Infrared and UV-visible ellipsometric study of WO 3 electrochromic thin films

Abstract

Infrared Fourier Transform phase-modulated and UV-visible ellipsometry techniques have been applied to study the colouring/bleaching process of WO 3 electrochromic thin films. The optical absorption of these films can be changed markedly in a reversible process induced by hydrogen incorporation. The colouring/bleaching process was carried out by using the sample as one electrode of an electrochemical cell filled with an electrolytic solution of H 2SO 4. The spectroscopic ellipsometric angles Ψ and Δ were obtained ex situ by an infrared Fourier Transform phase-modulated ellipsometer working in the wavenumber range from 800 to 4000 cm −1 and a rotating analyzer ellipsometer in the UV-visible range (1.7–4.8 eV). Significant optical absorption and vibrational changes associated with the colouring/bleaching process which follow the hydrogen incorporation in the film were observed in the UV-visible range from 1.7 to 3.8 eV and in the IR range at 1605 cm −1 and from 3000 to 3800 cm −1. The strong changes can be seen by changes in the spectroscopic dielectric function. For the coloured WO 3 layer, the main absorption contribution associated with the colour effect was found at 6032 cm −1, outside the analysed region.