Critical Interface States Controlling Rectification of Ultrathin NiO–ZnO p–n Heterojunctions

Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/oxide’ heterojunctions are of interest for UV optical sensing, gas sensing, photocatalysis, charge confinement layers, piezoelectric nanogenerators, and flash memory devices. These heterojunctions can also be used as current rectifiers and potentially as recombination layers in tandem photovoltaic stacks by making the two oxide layers ultrathin. In the ultrathin geometry, understanding and control of interface electronic structure and chemical reactions at the oxide/oxide’ interface are critical to functionality, as oxygen atoms are shared at the interface of the dissimilar materials. In the studies presented here the extent of chemical reactions and interface band bending is monitored using X-ray and ultraviolet photoelectron spectroscopies. Interface reactivity is controlled by varying the near surface composition of nickel oxide, nickel hydroxide, and ...