Radio Frequency Reactive Magnetron Sputtered Nickel Oxide Films and Their Electrochromic Behaviors

Author:Hou Shuai

Supervisor:geng hong bin


Degree Year:2018





In recent years,reducing the energy consumption is an efficient way to solve the energy problem which has become one of the key issues that hinder the development of the world.As a promising energy-saving and environmental protection technology,smart windows have been developed.The challenge for commercially smart windows is to prepare the electrochromic(EC)materials with long-term cyclic stability,short switching time and high coloration efficiency(CE).Among various electrochromic materials,nickel oxide has received a considerable amount of attention due to its outstanding EC properties.However,kinds of difficulties,such as slow switching speed,narrow optical modulation and poor cycling durability have hindered its application.From the perspective of crystallography,based on the dual injection/extraction model,the lattice structure of NiO can be modified by controlling the substrate temperature and doping with other element based on RF magnetron sputtering technology.Thus,the electrochromic performance of NiO can be improved by controlling the insertion/extraction of electrolyte ions into the film under an applied electric field.Orthogonal experiments under oxygen-rich and oxygen-free,low-pressure and high-pressure conditions were performed by RF reactive magnetron sputtering technology.Through testing the NiO films obtained under different conditions,we determined the best conditions for preparing nickel oxide films is oxygen-rich and high-pressure.Based on this result,by controlling the substrate temperature,NiO films with different degree of crystallinity were obtained.Electrochemical and electrochromic properties tests showed that the amorphous NiO film demonstrated better electrochemistry activity and capacity.Besides,amorphous film showed the best electrochromic performance with a short response time(coloring time of 0.8 s,fade time of 0.7 s)and a larger optical control range(43%at 550 nm).However,the cycling stability of amorphous NiO film is poor.On the contrary,crystalline NiO films showed the worse electrochromic performance and better cycling stability.Based on these,the double-layer structured NiO with amorphous layer on the top and the crystalline layer at the bottom was prepared by reactive rf magnetron sputtering technology.Compared with the single layer cystalline or amorphous NiO thin films,it exhibits improved electrochromic properties,such as short coloration/bleaching time(0.8 s/1.1 s),large transmittance modulation range(62.2%).However,at the end stage of cycling,there was a slight fluctuation at the bleaching state of double-layer structured NiO electrode.It has been repeatedly confirmed by experiments that this phenomenon is not a test error.The reason for this phenomenon may be attributed to the interface between the crystalline film and the amorphous film.When ions crosses this interface,they have to overcome the interface barrier energy,thus causing performance fluctuations.A experiment was performed to overcome this shortcoming.Based on reactive RF magnetron sputtering technology,by gradually controlling the temperature of the substrate from a high temperature to a low temperature,a NiO thin film with its crystalline nature change gradually from the crystalline phase at the bottom to amorphous phase on the top was obtained.The results showed that the film combined both amorphous and crystalline characteristic,such as large surface roughness(Ra and Rq are 4.67and 3.84)and the characteristic peak of NiO(200)can be detected through XRD test.Electrochemical tests show that the sample exhibited good chemical activity and its corresponding particle capacity(7.51 mC/cm2)and diffusion efficiency(oxidation 4.62×10-1212 cm2/s,reduction3.01×10-1212 cm2/s)was much higher than the crystalline film.The sample also showed excellent electrochromic properties.The coloraion time and bleaching time were 3.1 s and 2.1 s,respectively.The maximum change of the transmittance at 550 nm was 67.6%,and it overcomes the fluctuating transmittance in the bleached state of the double-layer structured NiO film.The cycling process is quite stable.Using RF reactive magnetron sputtering technology,a group of NiO films with different copper doping amounts were prepared by two targets co-sputtering method.During the sputtering process,sputtering power of the nickel target was fixed and the content of copper can be controlled by adjusting the power of the copper target.The electrochromic performance test showed that the transmittance adjustment range of the doped film can be improved compared with the undoped film.The NiO-P30 sample showed most significant increase,and its transmittance at 550 nm could reach 72.68%.In addition,the switching speed of the doped samples were also increased.The coloring time and bleaching time of the NiO-P60 sample were1.6 s and 0.7 s,respectively.Based on the experimental results above,an all solid state electrochromic device with ITO/Cu:NiO/LiTaO3/WO3/ITO structure was prepared.It exhibit good optical modulation ability with the transmittance adjustment range at 550 nm can reach about 50%.