Optical properties of in vacuo lithiated nanoporous WO3:Mo thin films as determined by spectroscopic ellipsometry

Maxime Lagier,Luc Burnier,Aurélien Bertinotti,Andreas Schüler,Olivia Bouvard

doi:10.1016/j.optmat.2021.111091

Maxime Lagier, Luc Burnier + Show 3 more

Open Access

https://doi.org/10.1016/j.optmat.2021.111091

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Abstract

Incorporation of Mo into pure WO3 thin films is of interest for electrochromic devices, as it improves colour neutrality in the dark state. Existing literature still lacks reliable quantitative data on the complex dielectric function of such coatings. In this study, we deposited WO3 and MoxW1−xO3 thin films by magnetron sputtering and subsequently characterised them by X-ray photoelectron spectrometry (XPS), grazing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM). In vacuo-lithiation was performed to incorporate lithium inside the thin films. The lithiated films were evaluated with variable-angle spectroscopic ellipsometry (VASE) from 340 to 990 nm and with transmission measurements from 330 to 2100 nm. The ellipsometric data were analysed with a straightforward model composed of the sum of Tauc-Lorentz and Lorentz oscillators dispersion laws. One Lorentz oscillator positioned around 1.3 eV is associated with the reduction of W6+ to W5+ by lithium incorporation. One Lorentz oscillator positioned around 2.3 eV is associated with additional states in the band-gap due to the presence of Mo in the lithiated film. The proposed model allows a better comprehension of the involved electronic transitions and allows to determine the refractive index and extinction coefficient of the materials precisely.

Highlights

Electrochromic (EC) materials have increasing prospects in smart windows, in anti-glare automobile rear view mirrors and in displays due to their low cost and energy consumption [1,2]
None of them per formed a quantitative analysis of the complex dielectric function of lithiated MoxW1− xO3 films. This is what we focus on in this work: we study the effect of Mo insertion on the refractive index and the extinction coefficient of sputtered tungsten trioxide films (WO 3)
The TaucLorentz dispersion law is associated with band-gap properties and de scribes the dispersion relation in the interband region, while the Lorentz oscillators are associated with electronic transitions with energy values lower than Eg

Summary

Introduction

Electrochromic (EC) materials have increasing prospects in smart windows (controlling solar heat gains or visual comfort), in anti-glare automobile rear view mirrors and in displays due to their low cost and energy consumption [1,2]. When these materials are employed in a suitable device, the optical properties can be modified (clear/dark state) with a relatively small voltage. The anodic electrochromic layer can change colour complementarily to the cathode. Transparent conductive elec trodes are needed to apply a tension, transport the current and carry out the colour changes according to the polarity of the applied voltage

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