Optical properties

Fig. 3: Samples of photoelectrochromic devices with solid electrolyte in the bleached state (left, short circuit) and coloured by illumination equivalent to one sun (right, open circuit).

Fig.3 shows a sample with solid electrolyte in the bleached state (left, short cicuit) and the coloured state (right, after illumination in a solar simulator). The corresponding transmittance spectra are shown in fig. 4. The transmittance in the coloured state depends strongly on the thickness of the WO3 layer. It should be noted that the thickness of the Pt

layer and the amount of the dye are small enough to allow a transmittance of 62% in the bleached state for the photopic response spectrum, and 41% for the solar spectrum. The main losses of transmittance are due to the TCO layers (especially in the infra-red range) and the redox electrolyte, which can be made thinner.

Transparent nanoporous TiO2 and WO3 layers were prepared using sol-gel techniques described in [9]. Ormosilane was used as a binder in WO3 and TiO2 sols. TCO-coated
(F:SnO2) glass plates from Pilkington were covered by dip-coating first with WO3, then with TiO2. The thickness of the TiO2 layers was about 150 nm, and of the WO3 layers about 500 nm. The diameter of the particles in the WO3 layers is around 20 to 30 nm, in the TiO2 10 nm, as displayed by SEM (Scanning Electron Microscopy) measurements (fig. 5a). The thin Pt layers were sputtered. The dye (Ru 535 bis-TBA from Solaronix) was deposited by soaking the TCO/WO3/TiO2 layers in a solution of the dye in ethanol.

We investigated the WO3-TiO2 layers with high-resolution transmittance electron spectroscopy (HRTEM), IR spectroscopy, Auger electron spectroscopy and energy dispersive X-ray spectroscopy (EDXS) [10]. The result was that the WO3 particles consist of a crystalline monoclinic core (m-WO3), which is surrounded by an amorphous phase (a — WO3, fig. 5b). Because of the preparation process, TiO2 and SiO2 are left inside the WO3 layer, mainly situated in the amorphous phase. The content of TiO2 increases inside the amorphous phase from the inner to the outer parts of the WO3 grain.

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