This article investigates the ionic transport in hybrid lead iodide perovskite solar cells, focusing on the activation energies for ionic migration in methylammonium lead iodide (CH₃NH₃PbI₃). Using first-principles calculations and kinetic data from solar cell experiments, the study identifies the microscopic transport mechanisms, revealing that iodide ions migrate easily with an activation energy of 0.6 eV, consistent with experimental measurements. The results suggest that hybrid halide perovskites are mixed ionic-electronic conductors, which has significant implications for solar cell device architectures. The study highlights the importance of ionic transport in explaining unusual behaviors such as current-voltage hysteresis and low-frequency giant dielectric response in perovskite solar cells. The research also shows that iodide ion vacancies play a key role in the transport process, with the activation energies for their migration being close to those measured in the solar cell. The findings support the idea that ionic transport is a primary cause of the anomalous hysteresis and giant switchable photovoltaic effect observed in these cells. The study combines computational and experimental approaches to understand the transport mechanisms in CH₃NH₃PbI₃. It identifies the migration pathways for iodide, Pb²⁺, and CH₃NH₃⁺ ions, and compares these with kinetic data from a working solar cell. The results indicate that iodide ions are the majority ionic carriers, and the level of intrinsic iodide ion vacancies in the material significantly affects the conductivity. The research also discusses the implications of these findings for the stability and performance of perovskite solar cells. It suggests that the migration of iodide ion vacancies under an electric field can influence the photogenerated charge collection efficiency, helping to explain hysteresis. The study further explores the influence of iodide ion vacancies on the band energies of perovskite thin film devices and their interfaces, providing a model to explain the chronophotoamperometry measurements. Overall, the study provides new insights into the ion transport mechanisms in hybrid perovskite solar cells, highlighting the importance of ionic transport in their performance and stability. The findings have major implications for the design and optimization of future perovskite solar cell architectures.This article investigates the ionic transport in hybrid lead iodide perovskite solar cells, focusing on the activation energies for ionic migration in methylammonium lead iodide (CH₃NH₃PbI₃). Using first-principles calculations and kinetic data from solar cell experiments, the study identifies the microscopic transport mechanisms, revealing that iodide ions migrate easily with an activation energy of 0.6 eV, consistent with experimental measurements. The results suggest that hybrid halide perovskites are mixed ionic-electronic conductors, which has significant implications for solar cell device architectures. The study highlights the importance of ionic transport in explaining unusual behaviors such as current-voltage hysteresis and low-frequency giant dielectric response in perovskite solar cells. The research also shows that iodide ion vacancies play a key role in the transport process, with the activation energies for their migration being close to those measured in the solar cell. The findings support the idea that ionic transport is a primary cause of the anomalous hysteresis and giant switchable photovoltaic effect observed in these cells. The study combines computational and experimental approaches to understand the transport mechanisms in CH₃NH₃PbI₃. It identifies the migration pathways for iodide, Pb²⁺, and CH₃NH₃⁺ ions, and compares these with kinetic data from a working solar cell. The results indicate that iodide ions are the majority ionic carriers, and the level of intrinsic iodide ion vacancies in the material significantly affects the conductivity. The research also discusses the implications of these findings for the stability and performance of perovskite solar cells. It suggests that the migration of iodide ion vacancies under an electric field can influence the photogenerated charge collection efficiency, helping to explain hysteresis. The study further explores the influence of iodide ion vacancies on the band energies of perovskite thin film devices and their interfaces, providing a model to explain the chronophotoamperometry measurements. Overall, the study provides new insights into the ion transport mechanisms in hybrid perovskite solar cells, highlighting the importance of ionic transport in their performance and stability. The findings have major implications for the design and optimization of future perovskite solar cell architectures.