The Effect Of Crystallinity On Electrochromic Properties Of Tungsten Trioxide Thin Films

Electrochromism is a phenomenon related to optical changes（such as color,transmittance,reflectivity,absorption rate,etc.）occurring in materialscaused by the insertion/extraction of ions/electrons under the influence of an applied electric field.Tungsten trioxide is a relatively common inorganic electrochromic material.Due to the loose structure of the amorphous tungsten trioxide,ion/electrons have a fast response during the deintercalation process,but the internal porous structure collapses after many cycles,therefore,the stability of amorphous tungsten trioxide is poor.Increasing the crystallinity can improve its cycle stability,but the denser and ordered atomic arrangement results in slower ion/electron deintercalation.Therefore,controlling and selecting the appropriate crystallinity has an extremely important influence on the electrochromic properties of the material.In this paper,the effect of crystallinity on the electrochromic properties such as response time and cycle stability was studied by adjusting tungsten trioxide films with different crystallinity,so as to obtain the optimized crystallinity and meet the requirements of the response time and cycle life in the process of use.First,amorphous tungsten trioxide thin films were prepared by electrodeposition,and the influence of deposition time on the electrochromic properties was studied.With the increase of electrodeposition time,the optical modulation range and coloring efficiency gradually increase,the bleaching time gradually increases,and the coloring time decreases,mainly due to the difference in deposition time leading to the difference in morphology.The tungsten trioxide thin film prepared at a deposition time of 1000 s exhibits a large optical modulation range of 83%,an average response time of 4s and a high coloring efficiency of 83 cm²·C^-1.In order to precisely control the degree of crystallinity,a tungsten trioxide film with different crystallinity was prepared by sol-gel method by adjusting the reaction temperature and time.The effect of different crystallinity films on the electrochromic properties was tested:as the crystallinity increases,the diffusion rate of Li⁺in the film decreases,so the response time increases with the increase of crystallinity.The higher the degree of crystallization,the smaller the amount of Li⁺trapped in the film due to the trapping,and the long-range ordering of the atoms helps maintain the structural stability.When the crystallinity is 72%,the utilization of embedded charges is the highest,the optical modulation range can reach 77%,and the coloring efficiency is 55 cm²·C^-1.Finally,the prepared film with 72%crystallinity with excellent electrochromic properties was assembled into a solid electrochromic device,and the device exhibited excellent performance.The faded transmittance reached 75%,the average optical modulation range in the range of 400¹100nm reached 60%,the response time was 25 s,the coloring efficiency was 52 cm²·C^-1,and the relative light brightness reached 45.5%.The device as a smart window application can fully realize the regulation of solar thermal radiation.