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For many low-power applications, solar cells are used as an environmentally friendly power supply. In order to provide electrical power also in the absence of solar radiation, we invented a device which unifies both a solar cell and a rechargeable battery in one unit. The main components are a photoactive layer on a charge-storage layer. Thus, this new device represents a flat battery, which charges itself on illumination. In principle it can be produced cheaply, e.g., by screen printing. The characteristics of charging and discharging are presented with special consideration of the variation of light intensity and ion concentration in the electrolyte. A relatively high reverse reaction at the WO 3 electrode still takes place. After charging for 1 h under an illumination of 1000 W/m 2 , the first samples yielded 1.8 C/cm 2 when discharged in the dark.The dye-sensitized solar cell represents an electrochemical solar cell in which an organic dye is excited by illumination with light and transfers an electron from a redox couple in an electrolyte ͑I Ϫ /I 3 Ϫ in an organic solvent͒ to a highly porous TiO 2 layer. 1 By this, the energy of the electron is increased and this energy can be used to supply an external load. The electrons are re-injected into the I Ϫ /I 3 Ϫ couple in the electrolyte via a catalyst like Pt. This dye sensitized solar cell already contains most of the components of a battery, one redox couple (I Ϫ /I 3 Ϫ ), an electrolyte ͑mostly an organic solvent͒, and two electrodes ͑TiO 2 and Pt͒. To complete the battery, only a second redox couple is needed. TiO 2 is usually not sufficient, because its reduction potential for Li ϩ intercalation is too high, as can be concluded from Ref. 2. The charge capacity of a typical dyesensitized solar cell is measured in Ref. 3 to be about 20 C/ cm 2 , which is due to surface adsorption phenomena. In this paper, we present the idea to add a second redox couple in the form of an additional layer (WO 3 ), which is able to store the energy generated by the solar cell. The reduction potential for Li ϩ -intercalation of WO 3 is close to 0 V vs. standard hydrogen electrode, 4 and therefore it is suitable for the potentials in a dye-sensitized solar cell, where the reodox potential of the I Ϫ /I 3 Ϫ couple is about 0.53 V vs.hydrogen, 5 and the open-circuit potentials of the dye-sensitized TiO 2 layer vs. the I Ϫ /I 3 Ϫ couple on the Pt electrode are about 0.7 V. 6The design of the photovoltaically self-charging battery ͑PSB͒ is shown in Fig. 1. It consists of a photoactive layer ͑dye-coated nanocrystalline TiO 2 ͒ which is situated on a charge-storage layer ͑e.g., nanocrystalline WO 3 ͒ on a glass substrate coated with a transparent conductive oxide ͑TCO͒. The opposing electrode is a platinized, TCO-coated glass substrate. The Pt layer is so thin that it is still transparent. The pores of the WO 3 and TiO 2 layers and the space between the two electrodes are filled with an electrolyte which contains Li ϩ and a redox couple ͑I Ϫ and I 3 Ϫ ͒ in an organic solvent ͑e.g.,pro...

We prepared photoelectrochromic devices, which can be switched from transparent to dark, by adding an electrochromic layer (WO 3 ) to a dye-sensitized solar cell. Layers in different orders are compared: first, the TiO 2 layer situated above the WO 3 layer; second, the WO 3 layer situated above the TiO 2 layer; and third, the TiO 2 and WO 3 layers situated on opposite electrodes, leaving out the Pt layer. For the first and second configuration, fast and intense coloring is possible together with fast bleaching, which is in principle not possible for the third. The comparison of the first and the second configurations demonstrated that it is possible to exchange the positions of the TiO 2 and WO 3 layers. The layers were prepared by dipping the samples in sols and we found that material of the second layer penetrates the first layer, which influences the kinetics of the coloring and bleaching. For the first configuration, we showed that it is possible to switch the transmittance not only from the illuminated to the nonilluminated state, but also during continuous illumination. For the first configuration and under 1 sun of illumination, the visual transmittance changed from 51 to 4.8%. Switching times are about 3 min for coloring and bleaching.

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