layer of perovskite film. We also realize that oxygen is difficult to incorporate into the lattice in the presence of amino acid additives. As a result, perovskite films with additives shows 20 times longer lifetime than their reference counterpart under oxygen and light exposure.