Studies On Electrochromic Properties Of Nickel Oxide And Tungsten Oxide Thin Films And Devices

Electrochromic technology has the ability to dynamically adjust near-infrared and visible light bands,which can be widely used in energy-saving windows,smart homes,non-emissive displays,aerospace,on-board systems,consumer electronics,etc.However,there are still many technical bottlenecks in the application of electrochromic technology,including short cycle life,low coloring efficiency,poor weather resistance,and high preparation cost.Therefore,how to develop high-performance and low-cost electrochromic materials and devices has become a hot research topic.Nickel oxide（NiO_x）and tungsten oxide（WO₃）are inorganic electrochromic materials,which are complementary and considered as electrochromic materials with large-scale application potential.However,at present,there are rare studies on the electrochromic mechanism of the anodic material（NiO_x）,resulting in the lack of theoretical support for the optimization of anodic material performance,and neither its cycle life nor storage capacity is adequate to match those of the cathodic material WO₃,which seriously hinders the industrial application.In addition,there is a lack of systematic research on the influence of the kinetic behavior of different chromogenic cations(Al³⁺,H⁺,Li⁺,etc.)on the electrochromic performance of cathodic material WO₃.In view of this,the electrochromic anodic thin film material（NiO_x）was prepared by electron beam evaporation and magnetron sputtering,and the radio frequency sputtering and medium frequency sputtering were used to prepare the electrochromic cathodic film material（WO₃）.The microstructure and elemental valence state of the films were characterized by XRD,HRTEM,XPS,AFM,EDS and SEM.The electrochromic performances of NiO_x/WO₃ were evaluated in situ by electrochemical workstation combined with UV-vis-NIR spectrophotometer.Time-of-flight secondary ion mass spectrometry（TOF-SIMS）,Raman and Fourier transform infrared spectroscopy were used to characterize and analyze the element distribution,chemical bonds,and functional groups on the surface of the film.The electrochromic kinetics and electrochemical properties of NiO_x/WO₃ thin films were analyzed by electrochemical constant current intermittent titration,cyclic voltammetry,chronoamperometry,chronopotentiometry and linear scanning voltammetry,etc.The main results of the thesis are as follows:The micro columnar structure of the NiO_x thin film prepared by electron beam evaporation method was optimized due to the auxiliary effect of ion source.The film had larger specific surface area.After 1000 cycles,the prepared thin film showed no attenuation of optical modulation,exhibiting anexcellent cycle stability.In addition,the anodic polarization resulted in an increase of active Ni in the thin film.After anodic polarization treatment,the coloring efficiency of NiO_x film reached 24.5 cm²/C.Compared with those of the films without anodic polarization treatment,the optical modulation and charge capacity were increased by 17.6%and 43.8%,respectively.The electrochromic reaction mechanism of NiO_x film was related to the diffusion behavior and surface capacitance behavior of color-causing Lithium ions in the film.The results of electrochemical tests showed that the reaction rate of Lithium ions in the film was mainly controlled by the diffusion of Lithium ions in the film,while the electrochromic performance was dominated by the surface capacitance behavior of the film.At the initial stage of electrochromic reaction,some of the chromogenic ions in the NiO_x film did not participate in it or irreversibly intercalated.The diffusion coefficients of NiO_x film under different colored states were calculated,and the relationship between the kinetics of color-causing ion and electrochromic performance during the cycle was illustrated.By adjusting the deposition temperature,the properties of NiO_x films such as grain size and Ni valence state ratio were changed,and the electrochromic properties of NiO_x film were affected.It was found that the change of the grain size of NiO_x thin film affected the ratio of the surface capacitance form and the internal bulk diffusion form of its electrochemical reaction in lithium perchlorate/propylene carbonate electrolyte（LiClO₄-PC）.The regulation of the microstructure of NiO_x thin film affected its"effective" contact with the electrolyte,and the optimized microstructure formed more active sites.When the deposition temperature T=100 ℃,the colored state of the film had the best light shielding effect;When T=200℃,the bleached state of the film had the best light transmission effect.At t=100℃-200℃,the average optical modulation at 550nm could reach as high as 40%.The cycle life of amorphous WO₃ films in conventional electrolyte solutions（HCl,LiCl,PC-LiClO₄,etc.）usually lasts only several hundred times,which is far from meeting the requirements of commercial applications.In the present study,a new mixed cationic electrolyte solution（1,2-PG-AlCl₃·6H₂O）was proposed.When the molar concentration of this electrolyte solution was 0.6M,the electrolyte conductivity reached the largest value of 0.7 mS/cm.The amorphous WO₃ film prepared by RF magnetron sputtering formed a solid electrolyte interface layer containing Al on its surface during the electrochromic process in this electrolyte solution.In the coloring/bleaching voltage range of-1.5/2 V,its cycle life lasted more than 20000 times,with a max optical modulation of 90%@ 633 nm.On the basis of optimized NiO_x and WO₃ electrochromic materials,a UV electrolyte-based complementary device with simple structure,low cost and excellent cycle stability was prepared,its optical modulation and cycle life exceeded 65%@632 nm and 1000 times@550 nm,respectively.Based on that,a complementary anti glare rearview mirror sample was prepared.The results of this paper are expected to provide experimental basis for the performance optimization of electrochromic materials and devices.