Transmittance correlated real-time resistivity modulation and insulator-metal transition of electrochromic WO₃ thin films

A quantitative relationship between optical and electrical properties of crystalline WO₃ films during Li ⁺ insertion has been established in this work. Detailly, the exponential relationship between transmittance and resistivity of crystalline WO₃ thin films in visible and near-infrared wavebands is determined, and the real-time characterization of the resistivity of WO₃ thin films during electrochromic process is realized. The experimental results show the resistivity of WO₃ film can be regulated in the range of 0.078–0.98 Ω m with rapid response (∼6 s) when applied ±1 V voltage for 30 s. By analyzing the fitted resistivity-transmittance curve of WO₃ thin film, a high-efficiency optical modulation of WO₃ film in near-infrared 1000 nm can be achieved. Combining the IMT phenomenon during Li⁺ insertion, when x of Li_xWO₃ changes from 0.08 to 0.13, the WO₃ film is in a transition state from the insulating state to the metallic state with the tetragonal crystalline phase. The symmetry structural evolution of crystalline WO₃ thin films during the Li ⁺ insertion is further demonstrated by transmission electron microscopy (TEM). These results contribute to the development of WO₃ based optoelectronic bifunctional electrochromic devices.