Preparation And Characterization Of WO₃ Thin Films For Electrochromic Devices

Electrochromism refers to the phenomenon that the color of a material can gradually transition from a certain state to another state with the insertion/extraction of cations under the condition of an applied current or voltage.After exchanging the direction of the electric field or current,the color state of the material can be stable and reversible.WO₃is currently the most commonly used material that can produce electrochromic phenomena,and has excellent performance in optical properties and electrochemical properties.The inorganic all-solid-state electrochromic devices developed using WO₃thin films also have the advantages of large optical modulation amplitude and low cost,good cycle stability and other advantages.However,there are many production process and technical problems in the preparation of WO₃thin films and devices,which are the key factors restricting the practical application and market development of electrochromic technology.In this paper,the preparation of WO₃thin films by magnetron sputtering was studied in detail,and the effects of different parameters（sputtering power,oxygen-argon flow ratio,working pressure）on the microscopic morphology,optical properties and electrochemical properties of WO₃thin films during the preparation process were discussed.Firstly,the preparation range of the parameters used is obtained through the pre-experiment,and then the deposition rate test,transmittance test,surface morphology test,and electrochemical performance test are carried out on the prepared film.Through research,the optimal preparation parameters of the thin film were successfully obtained,the sputtering power is 175W,the working pressure is 2.0Pa,and the oxygen-argon flow ratio is60:20.The transmittance of the films prepared under these parameters changed as high as 66%,the maximum transmittance was 85%,and the coloring/fading time was within 10s（9.6s/6.4s）.The electrochemical performance test of the film shows that the film has good electrochromic performance and good stability.Secondly,this paper studies the annealing experiments to prepare crystalline WO₃films,and explores the influence of crystalline WO₃films on optical properties and electrochromic properties.Through comparison,it is found that the light modulation range capability of the amorphous WO₃film is more excellent,and the film has a higher initial transmittance and a faster electrochromic rate.Heat treatment of the amorphous WO₃film to 400°C can convert the amorphous film into a monoclinic crystalline film.Due to the annealing treatment,the internal structure of the film will become more compact,and a small amount of Li⁺will penetrate deep into the film,the coloring degree of the film will be deeper,but the annealing treatment will reduce the transport channel of Li⁺,resulting in the decrease of cyclic voltammetry performance.Through the electrochemical performance test of the crystalline film,the current density of the film tends to be stable after 1500bipotential cycles,and the stability is better than that of the amorphous film.Finally,the WO₃film prepared under the best experimental conditions was used to prepare the Li PON electrolyte film and the all-solid-state electrochromic device.The transmittance of the device at a wavelength of 550 nm changed as high as 50.6%,which can basically meet the transmittance change range of the device.However,limited by the Li⁺conductivity of the solid electrolyte,the response rate of the device is slow,and the device has certain electrochemical activity in terms of cyclic voltammetry characteristics.