Ultraefficient Photovoltaics

The new class of materials enabling the world’s best solar cell has a bright future.

A solar cell more than twice as efficient as typical rooftop solar panels has been developed by Spectrolab, a Boeing subsidiary based in Sylmar, CA. It makes use of a highly customizable and virtually unexplored class of materials that could lead to further jumps in efficiency over the next decade, making solar power less expensive than grid electricity in much of the country.

Stress release: Graded buffer layers are the key to combining structurally incompatible semiconductors in high-efficiency solar cells, such as this one from the National Renewable Energy Laboratory. Strain from mismatched crystal structures cracks the cell’s eight-layered buffer (see n=8), relieving strain in the crystal and thereby protecting the active semiconductor layer above.

The cell, which employs new “metamorphic” materials, is designed for photovoltaic systems that use lenses and mirrors to concentrate the sun’s rays onto small, high-efficiency solar cells, thereby requiring far less semiconductor material than conventional solar panels. Last month Spectrolab published in the journal Applied Physics Letters the first details on its record-setting cell, initially disclosed in December, which converts 40.7 percent of incoming light into electricity at 240-fold solar concentration–a healthy 1.4 percent increase over the company’s previous world-record cell. Other groups are developing promising cells based on the new type of materials, including researchers at the Department of Energy’s National Renewable Energy Laboratory (NREL), in Golden, CO. The NREL researchers will soon publish results in the same journal showing that their NREL’s designs are tracking Spectrolab’s, improving from 37.9 percent efficiency in early 2005 to 38.9 percent efficiency today.

Metamorphic semiconductors resemble the high-efficiency cells used in space. Like the cells that grace satellites and planetary landers, they employ three layers of semiconductors, each tuned to capture a slice of the solar spectrum (solar panels have only one active layer). These semiconductor layers are assembled, one upon the next, by altering elements fed to a crystal growing in a vacuum. To avoid growing crystals filled with energy-trapping defects, device designers have until recently employed only a limited repertoire of semiconductors, such as germanium and gallium arsenide, which form similar crystal structures.

Metamorphic materials provide flexibility by throwing off this structural constraint, employing a wide range of materials, including those with mismatched structures. “The parameter space you can explore using mismatch opens up a whole world of possibilities,” says NREL principal scientist Sarah Kurtz.

What makes this possible is the addition of buffer layers between the semiconductor layers. This technique was employed in the early 1990s to make high-speed transistors combining silicon and germanium, and then introduced to photovoltaics later in the decade by Cleveland-based semiconductor developer Essential Research. Spectrolab has, however, seen the best results. Its 40.7 percent metamorphic cell improves on Spectrolab’s best conventional cells by incorporating new semiconductors in the top and middle layers that excel at capturing infrared light that was all but missed by the cell’s predecessors.

Such high output may be just the beginning. Raed Sherif, director of concentrator products at Spectrolab, says there is every reason to believe that these metamorphic solar cells will top 45 percent and perhaps even 50 percent efficiency. Sherif says those efficiencies, combined with the vast reduction in materials made possible by 1,000-fold concentrators, could rapidly reduce the cost of producing solar power. “Concentrated photovoltaics are a relatively late entry in the field, but it will catch up very quickly in terms of cost,” he predicts. (See “Solar Power at Half the Cost.”)

Sherif says that right now his company is focusing commercialization efforts on the older and better-known designs, which currently deliver 35 to 37 percent efficient modules and could improve to 40 percent efficiency within two to three years. But he says the metamorphic approach is more likely to achieve the 45 percent efficiency level the company hopes to hit within six to seven years. Sherif estimates that a 40 percent module would reduce overall cost by about 14 percent if Spectrolab holds at its current $10-per-square-centimeter module price, while a 45 percent cell would trim system costs by an additional 9 to 10 percent.

Boeing anticipates further cost reductions as other components improve or are mass-produced. Under a $29.8 million concentrated-photovoltaic development partnership with the Department of Energy announced this spring, Boeing promises to cut the delivered price of electricity via concentrated solar to 15 cents per kilowatt hour by 2010, from an estimated 32 cents per kilowatt hour today, and to cut that price in half again by 2015. That would make solar power less expensive than electricity from the grid in much of the United States, where the average price of electricity in recent months has been about 10 cents per kilowatt hour.

Spectrolab’s competitors, meanwhile, see metamorphic materials as a way to reduce the use of relatively exotic and expensive semiconductor wafers on which they are now produced. NREL’s design, for example, can be lifted off of the germanium wafers on which both NREL’s and Spectrolab’s cells are grown. The expensive wafers could then be reused. Metamorphic photovoltaic startup 4Power, of Windham, NH, proposes to employ metamorphic buffers to grow high-efficiency cells on the same wafers of silicon on which nearly all semiconductor chips are produced. Silicon wafers are cheaper to buy and process than germanium wafers. 4Power founder Eugene Fitzgerald, a materials engineering professor at MIT and a metamorphic-materials pioneer, claims that this would cut the cost of growing high-efficiency cells in half.

What remains to be demonstrated, notes NREL’s Kurtz, who leads the lab’s high-efficiency solar research, is whether solar concentrators–especially their sensitive optics–will prove reliable in the field.

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