Researchers Crash Through Solar Efficiency Barriers

Funded by a U.S. government research grant, a consortium led by the University of Delaware is making great strides forward in solar electric generation and is well on it's way to achieving its goal of 50 percent efficiency. The consortium announced on July 30, 2007 that in applying multiple innovations to a very high-performance crystalline silicon solar cell platform it has achieved a record-breaking combined solar cell efficiency of 42.8 percent from sunlight at standard terrestrial conditions.

That number demonstrates an important milestone on the path to the 50 percent efficiency goal set by the Defense Advanced Research Projects Agency (DARPA) Very High Efficiency Solar Cell (VHESC) program and is a significant advance from the current record of 40.7 percent announced in December. Combined with the demonstrated efficiency performance of the very high efficiency solar cells' spectral splitting optics, which is more than 93 percent, these recent results put the pieces in place for a solar cell module with a net efficiency 30 percent greater than any previous module efficiency and twice the efficiency of state-of-the-art silicon solar cell modules. If progress continues at this pace the new, more efficient modules could be in production as early as 2010.

The ground-breaking research was led by Allen Barnett, principal investigator and UD professor of electrical and computer engineering, and Christiana Honsberg, co-principal investigator and associate professor of electrical and computer engineering.

"The achievement of this benchmark is a major step forward in the ongoing development of low-cost solar photovoltaic technology," Rhone Resch, president of the Solar Energy Industries Association, said. "Furthermore, we applaud DARPA for making a strategic investment in American's energy security. We anticipate that this project will result in a wide range of commercial solar applications that will benefit the U.S. military and American consumers alike."

"DARPA has leapfrogged the 'conventional,' demonstrating that creativity and focus can significantly accelerate revolutionary research-bench concepts toward reality, demonstrating this does not have to take decades," Lawrence L. Kazmerski, director of the U.S. Department of Energy's National Center for Photovoltaics at the National Renewable Energy Laboratory in Golden, Colo., said. "This is a first step-but a significant one in making sure our energy future is what we know it should look like."

The highly efficient VHESC solar cell uses a novel lateral optical concentrating system that splits solar light into three different energy bins of high, medium and low, and directs them onto cells of various light sensitive materials to cover the solar spectrum. The system delivers variable concentrations to the different solar cell elements. The concentrator is stationary with a wide acceptance angle optical system that captures large amounts of light and eliminates the need for complicated tracking devices.

Barnett and Honsberg said that reaching the 42.8 percent mark is a significant advance in solar cell efficiency, particularly given the unique small and portable architecture being used by the consortium and the short time - 21 months - in which it was developed.

Modern solar cell systems rely on the concentration of the sun's rays, a concept similar to youngsters using magnifying glasses to set scraps of paper on fire. Honsberg said the previous best of 40.7 percent efficiency was achieved with a high concentration device that requires sophisticated tracking optics and features a concentrating lens the size of a table and more than 30 centimeters, or about 1 foot, thick.

The UD consortium's devices are potentially far thinner at less than 1 centimeter. "This is a major step toward our goal of 50 percent efficiency," Barnett said. "The percentage is a record under any circumstance, but it's particularly noteworthy because it's at low concentration, approximately 20 times magnification. The low profile and lack of moving parts translates into portability, which means these devices easily could go on a laptop computer or a rooftop."

Honsberg said the advance of 2 percentage points is noteworthy in a field where gains of 0.2 percent are the norm and gains of 1 percent are seen as significant breakthroughs.

"This achievement is the direct result of the new architecture we developed under the DARPA program," Barnett and Honsberg said. "By integrating the optical design with the solar cell design, we have entered previously unoccupied design space leading to a new paradigm about how to make solar cells, how to use them, and what they can do."

During the first 21 months of the VHESC program, a diverse team of academia, government lab and industrial partners, led by UD, was focused on developing the technology basis for a new extremely high efficiency solar cell. The rapid success of that effort has enabled the present transition to a focus on prototype product development.

"This is a solar cell that works," Barnett said, adding, "This technology has the potential to change the way electricity is generated throughout the world."

Barnett believes the 50 percent efficiency mark is just the beginning. "Our best inventions are in front of us," he said. "The consortium has been a super team, and has worked to develop new devices and architectures based on a breakthrough design paradigm."

Honsberg said the efficiency potential for solar cells is significantly higher still, which leaves "large room for improvement." She said she enjoys the engineering challenge, particularly in a field in which "we create things that make a difference."

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