A case study of optical properties and structure of sol-gel derived nanocrystalline electrochromic WO3 films

Sol-gel derived. as-deposited amorphous tungsten oxide (WO3) films become nanocrystalline with a pseudocubic triclinic structure upon annealing at 600 degrees C. The annealed films are constituted of nanorods along with interconnected nanoparticles and pores. While the high transmission modulation and colouration efficiency in the visible region and the fast bleaching kinetics as shown by the nanostructured film were typical of amorphous WO3, the absorption coefficient spread in the 300-2000 nm wavelength range. the reflectance modulation and the colouration efficiency peak in the NIR region were reminiscent of crystalline WO3. The larger maunitude of the indirect band gap shift to blue than the direct gap shift for the same level of lithium intercalation (18 mC cm(-2)) and the irreversibility of the direct gap shift upon bleaching are characteristic of the nanocrystalline structure of the film. The moderate decline in the anodic peak current maximum, the diffusion coefficient for lithium and the optical modulation at 632.8 nm upon repetitive switching between the coloured and bleached states ratified that nanostructured films sustain 1000 cycles without much deterioration.