<i>Ab Initio</i> Molecular Dynamics Simulations of Methylammonium Lead Iodide Perovskite Degradation by Water

Abstract

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 <i>ab initio</i> molecular dynamics simulations aimed at unraveling the atomistic details of the interaction between the methylammonium lead iodide (MAPbI₃) 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. PbI₂-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 PbI₂-terminated slab, which could represent the first step in the formation of an intermediate hydrated phase. Interestingly, PbI₂ defects on the PbI₂-terminated surface promote the rapid dissolution of the exposed facet. Surface hydration, which is spontaneous for both MAI- and PbI₂-terminated slabs, does not modify the electronic landscape of the former, while the local band gap of the PbI₂-exposing model widens by ∼0.3 eV in the interfacial region. Finally, we show that water incorporation into bulk MAPbI₃ 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