First-Principles Study of Ferroelastic Twins in Halide Perovskites

We present an ab initio simulation of 90 degrees ferroelastic twins that were recently observed in methylammonium lead iodide. There are two inequivalent types of 90 degrees walls that we calculate to act as either electron or hole sinks, which leads us to propose a mechanism for enhancing charge carrier separation in photovoltaic devices. Despite separating nonpolar domains, we show these walls to have a substantial in-plane polarization of similar to 6 mu C cm(-2), due in part to flexoelectricity. We suggest this in turn could allow for the photoferroic effect and create efficient pathways for photocurrents within the wall.