Inverse design approach to hole doping in ternary oxides: Enhancingp-type conductivity in cobalt oxide spinels

Holes can be readily doped into small-gap semiconductors such as Si or GaAs, but corresponding p-type doping in wide-gap insulators, while maintaining transparency, has proven difficult. Here, by utilizing design principles distilled from theory with systematic measurements in the prototype A₂BO₄ spinel Co₂ZnO₄, we formulate and test practical design rules for effective hole doping. Using these, we demonstrate a 20-fold increase in the hole density in Co₂ZnO₄ due to extrinsic (Mg) doping and, ultimately, a factor of 10⁴ increase for the inverse spinel Co₂NiO₄, the x $=$ 1 end point of Ni-doped Co₂Zn_1-xNiₓO₄.