Organic–inorganic halide perovskites are promising for use in solar cells because of their efficient solar power conversion. Current–voltage hysteresis and degradation under illumination are still issues that need to be solved for their future commercialization. However, why hysteresis and degradation occur in typical perovskite solar cell structures, with an electron transport layer of metal oxide such as SnO₂, has not been well understood. Here we show that one reason for the hysteresis and degradation is because of the localization of positive ions caused by hydroxyl groups existing at the SnO₂ surface. We deactivate these hydroxyl groups by treating the SnO₂ surface with a self-assembled monolayer. With this surface treatment method, we demonstrate hysteresis-less and highly stable perovskite solar cells, with no degradation after 1000 h of continuous illumination.