Coloration mechanism of electrochromic Na x WO3 thin films
Résumé
The coloration mechanism of tungsten trioxide (WO3)
upon insertion of alkali ions is still under debate after
several decades of research. This Letter provides new
insights into the reversible insertion and coloration
mechanisms of Na+ ions in WO3 thin films sputter-deposited on ITO/glass substrates. A unique model based
on a constrained spline approach was developed and
applied to draw out ε1+iε2 from spectroscopic
ellipsometry data from 0.6 to 4.8 eV whatever the state of
the electrochromic active layer, i.e. as-deposited, colored
or bleached. It is shown that electrochemically
intercalated sodium-tungsten trioxide, NaxWO3 (x=0.1,
0.2, 0.35), exhibits an absorption band centered at ca.
1.14 eV in 2 governing the coloration mechanism.
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