Frenkel defects promote polaronic exciton dissociation in methylammonium lead iodide perovskites

Abstract

Hybrid organic–inorganic perovskite materials, such as CH₃NH₃PbI₃, exhibit substantial potential in a variety of optoelectronic applications. Nevertheless, the interplay between the photoinduced excitations and iodine Frenkel defects which are abundant in CH₃NH₃PbI₃ films remains poorly understood. Here we study the light-triggered electronic and excitonic properties in the presence of iodine Frenkel defects in CH₃NH₃PbI₃ by using a combination of density functional theory (DFT) and time-dependent DFT approaches, the latter of which treats electron–hole and electron–nucleus interactions on the same footing. For isolated Frenkel defects, electrons are trapped close to the iodine vacancies and the electron–hole correlation brings the holes in close vicinity to the electrons, yielding tightly bound polaronic excitons. However, in the presence of multiple interactive Frenkel defects, the holes are pulled out from an electron–hole Coulomb well by the iodine interstitials, leading to spatially separated electron–hole pairs. The X-ray photoelectron spectra are then simulated, unravelling the light-triggered charge transfer induced by Frenkel defects at the atomistic level. We also find that the energy and spatial distributions of polaronic excitons at the Frenkel defects can be controlled by the dynamical rotation of organic cations.