Influence of crystallinity and copper dopant concentration in Cu:NiO on their heterojunction characteristics with Al:ZnO

We report the diode characteristics of solution-processed p-n heterojunction formed by NiO and copper-doped NiO with an aluminum-doped ZnO layer (AZO). The pure and copper-doped NiO thin films annealed at 300 °C and 450 °C are amorphous and crystalline, respectively. Their optical bandgap increased with Cu substitution and annealing temperature. It caused a substantial difference in the rectifying behavior of the Schottky junction due to different current transport mechanisms attributed to lattice mismatch, barrier tunneling, radiative recombination, and variations in the interface composition. The heterojunction of Cu:NiO film (annealed at 300 °C) with AZO exhibited a high rectification due to the direct tunneling of electrons and holes without radiative recombination. However, the heterojunction of the p-type film annealed at 450 °C exhibited very weak rectification ascribed to accelerated radiative recombination of electrons and holes in the depletion region.