Ab-initio Calculations of Surface Defect Migration in Halide Perovskites

Recently, interest in halide perovskites has been on the rise due to their high-power conversion efficiencies exceeding 20%. Nonetheless, a significant current-voltage hysteresis and poor stability are issues that are not fully understood yet. Ionic migration has been suggested as the cause for the current-voltage hysteresis [1]. An open question in this context is how grain boundaries and crystal surfaces affect ion migration.

The present work focuses on halide-related defects, the main contributors to ionic conductivity [2]. We use first principles density functional theory calculations to simulate the migration of halide vacancies in CsPbBr3 perovskite. The well-established nudged elastic band method [3] is used to compute minimum-energy pathways and energy barriers of halide-related defects. We systematically study the change of migration activation energies upon moving from the surface into the bulk. Our results show that ions migrate more easily in soft lattices showing a pronounced dependence on surface proximity, depending on the availability and size of interstitial sites.

[1] Adv. Mater. 2016, 28, 2446-2454
[2] Energy Environ. 2015, 8, 2118-2127
[3] J. Chem. Phys. 2000, 113, 9901-9904