Roll-Up Solar Panels  

Technology Review has an article on thin film solar vendor Xunlight's approach - making cells on flexible steel sheets - Roll-Up Solar Panels.

Xunlight, a startup in Toledo, Ohio, has developed a way to make large, flexible solar panels. It has developed a roll-to-roll manufacturing technique that forms thin-film amorphous silicon solar cells on thin sheets of stainless steel. Each solar module is about one meter wide and five and a half meters long.

As opposed to conventional silicon solar panels, which are bulky and rigid, these lightweight, flexible sheets could easily be integrated into roofs and building facades or on vehicles. Such systems could be more attractive than conventional solar panels and be incorporated more easily into irregular roof designs. They could also be rolled up and carried in a backpack, says the company's cofounder and president, Xunming Deng. "You could take it with you and charge your laptop battery," he says.

Amorphous silicon thin-film solar cells can be cheaper than conventional crystalline cells because they use a fraction of the material: the cells are 1 micrometer thick, as opposed to the 150-to-200-micrometer-thick silicon layers in crystalline solar cells. But they're also notoriously inefficient. To boost their efficiency, Xunlight made triple-junction cells, which use three different materials--amorphous silicon, amorphous silicon germanium, and nanocrystalline silicon--each of which is tuned to capture the energy in different parts of the solar spectrum. (Conventional solar cells use one primary material, which only captures one part of the spectrum efficiently.)

Still, Xunlight's flexible PV modules are only about 8 percent efficient, while some crystalline silicon modules on the market are more than 20 percent efficient. As a result, Xunlight's large modules produce only 330 watts, whereas an array of crystalline silicon solar panels covering the same area would produce about 740 watts.

United Solar Ovonic, based in Auburn Hills, MI, is already selling flexible PV modules. The company also uses triple-junction amorphous silicon cells, and its modules can be attached to roofing materials. But Xunlight's potential advantage is its high-volume roll-to-roll technique. "If their roll-to-roll process allows them to go to lower cost and larger area, that's the central advantage," says Johanna Schmidtke, an analyst with Lux Research, in Boston. "But they have to prove it with manufacturing."