Charge localization, stabilization and hopping in lead-halide perovskites: Competition between polaron stabilization and cation disorder

Metal halide perovskites, of general formula ABX₃, show a complex interplay of the inorganic BX₃ sub-lattice and the organic/inorganic A-site cations, which likely determines some of their peculiar optoelectronic properties. Comprehension of the physics underlying this interaction may reveal further means of fine-tuning their optoelectronic response. Here, we investigate in depth charge/lattice interactions associated to the formation of polarons in different models of the prototypical CH₃NH₃PbI₃ perovskite through advanced electronic-structure calculations. We demonstrate that charge localization, while induced by the disordered dipolar field of the organic cations, is only stabilized by distortions in the inorganic sub-lattice. Polaron hopping between neighbouring minima is related to the random reorientation of the organic cations and occurs via a delocalized transition state. Our simulations highlight a struggle between thermally induced disorder, driven by the motion of A-site cations, and polaron stabilization within the BX₃ sub-lattice, which explains the simultaneous low mobility and high diffusion length of charge carriers in lead-halide perovskites.