Instability of organic-inorganic halide perovskite solar cells (PSCs) under continuous light illumination in ambient air atmosphere has been studied. Efforts have been made to improve the PSC stability by chemical engineering of perovskite or device encapsulation. However, most of the attention has been directed to the perovskite material itself rather than its dynamic interaction with selective contacts. In this study we report on the impact of selective contacts on long-term stability. When the unencapsulated PSC with mesoscopic structure, bl-TiO2/mp-TiO2/CH3NH3PbI3 (MAPbI(3))/spiro-MeOTAD, was illuminated in ambient air atmosphere, photocurrent rapidly declined despite little change in absorbance. Impedance spectroscopic measurements confirmed that a new semicircle appeared at high frequency with exposure time, which was indicative of formation of a new layer at the MAPbI(3)/spiro-MeOTAD interface inhibiting charge collection. The device prepared in ambient atmosphere was encapsulated in N-2 using a UV-curable sealant, which demonstrated no deterioration in absorbance but continual decrease in photocurrent, photovoltage, and fill factor with exposure time up to 300 h. This indicates that the photoinduced barrier at the MAPbI(3)/spiro-MeOTAD interface could not be effectively overcome by encapsulation. Replacement of the HTM layer from spiro-MeOTAD to NiO was found to be an effective way to not create the barrier layer, which eventually improved long-term stability. Our results clearly suggest that the selective contacts play a critical role in long-term stability of PS Cs.