Investigations on niobium tungsten oxide thin films for optical modulation

The electrochromic parameters such as high color contrast, quick switching time, excellent modulation of percentage transmittance accompanied by long cyclic durability and stability are of utmost importance for researchers in the electrochromism. Till present, tungsten trioxide (WO3) is the overwhelming candidate as an electrochromic material because of its excellent coloring efficiency, chemical stability and acceptable switching time. Despite, there are many possibilities to further improve the performance of tungsten oxide thin films based electrochromic devices. In this study, niobium (Nb) is doped into WO3 lattice to further enhance its functionality and overall performance. It is found that adding minimal concentrations of Nb (0.64%) in WO3 results in instant coloring and bleaching times (~ 11.5 and ~ 2.9 s, respectively), which outwit the switching times of pristine WO3. For Nb-doped WO3 films, considering the percentage transmission difference between bleached and colored states, the ΔT value increased 10% from 50 to 60% at 700 nm and the maximum current density value of ~ 19.4 mAcm−2 has been achieved. In addition, an increase of 0.93 cm−1 path length in optical density has been found that is higher than the values of 0.74 cm−1 for only pristine WO3 films. The corresponding current density values of the Nb-doped samples were reduced to 99.4% (16.17 mAcm−2) and 98% (4.6 mAcm−2), to achieve better stability and durability. Based on the band gap study, crystal structure and surface morphology of the Nb-doped samples, the electrochromic mechanism is discussed in details.