Reversible Pb2+/Pb0 and I−/I3− Redox Chemistry Drives the Light‐Induced Phase Segregation in All‐Inorganic Mixed Halide Perovskites

Tunability of optoelectronic properties of lead halide perovskites achieved through halide mixing can potentially enable their multiple applications, for example, in tandem solar cells and light‐emitting diodes. However, mixed halide perovskites are unstable under illumination due to their segregation to Br‐rich and I‐rich phases, which negatively affects the performance and the operational stability of devices. Research efforts over the past years provided a substantial understanding of the factors influencing light‐induced halide phase segregation. While several mechanisms have been proposed, none of them could account for all available experimental data; and hence the origin of the effect is still under active debate. Herein, the photodegradation of CsPbI₂Br and Cs_1.2PbI₂Br_1.2 is thoroughly investigated using a set of complementary techniques. In situ atomic force microscopy provides a visualization of the real‐time halide phase segregation dynamics demonstrating that iodoplumbate is selectively expelled from the mixed halide perovskite grains and nucleates as a separate I‐rich phase at the grain boundaries. A mechanism based on the reversible Pb²⁺/Pb⁰ and I⁻/I₃⁻ redox (photo)chemistry is proposed, which explains the experimental findings and other previously reported results. Furthermore, it sheds new insights on the underlying mechanisms of multiple phenomena related to light‐ or electric field‐induced degradation of various lead halide perovskites.