Shallow halogen vacancies in halide optoelectronic materials

Halogen vacancies (VH) are usually deep color centers (F centers) in halides and can act as major electron traps or recombination centers. The deep VH contributes to the typically poor carrier transport properties in halides. However, several halides have recently emerged as excellent optoelectronic materials, e.g., CH₃NH₃PbI₃ and TlBr. Both CH₃NH₃PbI₃ and TlBr have been found to have shallow VH, in contrast to commonly seen deep VH in halides. In this paper, several halide optoelectronic materials, i.e., CH₃NH₃PbI₃, CH₃NH₃SnI₃ (photovoltaic materials), TlBr, and CsPbBr₃ (gamma-ray detection materials) are studied to understand the material chemistry and structure that determine whether VH is a shallow or deep defect in a halide material. It is found that crystal structure and chemistry of ns² ions both play important roles in creating shallow VH in halides such as CH₃NH₃PbI₃, CH₃NH₃SnI₃, and TlBr. The key to identifying halides with shallow VH is to find the right crystal structures and compounds that suppress cation orbital hybridization at VH, 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 VH. The results of this paper provide insight and guidance to identifying halides with shallow VH as good electronic and optoelectronic materials.