Reactive pulsed-DC sputtered Nb-doped VO2 coatings for smart thermochromic windows with active solar control.

Thermochromic VO2 thin films have successfully been grown on SiO2-coated float glass by reactive pulsed-DC magnetron sputtering. Different Nb doping amounts were introduced in the VO2 solid solution during the film growing which resulted in films with distinct semiconducting-metal phase transition temperatures. Pure VO2 showed improved thermochromic behavior as compared with VO2 films prepared by conventional DC sputtering. The transition temperatures were linearly decreased from 59 down to 34 degrees C with the increase in Nb content. However, the luminous transmittance and the infrared modulation efficiency were markedly affected. The surface morphology of the films was examined by scanning electron microscopy (SEM) and showed a tendency for grain sized reduction due to Nb addition. Moreover, the films were found to be very dense with no columnar microstructure. Structural analyses carried out by X-ray diffractometry (XRD) revealed that Nb introduces significant amount of defects in the crystal lattice which clearly degrade the optical properties.