NREL principal engineer Keith Emery.
Solar Cells Do Create Jobs
December 2011 / January 2012
Volume 9 Number 6
How small can a solar cell be and still be a powerhouse? How about six hundred microns wide—about the diameter of a dot made by a ballpoint pen? That’s the size of the small but powerful solar cells designed by North Carolina-based Semprius, a small company that, with a boost from the federal government, attracted private investment and now has a highly efficient solar module ready for market.
The National Renewable Energy Laboratory recently validated greater than 41 percent efficiency at a concentration of 1,000 suns for Semprius’s tiny cells, made from elements in the third and fifth column of the periodic table. The energy conversion efficiency of a solar cell is the percentage of sunlight converted by the cell into electricity. This is one of the highest efficiencies recorded at this concentration.
Seed money from DOE, together with the expertise and testing facilities at NREL, accelerated the technology and helped lift Semprius from a small electronics startup with a novel idea to an actual difference-maker in the photovoltaics world.
Semprius’ triple-junction cells are made of gallium arsenide. Low-cost lenses concentrate the sun light onto the tiny cells 1,100 times. Their tiny size means they occupy only one-one thousandth of the entire solar module area, reducing the module cost. In addition, the use of a large number of small cells helps to distribute unwanted heat over the cell’s structure, so there’s no need for expensive thermal management hardware such as heat fins.
Semprius engineers use the company’s patented micro-transfer printing process to allow the micro-cells to be transferred from the growth substrate to a wafer. In a massive parallel process, thousands of cells are transferred simultaneously. This allows the original substrate to be used again and again, dramatically cutting costs. It also provides a way to handle very small cells.
This low-cost approach, which Semprius executives say can cut manufacturing costs by 50 percent, caught the eye of energy giant Siemens, which this year took a 16 percent stake in Semprius, as part of a $20 million investment from venture capitalists.
SunShot Incubator program spurs private investment
Since 2007, DOE has invested $50 million for 35 solar startups to participate in the PV Incubator program—now the SunShot Incubator—at NREL. Private investment in those firms now totals more than $1.3 billion, a 25-to-1 multiple.
Semprius broke ground on a manufacturing plant in Henderson, N.C., last month, employing 48 construction workers in an area with a 13 percent unemployment rate. The state of North Carolina and local agencies kicked in $7.9 million for the 50,000-sq. ft. plant, which is expected to employ 256 people at full build-out.
DOE and NREL selected Semprius to be one of their PV Incubator companies in 2010. Incubator companies get $1 million to $3 million to develop their concepts into actual working prototypes or pilot projects. And they also get the expertise of NREL scientists to help overcome obstacles and test for reliability and validity.
Semprius’s back story, though, begins at the University of Illinois where Professor John Rogers and his team developed the transfer-printing process initially intended for flexible electronics. Soon, Rogers realized that applying the technology to a concentrated photovoltaic (CPV) design could be much more lucrative.
Semprius grows layer on the original gallium-arsenide substrate, and then grows the multi-junction solar cell structure on top of the layer. Then after wafer processing, the transfer printing process is used to remove the cells from the gallium-arsenide substrate and transfer them to an interposer wafer.
“We’re using a completely different approach to what has been practiced,” said Kanchan Ghosal, CPV applications engineering manager. “This approach uses micro-cells and transfer printing to significantly reduce the use of materials in highly concentrated PV modules. And it provides a highly parallel method to manufacture the module, based on established microelectronics processes and equipment.
Semprius uses inexpensive lenses to concentrate the power of 1,100 suns on each cell, so less than a tenth of 1 percent of the module’s surface area is semiconductor cells. “We use a lot less metal in the module, since we have a short optical path and don’t need heat sinks in the modules,” said Ghosal.
North Carolina Gov. Bev Perdue attended the groundbreaking of Semprius’s manufacturing plant in August, citing her state’s investments in education and job training as the reason the company chose to locate there. The plant is expected to start operating next year, with an initial capacity of 5 megawatts, eventually growing to 35 megawatts.
The available market for highly-concentrated photovoltaics is expected to double or more each year over the next nine years, reaching greater than 10 gigawatts of power by 2020, according to Semprius CEO Joe Carr.
Semprius first looked at using its micro-transfer printing for solar cells in 2007, with the help of a grant from DOE’s Office of Energy Efficiency and Renewable Energy. In 2010, Semprius earned one of four spots in the PV Incubator program funded by DOE and run out of NREL.
Kaitlyn VanSant, NREL’s technical monitor for Semprius, said the company “actually met the goals a lot faster than originally anticipated. The goals were definitely aggressive, but they accomplished them quicker than the timeline.”
NREL’s role was critical, Ghosal said .“A lot of the early benefits were from the testing NREL could do. NREL has an internationally recognized testing program,” he said. “It’s one thing to claim a particular output, but something different to say that it was validated at NREL. It gives that stamp of credibility.
“Also, we learned from NREL how vigorous we had to be in terms of the materials we are using,” Ghosal said. “We got an understanding of how it would perform in the field and got some important pointers of what to watch for.”
Bill Scanlon is a writer for NREL Public Affairs
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