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 WO3 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.