Single-layer, anti-reflective thin films of porous MgF2 for solar thermal applications

Abstract

Single-layer, anti-reflective coatings prepared via the sol–gel method can be scaled to large-area applications (e.g. solar collectors). In the specific case of evacuated tube solar thermal collectors, there are often three glass–air or glass–vacuum interfaces which can cause reflection. Thus, in the absence of any anti-reflective technology (i.e. most commercial products), a steep 15% or 22% reflective loss (e.g. transmission ~0.92N, where N = 2 or 3) occurs in evacuated tube solar collectors. Porous MgF2-based materials have been shown to provide low refractive index as a single layer, but they have not yet been applied to solar thermal collector technology—which requires elevated temperature stability and spectrum transmission. Importantly, these should be low cost at the m2 scale. Further, since solar technologies are considered ‘green’, the materials used in their manufacturing should be carefully considered. In this study, we have employed trifluoroacetic acid (instead of hydrofluoric acid) as the fluorine source to produce single-layer porous MgF2 coatings. These coatings were designed and modeled with OpenFilters, fabricated with dip-coating, and characterized with ellipsometetry, x-ray diffraction, Raman spectroscopy, and focused ion beam/transmission electron microscopy to validate the OpenFilters model. For high transmission substrates, the dip coated films produced in this study can reach a solar-weighted reflectance of 2.3%. Additionally, to test if these films are suitable for solar thermal collectors, durability tests were conducted, which revealed acceptable heat resistance. However, without a water-resistant outer layer, the films did not pass an ASTM D870-09 water immersion test. Overall, this study presents a low-cost approach to developing anti-reflective coatings which are suitable for large-area solar thermal applications.