Stress changes in electrochromic thin film electrodes:: Laser beam deflection method (LBDM) as a tool for the analysis of intercalation processes

The mechanical effects of the intercalation processes in electrochromic WO₃ thin films are reported here and discussed. In particular the electrochemical insertion of H, Li and Na ions in WO₃ was studied by means of laser beam deflection method (LBDM). Linear changes of WO₃ stress were observed for small amounts of the inserted charge and linearity was always associated with a reversible mechanical behavior of thin films. An explanation in terms of homogeneity of the insertion is given. An analogous trend was also verified in the absorbance vs. charge curves. As a consequence the constancy of the electrochromic efficiency values for the three different ions was found. In this way it was possible to determine a full reversible behavior for M_xWO₃ in the composition range 0x0.2. The onset of new phases formation when x exceeded previous upper limit, was observed during Li and Na intercalation. Such transitions brought about the loss of optical and mechanical reversibility. During prolonged hydrogen insertion a reversible slope inversion occurred in stress curve so that it was necessary to take into account different possible mechanisms of the WO₃ electrochromic reaction.