Probing Structural Distortions and Ionic Migration in CH₃NH₃PbI₃: The Role of Intrinsic Defects

Structural distortions play a decisive role in the stability and performance of organic–inorganic mixed halide perovskites. Therefore, understanding the role of inevitably present intrinsic defects in the structural distortions is essential for the defect engineering of halide perovskites. In this study, we present an in-depth analysis of the structural distortions induced by the presence of five intrinsic defects, namely, CH3NH3 (MA), Pb and I vacancies, Pb interstitial, and PbMA antisite in CH3NH3PbI3. Quantitative analysis of the octahedral distortions and MA molecular rotation showed that the Pb vacancy causes a maximum change in the isometric octahedral volume (∼14%) as well as MA molecule rotation (∼70°), while the asymmetric octahedral twisting and bond angle deviations are negligible. On the other hand, asymmetric octahedral distortions are highest in the I-vacant structure, where an average variation of ∼8° in the I–Pb–I bond angle was reported, and the effective coordination number of Pb drops to ∼5.2. Classical force-field based calculations revealed that the halide ion migration barrier varies proportionally with the isometric octahedral volume change and the MA molecule rotation, while the effect of asymmetric octahedral distortions was observed to be negligible. The total activation energy of halide vacancy diffusion increases by 0.3–1.1 eV due to the presence of intrinsic defects, where binding of the iodide vacancies with the intrinsic defects plays a dominating role. Inhibiting ionic migration through the interplay of defects is an effective strategy to suppress phase transitions and enhance the stability of perovskite-based devices, enabling their application in LEDs, photodetectors, and solar cells.