WO₃ And NiO_x Complementary Electrochromic Films Deposited By Mid-frequency Magnetron Sputtering Method

Amorphous WO₃ film is probably the most promising one of various electrochromic materials due to its excellent electrochromic properties.NiO_x film is the preferred typical anode electrochromic material as the counter electrode layer of electrochromic device.The devices with the above two electrochromic films greatly simplify the procedure of fabrication and exhibit better performance than the traditional devices,which offer wide prospect of application.In this paper,amorphous WO₃,Ti-doped WO₃ and NiO_x films were prepared by mid-frequency dual-target magnetron sputtering using pure metal targets.With the optimization of technical parameters,the films showed good electrochromic property.The microstructure,morphology composition and electroehemical property of the films were characterized with many technologies,and the primary conclusions were listed as follow:The WO₃ films deposited by mid-frequency dual-target magnetron sputtering method at room temperature are amorphous.XPS spectra show that there is only W⁶⁺ on the film surface, and contains simple substance and mixed valence tungsten in the inner of the films.The atomic ratio of O/W increases with the oxygen content in the atmosphere,and the electrochromic performance is improved,too.In this paper,the transparent WO₃ films prepared in a sputtering atmosphere at 95%oxygen content show good electrochromism.The variation of average transmittance difference between the bleached and colored state can reach up to 63%at the wavelength 633 nm.It is because the porous structure of the films is more convenient for Li⁺ ions injection and extraction.The study on the effect of heat treatment on the WO₃ films structure at different temperatures show that heat treatment does not change the binding energy of the elements,the WO₃ films remain amorphous after annealing at 200℃and keep good electrochromism, which the variation of average transmittance difference between the bleached and colored state can reach up to 70%at the wavelength 633 nm.The films crystallize after annealing at 350℃,the charge capacity and electrochromism decrease significantly because the structure of the crystallized film makes difficult for Li⁺ ions to inject and extract.The analysis on the Ti-doped WO₃ electrochromic films shows that the valence of titanium in films is +4,without mixed valence.Ti⁴⁺ can substitute W⁶⁺ due to the similar radius with each other.Ti-doping does not affect the valence of tungsten but considerably influence the structure and electrochromic performance of the WO₃ films.For instance,the particle size is refined and crystallization temperature increase,Ti-doped can enhance the response speed and service life,improve the reversibility and reduce the value of drive voltage,as the grain growing can be restrained by Ti-doping,and the sructure is more convenient for Li⁺ ions to inject into films corresponding to the doped films than un-doped ones.In this paper,the optimum amount of doped titanium is 5.1%,and the electrochromic performance degrades with the increase of Ti-doping content.The deposited nanosized nickel oxide(NiO_x) films are of relatively clean surface that the average grain size is about 4.5 nm and the roughness(Ra) was 1.659 nm.Under±3V,the films show good electrochromic properties whose different transmittance between the bleached and colored state can reach up to 30%using Li⁺ as colouring ions in the visible region.The structure of as-deposited nanosized NiO_x films is beneficial for ions' injection and extraction.The NiO_x films have fast response speed and good regulatory effect in visible region,which served as a supplement to WO₃ films in complementary electrochromic device. The laminated Glass/ITO/WO₃/LiClO₄-PC/NiO_x/ITO/Glass complementary electrochromic device was studied.The results indicate that the complementary device has more excellent electrochromic characters than the structure of Glass/ITO/WO₃/LiClO₄-PC/ITO/Glass device, and show good electrochromism and fast response speed with the variation of average transmittance difference between the bleached and colored state can reach up to 81%at the wavelength 633 nm.