Low Temperature Annealing Improves the Electrochromic and Degradation Behavior of Tungsten Oxide (WOx) Thin Films

This research aims to understand the fundamental aspects of annealing on the electrochromic performance of tungsten oxides, using as-synthesized W18O49 substoichiometric bundled nanowires benchmarked against commercial WO3 nanoparticles. Linking detailed structural analyses with the electrochromic measurement results, we have investigated the electrochromic performance effects of low temperature annealing, up to 350 degrees C, on tungsten oxide (WOx) thin films, trying to establish the fundamental heat treatment structure-performance-loop. We have found that the annealing treatment at low temperature improved the optical modulation and long-term durability of the WOx thin films, without changing the structure and morphology of the as-synthesized samples. The 350 degrees C annealing was found to have the best stability improvement for the WO3 nanoparticle films during the electrochromic assessments, with a 4% improvement for Li+ intercalation and a 12% improvement for deintercalation, compared with the untreated WO3 samples. Further improvements have been achieved for the W18O49 nanowire thin films, with a stability improvement of 36% for Li+ intercalation and 60% for deintercalation against the as-prepared W18O49 nanowire samples during the electrochromic performance testing.