Halide Anion-Exchange-Driven Structural Phase Transition in CsPbX₃ Perovskites: Alloys versus Heterostructures

All-inorganic lead halide perovskites CsPbX 3 (X = Cl, Br, I) hold great promise for developing integrated optoelectronic devices with on-demand optical and electronic properties. The soft crystal lattice and strong ionic-bonding nature allow us to tailor the chemical composition and phase structure of CsPbX 3 via anion-exchange reactions at the vapor–solid or the solid–solid interface. The understanding of anion diffusion and exchange mechanisms in CsPbX 3 is crucial for the precise control of their structural transformation and promotion of device performance. Herein, we report a controllable growth of CsPbX 3 microstructures from alloys to heterostructures by an improved two-step vapor epitaxial method. We demonstrate that the surface reaction of Cl-based precursors on CsPbBr 3 microplates facilitates the diffusion and incorporation of Cl ions into the perovskite lattice, leading to the structural transition into CsPb(Br x Cl 1– x ) 3 alloys with a tunable composition and photoluminescence emission. On the contrary, Br ions are more difficult to incorporate into the CsPbCl 3 lattice for the reaction of Br-based precursors on CsPbCl 3 microplates, which contributes to the formation of CsPbBr 3 /CsPb(Br x Cl 1– x ) 3 heterojunctions via a diffusion-limited growth mechanism. Density functional theory calculations reveal that the lower dopant formation energy and smaller diffusion barrier of Cl ions in CsPbBr 3 as compared to those of Br doping in CsPbCl 3 are responsible for the large growth difference. This work not only offers deep insights into the anion-exchange mechanisms but also opens an efficient route to growing various CsPbX 3 microstructures with the desired halide compositions and properties for functional device applications.