Each interface is crucial to perovskite solar cells (PSCs), so the interface modification is often used to improve the PV performances. In this manuscript, an environment-friendly cesium iodide (CsI) interface layer between the mesoporous TiO2 (mp-TiO2) and the MAPbI(3) perovskite active layer was introduced to improve the PV performances and stabilities of PSCs and significantly reduce hysteresis of the device. After optimizing, the PSC employing a CsI-modified mp-TiO2 as electron transporting layer (ETL) could obtain a high fill factor (FF) of 0.79 with an increased power conversion efficiency (PCE) of 17.10% and a low hysteresis index (HI) of 0.10 under illumination of AM 1.5G, while FF of 0.75 and PCE of 14.38% for PSC without CsI interface modification. The enhanced PV performances can be attributed to the improved surface morphology of mp-TiO2, the lower work function (WF) of TiO2, the passivation of surface trap states, the improved perovskite crystallization and reduced grain boundaries. Besides, the reduced WF of TiO2 results in the enhanced built-in potential, which promotes the charge separation, transport and collection. (C) 2017 Elsevier Ltd. All rights reserved.