Electrochromic Stability of WO₃ Thin Films with Nanometer-Scale Periodicity and Varying Degrees of Crystallinity

Abstract

Electrochromic tungsten oxide (WO 3 ) thin films with nanometer-scale porosity have been synthesized via a sol-gel procedure making use of evaporation-induced self-assembly. According to wide-angle X-ray scattering data combined with electrochemical analyses, the degree of crystallinity ranging from fully amorphous to 100% crystalline can be adjusted by straightforward annealing. The three-dimensional cubic pore structure is thereby almost not affected. Aside from the material characterization, in this work we specifically focus on the overall electrochemical and electrochromic behavior (coloration efficiency, charge capacity, etc.) upon changes in the operating temperature. As a main result, only the mesoporous highly crystalline WO 3 films display long-term cycling stability under realistic environmental conditions. We further demonstrate that sufficient crystallinity is needed to ensure stability of the inherent electrochemical properties at high operating temperatures (up to 70 °C). Thus, only the WO 3 films with a highly crystalline framework exhibit almost unchanged electrochemical/electrochromic characteristics after prolonged potentiostatic cycling and exposure to elevated operating temperatures. In contrast, amorphous and partially crystalline films suffer from irreversible performance degradation due to structural modifications.

Electrophoretic Deposition of Nanostructured Electroactive Materials

“…The switching time, namely bleaching (tb) and coloration i = KC/(π Dt) 1/2 (tc) time, are calculated based on the standard time required for 70% optical modulation. Coloration time, tc70%, is found to be 28.8 s, which is comparable to the reported values (~ 25 s) [28]. Nevertheless, it should be noted that the bleaching time, tb70%, of 4.5 s is much superior to the crystalline (~ 22 s) and amorphous WO 3 (~ 18-36 s) reported [28,29].…”

“…Coloration time, tc70%, is found to be 28.8 s, which is comparable to the reported values (~ 25 s) [28]. Nevertheless, it should be noted that the bleaching time, tb70%, of 4.5 s is much superior to the crystalline (~ 22 s) and amorphous WO 3 (~ 18-36 s) reported [28,29]. This improvement is attributed to the porous nature of the oxide layer, which consists of nanorods.…”

Evaporation‐Induced Self‐Assembly for the Preparation of Porous Metal Oxide Films

“…The infl uence of the porosity on the switching times was studied by Sallard et al, who prepared electrochromic WO 3 layers with regular cubic arrangement of the pores, using KLE as a template. 57,73 As expected, the electrochromic response times for both the coloration and the bleaching steps were signifi cantly shortened as compared with nonporous WO 3 layers prepared by similar sol -gel methodology. Moreover, the used synthesis procedure enabled preparation of fi lms with an adjustable degree of crystallinity while maintaining the same pore structure.…”

Efficient electrochromic device based on sol–gel prepared WO3 films

“…The structural parameters are well along with PDF card number 20-1323, indicating that the films are of crystalline triclinic structure [15]. It is well known that the EC performance of WO 3 is closely related to its level of crystallization [22]. However, the high crystallinity of metallic oxide exhibited inferior performance to the amorphous one, because of its ordered and dense structure, which is known to inhibit insertion and desertion of ions and maybe lead to a long response time [23,24].…”