SnO₂ has attracted significant attention as an electron transport layer (ETL) because of its wide optical bandgap, electron mobility, and transparency. However, the annealing temperature of 180 °C–200 °C, as reported by several studies, for the fabrication of SnO₂ ETL limits its application for flexible devices. Herein, we demonstrated that the low-temperature deposition of SnO₂ ETL and further surface modification with oxygen plasma enhances its efficiency from 2.3% to 15.30%. Oxygen plasma treatment improves the wettability of the low-temperature processed SnO₂ ETL that results in a larger perovskite grain size. Hence, oxygen plasma treatment effectively improves the efficiency of perovskite solar cells at a low temperature and is compatible with flexible applications.