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. (Sandia/Randy Montoya)
Sandia National Laboratories scientists have developed tiny glitter-sized photovoltaic cells that are expected to be less expensive and have greater efficiencies than current photovoltaic collectors pieced together with 6-inch-square solar wafers.

Fabricated of crystalline silicon and using microelectronic and microelectromechanical systems (MEMS) techniques, the solar particles hold the potential for a variety of new applications.

“Eventually units could be mass-produced and wrapped around unusual shapes for building-integrated solar, tents, and maybe even clothing,” said Sandia lead investigator Greg Nielson. "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.

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.

Representative thin crystalline-silicon photovoltaic cells. (Sandia/Murat Okandan)
“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,” said Sandia field engineer Vipin Gupta.

According to Sandia researcher Murat Okandan, one of the biggest scale benefits for large-scale power generation is a reduction in manufacturing and installation costs compared with current PV techniques.

Microcells require relatively little material to form well-controlled and highly efficient devices. At 14 to 20 micrometers thick, they are 10 times thinner than conventional 6-inch-by-6-inch brick-sized cells, yet perform at about the same efficiency. Presently, electricity can be harvested from the Sandia-created cells with 14.9 percent efficiency, and off-the-shelf commercial modules range from 13 to 20 percent efficient.

“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,” said Okandan.

From left to right: Murat Kandan, Greg Nielson, and Jose Luis Cruz-Campa, holding samples containing arrays of microsolar cells. (Sandia/Randy Montoya)
The cells can be fabricated from commercial wafers of any size, including today’s 300-millimeter (12-inch) diameter wafers and future 450-millimeter (18-inch) wafers. If one cell proves defective in manufacture, the rest can still be harvested - while if a brick-sized unit goes bad, the entire wafer may be unusable.

Brick-sized units fabricated larger than the conventional 6-inch-by-6-inch cross section to take advantage of 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.

“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.”

Each cell is formed on silicon wafers, etched, and then released in hexagonal shapes, with electrical contacts prefabricated on each piece - by borrowing techniques from integrated circuits and MEMS.

A widely used commercial tool called a pick-and-place machine — the current standard for the mass assembly of electronics — can place up to 130,000 pieces of glitter per hour at electrical contact points preestablished on the substrate. The cost is approximately one-tenth of a cent per piece with the number of cells per module determined by the level of optical concentration and the size of the die, likely to be in the 10,000 to 50,000 cell per square meter range.

Solar concentrators can be placed directly over each cell to increase the number of photons arriving to be converted via the photovoltaic effect into electrons. The small cell size means that cheaper and more efficient short focal length microlens arrays can be fabricated for this purpose.

(Sandia National Laboratories)