Today's perovskite solar cells's power conversion efficiency(PCE) has been increased up to 22.1%. Most of perovskite solar cells use TiO2 for electron transport layer (ETL). But in this studies we used ZnO for ETL. ZnO has larger band gap than TiO2, so ZnO films can transmit more sunlight than TiO2 films. Also ZnO films can be obtained at lower temperature (~350°C) than TiO (~550°C). Despite of these benefit of ZnO films the reason why we did not use ZnO is because of unstability of perovskite films deposited on ZnO films. The ZnO films' surface has lots of hydroxyl groups. These hydroxly groups disassemble the perovskite into Methylammoniumiodide (MAI) and Leadiodide (PbI ) because of this problem we don't use ZnO for ETL in perovskite solar cells. But we can solve these problems by pre-annealing. Pre-annealing can reduce the hydroxyl groups on the perovskite films, and it helps to stabilize the perovskite films deposited on ZnO films. Moreover, pre-annealing can help to increase the size of the perovskite grains about 1 , and it contributes to increase mobility of electron. Increasing of mobility carries over to rise the PCE of perovskite solar cells. As a result, pre-annealing on 240°C devices exhibited overall PCE up to 3.49% with improvements of a V by 60.31%, a Fill factor (FF) by 45% ,a J by 165.01% and a PCE by 512.28%, as compared to that of pristine perovskite. The stability of perovskite also increased. Compared to pristine perovskite, pre-annealing perovskite shows 150% better stability at the atmosphere. In the presentation, details of morphology investigations, optoelectronic properties of ZnO, and its devices will be discussed.