The Halogen Exchange Equilibrium in Halide Perovskites—Halide Diffusion, Spontaneous Electronic Doping, and Implications Towards Stability

A renewed interest in lead-halide perovskites and lead-free halide double perovskites, fueled by their successful incorporation into various optoelectronic devices and technologies, has come with a recognition of the characteristic instabilities of this class of materials. Underlying many degradation mechanisms and operational instabilities are ionic point defects, or localized intrinsic (e.g., vacancies) or extrinsic (e.g., dopants) disorder in the crystal. For instance, current-voltage hysteresis and space-charge formation at perovskite interfaces are attributed to mobile ions resulting from high equilibrium point defect concentrations. A broad and critical evaluation of the mixed ionic-electronic conductivity and defect chemistry of the halide perovskites is thus warranted, along with other efforts ranging from bulk to device-level stability. Here, we draw analogy to the established defect chemistry of the oxide perovskites and characterize the halogen exchange equilibrium in single crystals of two halide double perovskites.<br/><br/>We observe reversible halogen exchange, which is a defect equilibrium involving halide vacancies, free electrons, and the molecular halogen, in the bromide and iodide perovskites, notably occurring at or near room temperature. Single-crystal electronic conductivity measurements in the diffusion-limited regime allow for the determination of the diffusivity of halide vacancies and the activation energy associated with this ionic conductivity. Starting from the pristine state, halogen off-gassing is spontaneous and the equilibrium drives the formation of halide vacancies accompanied by n-type doping via charge compensation. We discuss the implications of this spontaneous self-doping across the family of perovskites and propose approaches to stabilizing the defect chemistry and electronic structure. Finally, we introduce complementary efforts to quantify and correlate structural defects using a suite of electron and X-ray scattering methods.