Coloration mechanism of electrochromic NaWO thin films.

The coloration mechanism of tungsten trioxide (WO) 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 WO thin films sputter-deposited on ITO/glass substrates. A unique model based on a constrained spline approach was developed and applied to draw out ε+iεfrom 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, NaWO(x=0.1,0.2,0.35), exhibits an absorption band centered at ca. 1.14 eV in ε governing the coloration mechanism.