Metal halide perovskites have excellent features, such as very high power conversion efficiency, solution processability and light weight. SnO2 is used as the electron transport layer (ETL) in perovskite solar cells (PSCs). In this study, we optimized spin-coating conditions of the SnO2 ETL in terms of relative humidity (RH). We found that using lower RH during SnO2 spin-coating made surfaces of the SnO2 ETL smoother and, therefore, led to greatly improved PSC performance. Additionally, we demonstrated that using two-layer SnO2, in which a second rough SnO2 layer is staked on a first smooth SnO2 layer, could improve PSC performance because of suppressed shunting path formation and efficient electron extraction.