Optical constants of sputtered Ni/NiO solar absorber film—depth-profiled characterization

An optical analysis of a solar absorber with a graded index coating of nickel–nickel oxide deposited in a continuous process for reactive DC magnetron sputtering is reported. The optical constants were determined by a set of 14 samples deposited under the same sputtering conditions as in the continuous process representing the composition at 14 different depths in the continuously graded index coating solar absorber. The optical constants of the 14 layers were derived from optical measurements of reflectance and transmittance in the wavelength range 300–2500 nm. The optical constants were used to construct a model for the graded refractive index in the solar-absorbing coating. It was found that the graded index was not linear in depth profile and hence could be modelled with four layers: two metallic layers in the base closest to the substrate, a dielectric layer at the top of the coating and one intermediate layer representing the change from metallic to dielectric properties in the middle part of the coating.