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  • Full spectrum boosts solar cell power

Photonics Spectra
May 2012
“… in the last decade, revolutions in photonic material design and large-area nanostructure fabrication have given researchers and technologists tools to enable a new era of ultrahigh-efficiency photovoltaics.” – Harry Atwater, California Institute of Technology

Solar cell efficiency, currently hovering in the 15 to 20 percent range, can theoretically be boosted to as high as 70 percent by printing specially engineered nanostructures on the cells, researchers say.

Conversion efficiency of solar cells has long been thought limited to 34 percent for a single material or less than about 45 percent for the most efficient cells. A thermodynamic limit is responsible for this practical impediment.

The conversion process of solar cells is typically not very efficient: A conventional silicon solar cell commercially available today converts only 15 to 20 percent of the energy of the sunlight to electricity, with the balance lost as heat.

Blue and green lightwaves are converted to electricity with an efficiency of less than 50 percent, while infrared light is not absorbed by a silicon solar cell at all. The highest efficiency record realized by a silicon solar cell was 27 percent.

Light that is not converted in the solar cell leads to thermodynamic disorder known as entropy and, as a result, to reduced cell voltage. As a result, the maximum achievable efficiency is limited to 34 percent, also known as the Shockley-Queisser limit.

The incomplete trapping of light inside the solar cell and defects in the solar cell material’s crystal structure also cause efficiency loss.

But Harry Atwater, an applied physicist at California Institute of Technology in Pasadena, and his colleague Albert Polman of the FOM Institute for Atomic and Molecular Physics in Amsterdam say that it should be possible to achieve efficiency as high as 70 percent by managing the light with specially engineered nanostructures printed on the surface of the solar cells; then it can be better trapped, and many of the efficiency problems can be solved.

In their Nature Materials paper, Atwater and Polman describe several recipes for achieving these improvements. The inspiration for some of these ideas, which are based on the integration of photonic nanostructures and circuits on the solar cell, comes from optical integrated circuit technology, where structures to guide and control light are routinely made.

“Before 2000, scientists and technologists had developed materials and devices with high electronic quality, but little attention was paid to optical design,” Atwater said. “But in the last decade, revolutions in photonic material design and large-area nanostructure fabrication have given researchers and technologists tools to enable a new era of ultrahigh-efficiency photovoltaics.”

He noted that the solar cell community historically has assumed that cells could be made either with low efficiency at a low cost or with high efficiency at a high cost. Now, however, high-efficiency, low-cost solar cells are achievable.

“The solar cell community has driven down solar panel cost, yet is very conservative and has not boosted efficiency significantly,” Polman said. “But solar panels with a high efficiency take up much less space because you need fewer panels to generate the same amount of power. That saves costs of land, installation and infrastructure.

“With a slightly more complex solar cell, it becomes possible to convert all colors of the light from the sun to electricity, and an efficiency of up to 70 percent is achievable.”


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