Shallow halogen vacancies in halide optoelectronic materials

Halogen vacancies (${\mathit{V}}_{\mathrm{H}}$) are usually deep color centers (F centers) in halides and can act as major electron traps or recombination centers. The deep ${\mathit{V}}_{\mathrm{H}}$ contributes to the typically poor carrier transport properties in halides. However, several halides have recently emerged as excellent optoelectronic materials, e.g., $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ and TlBr. Both $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ and TlBr have been found to have shallow ${\mathit{V}}_{\mathrm{H}}$, in contrast to commonly seen deep ${\mathit{V}}_{\mathrm{H}}$ in halides. In this paper, several halide optoelectronic materials, i.e., $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$ (photovoltaic materials), TlBr, and $\mathrm{CsPbB}{\mathrm{r}}_{3}$ (gamma-ray detection materials) are studied to understand the material chemistry and structure that determine whether ${\mathit{V}}_{\mathrm{H}}$ is a shallow or deep defect in a halide material. It is found that crystal structure and chemistry of $n{s}^{2}$ ions both play important roles in creating shallow ${\mathit{V}}_{\mathrm{H}}$ in halides such as $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$, and TlBr. The key to identifying halides with shallow ${\mathit{V}}_{\mathrm{H}}$ is to find the right crystal structures and compounds that suppress cation orbital hybridization at ${\mathit{V}}_{\mathrm{H}}$, such as those with large cation-cation distances and low anion coordination numbers and those with crystal symmetry that prevents strong hybridization of cation dangling bond orbitals at ${\mathit{V}}_{\mathrm{H}}$. The results of this paper provide insight and guidance to identifying halides with shallow ${\mathit{V}}_{\mathrm{H}}$ as good electronic and optoelectronic materials.