(543d) Dominant Halogen Defect Chemistry and Spontaneous Self-Doping in Halide-Perovskite Semiconductors

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.

The critical role of the halogen vacancy point defect in halide-perovskite semiconductors is discussed, particularly through recent experimental high-pressure studies elucidating precise energy levels of the predominant intrinsic defect. Further, we draw analogy to the established defect chemistry of the oxide perovskites and characterize the halide-halogen exchange equilibrium in single crystals of two halide double perovskites.

We observe and characterize reversible halogen exchange, which is a defect equilibrium involving halogen 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 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 halogen vacancies and compensating charge carriers, resulting in n-type doping. We present the first thermodynamic quantities on this equilibrium and rationalize the spontaneity of the self-doping off-gassing reaction. The implications of this self-doping across the broad family of technologically relevant perovskites—and proposed approaches to stabilizing the defect chemistry and electronic structure—will be discussed. Finally, we introduce complementary efforts to quantify static and dynamic crystallographic disorder using X-ray scattering methods.