A low-temperature solution process technique is employed to develop an inorganic cesium carbonate (Cs2CO3) as an electron transport material for inorganic–organic hybrid double cation (FAPbI3)0.85(MAPbBr3)0.15 perovskite solar cells, as an alternative to the conventional thick and meso-TiO2. A device structure of compact-TiO2/Cs2CO3 (0.2 wt. %)/perovskite/spiro-OMETAD leads to enhanced performance of the photovoltaic device, achieving a short-circuit current density (Jsc) of 22.26  mA/cm2, an open-circuit voltage (Voc) of 1054 mV, a fill factor (FF) of 71.6%, and a power conversion efficiency (PCE) of about 17% under one sun illumination, whereas the controlled device structure shows an efficiency of 16.58% without such surface modification layer. Additionally, a device structure of Cs2CO3 (6 wt. %)/perovskite/spiro-OMETAD without any