Atomic layer deposition of aluminum oxide (ALD-AlOx) layers has been extensively studied for stabilizing perovskite solar cells (PSCs) against environmental stressors, such as humidity and oxygen. [1,2,3] In addition, the ALD-AlOx layer acts as a protective barrier, mitigating the pernicious halide ion migration from the perovskite toward the hole transport interface. However, its effectiveness in preventing the infiltration of ions and additives from the hole-transport layer into perovskites remains insufficiently understood. Herein, we demonstrate the in-situ growth mechanism of a compact ultrathin (~0.75 nm) ALD-AlOx layer using in-situ ellipsometry and conductive-atomic force microscopy. Our result highlights the conformal growth of <1 nm of the ALD-AlOx layer follows the morphology of a triple-cation perovskite film over a large area. This promotes effective mechanical adhesion of the spiro-OMeTAD layer on top of the perovskite. The electronic and chemical structure of the ALD-AlOx is explored via in situ XPS and reflective electron energy loss spectroscopy uncovering the underlying reasons for improved charge carrier collection between these two layers. Upon systematically investigating the layer-by-layer structure of the PSC stack, we discovered that ALD-AlOx also acts as a diffusion barrier layer for the degraded species from the adjacent transport layer into the perovskite. In addition to all protection capabilities, ALD-AlOx impedes the transition of crystalline perovskites to an undesired amorphous phase instead of a yellow delta phase. Consequently, the dual functionality (i.e., enhanced mechanical adhesion and diffusion barrier) of the ALD-AlOx protection enhanced the device performance from 19.1% to 20.5%, retaining 85% of its initial power conversion efficiency (PCE) compared to 10% for pristine devices after 180 days of shelf-aging, followed by 1000 min of maximum power point tracking under ambient conditions. In conclusion, our outdoor testing (ISOS-O-2 protocol) for 3000 hrs of our encapsulated devices shows the ALD-AlOx device maintains up to 85% of its initial as compared to 20% for the pristine devices. Finally, this study deepens our understanding of the mechanism of ALD-Al₂O₃ as a two-way diffusion barrier, highlighting the multifaceted role of buffer layers in interfacial engineering for the long-term stability of PSCs.