Infrared Optical Properties of Doped and Pure Thermochromic Coatings for Solar Thermal Absorbers

A new generation of solar collectors is based on thermochromic thin film technology, where the remarkable optical changes of vanadium dioxide, through its reversible semiconductor-to-metal phase transition, are exploited. This study reports on the infrared optical properties of VO2 based coatings deposited on Al substrates by reactive magnetron sputtering. Fourier transform infrared (FTIR) spectroscopy is used to determine the emittance of a VO2 coated sample before (ε = ~6%) and after (ε = ~33%) the phase transition. The angular dependence of the thermal emittance for such coating is investigated. It is found that the emittance increases with increasing the angle from normal incidence. In literature it is widely reported that the transition temperature of vanadium dioxide can be tuned by doping with different elements. An increase in the transition temperature is desired for solar thermal applications and it is reached with the addition of Ge into the VO2 matrix. Here we show that through Ge doping, the emittance switch, especially the emittance in the high temperature state, can be optimized as well. An increase of ~10% in emittance is recorded.