Research progress of electrochromic performances of WO3

Abstract

From the aspects of both experimental studies and first-principles calculation, we review the research progress of improving the electrochromic performances of WO3, and analyze the transformation tendency in applied field, performance requirement and research focus. Due to the low color-switching, the application field of WO3 shifts from display devices to smart windows or other energy-saving devices. According to the requirement for electrochromic performance, the concerned WO3 morphology changes from amorphous form to nanostructure. For the high desire of smart windows in large-area curtain walls, the solid state inorganic electrochromic materials with lithium ion conductors are used as substitutes for the organic electrochromic films in hydrogen ion electrolytic solution. Correspondingly, response time and cycle life are regarded as the most important performance indices. Doping and synthesizing nanostructure are considered to be the main methods to improve electrochromic performance by introducing the pores into the crystals as the ion diffusion path. Especially, the nano-crystalline WO3 attracts much attention, due to its high stability and quick color switching. In the respect of the first-principles calculation, the simple cubic WO3 is a widely used model for calculation, because of its simple structure and high symmetry. However, there always occur the underestimation of band gap and the incorrect relationship between the cell sizes of WO3 and LiWO3. In response to the problem, by analyzing the Li-intercalated WO3 configuration, it is found that the lattice parameter is closely associated with the interaction between lithium and oxygen. The large discrepancy between the experimental and calculated band gaps is primarily due to the omission of the structural distortion in the calculation, including tilting of WO6 octahedra, as well as the off-centering of W in octahedral caused by the second-order Jahn-Teller effect. According to this, we propose a distorted cubic WO3 model (Im3 space group) to better explain the relevant experimental results. In light of the achieved results and the encountered problems in recent researches, it is generally received that the industrialization of nano-crystalline WO3 and systematic calculation on the lithium diffusion in WO3 deserve the serious consideration. In addition, possessing the function of blocking near-infrared and visible light selectively is the trend for the next generation electrochromic materials. Therefore, the noteworthy development directions on the aspect of both experimental studies and first-principles calculation are pointed out to provide some valuable references for the further researches.