POSS® Coatings as Replacements for Solar Cell Cover Glasses

Presently, solar cells are covered with Ce-doped microsheet cover glasses that are attached with Dow Corning DC 93500 silicone adhesive. This general approach has been used from the beginning of space exploration, however, it is expensive and time consuming. Furthermore, as the voltage of solar arrays increases, significant arcing has occurred in solar arrays, leading to loss of satellite power. This problem could be ameliorated if the cover glass extended over the edges of the cell, but this would impact packing density. An alternative idea that might solve these issues and be less expensive and more protective is to develop a coating that could be applied over the entire array. Such a coating must be resistant to atomic oxygen for low earth orbits below about 700 km, it must be resistant to ultraviolet radiation for all earth and near-sun orbits and it must withstand the damaging effects of space radiation. Coating flexibility would be an additional advantage. We have been exploring the use of newly discovered polyoligomericsilsesquioxane (POSSreg) materials with metallic additives for these applications. This technology has several significant advantages: the glass-like composition of POSSreg provides excellent resistance to radiation and VUV and the POSS nano-building blocks can be incorporated into all known plastics using conventional polymerization or compounding techniques that can lead to tailored optically transparent materials with entirely new performance levels. We will report on the results of POSS coatings containing various additives (e.g. organic and metallic). Thick samples (150 mum) are being applied to various substrates and have been exposed to 2 MeV protons up to 10 ¹⁵ P⁺/cm² and UV/VUV irradiation up to 1000 hrs. The 2 MeV protons are absorbed within about 85 mum depth with ~2 mum straggle so the damage is contained entirely within the layer. Results of these tests with several POSSreg matrices will be presente- d