Ab Initio Molecular Dynamics Simulations of Methylammonium Lead Iodide Perovskite Degradation by Water

Protecting organohalide perovskite thin films from water and ambient humidity represents a paramount challenge for the commercial uptake of perovskite solar cells and, in general, of related optoelectronic devices. Therefore, understanding the perovskite/water interface is of crucial importance. As a step in this direction, here we present ab initio molecular dynamics simulations aimed at unraveling the atomistic details of the interaction between the methylammonium lead iodide (MAPbI<inf>3</inf>) perovskite surfaces and a liquid water environment. According to our calculations, MAI-terminated surfaces undergo a rapid solvation process, driven by the interaction of water molecules with Pb atoms, which prompts the release of I atoms. PbI<inf>2</inf>-terminated surfaces, instead, seem to be more robust to degradation, by virtue of the stronger (shorter) Pb-I bonds formed on these facets. We also observe the incorporation of a water molecule into the PbI<inf>2</inf>-terminated slab, which could represent the first step in the formation of an intermediate hydrated phase. Interestingly, PbI<inf>2</inf> defects on the PbI<inf>2</inf>-terminated surface promote the rapid dissolution of the exposed facet. Surface hydration, which is spontaneous for both MAI- and PbI<inf>2</inf>-terminated slabs, does not modify the electronic landscape of the former, while the local band gap of the PbI<inf>2</inf>-exposing model widens by ~0.3 eV in the interfacial region. Finally, we show that water incorporation into bulk MAPbI<inf>3</inf> produces almost no changes in the tetragonal structure of the perovskite crystal (~1% volume expansion) but slightly opens the band gap. We believe that this work, unraveling some of the atomistic details of the perovskite/water interface, may inspire new interfacial modifications and device architectures with increased stabilities, which could in turn assist the commercial uptake of perovskite solar cells and optoelectronic devices. (Figure Presented).