Influence of PEG additive and annealing temperature on structural and electrochromic properties of sol–gel derived WO₃ films

Sol–gel derived tungsten oxide (WO₃) films have been deposited by spin coating route using acetylated peroxotungstic acid (APTA) or a mixture of APTA and polyethylene glycol (PEG) dissolved in ethanol as the precursor solution, followed by thermal treatment in air. The influence of PEG additive and annealing temperature on the structural and electrochromic (EC) behavior of the films have been investigated. For films annealed at 300 °C, a porous nanocrystalline/amorphous microstructure was obtained in the WO₃-PEG film, while monoclinic microstructure was formed in the pure WO₃ film. Moreover, for the WO₃-PEG films, the film microstructure was found to depend on the annealing temperature. Electrochemical studies indicate that the WO₃-PEG film annealed at 300 °C (WP-300) exhibits superior EC properties, which produces faster switching speed (t _c = 19 s, t _b = 3 s),better reversibility (K = 0.97) as well as higher optical modulation (ΔT = 32% at 550 nm) and coloration efficiency (η = 22 cm²/C at 550 nm). Our results suggest that PEG addition in combination with an appropriate annealing treatment can benefit the EC properties, arising from the ease of ion diffusion within the EC material, as evident from the nanocrystallines embedded into the amorphous matrix with a porous character.