The Electronic States of Copper Oxides Photoactive Layers Prepared by Electrodeposition followed by Annealing

Abstract

The copper oxides are best performing metal oxide materials regarding solar energy conversion, and due to their abundance, scalability, ease of fabrication and subsequently being low-cost, Cu2O and CuO have attracted extensive attraction. In this research, Cu2O layers were electrodeposited on Ga:ZnO (GZO) and annealed to form directly-stacked GZO/Cu2O/CuO. The external quantum efficiency (EQE) of the devices was evaluated which demonstrated peculiar negative regions around 500 nm besides the typical 410 nm originating from the Cu2O. Reversed bias voltages were applied to investigate the changes, and along with the disappearance of negative regions, a peak around 550 nm and an absorption edge at 850 nm appeared, which increased with the increment of bias voltage. To elucidate the mechanism, the electronic states were investigated by x-ray photoelectron spectra (XPS). Species identification was also successful which showed the presence of CuO on the topmost layer and the underlying Cu2O, which was difficult to analyze by XRD due to its thinness. The schematics of the band-alignments were drawn based on the calculations from the XPS measurements. The conduction band of the Cu2O appeared elevated, which contributed to the speculated two-way flow of the charge when illuminated, and explains the appearance of the negative EQE regions. The charge transportation was successfully controlled and aligned when bias voltages were applied, which caused the disappearance of negative regions and the appearance of the absorption edge of the CuO.