Editor’s Note: EarthTechling, always looking to inform its readers on the latest in cleantech innovation, is proud to republish this story from the National Renewable Energy Laboratory. Author credit goes to Bill Scanlon.
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?
The U.S. Department of Energy’s National Renewable Energy Laboratory recently validated greater than 41 percent efficiency at a concentration of 1,000 suns for tiny cells made by Semprius — one of the highest efficiencies recorded at this concentration. The energy conversion efficiency of a solar cell is the percentage of sunlight converted by the cell into electricity.
Seed money from DOE, together with the experts at the NREL-based SunShot Incubator Program, lifted Semprius from a small electronics start-up with a novel idea to a real difference-maker in the solar cell 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 expense 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 start-ups 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.
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.
Transfer Printing Technology Evolves to Innovative Solar-Cell Use
Semprius’ 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 a temporary layer on the original gallium-arsenide substrate, and then grows the multi-junction solar cell structure on top of that layer. Then, after the wafer is processed, 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 and the principal investigator for Semprius’ PV Incubator Award. “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.”
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