Direct experimental evidence of halide ionic migration under bias in CH₃NH₃PbI_3-xCl_x-based perovskite solar cells using GD-OES analysis

In recent decades, the development of organic-inorganic hybrid perovskite solar cells (PSCs) has been increasing very quickly due to their high initial efficiency and low-cost process. However, key points such as crystal growth mechanisms, current-voltage hysteresis, and instability remain still unexplained or misunderstood. Among several possibilities, ionic migration in PSCs has been suggested to explain the hysteresis effect. However, direct experimental evidence of ionic migration under operation or measurement conditions of PSCs is still missing. This work shows directly the ionic migration of halogen components (I^- and Cl^-) of a CH₃NH₃PbI_3-xCl_x perovskite film under an applied bias using glow discharge optical emission spectrometry (GD-OES). Furthermore, no migration of lead and nitrogen ions is observed on a polarization time scale less than 2 min. The ratio of fixed to mobile iodide ions is deduced from the evolution of the GDOES profile lines as a function of the applied bias. The average length of iodide and chloride ion migration is deduced from the experimental results.