Copper oxide coatings deposited by reactive radio-frequency sputtering for solar absorber applications

Abstract

Submicron copper oxide coatings were deposited on glass and copper substrates using reactive radio-frequency sputtering of a copper target in argon-oxygen atmospheres. Morphological, structural, and optical properties of the deposited films were examined using X-ray reflectometry and diffraction, optical transmission measurements, Fourier transform infrared spectroscopy, transmission electron microscopy, atomic force microscopy, electrical resistivity, and thermal emissivity measurements. The obtained coatings consist of a major phase of cupric oxide (Cu2O) at 10% oxygen, which transforms into monoclinic CuO through an intermediary tetragonal Cu4O3 phase for greater oxygen flow rates. The morphology of the Cu2O films is interpreted by an original fish-scale-like growth mode based on the nucleation of single crystallites at cluster boundaries of the underlying layer, which promotes the formation of aligned grain strips and yields coalescing clusters of a height similar to the crystallite size. The deposited films behave as p-type semiconductors with a direct bandgap of 1.9 eV and an indirect bandgap of 1.15 eV for Cu2O and CuO, respectively. The oxygen flow rate-dependent crystallite size exhibits a maximum at 30% oxygen for which the CuO film grows in column-shaped structures of continuous sub-100-nm-wide rods with enhanced surface roughness, solar absorptivities of 0.93 and 0.83, and thermal emissivities of 0.07 and 0.17 in the temperature ranges 50–150 °C and 150–250 °C, respectively.