To modify the stoichiometry and enhance electronic transportation of electron transport layers (Ells) potentially for the large-scale high performance planar perovskite solar cells (PSCs) on the flexible substrate, ZnO films as ETLs with controlled stoichiometry are fabricated by magnetron sputtering method under a mixture working gases of Ar and O-2 at room temperature. The impact of Ar/O-2 ratios on the structural, electrical, and optical properties of ZnO films is systematically investigated. The X-ray photoelectron spectroscopy results indicate that the sputtered ZnO films with controlled stoichiometry are successfully achieved. For the proper deposition condition with Ar/O-2 ratio of 1:4, ZnO film exhibits large-size grains, low defect states density, and good optical crystalline quality. Furthermore, the sputtered ZnO films deposited under various Ar/O-2 ratios are introduced into the PSCs as ETLs. The photovoltaic performance of the PSCs is strongly dependent on the properties of the sputtered ZnO Ells. Compared with the ZnO ETLs prepared under various Ar/O-2 ratios, the PSCs based on the ZnO ETL deposited under the Ar/O-2 ratio of 1:4 demonstrate the improved short-circuit current and fill factor, contributing to a maximum power conversion efficiency of 16.60%. This study highlights that the sputtered ZnO film is one of promising Ells for high-efficiency PSCs under low-temperature processing.