Perovskite materials are drawing tremendous interest for photovoltaic solar cell applications, but are hampered by intrinsic material and device instability issues. Such issues can arise from environmental influences as well as from the chemical incompatibility of the perovskite layer with charge transport layers and electrodes used in the device stack. Several attempts have been made to address the instability issue, mostly concentrating on the substitution of the organic cations in the perovskite lattice, and on alternatives for the organic charge extraction layers, without laying much emphasis on stabilising the existing, conventional high efficiency methylammonium lead iodide/spiro-OMeTAD based devices. To address the latter issue, we utilized atomic layer deposition (ALD) as a straightforward and soft deposition process to conformally deposit Al2O3 on top of the perovskite absorber. An ultra-thin ALD Al2O3 film effectively protects the perovskite layer while it is sufficiently thin enough to provide a tunnel contact. The fabricated perovskite solar cells (PSCs) exhibit superior device performance with a stabilised power conversion efficiency (PCE) of 18%, a significant reduction in hysteresis loss, and enhanced long-term stability (beyond 60 days) as a function of the unencapsulated storage time in ambient air, under humidity conditions ranging from 40 to 70% at room temperature. PCE measurements after 70 days of humidity exposure show that the devices incorporating 10 cycles of ALD Al2O3 could significantly retard the humidity-induced degradation thereby retaining about 60-70% of its initial PCE, while that of the reference devices drops to a remaining 12% of their initial PCE. This work successfully addresses and tackles the problem of the hybrid organic-inorganic IV-halide perovskite solar cell's instability in a humid environment, and the key findings pave the way to the upscaling of these devices.