The success of a photovoltaic power supply is determined largely by the lighting conditions: Outdoors, a device is exposed to solar intensities of up to 1000 W/m2, but on a desk under artificial lighting or during overcast weather, the value is only 1 – 10 W/m2. The demands on the solar cells reflect this: For a given area, the output power must be as high as possible, the cells should provide close to maximum voltage already for very low illumination intensities, and good use should be made of artificial light.

These specifications are met very well by high-efficiency cells from Fraunhofer ISE, with efficiency values exceeding 20 %. To demonstrate their advantages, we developed a solar charging device for a commercially available mobile phone, which together form a unit. The charging device consists of an 8-cell solar generator connected in shingle technology, with an area of 13 x 4 cm2 and 1 W rated power. A low-loss charge controller charges the Li-ion battery integrated in the mobile phone. Energy balance calculations and practical tests demonstrated that stand-by operation with simultaneous recharging is possible without difficulties during summer and the transitional seasons. Under artificial lighting or during overcast weather, the stand-by duration is considerably longer than is possible with battery operation alone.