Point defects and transport mechanisms in transparent conducting oxides of intermediate conductivity

The layered delafossite structure p-type transparent conducting oxides (TCOs) and the mayenite cage-structure n-type transparent conducting oxides represent enabling materials for novel technological applications. In the present work, isovalent replacement of the delafossite B-site cation (i.e., B=Sc and Y for Al in CuBO₂) and isovalent substitution of the mayenite Ca-cations by Mg in C₁₂A₇ (12CaO•7Al ₂O₃) were undertaken to probe the conduction mechanisms and defect structures of these novel materials. Both classes of materials exhibit small polaron conduction with comparable activation energies and conductivities. In the delafossites, increasing B-cation radius increases the hopping energy without changing the pre-exponential factors. An upper limit for mobility is estimated at ∼1 cm² V^-1 s^-1 for these materials. In terms of carrier generation mechanisms, there is a changeover from aluminum anti-site/oxygen interstitial associates in CuAlO ₂ to oxygen interstitials in CuScO₂ and CuYO₂. In Mg-doped mayenite, substitution produces no change in activation energy but a precipitous drop in the pre-exponential factor. This behavior is linked to magnesium ions blocking critical conduction paths in the mayenite structure.