The effect of the TiO2 interface layer on the electrochromic properties of WO3-based devices

Abstract

Herein, using DC magnetron sputtering technology, smooth and dense WO₃ films and WO₃/TiO₂ composite films were successfully prepared under the optimized preparation conditions. Subsequently, electrochromic devices (EC) and photoelectrochemical devices (PEC) were fabricated with a sputtering deposition time of 30 min for the WO₃ film and different deposition times for the TiO₂ interface layer. The TiO₂ interface layers deposited with a sputtering time of 2–3 min were proven to improve the electrochromic properties of the devices. Specifically, compared with the WO₃-based EC devices, the WO₃/TiO₂-based EC devices exhibited a higher transmittance in the bleached sate and much lower transmittance in the coloured state, resulting in a wider transmission modulation range. For instance, the modulation range of the WO₃/TiO₂-based EC devices reached 73.9% at 600 nm, which is larger than that of the WO₃-based EC devices (62.5%). Optical measurements revealed that the TiO₂ interface layer reduced the amount of undesirable localized states by protecting WO₃, while increasing the amount of Li⁺ ions embedded in WO₃ for device colouration. This was further demonstrated by cyclic voltammetry measurements. The optimized TiO₂ interface layer significantly increased the inserted Li⁺ ion density and extracted Li⁺ ion density. Interestingly, the difference between the inserted and extracted charge density was obviously reduced after the introduction of the TiO₂ interface layer. This indicated that the WO₃/TiO₂-based EC devices possessed good cycling stability, which is due to the fact that the surface of WO₃ was passivated by TiO₂, and thus its surface state was not subject to corrosion by the electrolyte and the formation of irreversible interface states was suppressed. Additionally, PEC devices were successfully fabricated based on WO₃/TiO₂ films to investigate their electrochromic characteristics under a small self-driving power. Consequently, driven by the integrated photovoltaic film, the PEC device exhibited a large transmission modulation in the wavelength range from 500 nm to 1800 nm, and especially in the range from 545 nm to 1077 nm, its transmission modulation range exceeded 80%.