The thermodynamics and kinetics of iodine vacancies in the hybrid perovskite methylammonium lead iodide

Abstract

The current picture of ion transport in the solar-cell absorber material CH₃NH₃PbI₃ (MAPbI₃) suffers from a disturbing lack of clarity. In this study, we demonstrate that, with knowledge of the jump rate of iodine vacancies and with a defect chemical model, various experimental data reported in the literature for the ionic conductivity of MAPbI₃ can be reconciled. A quantitative expression for the vacancy jump rate was obtained by studying iodine tracer diffusion as a function of temperature and iodine-vacancy concentration by means of classical molecular-dynamics simulations. The defect-chemical model yields acceptor concentrations in experimental samples of 10¹⁵ cm⁻³ and lower, and the enthalpy and entropy of anti-Frenkel disorder. We also demonstrate that the generation of additional iodine vacancies can explain quantitatively the increase in the ionic conductivity under illumination. Finally, the consequences for devices under bias and for grain-boundary transport are discussed.