Fast Diffusion of Native Defects and Impurities in Perovskite Solar Cell Material CH₃NH₃PbI₃

CH₃NH₃PbI₃-based solar cells have shown remarkable progress in recent years but have also suffered from structural, electrical, and chemical instabilities related to the soft lattices and the chemistry of these halides. One of the instabilities is ion migration, which may cause current–voltage hysteresis in CH₃NH₃PbI₃ solar cells. Significant ion diffusion and ionic conductivity in CH₃NH₃PbI₃ have been reported; their nature, however, remain controversial. In the literature, the use of different experimental techniques leads to the observation of different diffusing ions (either iodine or CH₃NH₃ ion); the calculated diffusion barriers for native defects scatter in a wide range; the calculated defect formation energies also differ qualitatively. These controversies hinder the understanding and the control of the ion migration in CH₃NH₃PbI₃. In this paper, we show density functional theory calculations of both the diffusion barriers and the formation energies for native defects (VI⁺, MAi⁺, VMA–, and Ii–) and the Au impurity in CH₃NH₃PbI₃. VI⁺ is found to be the dominant diffusing defect due to its low formation energy and the low diffusion barrier. Ii– and MAi⁺ also have low diffusion barriers but their formation energies are relatively high. The hopping rate of VI⁺ is further calculated taking into account the contribution of the vibrational entropy, confirming VI⁺ as a fast diffuser. We discuss approaches for managing defect population and migration and suggest that chemically modifying surfaces, interfaces, and grain boundaries may be effective in controlling the population of the iodine vacancy and the device polarization. We further show that the formation energy and the diffusion barrier of Au interstitial in CH₃NH₃PbI₃ are both low. As a result, it is thus possible that Au can diffuse into CH3NH3PbI3 under bias in devices (e.g., solar cell, photodetector) with Au/CH₃NH₃PbI₃ interfaces and modify the electronic properties of CH₃NH₃PbI₃.