Structural, optical, and electrochromic properties of Nb-doped WO3 thin films

The influence of Nb doping on the structural, optical, and EC properties of WO3 thin films was evaluated with the goal of enhancing the long-life electrochromic stability of WO3 films. Pure and Nb-WO3 thin films, with doping ratios ranging from 2.0 % to 3.9 %, were deposited by RF magnetron sputtering. The thickness of the films was controlled between 130 and 145 nm. Refractive indices were found to be in the range of 2.05-1.99 at 630 nm from the spectral ellipsometer measurements and modelling. Optical band gap energies were calculated as 3.29 eV, 3.24 eV, 3.25 eV and 3.16 eV, respectively. Electrochromic characterization revealed that the 3.9 % NbWO3 film exhibited the maximum optical modulation of 54.9 % at 630 nm. However, in long-term stability testing, it was evident that the sample with the lowest Nb doping content (2.0 %) showed superior stability. Specifically, the lowest doped sample endured up to 1600 cycles, whereas the undoped and higher-doped samples degraded after approximately 150 cycles.