The effect of Cu content on structural, optical and photo-electrical properties of sol-gel derived Cu Co3-O4 thin films

Abstract

Abstract CuxCo3-xO4 thin films with different x values ranging from 0 to 1.25 were successfully deposited onto soda-lime glass substrates by sol-gel technique. Structural analyses revealed that mono-phase cubic spinel structure forms for the low concentrations of Cu, while it is not case when the Cu content exceeds the CuCo2O4 stoichiometry, which led to formation of a second phase of copper oxide compound on the surface region of the fabricated thin film, evidenced by XPS depth profiling analysis. The oxidation state of the fabricated films was also studied by XPS measurements, which revealed the formation of mixed spinel structure in the structure for x > 0.25 values. Addition of Cu led to dramatic changes in both electrical and optical properties of the films. Optical analysis showed that with incorporation of Cu not only the low energy band gap decreases but also the Urbach energy changes. From the electrical measurements it was deduced that with addition of Cu (x = 1) there was a two order of magnitude reduction in room temperature resistivity compared to Co3O4 (x = 0) film, which was attributed to the increase of the charge carriers as a results of the increase in the density of localized states in the band gap. From the photoconductivity measurements it was observed that the prepared thin films exhibited good photo-response under different illumination intensities. As a device application of the prepared films, hydrothermally grown ZnO nanowires were completely decorated with a thin layer of Co3O4 to construct a nanowire based p-n heterojunction for the first time as a low cost all-oxide photovoltaic cell, the rectifying behavior of which was revealed by current-voltage characteristics.