Fabricating electron transport layers at low temperatures is challenging but highly desired in the field of flexible perovskite solar cells (f-PSCs). In this study, highly uniform cerium oxide (CeO_x) films prepared by the UV–O₃ treatment have been successfully applied as the electron transport layer (ETL) in methylammonium lead halide (CH₃NH₃PbI₃) perovskite-based f-PSCs. Under AM 1.5 G sunlight with 100 mW cm⁻², these cells exhibited an open-circuit voltage (V_oc) of 0.98 V, a short-circuit current density (J_sc) of 19.42 mA cm⁻², a fill factor (FF) of 0.72 and power conversion efficiency (PCE) of 14.63%. The PCE was much higher than that of the control planar CeO_x ETL (PCE ∼ 9.08%) prepared at a low temperature (80 °C) without the UV–O₃ treatment, and this was ascribed to the improved CeO_x film, enhanced light absorption and suppressed charge recombination. The cells that bend at 15 mm of radius showed excellent stability with less than 10% reduction in PCE after 500 cycles of repeated bending at ambient temperature. The charge-transmission kinetic parameters and long-term stability of the CeO_x-based f-PSCs were analyzed as well.