Unraveling the critical roles of surface chemical states on anion exchange of halide perovskites

Anion exchange reactions offer a unique approach to modulate the optical properties of perovskite nanocrystals (NCs) through post-synthesis processes. However, the relationship between the surface ligand dynamic equilibrium and the rational regulation of the anion-exchange process is seldomly studied. In this study, we investigate the kinetics of anion exchange between CsPbBr3 NCs and potassium iodide (KI). We find that the exchange process can be either accelerated or decelerated by using different accompanying ligands during the posttreatment process, and also find the anomaly that an increase in temperature slows down the anion exchange rate. We confirm that accompanying ligands affect anion exchange by disrupting the dynamic equilibrium of native surface ligands or by closing the shuttling channel due to steric hindrance, while the abnormal temperature effect is not dominated by the diffusion process, but by the surface adsorption equilibria. Furthermore, the slow anion exchange process in the case of post-treatment with sodium dodecylbenzene sulfonate (SDSA) would result in a homogeneous halide distribution as well as a large steric hindrance for halide ions, which enable a suppressed ion migration in the device under the stress of electrical field.