Аннотация: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 Cs1.2PbI2Br1.2 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.