Revealing the mechanism of electrochromic sticking image behavior in tungsten oxide film prepared from tungstate precursors

Ammonium tungstate precursor system are one of promising preparation routes for sol-gel tungsten oxide (WO3) film because of its low-cost preparation, good solution stability, and good solvent compatibility. Nevertheless, we notice that an incomplete ion extraction (called electrochromic sticking image) behavior of electrochromic WO3 film prepared from tungstate precursor, which weakens its electrochromic performance. In this work, some key factors such as residual precursor ammonium ions and oxygen vacancy defects are investigated to understand the electrochromic sticking image behaviors. For the amorphous WO3 film, the film components consist of WO3 and tungstate. The endogenic ammonium ions can preferentially participate in the coloring process to occupy the WO3 lattices. However, in the bleaching process, there is always part of ions remaining in the film due to electrical neutrality principle (between tungstate anions and positive ions), resulting in the electrochromic sticking image. For the crystalline WO3 film, the increasing concentration of oxygen vacancy and film density become a main factor that inhibits the extraction of Li+ ions, which is reflected by the low diffusion coefficient and high charge transfer resistance. For this case, a long-term bleaching bias is valid to eliminate the electrochromic sticking images for crystalline WO3 film. This work provides a mechanism understanding for improving the electrochromic memory of sol-gel WO3 film.