A facile process to prepare copper oxide thin films as solar selective absorbers

Copper oxide thin films as solar selective absorbers were conveniently prepared by one-step chemical conversion method. X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis-NIR spectra and Fourier transform infrared (FTIR) spectra were employed to characterize the composition, structure and optical properties of thin films. The results indicated that the composition, structure and optical properties of thin films were greatly influenced by reaction temperature, time and concentration of NaOH. When reaction temperature was fixed at 40 °C, the as-prepared films consist of pure cubic Cu ₂ O. The surface morphology of thin films was changed from square-like structure (reaction time ≤ 25 min) to porous belt-like structure (reaction time ≥ 30 min) with the elongation of reaction time. While for thin films prepared at 60 °C and 80 °C, single Cu ₂ O was observed after 5 min reaction. When reaction time is longer than 5 min, CuO appears and the content of CuO is increasing with the elongation of reaction time. With the increase of reaction temperature, the belt-like structure was easily formed for 60 °C/10 min and 80 °C/5 min. Decreasing concentration of NaOH also could result in the formation of CuO and porous belt-like structure. Simultaneously, the film thickness is increasing with the increase of reaction time, temperature and concentration. Films containing CuO with belt-like structure exhibited high absorptance (>0.9), and the emissivity of films increased with elongation of reaction time. Combination of the composition, structure and optical properties, it can be deduced that the porous belt-like structure like as a light trap can greatly enhance absorbance (α), while the composition, thickness and roughness of thin films can greatly influence the emissivity (ε). The highest photo-thermal conversion efficiency was up to 0.86 (α/ε = 0.94/0.08) for thin films prepared at 80 °C/5 min, which proved that the CuO _x thin films can be served as high performance solar selective absorbers.