Metal oxide heterojunctions using a printable nickel oxide ink†
Wide band gap metal oxides are ideally suited for inorganic optoelectronic devices. While zinc oxide is a commonly used n-type material, there is still a lot of ongoing work for finding suitable p-type oxides. In this work, we describe a two-step route to formulate a stable and conducting p-type nickel oxide (NiO) nanofluid. NiO nanoparticles were synthesised using a bottom-up wet chemical approach and dispersed in ethylene glycol to form a nanofluid. The viscosity and surface tension of the nanofluid were optimised for printing. The printing was done using an extrusion-based direct writer. The NiO nanofluid was printed onto an aluminum-doped zinc oxide layer and annealed at different temperatures. Electrical characterisation of the junction was used to extract the junction barrier for carriers across the interface. The resulting heterojunction was found to exhibit rectifying behaviour, with the highest rectification ratio occurring at an annealing temperature of 250 °C. This annealing temperature also resulted in the lowest junction barrier height, and was in excellent agreement with theoretically predicted values. The development of a printed p-type ink will help in the realisation of oxide-based printed electronic devices.