Enhancement of electrochromic response and cyclic durability of WO3 thin films by stacking Nb2O5 layers

Abstract

This article reports on the control of interface morphology degradation between acid-based electrolytes and electrochromic WO3 thin films by stacking Nb2O5 layers to develop highly sustainable and durable electrochromic devices. Thin films of Nb2O5 sputtered at room temperature did not exhibit electrochromic properties, whereas WO3 thin films showed a typical electrochemical response by manifesting reversible color changes from dark blue (under negative potential) to transparent (under reverse potential). As the number of electrochromic cycles in the H2SO4 electrolyte increased, the columnar-structured WO3 thin film considerably changed to a nanoflake structure; however, the microstructure did not significantly change when Nb2O5 layers were stacked on the WO3 layers. The electrochromic performance of bilayer thin films was critically dependent on the stacking order and thickness of each layer. The thin film stack of 100-nm thick Nb2O5/200-nm thick WO3 exhibited enhanced transmittance modulation, response speed, and coloration efficiency compared with that of the 300-nm thick WO3 thin film. Furthermore, up to 1300 cycles, the bilayer thin film exhibited transmittance modulation exceeding 40%; it lost electrochromic switching at the 3500th cycle. In contrast, the single-layer WO3 lost its electrochromic properties at the 700th cycle. This study provides an excellent opportunity for fabricating devices with enhanced electrochromic properties and long-term cyclic durability that can be achieved by stacking Nb2O5 layers.