Ion\nMigration and Redox Reactions in Axial Heterojunction\nPerovskite CsPb(Br<sub>1–<i>x</i></sub>Cl<i>x</i>)<sub>3</sub> Nanowire Devices Revealed by Operando Nanofocused\nX‑ray Photoelectron Spectros

Metal-halide perovskites\n(MHPs) have gained substantial interest\nin the energy and optoelectronics field. MHPs in nanostructure forms,\nsuch as nanocrystals and nanowires (NWs), have further expanded the\nhorizons for perovskite nanodevices in geometry and properties. A\npartial anion exchange within the nanostructure, creating axial heterojunctions,\nhas significantly augmented the potential applications. However, surface\ndegradation and halide ion migration are deteriorating device performance.\nQuantitative analysis of halide metal concentration and mapping of\nthe electrical surface potential along the operating NW device are\nneeded to better understand ion transportation, band structure, and\nchemical states, which have not been experimentally reported yet.\nThis requires a characterization approach that is capable to provide\nsurface-sensitive chemical and electrical information at the subμm\nscale. Here, we used operando nanofocused X-ray photoelectron spectroscopy\n(nano-XPS) to study CsPbBr<sub>3</sub>/CsPb(Br<sub>1–<i>x</i></sub>Cl<sub><i>x</i></sub>)<sub>3</sub> heterojunction\nNW devices with a spatial resolution of 120 nm. We monitored Br<sup>–</sup> and Cl<sup>–</sup> ion migration and comprehended\nthe potential drop along the device during operation. Ion migration\nand healing of defects and vacancies are found for applied voltages\nof as low as 1 V. We present a model delineating band bending along\nthe device based on precise XPS peak positions. Notably, a reversible\nredox reaction of Pb was observed, that reveals the interaction of\nmigrating halide ions, vacancies, and biased metal electrodes under\nelectrical operation. We further demonstrate how X-ray-induced surface\nmodification can be avoided, by limiting exposure times to less than\n100 ms. The results facilitate the understanding of halide ion migration\nin MHP nanodevices under operation.