Solution processed WO3 for electrochromic applications. Structure and properties

Hadjittoouli, Nicholas and Antoniou, Giorgos and Halcovitch, Nathan Ross and Milne, W.I. and Adamopoulos, George (2018) Solution processed WO3 for electrochromic applications. Structure and properties. In: 7th International Symposium on Transparent Conductive Materials, 2018-10-14 - 2018-10-19, Chania, Crete, Greece.

Abstract

Electrochromic properties of transition metal oxides have extensively been investigated over the last couple of decades mainly due to their potential applications in “smart” windows. Amongst them, tungsten trioxide (WO3) outperforms the vast majority of transition metal oxides, making it the most attractive candidate for electrochromic applications. WO3 films can be deposited by a wide range of techniques, including chemical vapor deposition, e-beam evaporation, sputtering, thermal vacuum evaporation, as well as sol-gel. At present, reactive sputtering is method of choice for large area applications such as building glass panels. Sputtered WO3 films are denser than those obtained by other techniques however denser WO3 films with low porosity and smooth surfaces are unsuitable for electrochromic applications as a rough and porous surface structure would be more conducive to the intercalation and de-intercalation of proton during the electrochromic process, significantly minimising the optical modulation time To that end and in order the costly character of the vacuum-based deposition techniques to be equally addressed, we have investigated the deposition of WO3 by a solution-processed technique namely the spray pyrolysis in order to increase the surface roughness of a desired phase WO3. The latter has been achieved by choosing suitable precursors and solvents as well as substrate temperatures. Amongst the various tungsten precursors, tungsten hexachloride was the precursor of choice as it decomposes at temperatures as low as 280 oC. This was independently confirmed by TGA. Aerosols of WCl6 solutions in ethanol and acetylacetone were spray coated on preheated substrates in the range between 200 oC and 550 oC. Analyses revealed that for an optimal substrate temperature of about 400 oC, orthorhombic WO3 films of an average crystal size of about 5 nm and optical band gap of 3.7 eV were deposited. The WO3 structure was further confirmed by FTIR where the infrared active phonon modes were illustrated. Additionally, the films were of a remarkably low Urbach tail energy of less than 380 meV, high optical transparency in excess of 80 % in the visible spectrum and an RMS roughness of about 8 nm. The latter, combined with the remarkably large grain size of about 100 nm as illustrated by AFM, constitute a promising finding for the application of this simple and cost-effective deposition technique in electrochromic devices manufacturing over large area.