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The dependence of the chemical potential of WO3 films on hydrogen insertion

: Georg, A.; Schweiger, D.; Graf, W.; Wittwer, V.


Solar energy materials and solar cells 70 (2002), No.4, pp.437-446
ISSN: 0927-0248
Journal Article
Fraunhofer ISE ()
chemical potential; hydrogen insertion; electrochromic film; electrochromic device; colouring mechanism; microscopic structure; hydrogen concentration; tungsten oxide film; crystallinity; water content; catalytic coloration; hydrogen gas; electromotive force; proton binding energy; hexagon; amorphous sputtered film; hexagonal structure; hexagon centre site

Knowledge of the chemical potential of electrochromic films coloured by hydrogen is important for matching the elements of an electrochromic device, for understanding the colouring mechanism and for obtaining information about the microscopic structure of the film. The dependence of the chemical potential on the hydrogen concentration was measured electrochemically for tungsten oxide films of different crystallinity and water content. A new method for determining the chemical potential by catalytic coloration by hydrogen gas is introduced. It revealed that the increase in electromotive force with increasing crystallinity is due only to different binding energies of the protons. We expect the protons to be located in the centres of hexagons, which are created by WO/sub 6/ octahedra. According to our model, amorphous sputtered films show a hexagonal structure which is similar to that of evaporated films, but the hexagons are connected, leading to more hexagon centre sites, which increases the electromotive force.