In situ photoelectron spectroscopy: a powerful tool to develop electrochromic materials

Abstract

In the building sector, a highly glazed envelope can lead to overheating in summer. In return, it increases daylighting and provides passive solar heating in winter. In order to make the best use of the solar resource, the use of a window with dynamic solar gain would be advantageous. Electrochromic coatings can darken on demand and thus modulate the solar gains. Some products are on the market but there are still several drawbacks with existing technologies such as the dynamic range, the switching speed, the color, the energy efficiency and the durability. Nanomaterials can improve the properties of such coatings. In this work, thin film materials for electrochromic glazing are studied in detail using X-ray photoelectron spectroscopy. The thin films are deposited via reactive magnetron sputtering and transferred to the analysis equipment without breaking the vacuum. The surface of the samples is therefore characterised as deposited, ex situ contamination is avoided and the surface state is not modified because no additional argon sputtering is required. X-ray photoelectron spectroscopy (XPS) provides information on the electronic structure of the sample. The quantification of the chemical composition can be determined through integration of peak areas. Information on stoichiometry can thus be obtained precisely on the same day of sample preparation and provides feedback to improve deposition parameters rapidly. In this study, an electrochromic material, tungsten oxide, is deposited by reactive magnetron sputtering and studied by XPS. Elemental compositions of the deposited films have been determined by in situ core-level photoelectron spectroscopy. The influence of deposition parameters, such as working pressure and O2/Ar mass flow ratio, on the electronic, electrochromic and optical properties has been studied. It was shown that in situ analysis by X-ray photoelectron spectroscopy is a powerful tool to investigate and develop new nanomaterials for electrochromic windows.