Applications of thin graded-index films to solar absorbers

The solar absorptance of a single ideal film on an idealized metallic substrate has been calculated as a function of its optical constants to determine the optimum parameters for selective absorber applications. High normal solar absorptances (>0.9) are possible for single uniform interference films if n < 1.5 and 0.2 < k < 0.8, values that are achievable using cermet materials. Grading the refractive index of single interference films improves the absorptance, with optimum results given by a uniformly increasing dielectric constant through the layer. With such a linear grading to air, normal solar absorptances of greater than 0.9 can be achieved for n < 1.5 and 0.1 < k < 5. Grading an absorbing material with a dielectric also improves the performance of the absorbing material. For bulk films, the optimum profile with minimum material is one where the dielectric constant increases slowly at the surface of the film.