# Unraveling the water degradation mechanism of CH$_3$NH$_3$PbI$_3$

## TL;DR

This study uses ab initio metadynamics to elucidate the water-induced degradation mechanism of CH$_3$NH$_3$PbI$_3$ perovskite, revealing a low energy barrier and spontaneous dissolution process that explain its instability in water.

## Contribution

It provides a detailed molecular-level understanding of water degradation in MAPbI$_3$, including a thermodynamic cycle and comparison with inorganic perovskites, advancing the knowledge of perovskite instability.

## Key findings

- Initial dissolution involves I$^-$ ion departure from the surface.
- Water dissolution has a low energy barrier.
- Dissolution in water is thermodynamically spontaneous.

## Abstract

Instability of perovskite photovoltaics is still a topic which is currently under intense debate, especially the role of water environment. Unraveling the mechanism of this instability is urgent to enable practical application of perovskite solar cells. Here, ab initio metadynamics is employed to investigate the initial phase of a dissolution process of CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) in explicit water. It is found that the initial dissolution of MAPbI$_3$ is a complex multi-step process triggered by the departure of I$^-$ ion from the CH$_3$NH$_3$I-terminated surface. Reconstruction of the free energy landscape indicates a low energy barrier for water dissolution of MAPbI$_3$. In addition, we propose a two-step thermodynamic cycle for MAPbI$_3$ dissolution in water at a finite concentration that renders a spontaneity of the dissolution process. The low energy barrier for the initial dissolution step and the spontaneous nature of MAPbI$_3$ dissolution in water explain why the water immediately destroys pristine MAPbI$_3$. The dissolution thermodynamics of all-inorganic CsPbI$_3$ perovskite is also analyzed for comparison. Hydration enthalpies and entropies of aqueous ions play an important role for the dissolution process. Our findings provide a comprehensive understanding to the current debate on water instability of MAPbI$_3$.