Kinetic Properties

The photoelectrochromic device, as it is described in this paper, allows various switching modes (fig.6). The device colours on illumination with open circuit and it bleaches in the dark with short circuit within about 10 minutes with an solid ion conductor. It is possible to adjust the electrolyte such that the device bleaches with short circuit under illumination or that it retains its colour. Slow bleaching occurs with open circuit conditions in the dark (about 10 hours for liquid electrolyte, up to 100 hours for solid electrolyte).

Fig. 6: Various switching modes under illumination (sun) or in the dark (cloud) with open circuit and short circuit. The top three modes take about 10 minutes, whereas the last one (dark, open circuit) takes about 10 to 100 hours.

Thus, the only impossible process seems to be a colouring in the dark, but this is usually not needed. However, if needed, the device still acts as an electrochromic device, i. e. it can be coloured and bleached by applying an external voltage, independent of the conditions of illumination.

The dominating kinetic processes were investigated in detail, with a focus on liquid electrolytes as a model system. The results will be published soon. For solid ion conductors, the colouring and bleaching is shown in fig. 7. For open circuit, one can measure the voltage of the device, which reaches about 0.5V. For short circuit, one can measure the current density with respect to the area of the device, and integrate to get the

charge. The charge is proportional to the optical density, the coefficient of proportionality being the coloration efficiency.

Fig. 8: Bleaching in open circuit in the dark. (liquid electrolyte)

The curves displayed in fig. 8 and 9 were measured applying a liquid electrolyte. Fig.8 shows the bleaching in open circuit in the dark due to loss reactions, which are mainly electron transfers from the WO3 to the I3- in the electrolyte. For solid ion conductors, an even longer time (100 hours instead of 10 hours) of self bleaching can be achieved. Fig. 9 demonstrates the switching during constant illumination by switching between open and short circuit conditions, with open circuit voltage and short circuit current density, respectively.

Fig. 9: Colouring in open circuit and bleaching in short circuit with constant illumination (1 sun). (liquid electrolyte)

Conclusions

A photoelectrochromic device has been presented, which combines an electrochromic layer of WO3 with a dye solar cell. The layers show a complex nanostructure. The high porosity allows the electrolyte to penetrate into the layers of WO3 and TiO2, even for polymer ion conductors. The device can be switched under illumination as well as in the dark. For a cell with solid electrolyte, the visible (solar) transmittance changes from 62% (41%) to 2% (1%) in roughly 10 minutes.

Acknowledgements:

This work was supported financially by the University of Freiburg, Germany and by the German Ministry of Education and Research BMBF.

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