High electric insulation and hard protective waterproof coating on metal foil for flexible solar cell and smart paper
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High electric insulation and hard protective waterproof coating on metal foil for flexible solar cell and smart paper Masataka Murahara1,2,3 and, Yuji Sato2, and Toshio Ohkawara3 1 Professor Emeritus of Tokai University, Hiratsuka, Kanagawa, Japan 2 Innovative Research Initiatives, Tokyo Institute of Technology, Tokyo, Japan 3 M Hikari & Energy Laboratory Co., Ltd., Kamakura, Kanagawa, Japan
ABSTRACT An electric insulation resistant coating has been developed by using photo-oxidized silicone oil. The silicone oil was exposed to ultraviolet light in the air; which transformed the organic silicone oil into an inorganic amorphous glass film. Conventional large-area solar panels and smart papers are made of glass or plastic; in addition to being well insulated, waterproof, transparent, and hard, the panels are desirable to be light in weight and flexible. Here, metal foil is dipped into dimethylsiloxane silicone oil, and the silicone oil on the foil surface is irradiated with Xe2 excimer lamplight. Oxygen adsorbed on the sample surface is photo-excited by the irradiation to produce active oxygen. This active oxygen reacts with the silicone oil photo-excited and forms inorganic glass. With the process of the photo-oxidation, the silicone oil is vitrified, being inorganic. INTRODUCTION Silicon dioxide (SiO2) coatings are widely used as protective coatings for optical components and solar cell. Such optical thin coatings must be transparent, hard, and resistant to environmental elements. One method of forming a SiO2 coating is Plasma Vapor Deposition (PVD), in which SiO2 is deposited onto a substrate heated to around 400 degree C by means of vapor deposition, high-frequency ion plating, or sputtering[1]. Although this produces a hard coating, its refractive index depends on the oxygen concentration and is reduced with increasing oxygen partial pressure. The reason for this is that the particles to be vapor-deposited collide with the gas molecules and are scattered more frequently at higher oxygen partial pressure, which causes them to lose energy upon deposition to create a porous structure. Meanwhile, low-temperature plasma enhanced Chemical Vapor Deposition (CVD) produces a SiOx coating with high density, but it is soft[2-4]. To increase the hardness, the coated sample can be heated to 800-1000 degree C, which raises the density and creates a coating of oxygen-rich SiO2. In general, therefore, the substrate had to be heated to improve the transparency and hardness of the coating. However, many applications are not amenable to high temperature treatment, including plastics which are deformed by heat, metal mirrors that undergo heat oxidation, and coated films which can become heat distorted. In an effort to create transparent coatings at room temperature, Hozumi et al. irradiated tetraethylorthosilicate (TEOS) with a Xe2 excimer lamp to form an ultra-thin SiO2 coating on poly methyl methacrylate (PMMA) at low temperature[5]. Yet, the coating had a low hardness. Meanwhile, coatings formed by chemical reactions alone, with
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