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VOLUME -25 NUMBER 2
Publication Date: 02/1/2010
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ARCHIVE >  February 2010 Issue >  Front Page News > 

Micro-Size Si PVs Developed at Sandia
Sandia project lead Greg Nielson holds a solar cell test prototype with a microscale lens array fastened above it. Together, the cell and lens help create a concentrated photovoltaic unit. (Photo by Randy Montoya).
Albuquerque, NM — Almost microscopic in size, tiny photovoltaic cells have been developed by Sandia National Laboratories scientists and may well revolutionize the solar cell industry. The tiny glitter-sized photovoltaics could turn a person into a walking solar battery charger if they were fastened to flexible substrates molded around unusual shapes, such as clothing.

The solar particles, made of crystalline silicon, hold the potential for a variety of new applications. They are expected eventually to be less expensive and provide greater efficiencies than current PV collectors that are pieced together with 6-in. square solar wafers. The cells are fabricated using microelectronic and microelectromechanical systems (MEMS) techniques common to today's electronic foundries.

Sandia lead investigator Greg Nielson said the research team has identified more than 20 benefits of scale for its microphotovoltaic cells. These include new applications, improved performance, potential for reduced costs and higher efficiencies.

"Eventually units could be mass-produced and wrapped around unusual shapes for building-integrated solar, tents and maybe even clothing," he said. This would make it possible for hunters, hikers or military personnel in the field to recharge batteries for phones, cameras and other electronic devices as they walk or rest.

Microengineered Panels
Even better, such microengineered panels could have circuits imprinted that would help perform other functions customarily left to large-scale construction with its attendant need for field construction design and permits.

Said Sandia field engineer Vipin Gupta, "Photovoltaic modules made from these microsized cells for the rooftops of homes and warehouses could have intelligent controls, inverters and even storage built in at the chip level. Such an integrated module could greatly simplify the cumbersome design, bid, permit and grid integration process that our solar technical assistance teams see in the field all the time." For large-scale power generation, said Sandia researcher Murat Okandan, "One of the biggest scale benefits is a significant reduction in manufacturing and installation costs compared with current PV techniques."
Representative thin crystalline-silicon photovoltaic cells, from 14 to 20µm thick and 0.25 to 1mm across. (Image by Murat Okandan).


Part of the potential cost reduction comes about because microcells require relatively little material to form well-controlled and highly efficient devices.

From 14 to 20 micrometers thick (a human hair is approximately 70 micrometers thick), they are 10 times thinner than conventional 6 x 6-in. brick-sized cells, yet perform at about the same efficiency.

100 Times Less Silicon
"So they use 100 times less silicon to generate the same amount of electricity," said Okandan. "Since they are much smaller and have fewer mechanical deformations for a given environment than the conventional cells, they may also be more reliable over the long term."

Another manufacturing convenience is that the cells, because they are only hundreds of micrometers in diameter, can be fabricated from commercial wafers of any size, including today's 300mm (12-in.) diameter wafers and future 450mm (18-in.) wafers. Further, if one cell proves defective in manufacture, the rest still can be harvested, while if a brick-sized unit goes bad, the entire wafer may be unusable. Also, if brick-sized units were fabricated larger than the conventional 6 x 6-in. cross-section, the larger wafer size would require thicker power lines to harvest the increased power, creating more cost and possibly shading the wafer. That problem does not exist with the small-cell approach and its individualized wiring.

Other features are available because the cells are so small. "The shade tolerance of our units to overhead obstructions is better than conventional PV panels," said Nielson, "because portions of our units not in shade will keep sending out electricity where a partially shaded conventional panel may turn off entirely." Because flexible substrates can be easily fabricated, high-efficiency PV for ubiquitous solar power becomes more feasible, said Okandan.

Each cell is formed on silicon wafers, etched and then released inexpensively in hexagonal shapes, with electrical contacts prefabricated on each piece, by borrowing techniques used in producing ICs and MEMS.

The project combines expertise from Sandia's Microsystems Center; Photovoltaics and Grid Integration Group; the Materials, Devices, and Energy Technologies Group; and the National Renewable Energy Lab's Concentrating Photovoltaics Group.

Involved in the process, in addition to Nielson, Okandan and Gupta, are Jose Luis Cruz-Campa, Paul Resnick, Tammy Pluym, Peggy Clews, Carlos Sanchez, Bill Sweatt, Tony Lentine, Anton Filatov, Mike Sinclair, Mark Overberg, Jeff Nelson, Jennifer Granata, Craig Carmignani, Rick Kemp, Connie Stewart, Jonathan Wierer, George Wang, Jerry Simmons, Jason Strauch, Judith Lavin and Mark Wanlass (NREL). The work is supported by DOE's Solar Energy Technology Program and Sandia's Laboratory Directed Research & Development program, and has been presented at four technical conferences this year.

Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, an autonomous Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.

For more info:
http://www.sandia.gov

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