The insufficient stability of CH3NH3PbI3 (MAPbI3)-based perovskite solar cells (PSCs) remains a significant concern in this field of research, so a device engineering approach is required to obtain efficient, stable PSCs. The present work used a sequential deposition process to alternately fabricate thin MAPbI3-based perovskite and cesium iodide (CsI) layers with precise control over the CsI intercalation, producing high quality cesium containing perovskite films. The optimal CsI film thickness when applied to either the up, down or both layers of the MAPbI3 perovskite film was also assessed. Interestingly, the application of a double layer CsI layer greatly altered the perovskite morphology to produce large grain sizes, as a result of the precise intercalation of the CsI molecules into the host MAPbI3. Furthermore, PSCs made with double layer CsI intercalation exhibited power conversion efficiencies as high as 18.43%. These exceeded the values obtained from devices made with pristine MAPbI3, or with CsI intercalation on either the bottom or top of the perovskite (16.14%, 17.92% and 17.26%, respectively). A double layer CsI intercalation device was stored in the dark at relative humidities of 40–50% for more than 4000 h and retained over 83% of its initial efficiency. Sequential deposition technique was used to alternately fabricate CH3NH3PbI3 (MAPbI3) and cesium iodide (CsI) thin layers with precise control over the CsI intercalation, producing large grain size, and high-quality cesium containing perovskite films. When the CsI double-layer intercalate into the MAPbI3 framework effectively improves the efficiency and long-term stability of the perovskite solar cells. Display omitted •Double layer CsI intercalation greatly altered the perovskite morphology.•The optimal CsI film thickness when applied to either the up, down or double layers of the MAPbI3 film were assessed.•Double layer CsI intercalation significantly improve the long-term stability of perovskite solar cells.