Effect of oxalic acid dihydrate on optical and electrochemical properties of sol–gel derived amorphous electrochromic WO3 films

Tungsten oxide (WO3) films have been prepared from an ethanolic acetylated peroxotungstic acid sol containing different amounts of an organic moiety, namely oxalic acid dihydrate (OAD) by sol–gel technique. Changing the precursor solution chemistry by varying the dopant proportion from 0 to 5 wt.% modifies the microstructure of the films, which are otherwise amorphous to X-rays. The microstructural differences as a function of OAD content are reflected in the transmission modulation in the photopic and solar regions, NIR reflectance, optical absorption coefficient, band gap, refractive index, switching kinetics, charge storage capacity and electrochemical activity of the films. The films derived from 3, 4 and 5 wt.% OAD sols exhibit superior electrochromic characteristics than the ones derived from 0, 1 and 2 wt.% OAD sols. The absorption band structure for the colored films peaks at ∼1.4 eV and the band gap corresponding to indirect transitions between the parabolic bands narrows upon lithium ion insertion as it varies between 3.0 and 3.2 eV for virgin films and lies in the range of 2.6–3.1 eV for the electrolytically colored films. The films modulate transmission primarily by absorption; films with 3, 4 and 5 wt.% OAD are characterized by ΔT ≥ 70% and ΔR ≤ 10% at photopic wavelengths (λ ∼ 650–700 nm). As a paradigm illustrative of excellent electrochromic performance is the film obtained from a sol with 3 wt.% OAD which shows a coloration efficiency of 102 C cm−2 (at 800 nm), fast color–bleach kinetics (tc ∼ 192 s and tb ∼ 28 s) and an ion storage capacity of 18.6 mC cm−2, thereby rendering it suitable for smart window applications.