Enhancing the Spectroelectrochemical Performance of WO 3 Films by Use of Structure-Directing Agents during Film Growth.

Thin, porous films of WO₃ were fabricated by solution-based synthesis via spin-coating using polyethylene glycol (PEG), a block copolymer (PIB₅₀-b-PEO₄₅), or a combination of PEG and PIB₅₀-b-PEO₄₅ as structure-directing agents. The influence of the polymers on the composition and porosity of WO₃ was investigated by microwave plasma atomic emission spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, X-ray diffraction, and gas sorption analysis. The electrochromic performance of the WO₃ thin films was characterized with LiClO₄ in propylene carbonate as electrolyte. To analyze the intercalation of the Li⁺ ions, time-of-flight secondary ion mass spectrometry, and X-ray photoelectron spectroscopy were performed on films in a pristine or reduced state. The use of PEG led to networks of micropores allowing fast reversible electrochromic switching with a high modulation of the optical transmittance and a high coloration efficiency. The use of PIB₅₀-b-PEO₄₅ provided isolated spherical mesopores leading to an electrochromic performance similar to compact WO₃, only. Optimum characteristics were obtained in films which had been prepared in the presence of both, PEG and PIB₅₀-b-PEO₄₅, since WO₃ films with mesopores were obtained that were interconnected by a microporous network and showed a clear progress in electrochromic switching beyond compact or microporous WO₃. [ABSTRACT FROM AUTHOR]