Thermochromic Properties Of Tungsten Doped VO₂ Films Regulated By Sandwich Structure

Vanadium dioxide（VO₂）is a strongly correlated electronic material with a first-order reversible semiconductor-metal phase transition（SMT）from monocline to tetragonal rutile phase at critical temperature（68℃）.At the same time,the resistivity dramatically changes by 1-5 orders of magnitude,accompanied by a change from transparency at low temperature to high reflection at high temperature in the near-infrared region.These properties make VO₂ideal as a coating material for energy-efficient smart windows.However,the problems of high phase transition temperature（T_c）,balancing high visible light transmittance(T_lum)and solar modulation ability(?T_sol),and unsatisfactory service life still exist in the practical application of VO₂-based thermochromic smart windows.In this paper,Hf O₂/W-doped VO₂（W-VO₂）/Hf O₂and Sn O₂/W-VO₂/Sn O₂sandwich structure composite films were designed and fabricated on quartz glass by pulse laser deposition method,using W-VO₂film as intermediate layer and Hf O₂and Sn O₂films as the buffer layer and anti-reflection layer.The synergistic effect of element doping and sandwich structure was used to optimize the thermochromic performance of VO₂films.The effects of thickness of Hf O₂and Sn O₂buffer layer and anti-reflection layer and annealing time on the photoelectric properties and service life of W-VO₂films were investigated.The main research contents are as follows:（1）The Hf O₂/W-VO₂/Hf O₂symmetrical sandwich structure was designed and fabricated,and the effects of the thickness of Hf O₂buffer layer and anti-reflection layer on the optical properties,T_cand hydrophobicity of W-VO₂were studied.The?T_solof Hf O₂/W-VO₂/Hf O₂symmetrical sandwich structure was nearly three times higher than that of W-VO₂single-layer film.When Hf O₂buffer layer and anti-reflection layer were both 100 nm,Hf O₂/W-VO₂/Hf O₂sandwich structure obtained excellent comprehensive performance,T_lum,?T_soland T_cwere 48.9%,7.2%and 38.6℃,respectively.On this basis,the buffer layer thickness was kept at 100 nm,and the optical performance of Hf O₂/W-VO₂/Hf O₂sandwich structure was further improved by optimizing the thickness of Hf O₂anti-reflection layer.It was found that the Hf O₂/W-VO₂/Hf O₂composite films achieved the best properties when the structure was symmetrical.（2）By adjusting the thickness of Sn O₂buffer layer and anti-reflection layer,the influence of Sn O₂buffer layer and anti-reflection layer on the structure and photoelectrical properties of W-VO₂film was studied.Compared with Hf O₂film,the T_lumand?T_solof W-VO₂film were further improved by using the rutile phase Sn O₂matching VO₂（R）as buffer layer and anti-reflection layer.The results showed that when the thickness of Sn O₂buffer layer and anti-reflection layer was both 260 nm,the Sn O₂/W-VO₂/Sn O₂sandwich structure achieved the good balance of optical properties and T_c(T_lum=54.4%,?T_sol=10.7%,T_c=39.0℃).Furthermore,the effect of annealing time on the properties of Sn O₂（260 nm）/W-VO₂（60 nm）/Sn O₂（260 nm）sandwich structure was studied.The results showed that the unannealed Sn O₂/W-VO₂/Sn O₂sandwich structure obtained remarkable photoelectric properties and T_c.（3）The service life of W-VO₂film,Hf O₂/W-VO₂/Hf O₂sandwich structure and Sn O₂/W-VO₂/Sn O₂sandwich structure was investigated by boiling water test.The results showed that the absorption edges of W-VO₂monolayer film and Sn O₂/W-VO₂/Sn O₂sandwich structure blue shifted and T_lumincreased with the increase of boiling water treatment time.The absorption edge of Hf O₂/W-VO₂/Hf O₂sandwich structure first appeared red shift with the treatment time,and blue shift occurred when the treatment length was higher than 52 h,and T_lumdecreased first and then increased.In addition,W-VO₂film and Sn O₂/W-VO₂/Sn O₂composite films failed after being treated in boiling water for 0.67 h and 4 h respectively,while?T_solof Hf O₂/W-VO₂/Hf O₂sandwich structure still remained～1%after 72 h treatment in boiling water.