Abstract
The optical absorption of amorphous tungsten oxide thin films was studied at low intercalation levels. The electronic density of states was obtained by an electrochemical method and bias potential regions were assigned to the conduction band and gap states according to the experimentally estimated conduction band edge. Differences between the coloration in the conduction band and gap states were observed when comparing the experimental results to a theoretical site-saturation model that considers electronic transitions between localized tungsten sites. The model could reproduce the optical response due to conduction band states. However, it underestimated the rate of change of the optical absorption coefficient with intercalation level for gap states. This discrepancy is discussed in the context of small-polaron optical absorption.
Highlights
Electrochromic (EC) materials are able to change their optical properties upon the application of an electrical voltage [1]
The potential obtained by CP and the simultaneously measured transmittance of the aWO3 sample are depicted in Fig. 1(a) for low intercalation levels
The electrochemical density of states of aWO3 was studied and conduction band and gap states could be distinguished by determining the flatband potential using the Mott-Schottky equation
Summary
Electrochromic (EC) materials are able to change their optical properties upon the application of an electrical voltage [1]. EC thin films are functional components in devices providing variable transmission of visible light and solar radiation. Their largest application lies in energy-efficient and comfort-enhancing smart windows, that today are being installed in modern buildings [2]. Different methods can be used to measure this so-called electrochemical density of states (EDOS), but chronopotentiometry (CP) appears versatile [6]. This method has provided data with strong resemblance to the shape of the electronic DOS for various EC oxides—for example, WO3 and TiO2 [6], SnO2 [7], and V2O5 [8].
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