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Dongxu Pan1 Yongping Fu1 Jie Chen1 2 Kyle Czech1 John Wright1 Jin Song1

1, University of Wisconsin, Madison, Madison, Wisconsin, United States
2, Xi’an Jiaotong University, Xi'an, , China

The facile chemical transformation of halide perovskites via ion exchange has been attributed to the characteristic of "soft" crystal lattice that enables ions to readily migrate in the lattice. Kinetic studies of such process could provide mechanistic insights on the ion migration dynamics, yet from the experimental aspect little effort has been made. Herein, we present a general method to in situ visualize and monitor the kinetics of cation or anion interdiffusion in a specifically designed perovskite heterostructure via spatially resolved photoluminescence measurement. A CsPbCl3/CsPbBr3 heterostructure was fabricated by stacking single-crystal CsPbCl3 microplates on top of single-crystal CsPbBr3 nanowires to study chlorine-bromine interdiffusion behavior. Time-dependent confocal photoluminescence microscopy and energy-dispersive X-ray spectroscopy showed the solid-state anion interdiffusion can readily occur, leading to the formation of halide concentration gradients along the nanowire. Quantitative analysis of composition profiles across the heterojunction using Fick's law allowed us, for the first time, to extract interdiffusion coefficients of the couple and an activation energy of 0.44±0.02 eV for ion diffusion from temperature-dependent studies. Moreover, comparative studies on MAPbI3/CsPbBr3 heterostructure revealed limited extent of iodine-bromine interdiffusion likely due to the complex phase diagram of mixed alloys of CsPb(Br,I)3. In contrast to the relatively mobile anions, A-site cation interdiffusion across the interface in MAPbBr3/CsPbBr3 junction was barely observed at room temperature. Our results present an important step for developing a model system to investigate the kinetics of the solid-state ion migration. Moreover, the gained insights can provide guidelines for rationally designing perovskite heterostructures that could potentially give rise to new intriguing properties for both fundamental studies and technological applications.

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