Joong Il Jake Choi

Affiliation: Center for Nanomaterials and Chemical Reactions, IBS

Research Interests: Surface Science
Title: Lead-Halide Perovskite Surface Degradation: Insights from In Situ Atomic-Scale Analysis
Abstract: While organic-inorganic hybrid perovskites are emerging as promising materials for next-generation photovoltaic applications, the origins and the pathways of the instability of perovskites remain speculative. In particular, the degradation of perovskite surfaces by ambient water is a crucial sub-ject for determining the long-term viability of perovskite-based solar cells. Herein, we employ vari-able-pressure atomic force microscopy (VP-AFM) and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) to carry out surface characterization and atomic-scale analysis of the reac-tion mechanisms for methylammonium lead bromide (MA(CH3NH3)PbBr3) single-crystal surfaces in environments ranging from ultra-high vacuum (UHV) to ambient pressures. MAPbBr3 single crystals grown in a solution process are mechanically cleaved at UHV to obtain an atomically clean surface. We observe surface inhomogeneity on the freshly cleaved MAPbBr3 surface: the coexist-ence of MA-terminated layers with cubic layer heights, and full and partial coverage of PbBr2-terminated defective layers with lower layer heights. Consecutive topography and friction force measurements in low pressure water (pwater ≈ 10–5 mbar) show the creation of degraded patches that are one atomic layer deep, gradually increasing their coverage until fully covers the surface at water exposure of 4.7 × 104 Langmuir. At the perimeters of these degraded patches, a higher friction coef-ficient was observed, along with an interstitial step height, which we attribute to a structure equiva-lent to that of the MA–Br terminated surface. Combined with NAP-XPS analysis, our results demonstrate that water vapor induces the dissociation of surface methylammonium ligands, eventu-ally resulting in the depletion of the surface MA and the full coverage of hydrocarbon species after exposure to 0.01 mbar of water vapor.