Influence of Nb doping on the electrochromic properties of WO3 films

The effect of Nb doping on the electrochromic properties of WO3 thin films is investigated. The systematics of different properties such as the structural phase change, the colour-bleach kinetics, colouration efficiency, reversibility, stability, ion insertion/extraction capacity and change in charge states from W+6 to W+5 are found to depend on the Nb doping concentration. The x-ray diffraction results reveal the solid solution WNb2O8 phase with mixed WO3 - Nb2O5 (monoclinic epsilon-WO3 and orthorhombic Nb2O5) phases at lower Nb concentration (2 at.%) and mixed WO3 - Nb2O5 phases at higher Nb concentrations (6 and 10 at.%). The scanning electron microscopy results show a drastic change in surface morphology as the doping levels increase. The surface morphology changes from the fibrous-reticulate present in undoped films to a rough and granular structure for doped films. The cycle stability, charge storage capacity and reversibility of the films are improved upon doping; however, colouration efficiency decreases for doped films and is minimum for 6 at.% Nb doped film. The x-ray photoelectron spectroscopy measurements show that the valance state of Nb is not changed after colouration, indicating no direct contribution of Nb in the colouration process. These results are discussed in terms of the effects of doping induced compositional inhomogeneity and disorder structure on the film properties.