Graded-index Pt-Al2O3 composite solar absorbers

The solar absorption properties of thin composite films of coevaporated Pt and Al2O3, formed with a graded composition-depth profile, are compared to those of an identical film overcoated with a microscopically textured layer of SiOx. The composition grade of the metal-dielectric composite and the surface texturing of the SiOx produces a film with a refractive index slowly varying to unity at the front surface. This results in a very low reflectance of the film. The solar absorptivity of this graded refractive index surface is shown to be a high as 0.98. The production and structure of these surfaces are described as well as a general method of absorptivity enhancement using a microscopically textured surface formed by reactive ion etching.