Structural and optical properties of CuMnCoOx spinel thin films prepared by a citric acid-based sol–gel dip coating route for solar absorber applications

CuMnCoOx thin films for solar thermal absorber applications were prepared by a citric acid-based sol–gel dip coating method. The citric acid (CA) and polyethylene glycol 200 (PEG 200) were selected as a complexing agent and an esterifying agent respectively. The influence of different preparation conditions, i.e. (Cu + Mn + Co)/CA molar ratios and PEG/CA molar ratios, on film structure and optical properties were studied. Texture properties (crystallinity, grain size, surface topography) were obtained by using X-ray diffraction, field emission scanning electron microscopy and atomic force microscopy. Adhesion strength and thermal stability of the films were also tested. Absorption study indicated that pure spinel phase with a particle size of ~18 nm was a decisive factor in improving the intrinsic solar-absorbing capacity. Meanwhile, the generation of the uneven pores (pore size ~300 nm) on the film surface further lengthened the absorb path in UV–Vis spectrum. The single films displaying the best reflectance spectra and the highest solar absorptance (α > 0.93) were deposited from solutions containing molar ratio (Cu + Mn + Co)/CA/PEG = 1, which were much higher than that reported in the literature (88 %). The study showed that the intrinsic absorbing ability of the transition-metal oxide absorbers could be expanded by using a citric acid-based sol–gel route.