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 (V-I(+), MA(t)(+), V-MA(-), and I-t(-)) and the Au impurity in CH3NH3PbI3. V-I(+) is found to be the dominant diffusing defect due to its low formation energy and the low diffusion barrier. I-i(-) and MA(i)(+) also have low diffusion barriers but their formation energies are relatively high. The hopping rate of V-I(+) is further calculated taking into account the contribution of the vibrational entropy, confirming V-I(+) 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. 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.