Shallow halogen vacancies in halide optoelectronic materials

Halogen vacancies (V-H) are usually deep color centers (F centers) in halides and can act as major electron traps or recombination centers. The deep V-H 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 V-H, in contrast to commonly seen deep V-H 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 V-H is a shallow or deep defect in a halide material. It is found that crystal structure and chemistry of ns(2) ions both play important roles in creating shallow V-H in halides such as CH3NH3PbI3, CH3NH3SnI3, and TlBr. The key to identifying halides with shallow V-H is to find the right crystal structures and compounds that suppress cation orbital hybridization at V-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 V-H. The results of this paper provide insight and guidance to identifying halides with shallow V-H as good electronic and optoelectronic materials.