Unlocking p-type conductivity in Cu:ZnO thin films: a first-principles investigation on the role of Cu 3d states and self-compensation.

ZnO is a promising wide bandgap semiconductor for photovoltaics, light emitting diodes (LEDs) and sensors, but its intrinsic n-type conductivity limits its applicability where p-type behaviour is essential. Copper (Cu) is a leading acceptor dopant candidate, yet the microscopic origin of its electronic effects remains contested. We performed density functional theory (DFT) calculations on Cu-doped ZnO thin film models with dopant concentrations of 0, 2, and 4 at%, benchmarked against our previously published experimental data. We demonstrated that Cu substituting for Zn (CuZn) introduces shallow acceptor states near the valence band maximum, while Cu 3d orbital hybridization with O 2p states drives band gap narrowing. Self-compensation by an oxygen vacancy donor limits the net hole concentration at concentrations above 2 at%. The computed Fermi energy decreases by 0.844 eV at 2 at% Cu, providing direct evidence of p-type character onset. These results provide a unified mechanistic picture of Cu-induced p-type conductivity in ZnO thin films and establish doping windows for device-relevant carrier control.