title: Polarization Dependence of Water Adsorption to CH3NH3PbI3 (001) Surfaces.
abstract: The instability of organometal halide perovskites when in contact with water is a serious challenge to their feasibility as solar cell materials. Although studies of moisture exposure have been conducted, an atomistic understanding of the degradation mechanism is required. Toward this goal, we study the interaction of water with the (001) surfaces of CH3NH3PbI3 under low and high water concentrations using density functional theory. We find that water adsorption is heavily influenced by the orientation of the methylammonium cations close to the surface. We demonstrate that, depending on methylammonium orientation, the water molecule can infiltrate into the hollow site of the surface and get trapped. Controlling dipole orientation via poling or interfacial engineering could thus enhance its moisture stability. No direct reaction between the water and methylammonium molecules is seen. Furthermore, calculations with an implicit solvation model indicate that a higher water concentration may facilitate degradation through increased lattice distortion.
authors.affiliation: The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania , Philadelphia, Pennsylvania 19104-6323, United States.
authors.forename: Nathan Z
authors.initials: NZ
authors.lastname: Koocher
authors.affiliation: The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania , Philadelphia, Pennsylvania 19104-6323, United States.
authors.forename: Diomedes
authors.initials: D
authors.lastname: Saldana-Greco
authors.affiliation: The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania , Philadelphia, Pennsylvania 19104-6323, United States.
authors.forename: Fenggong
authors.initials: F
authors.lastname: Wang
authors.affiliation: The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania , Philadelphia, Pennsylvania 19104-6323, United States.
authors.forename: Shi
authors.initials: S
authors.lastname: Liu
authors.affiliation: The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania , Philadelphia, Pennsylvania 19104-6323, United States.
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Rappe
keywords: hybrid perovskite; methylammonium lead iodide; moisture; stability; water adsorption