Solar Cells Operating at Double the Usual Output

Solar cells that may be able to convert up to half of the sunlight that falls on them into electricity are undergoing tests in North Carolina. Even the best cells today only operate at 25 percent efficiency.

John Rogers, of the University of Illinois, Urbana-Champaign has devised cells in panels that are deployed in pilot projects by Semprius, The Economist reports.

The cells alone can convert 42.5 percent of sunlight into electricity he told this year’s meeting of the American Association for the Advancement of Science. When they are part of a panel, efficiency drops to 35 percent, still well above the industry standard. When the design is amended, he believes, their efficiency could rise to 50 percent. Their secret is that they are actually not one cell, but four, stacked one on top of another. Concentrating photovoltaic (CPV) modules work in much the same way as traditional PV modules. However, they use optics to concentrate the sunlight onto solar cells that do not cover the entire module area.

Rogers chooses materials so that the bottom of the band gap of the top layer matches the top of the band gap of the one underneath, and so on down the stack, says The Economist, adding that “Each layer thus chops off part of the spectrum, converts it into electrical energy and passes the rest on.”

Materials needed to make these semiconductors are problematic. They include expensive materials like arsenic, gallium and indium. But Rogers has devised a way to overcome this, says The Economist. Normal solar-cell modules are completely covered by semiconductor, but in his only 0.1 percent of the surface is.

Semprius’s panels are now being tested at 14 sites around the world. How much they will cost to make when mass produced is unknown.