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(3)) 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 similar to 0.3 eV in the interfacial region. Finally, we show that water incorporation into bulk MAPbI(3) produces almost no changes in the tetragonal structure of the perovskite crystal (similar to 4% 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.