Ultrathin metal oxides prepared by atomic layer deposition (ALD) have gained utmost attention as moisture and thermal stress barrier layers in perovskite solar cells (PSCs). We have recently shown that 10 cycles of ALD Al₂O₃ deposited directly on top of the CH₃NH₃PbI_{3- x}Cl _x perovskite material, are effective in delivering a superior PSC performance with 18% efficiency (compared to 15% of the Al₂O₃-free cell) with a long-term humidity-stability of more than 60 days. Motivated by these results, the present contribution focuses on the chemical modification which the CH₃NH₃PbI_{3- x}Cl _x perovskite undergoes upon growth of ALD Al₂O₃. Specifically, we combine in situ Infrared (IR) spectroscopy studies during film growth, together with X-ray photoelectron spectroscopy (XPS) analysis of the ALD Al₂O₃/perovskite interface. The IR-active signature of the NH₃⁺ stretching mode of the perovskite undergoes minimal changes upon exposure to ALD cycles, suggesting no diffusion of ALD precursor and co-reactant (Al(CH₃)₃ and H₂O) into the bulk of the perovskite. However, by analyzing the difference between the IR spectra associated with the Al₂O₃ coated perovskite and the pristine perovskite, respectively, changes occurring at the surface of perovskite are monitored. The abstraction of either NH₃ or CH₃NH₂ from the perovskite surface is observed as deduced by the development of negative N-H bands associated with its stretching and bending modes. The IR investigations are corroborated by XPS study, confirming the abstraction of CH₃NH₂ from the perovskite surface, whereas no oxidation of its inorganic framework is observed within the ALD window process investigated in this work. In parallel, the growth of ALD Al₂O₃ on perovskite is witnessed by the appearance of characteristic IR-active Al-O-Al phonon and (OH)-Al═O stretching modes. Based on the IR and XPS investigations, a plausible growth mechanism of ALD Al₂O₃ on top of perovskite is presented.

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