DARPA-Funded Effort Achieves New Record Solar Cell Efficiency

August 15, 2007

In late July, a consortium led by the University of Delaware (UD) announced that it has created a solar cell that can convert 42.8% of the sunlight that hits it into electricity, besting a record set by Spectrolab and DOE's National Renewable Energy Laboratory (NREL) in December 2006. Under a program funded by the Defense Advanced Research Projects Agency (DARPA), the UD-led consortium employed a novel optical system that splits sunlight into three components while concentrating it by about a factor of 20. Three separate solar cells—made by UD, NREL, and Emcore Corporation—convert each piece of the solar spectrum into electricity to achieve the record conversion efficiency. Unlike typical concentrating solar cells, the new device features optics that are less than one centimeter thick and that accept sunlight coming from a wide range of angles, allowing the solar device to be mounted in a fixed position. See the UD press release.

Based on the success of the UD-led effort, DARPA announced in late July the start of a new three-year effort to drive the efficiency of solar cells to more than 50%. The DARPA project will also develop pilot-scale technologies to produce the high-efficiency solar cells at a cost of less than $5,000 per square meter, which is the current cost of commercial solar cells. The new consortium, led by DuPont and UD, has been awarded $12.2 million by DARPA, and the total cost of the project could reach $100 million, according to DuPont. See the press releases from DARPA (PDF 81 KB) and DuPont. Download Adobe Reader.

While the DARPA effort shows great promise, researchers continue to make headway in alternative approaches to building solar cells. For instance, Emcore, a participant in the UD-led consortium, achieved a 39% conversion efficiency for a triple-junction solar cell—a cell with three layers, each of which captures a different part of the solar spectrum—under sunlight concentrated by a factor of 1000. Emcore claims that as a record for a product that is in high-volume production, as opposed to a cell created in a research laboratory. Plextronics, Inc. has achieved a record 5.4% conversion efficiency with an organic solar cell, a result certified by NREL. The company aims to make low-cost solar cells by printing solar "inks" onto plastic or glass. Researchers at the New Jersey Institute of Technology (NJIT) aim to achieve a similar product by combining polymers with nanoscale carbon structures, that is, structures on the scale of billionths of a meter. And NREL finds great potential in nanoscale pieces of silicon, called quantum dots, which can generate two electrons from a single photon of sunlight. Incorporating such quantum dots into solar cells could boost their conversion efficiencies. See the press releases from Emcore, Plextronics, NJIT, and NREL.