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Forschungszentrum Jülich - Annual Report 2012

Annual Report 2012 | Forschungszentrum Jülich 21 A t the end of 2012, it cost no more than 20 cents to gen- erate a kilowatt hour of solar electricity, which is less than what households pay on average for the same quantity of electricity. In order to reduce the costs even more, research- ers throughout the world are working on saving material and energy in solar cell production and on improving the current yield. To this end, different concepts are being pursued, including thin-film and wafer solar cells. Last year, scientists from Jülich’s Institute of Energy and Climate Research showed that both types of cell benefit from an innovative material. The material is a mixture of micro- crystalline silicon and amorphous silicon oxide. Microcrystalline substances com- prise tiny crystals in which the atoms are regularly ordered, while amorphous materials do not have a regular struc- ture. “According to our investigations, layers of the material mixture form channels of microcrystalline silicon which conduct electric current, while the amorphous silicon oxide ensures that the layers have a high permeability to light,” says Jülich photovoltaic expert Dr. Friedhelm Finger. Mixtures of silicon and silicon oxide can be used in the intermediate reflec- tor and in the semiconducting layers of a thin-film solar cell where small amounts of impurities like boron or phosphorus increase the number of mobile electric charge carriers. These ‘doped’ layers are like a sandwich enclosing the semiconducting layer with no impurities. And the more light they let through to this intermediate layer, the better. The Jülich researchers have demonstrated that their innovative material is beneficial in this regard. They have also prepared dozens of thin-film solar cells with the material and determined their stable efficiency – a measure of how effectively a cell in continuous operation converts light into electric current. With a value of 11.8%, these solar cells performed much better than other Jülich cells of a similar design without the new material and than commercially available modules, which achieve an efficiency of at best 10%. More material and energy are required to produce solar cells with crystalline silicon wafers compared to thin-film solar cells, but they also achieve much higher efficiencies. Using a special type of construction, referred to by the specialists with the abbrevia- tion ‘HIT’ (heterojunction with intrinsic thin layer), Jülich scientists achieved a maximal efficiency of 19% in 2012. All contact layers were composed of the innovative material mixture. “This demonstrates the potential of silicon oxide materials,” says Finger’s colleague Kaining Ding. And there are still ways of increasing efficiency even further – for example, texturing the wafer surfaces. The perfect material for many layers in a solar cell would be electrically conductive like silicon and simultaneously transparent like quartz glass. Jülich scientists incorporate a mixture of substances in their high-tech solar cells that bring them much closer to achieving this ideal. Smart Material for Solar Cells Kaining Ding (left) with the prototype of a small wafer solar cell containing layers made from an innovative mixture of materials. Thin-film solar cells (in the foreground) could also benefit from this material in the future. Institute