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A new generation of smart solar thermal collectors is based on thermochromic thin film technology. Thermochromic absorber coatings change their optical properties with temperature, particularly in the infrared range. In order to prevent overheating, the collector selectivity is deliberately hindered, through an increase of the thermal emittance above a critical temperature. The transition temperature of pure thermochromic VO2 (68°C) needs to be adjusted for solar thermal applications and to do that Si doping is attempted. Thus, pure and Si doped vanadium dioxide thin films are deposited by reactive magnetron sputtering and the films are characterized by four point probe and in-situ X-ray and UV photoelectron spectroscopic measurements. While a modest increase in the transition temperature with doping is reached (up to a critical Si concentration when the transition is lost), the insulating character of the films is also enhanced (decreased density of states at the Fermi level).