Optical performance of absorber-reflector combinations for photothermal solar energy conversion

For a number of absorber-reflector combinations the reflectance spectrum as well as the figure of merit a/e has been computed numerically in order to find an optimum set of parameters. Values of a/e in excess of 20 result for an absorber consisting of 0.5 μm Ge on a silver reflector, overlaid by 2.0 μm Si carrying a four-layer antirereflection sequence in which the refractive index varies from 2.79 to 1.43. To maximize the optical acceptance angle the antireflection coatings must vary the refractive index in a graded profile from the value of index of silicon to the one of air.