Optical Films for Solar Energy Applications

A number of solar energy conversion materials and coatings are considered stratified media. They are generally classified as graded-index media or layered media. With index coatings, two components (such as air and SiO₂ or Cr and Cr₂O₃) are created in a non-linear fashion with depth into the coating. By simple materials admixing, a coating is formed with varying optical constants (n, k). Layered media generally consist of interference films, films with thicknesses below the wavelength of light, made of alternating dissimilar media such as a dielectric and metal combination. This paper presents details of the properties of stratified coatings. Coatings that serve as antireflection films, transparent optical insulation (silica aerogel), thermal heat mirrors, or selective absorbers are also discussed. Both interference and semiconductor types of heat mirrors are evaluated. Four types of selective absorbers are also covered: dendritic optical trapping, graded composite, metalldielectric tandems, and optical interference techniques.