Realizing an Optical Micro‐Cavity in a CuCo2O4‐W‐CuCo2O4 Thin Film Stack for Spectrally Selective Solar Absorbers

Tapping the potential of solar thermal energy requires spectrally selective solar absorber coatings, which lead to high photothermal efficiency. Micro‐cavities are known to dramatically enhance or suppress light absorption and emission in a directional manner and are used in nanophotonics, photodetectors, lasing, etc. Here, a dielectric‐metal‐dielectric multilayer stack with an optical micro‐cavity of a 12 nm tungsten layer formed between two dielectric layers of transition metal oxide CuCo2O4 (CCO) is designed. The CCO‐W‐CCO thin film multilayers deposited on stainless steel substrate by DC‐radio frequency magnetron sputtering achieve a 90% solar absorptance for the entire solar spectrum and a thermal emittance of 19% for the infrared spectrum. Optimization of thicknesses of individual layers is achieved by numerical simulations done in COMSOL Multiphysics, thereby reproducing the optical properties of each layer. The simulation results satisfactorily reproduce the experimental reflectance and demonstrate maximum power dissipation in the metallic layer in visible as well as NIR regions. Nevertheless, CCO material characterizations reveal that the enhanced absorption over the entire solar spectrum could be attributed to the underlying spinel crystal structure and the nanostructure film morphology.