[en] Highlights: • Ambient-air-solution-processed perovskite solar cells based on CH₃NH₃PbI_3−xCl_x–NiO with Al₂O₃/NiO interface engineering. • High quality and excellent crystallinity of MAPbI_3−xCl_x–NiO composite film achieved by Al₂O₃/NiO interfacial engineering. • High power conversion efficiency of 18.14%, small hysteresis and excellent reproducibility. • Remarkable air-stability over 210 days with retaining ~100% of original V_oc, ~94% of J_sc, ~91% of FF and ~86% of PCE. The poor air-stability and reproducibility of perovskite solar cells (PSCs) have prevented the practical applications of the devices that can withstand sustained operation under ambient air conditions. Here, we report all-ambient-air-solution-processed PSCs based on CH₃NH₃PbI_3−xCl_x-NiO composite film with inserting Al₂O₃/NiO at the TiO₂/perovskite interface in a cell configuration of FTO/c-TiO₂/mp-TiO₂/Al₂O₃/NiO/MAPbI_3−xCl_x-NiO/spiroOMeTAD/Au. The interface engineering with Al₂O₃/NiO not only improves crystalline quality of perovskite films and enhances charge transport, but also effectively suppresses carrier recombination. This composite-based interface engineering PSCs showed a high power conversion efficiency (PCE) of 18.14% and excellent reproducibility with average 16–18% PCE for 35 devices. More importantly, the devices without encapsulation showed a significant enhancement in long-term air-stability; the device photovoltaic parameters stabilized after 20 days and sustained its stability over 210 days with retaining ~100% of its original V_oc, ~94% of J_sc, ~91% of FF and ~86% of PCE in an ambient environment.