Coloration mechanism of electrochromic NaxWO3 thin films

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+iε₂from 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, Na_xWO₃(x=0.1,0.2,0.35), exhibits an absorption band centered at ca. 1.14 eV in ε₂ governing the coloration mechanism.