Solar cell breaks efficiency record

It converts almost 41 percent of light that hits it into electricity, almost double the efficiency of current commercial cells.

Boeing-Spectrolab has developed a solar cell that can convert almost 41 percent of the sunlight that strikes it into electricity, the latest step in trying to drop the cost of solar power.

Potentially, the solar cell could bring the cost of solar power down to around $3 a watt, after installation costs and other expenses are factored in, over the life of the panel. The new cost information comes from Boeing, whose Spectrolab unit supplies searchlights and solar simulators, and the Department of Energy, which sponsored the project. Current silicon solar cells provide electricity at about $8 a watt, before government rebates. The goal is to bring it to $1 a watt without rebates or incentives.

The cell achieves 40.7 percent efficiency. The Department of Energy has been sponsoring research to find ways to get solar cells past the so-called 40 percent barrier.

Earlier this year, researchers at Lawrence Berkeley National Laboratories reported that cells made of a new type of semiconductor, zinc-manganese-tellurium, combined with a few atoms of oxygen, could convert around 45 percent of sunlight into electricity. That technology, also partly sponsored by the Department of Energy, has been licensed to RoseStreet Labs in Arizona. It remains to be seen whether this material can be made into solar cells economically.

Sharp Solar, one of the biggest solar companies in the industry, showed a solar cell offering 36 percent efficiency earlier this year. The Sharp cell includes a concentrator--a thin lens that focuses sunlight on the cell--but is not made of silicon. It instead is made out of III-V compounds, molecules made from elements in the III and V columns of the periodic table of elements. (The metallic element gallium--used in semiconductors and optoelectronic devices--is from this neighborhood.)

Currently, the best commercial silicon solar cells can , and physics dictates that maximum efficiency for these cells will come at around 26 percent.

Boeing got around that barrier by integrating two technologies. One, the solar cell, contains a layer of concentrators. From a practical point of view, using a concentrator is like adding extra surface area to the cell.

The solar cell also contains more than one material. Silicon cells interact with only a limited part of the light spectrum. Additional layers of gallium arsenide or other materials can convert light in other portions of the spectrum into electricity. Making so-called multi-junction solar cells is more expensive than making single-junction silicon cells. Still, many companies believe the higher manufacturing expense can be offset by cost savings from the cells' greater electrical output.

Boeing, however, did not state what materials it used in its cell.

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