Fabrication And Performance Modulation Of VO₂ Films For Smart Windows

Smart windows are widely used in the field of energy saving to keep warm in winter and cool in summer.Normally,a smart window is realized by a thin film that can dynamically adjust the optical transmittance to regulate the thermal radiation.Vanadium dioxide（VO₂）is a phase change metal oxide that is susceptible to phase change under thermal and electrical stimulation.At critical temperatures,VO₂ films can block the transmission of near-infrared light as the temperature rises,and allow the transmission of infrared and visible light when the temperature decreases.This phase change property coincides with smart window applications.However,the application of VO₂ in the field of smart windows still has drawbacks.For example,the phase change temperature is high（intrinsic 68℃）,the visible light transmission rate is low,and the synthesis of high-quality films is difficult,etc.Therefore,to address the defects of phase transition temperature and visible light transmission properties of VO₂,this paper systematically investigates the VO₂ films from synthesizing pure VO₂ film,firstly,the effects of annealing temperature,time,substrate and other synthesis parameters on the film properties were investigated,and the doping of metal cations could reduce the band gap and bonding stability of VO₂ through first principle analysis,thus affecting the phase transition temperature and visible light transmittance of VO₂.Based on this theory,experiments were designed to further optimize the performance of VO₂ through doping modification（single doping,co-doping）and composite film design.First,starting from the preparation of pure VO₂ films,magnetron sputtering and sol-gel were used to explore the synthesis of different methods.In order to ensure the homogeneity of VO₂ films,,the effects of annealing temperature,time and substrate on the properties of VO₂films were investigated.The results show that annealing can effectively improve the lattice structure and reduce film defects;substrate changes will affect the film adhesion ability and light transmission and other properties.Considering both methods,the sol-gel method was selected to prepare pure VO₂ films for subsequent study,and the films prepared after parameter optimization had low roughness with preferred crystal orientation（011）.In order to realize the reduction of phase transition temperature and the improvement of visible light transmission rate of VO₂ thin film,this paper adopts the first principle to simulate the relaxation and energy band change of VO₂ cell.By introducing tungsten（W）,magnesium（Mg）and zinc（Zn）doping,the effect of doping on the stability and energy band of the VO₂cell are calculated.The calculation results show that the doping of metal cations is feasible to reduce the phase transition temperature of VO₂.The doping atoms stabilize bonding,and the introduction of defects will produce lattice deformation and promote the phase transition of VO₂ at low temperatures;Meanwhile,the doping will increase the energy band density,reduce the VO₂ band gap and lower the phase transition temperature,and the band gap reduction is W>Mg>Zn.The experiments of single doping and co-doping of W and Mg are designed and carried out based on the first principle calculations.It is shown that W doping can significantly reduce the phase transition temperature.At the W doping ratio of 2%,the phase transition temperature（Tc）of the films was reduced to 29°C and the near-infrared modulation(ΔT_sol)was8%.On this study,the visible transmittance(T_lum)can be significantly improved by W and Mg co-doping,and 80%visible transmittance can be achieved in the spectral range of 350-780 nm when the W and Mg doping ratios are 2%and 4%,respectively.In addition,the performance of VO₂-Zn O composite films was also studied in this paper.By introducing Zn O as a buffer layer and adjusting the thickness,relative position of Zn O film,the visible transmittance is improved from 75%to 80%.The results show that the thickness of the Zn O film affects the transmittance of the VO₂-Zn O composite film,and the transmittance in the visible band can reach more than 80%,with a maximum transmittance of 85%at about520 nm.