Unraveling the Water Degradation Mechanism of CH3NH3PbI3

Instability of perovskite photovoltaics is still a topic that is currently under intense debate, especially the role of the 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 CH3NH3PbI3 (MAPbI3) in explicit water. It is found that the initial dissolution of MAPbI3 is a complex multistep process triggered by the departure of I– ion from the CH3NH3I-terminated surface. Reconstruction of the free-energy landscape indicates a low energy barrier for water dissolution of MAPbI3. In addition, we propose a two-step thermodynamic cycle for MAPbI3 dissolution in water at a finite concentration that renders spontaneity to the dissolution process. The low energy barrier for the initial dissolution step and the spontaneous nature of MAPbI3 dissolution in water explain why water immediately destroys pristine MAPbI3. The dissolution thermodynamics of all-inorganic CsPbI3 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 MAPbI3.