Zhaojian Xu1,Ross Kerner2,Steven Harvey2,Kai Zhu2,Joseph J. Berry2,3,4,Barry Rand1,5
Princeton University1,National Renewable Energy Laboratory2,Renewable and Sustainable Energy Institute, University of Colorado Boulder3,University of Colorado Boulder4,Andlinger Center for Energy and the Environment, Princeton University5
Zhaojian Xu1,Ross Kerner2,Steven Harvey2,Kai Zhu2,Joseph J. Berry2,3,4,Barry Rand1,5
Princeton University1,National Renewable Energy Laboratory2,Renewable and Sustainable Energy Institute, University of Colorado Boulder3,University of Colorado Boulder4,Andlinger Center for Energy and the Environment, Princeton University5
Mixed-halide perovskites, owing to straightforward bandgap tuning through halide stoichiometry variation, have been considered in many optoelectronic devices. However, unwanted halide segregation under operational conditions, including light illumination and voltage bias, restrict practical applications. While light-induced halide segregation has been heavily studied, voltage-induced halide segregation still lacks in-depth investigation and the mechanism behind remains unclear. Herein, we systematically study the impact of voltage bias on mixed bromide/iodide perovskite devices across the full range of bromide/iodide ratios by conducting a series of long-time voltage biasing tests, and observe three voltage thresholds for an optically stable, mixed-halide perovskite composition with a low bromide ratio, and assign them, in order of increasing voltage, as the hole transport material doping threshold, halide segregation threshold, and degradation threshold, respectively. These empirical threshold voltages are minimally affected by composition until very Br-rich compositions, revealing the dominant role of iodide/triiodide/iodine electrochemistry in voltage-induced halide separation in mixed-halide perovskite devices. Furthermore, voltage-induced halide redistribution is directly visualized by cross-sectional scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM–EDX). By changing the metal electrodes (Au and Ag) and testing different voltage polarities (forward and reverse bias), we demonstrate the mechanism of voltage-induced halide segregation: iodide oxidation at the anode initiates unbalanced bromide, iodide, and oxidized iodine mass transport fluxes, resulting in vertical halide redistribution. The suppression of halide segregation by Ag anode indicates that halide perovskite devices operate as solid-state electrochemical cells when threshold voltages are exceeded, and the species/reaction with the lowest oxidation potential and the species/reaction with the lowest reduction potential in the metal halide device dictate the stability behavior under voltage bias.