Optical characterization of industrially sputtered nickel-nickel oxide solar selective surface

Tandem absorbers are often used in the design of solar absorbers for photo thermal conversion. They consist of a thin coating, selectively absorbing i n the wavelength range of the solar spectrum, on a metal substrate. The opt ical performance of a tandem absorber depends on the optical constants and thickness of the absorbing coating and also on the reflectivity of the unde rlying metal. A very high solar absorptance is achieved when the coating ha s a non-uniform composition in the sense that the refractive index is highe st closest to the metal substrate and then gradually decreases towards the front surface. This type of composition suppresses coating interference and gives a low front surface reflection if the refractive index at the front surface is low. We report on optical analysis of a; Solar absorber with a g raded index coating of sputtered nickel-nickel oxide deposited on aluminium . The optical constants have been determined from reflectance, transmittanc e and ellipsometry data by fitting the data to a two-layer model of the coa ting. The optical constants of the two layers can be regarded as effective optical constants for the lower and upper part of the graded index coating respectively. It is found that the effective refractive index of the top la yer is somewhat tower than for the base layer. The extinction coefficient i s higher in the lower part of the coating. Both effective refractive index and extinction coefficient of the base layer increase monotonically with in creasing wavelength as for metallic materials. (C) 2000 Elsevier Science Lt d. All rights reserved.