Iodine Electrochemistry Dictates Voltage-Induced Halide Segregation Thresholds in Mixed-Halide Perovskite Devices

Owing to straightforward stoichiometry-bandgap tunability, mixed-halide perovskites are ideal for many optoelectronic devices. However, unwanted halide segregation under operational conditions, including light illumination and voltage bias, restricts practical use. Additionally, the origin of voltage-induced halide segregation is still unclear. Herein, a systematic voltage threshold study in mixed bromide/iodide perovskite devices is performed and leads to observation of three distinct voltage thresholds corresponding to the doping of the hole transport material (0.7 +/- 0.1 V), halide segregation (0.95 +/- 0.05 V), and degradation (1.15 +/- 0.05 V) for an optically stable mixed-halide perovskite composition with a low bromide content (10%). These empirical threshold voltages are minimally affected by composition until very Br-rich compositions, which reveals the dominant role of iodide/triiodide/iodine electrochemistry in voltage-induced Br/I phase separation and transport layer doping reactions in halide perovskite devices.