Herein, we achieved, air-stable low-temperature processed PSC (L-PSC) using alkali-metal modified ZnO ETLs. Using a simple chemical alkali-metal modification method, the surface defects of the ZnO were effectively passivated. As a result, the interfacial decomposition reactions were suppressed, while raising the Fermi energy level and enhancing electron mobility. The improved interfacial charge transfer and internal electric field in the developed L-PSC using K modified ZnO (ZnO-K) exhibited an improved power conversion efficiency (PCE) of 19.90% with negligible hysteresis, while a pristine ZnO based L-PSC exhibited a PCE of 16.10% with significant hysteresis. The ZnO-K based L-PSC also exhibited remarkably higher long-term air-storage stability (91% retention after 800 h) than pristine ZnO based L-PSCs (36% retention after 800 h) due to the suppressed decomposition reactions. The PCE and air stability of our L-PSC with the modified ZnO are among the highest reported for PSCs processed at ≤150 °C.