(327a) Reversible Halogen Off-Gassing from Halide Perovskites: Connecting Point Defect Chemistry, Electronic Self-Doping, and Structural Disorder

Renewed interest in lead-halide perovskites and lead-free halide double perovskites, fueled by their successful incorporation into optoelectronic devices, has come with a recognition of the characteristic instabilities of this class of materials. Underlying 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 and 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 modeling efforts to identify stabilizing modifications to the crystal. 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.

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 the halogen exchange equilibrium. Starting from the pristine state, halogen off-gassing is spontaneous, and the equilibrium drives the formation of halide vacancies and compensating charge carriers, resulting in n-type doping. 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 crystallographic defects and study local order using electron and X-ray scattering methods. We observe a bulk lattice response under various partial pressures of halogen near room temperature, validating the halogen exchange mechanism, and further correlate local structure and disorder with charge carrier transport between 10 K and room temperature.