Polarization Dependence of Water Adsorption to CH$_3$NH$_3$PbI$_3$ (001) Surfaces

TL;DR

This study uses density functional theory to explore how water interacts with CH$_3$NH$_3$PbI$_3$ (001) surfaces, revealing that methylammonium orientation affects water infiltration and suggesting strategies to improve moisture stability in perovskite solar cells.

Contribution

It provides an atomistic understanding of water adsorption mechanisms on perovskite surfaces, highlighting the role of methylammonium orientation and water concentration in degradation.

Findings

Water adsorption depends on methylammonium orientation.

Water can infiltrate and become trapped depending on cation orientation.

Higher water concentration may promote degradation.

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 CH$_3$NH$_3$PbI$_3$ under both 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. It is demonstrated 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. We do not see a direct reaction between the water and methylammonium molecules. Furthermore, calculations with an implicit solvation model indicate that a higher water concentration may facilitate degradation.