Hybrid density functional calculations of the defect properties of ZnO:Rh and ZnO:Ir

• Rh/Ir incorporation into ZnO at low doping induces local distortions in the lattice.
• Localized levels appear in the gap of ZnO:Rh/ZnO:Ir near band edges.
• Hole trapping is found in ZnO:Rh/ZnO:Ir and in the ZnRh2O4/ZnIr2O4 spinels.
• Hole diffusion barriers in the spinels are very small.

We report density functional calculations of the atomic and electronic structure of the spinel phases ZnRh2O4 and ZnIr2O4 as well as crystalline ZnO lightly doped (1 at.%) with Rh and Ir ions using the B3LYP hybrid functional. Calculations for the spinels show band gaps (~ 3 eV) and lattice parameters (~ 2% difference) in reasonable agreement with experimental data. Incorporation of the transition metals into ZnO induces local distortions in the lattice and the appearance of metal d levels in the low gap region and near the conduction band minimum, with a d–d splitting of about 2 eV, which reduces the effective transparency of the material. Addition of a hole to the simulation cell of both spinels and doped ZnO leads to charge localization in the neighbourhood of Rh/Ir accompanied by local lattice deformations to form a small polaron which may lead to low hole mobility. We calculate polaron diffusion barriers in the spinels and obtain values around 0.02–0.03 eV. These very low barrier energies suggest that at high Rh/Ir concentrations polaron hopping is not going to be detected at room temperature.