Optimized low-temperature fabrication of WO₃ films for electrochromic devices

We investigated the effect of process conditions on the electrochromic (EC) properties of tungsten trioxide (WO₃) films. When WO₃ films deposited using a sol-gel method were thermally treated in air at 150 °C, the majority of tungsten species in the films became W⁶⁺, which is important for the realization of an optically transparent bleached state. On the other hand, annealing in a vacuum required only 60 °C to induce a similar level of W⁶⁺ in the WO₃ films. However, a cracked film morphology was observed at higher temperatures, regardless of whether the films were annealed in air or vacuum. Using the WO₃ films prepared under various conditions, EC devices (ECDs) were fabricated to evaluate EC properties. We concluded that the optimal annealing conditions for WO₃ films for ECDs are 60 °C in vacuum, at which the highest coloration efficiency, largest transmittance difference, and fastest bleaching/coloration dynamics were obtained. These mild fabrication conditions at a low temperature (60 °C) provide the opportunity to utilize flexible electrodes on plastic. Therefore, we successfully demonstrated a flexible WO₃-based ECD.