Electrochromic properties of N-doped tungsten oxide thin films prepared by reactive DC-pulsed sputtering

In depositing nitrogen doped tungsten oxide thin films by using reactive DC-pulsed magnetron sputtering process, nitrous oxide gas (N2O) was employed instead of nitrogen (N2) as the nitrogen dopant source. The nitrogen doping effect on the structural and electrochromic properties of WO3 thin films was investigated. X-ray diffraction (XRD) results show that the films are amorphous. Morphological images reveal that the films are characterized by a hybrid structure comprising nanoparticles embedded in amorphous matrix and open channels between the agglomerated nanoparticles, which promotes rapid charge transport through the film. Increasing the nitrogen doping concentration is found to decrease the nanoparticle size and the band gap energy. The electrochromic properties were studied using cyclic voltammetric and spectroeletrochemical techniques. The film with N content of ~ 5 at.% exhibits higher optical modulation and coloration efficiency as well as faster ion transport kinetics. The results reveal that electrochromic and lithium ion transport properties are moderately enhanced relative to the un-doped tungsten oxide thin films by appropriate content of dopant, due to the effects of nitrogen doping.