Effect of Halide Ion Migration on the Electrical Properties of Methylammonium Lead Tri-Iodide Perovskite Solar Cells

Ionic migration in halide perovskite materials is now well recognized to affect the electrical properties of perovskite solar cells. Native point defects such as vacancies are considered as responsible for migration of ions. In order to interpret the unusual behavior of current-voltage curves in methylammonium lead tri-iodide perovskite (MAPI) solar cells, we combined two independent experiments: temperature-dependent conductivity measurements (213-363 K) under dark and glow discharge optical emission spectroscopy (GD-OES) under electrical bias conditions. From the film conductivity measurements, we observed two different conduction regimes: an ionic conduction regime at temperatures higher than 263 K, associated with an activation energy of 0.25 eV, and an electronic conduction regime below 263 K. At room temperature, both conductivities are around 10^-7 S/cm. From GD-OES, we observed directly a reversible migration of iodide ions under positive or negative bias (minute time scale) at room temperature and deduced an iodide ion diffusion coefficient of 1.3 × 10^-12 cm²·s^-1 and a mobility of 5 × 10^-11 cm²·V^-1·s^-1.