Switching electrochromic performance improvement enabled by highly developed mesopores and oxygen vacancy defects of Fe-doped WO ₃ films

In recent years, owing to the capability to reversibly adjust transparency, reflection, and color by the low electric field, electrochromic devices (ECDs) have received an extensive attention for their potential use in optoelectronic applications. However, considering that the performances of the ECDs, including coloration efficiency (CE, <30.0 cm ² /C) and switching speed (>10.0 s), are still low for an effective applied use, critical efforts are needed to push the development of a unique nanostructure film to improve electrochromic (EC) performances. Specifically, as the large-scale applications (e.g. refrigerators, vehicles, and airplanes) of the ECDs have been recently developed, the study for improving switching speed is urgently needed for commercialization of the devices. In this context, the present study reports a novel nanostructure film of Fe-doped WO ₃ films with highly developed mesopores and oxygen vacancy defects, fabricated using the Fe agent and the camphene-assisted sol-gel method. Fe-doped WO ₃ films with highly developed mesopores and oxygen vacancy defects show remarkable EC performances with both fast switching speed (2.8 s for the coloration speed and 0.3 s for the bleaching speed) and high CE (71.1 cm ² /C). These two aspects contribute to the synergistic effects of optimized Fe doping and camphene on the films and have outstanding values as compared to previously reported results of WO ₃ -based materials. Specifically, the fast switching speed is attributed to the shortened Li ⁺ diffusion pathway of the highly developed mesopores; and the other is the improved electrical conductivity of the highly increased oxygen vacancy defects. In addition, the high CE value is due to an efficient charge transport as the result of a more effective electroactive contact of the morphology with highly developed mesopores, resulting in a large transmittance modulation with a small intercalated charge density.