Different attempts have been made to produce anti-reflection (AR) layers for solar applications. The most promising solutions are based on the principle of mixing bulk material with air on a subwave-length scale in order to obtain the very low effective refractive indices suitable as AR coatings for glazing. Possibilities for achieving this are given by porous media and subwavelength surface-relief gratings. Subwavelength grating structures, which were embossed in organically modified sol-gel materials or acrylic materials, and porous sol-gel coatings were investigated and compared theoretically and experimentally by the Fraunhofer Institutes. It is shown, theoretically and experimentally, that an increase of solar transmittance of ≈3% per glass surface can be achieved with a porous sol-gel coating.
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