Strong coloration of nanoporous tungsten oxides by in-vacuo lithiation for all-solid-state electrochromic devices

Electrochromic windows allow for an improved management of solar heat gains in buildings and enhanced visual comfort of building users. All-solid-state electrochromic devices can be especially durable. During their fabrication lithium has to be inserted, preferably in-vacuo. For such a dry lithiation process it is essential to determine to which extent sufficiently thick electrochromic layers can be colored and with which efficiency. In this study, we focus on the dry lithiation of sputtered nanoporous tungsten trioxide films and the induced optical response. The porosity of the WO3 films was in the order of 18 %, as determined from variable angle spectroscopic ellipsometry and effective medium approximation. Films with a thickness above 1 mu m were dry-lithiated yielding a deep coloration, with a lithiation efficiency LE up to 1.5 center dot 106 cm2 center dot mol- 1. Hereby the light transmittance was reduced from 85.7% to 2.0%, with the spectral transmittance approaching 0% in the range from 600 nm to 1500 nm. These results suggest that lithium can diffuse over a large distance in nanoporous WO3. The columnar morphology and nanoporous structure of the thin films deposited at high working pressure is beneficial for the displacement of the lithium ions. All-solid-state devices were prepared using in-vacuo lithiation, tantalum pentoxide as a solid electrolyte and nickel oxide as a counter electrode. During cycling the solar transmittance varied between 8 % and 52 % and the light transmittance from 7.8% to 53.1%, the latter with a coloration efficiency CE up to 63 cm2 center dot C-1. Accordingly, switching times of 17 s for coloration and 13 s for bleaching were determined. The gained insights are of high practical relevance for the fabrication of all-solid-state electrochromic devices.