| Electrochromic smart glass can actively modulate the visible transmittance and solar shading performance of glass and realize intelligentization.It is considered as the future development direction of building energy-saving glass.Compared with other electrochromic materials,tungsten oxide has attracted much attention because of its high optical modulation and coloring efficiency.Considering the application in the large-scale smart windows,the electrochromic materials must meet the following requiremenets,including large modulation amplitude,high coloring efficiency,low driving voltage,excellent cycle stability,and environmental durability.However,simple tungsten oxide films can not satisfy the above requirements at the same time.Therefore,as the most promising material for large-scale EC smart windows,how to improve its electrochromic reversibility and other related main properties is still challenging and need more further research.In this study,WO3 and WO3:Ti films were deposited by pulsed direct current reactive magnetron sputtering using tungsten metal and tungsten-titanium alloy targets,respectively.XRD,Raman,SEM,AFM,XPS and electrochemical workstation,et al.were used to study effects of the film thickness and process parameters including substrate temperature and oxygen partial pressure on their crystal structure,surface morphology and electrochromic properities?for example modulation amplitude,response time,color change efficiency,cycle life,etc.?.And electrochromic mechanism was also proposed according to the comparison of the both WO3 thin films with and without Ti dopant.The results show that both the lattice strain and the surface roughness change when the WO3 film is colored.Their change values originally decrease and then increase with film thickness increasing,the minimum values are obtained both at the film thickness of 250 nm.Meanwhile,the optical modulation amplitude and color change efficiency reach their maximums at this thickness.Therefore,the thickness of250 nm is the optimum value for the WO3 film when it acting as electrochromic layer.The relatively loose structure of the amorphous WO3 film facilitates the insertion and extraction of lithium ions,but the film structure is gradually destroyed after multiple cycles,and the optical modulation amplitude decreases from 47.9%to 2.9%after 200 cycles.And,WO3 becomes brownish yellow,its reversibility no longer remains;For the crystalline WO3 film deposited at 350°C,of the optical modulation decreases from 61.8%to 49.5%after 200 cycles;therefore,although the crystalline WO3 film possesses the better cycle stability compared with the amorphous one,but still need to be improved.When the substrate temperature was increased to 300°C,the WO3:Ti film began to change from amorphous to cubic phase;The modulation amplitude of the WO3:Ti film prepared at 350°C of remains unchanged 62%after 200 cycles.Increasing oxygen partial pressure,the crystalline structure of the film transforms from monoclinic phase to cubic phase;the transmittance and the density of the film increase,the deposition rate decreases.Meanwhile,the oxygen partial pressure has little effect on the modulation amplitude of the film,and they are all in the range of60%64%;and the cyclic stability increases first and then decreases with increasing oxygen partial pressure,and reaches the optimum value when the oxygen flow rate is13 sccm;at the same time,the film coloring and fading response time decreases with increasing oxygen partial pressure,and then the rate of color change of the film increases.There is a reversible transition between monoclinic phase and cubic phase during WO3 film bleaching and coloring,and when doped with Ti,same cubic phase is found before and after coloration.The change of lattice strain and surface roughness during coloration cycle of WO3:Ti film is smaller compared with that of WO3 film.,indicating that the energy barrier has to be overcomed during coloration cycle is small in the former.In terms of performance,comparing WO3:Ti with WO3,the optical modulation amplitude is similar with each other,but the former's coloration and fading response speed is significantly faster,and the charge stored in the reaction is larger;the current decay is smaller after a number of cycles;therefore the cycle stability increases,and the cycle life of the filmis obviously improved after WO3doped with Ti. |
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