Chemical Analysis of the Interface between Hybrid Organic–Inorganic Perovskite and Atomic Layer Deposited Al ₂O ₃

Abstract

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 Al ₂O ₃ deposited directly on top of the CH ₃NH ₃PbI _{3– x} Cl _x perovskite material, are effective in delivering a superior PSC performance with 18% efficiency (compared to 15% of the Al ₂O ₃-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 CH ₃NH ₃PbI _{3– x} Cl _x perovskite undergoes upon growth of ALD Al ₂O ₃. Specifically, we combine in situ Infrared (IR) spectroscopy studies during film growth, together with X-ray photoelectron spectroscopy (XPS) analysis of the ALD Al ₂O ₃/perovskite interface. The IR-active signature of the NH ₃ ⁺ stretching mode of the perovskite undergoes minimal changes upon exposure to ALD cycles, suggesting no diffusion of ALD precursor and co-reactant (Al(CH ₃) ₃ and H ₂O) into the bulk of the perovskite. However, by analyzing the difference between the IR spectra associated with the Al ₂O ₃ coated perovskite and the pristine perovskite, respectively, changes occurring at the surface of perovskite are monitored. The abstraction of either NH ₃ or CH ₃NH ₂ 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 CH ₃NH ₂ 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 Al ₂O ₃ 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 Al ₂O ₃ on top of perovskite is presented.