Halide Anion-Controlled Transition from Charge Transfer to Energy Transfer in Type-II 2D Perovskite Heterostructures.

Interlayer carrier relaxation in type-II heterostructures is typically governed by charge transfer (CT) due to the energetically favorable spatial separation of electrons and holes across the interface. Here, we reveal an unusual transition from CT-dominated to energy transfer (ET)-dominated relaxation in type-II heterostructures of two-dimensional (2D) organic-inorganic hybrid perovskites enabled by halide anion substitution. In heterostructures composed of identical halide anions, CT dominates the carrier relaxation process. Remarkably, substituting I- with Br- on one side of the heterostructure, such as BA2PbBr4/BA2PbI4, switches the dominant relaxation pathway from CT to ET. Density functional theory calculations further demonstrate that halide substitution reconstructs the interfacial electronic structure which suppresses interlayer charge separation, thereby favoring long-range dipole-dipole coupling over CT. This work establishes halide anion substitution as an effective strategy for tailoring interfacial excitonic dynamics in 2D perovskite heterostructures and provides new insights for the design of perovskite-based excitonic and optoelectronic devices.