Sandia National Laboratories
scientists have developed tiny glitter-sized photovoltaic cells that could revolutionize
the way solar energy is collected and used.
Representative thin crystalline-silicon photovoltaic cells – these are from 14 to 20 micrometers thick and 0.25 to 1 millimeter across. (Image by Murat Okandan)
The tiny cells 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, fabricated of crystalline silicon, hold the potential
for a variety of new applications. They are expected eventually to be less expensive
and have greater efficiencies than current photovoltaic collectors that are
pieced together with 6-inch- 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.
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.”
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-inch-by-6-inch brick-sized
cells, yet perform at about the same efficiency.
100 times less silicon generates same amount of electricity
“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 300-millimeter (12-inch) diameter wafers
and future 450-millimeter (18-inch) 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, 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.
Other unique 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.
A commercial move to microscale PV cells would be a dramatic change from conventional
silicon PV modules composed of arrays of 6-inch-by-6-inch wafers. However, by
bringing in techniques normally used in MEMS, electronics and the light-emitting
diode (LED) industries (for additional work involving gallium arsenide instead
of silicon), the change to small cells should be relatively straightforward,
Gupta said.
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 from integrated circuits and MEMS.
Offering a run for their money to conventional large wafers of crystalline
silicon, electricity presently can be harvested from the Sandia-created cells
with 14.9 percent efficiency. Off-the-shelf commercial modules range from 13
to 20 percent efficient.
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 placement takes place at cooler temperatures. 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. An alternate technology,
still at the lab-bench stage, involves self-assembly of the parts at even lower
costs.
Solar concentrators — low-cost, prefabricated, optically efficient microlens
arrays — can be placed directly over each glitter-sized 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.
High-voltage output is possible directly from the modules because of the large
number of cells in the array. This should reduce costs associated with wiring,
due to reduced resistive losses at higher voltages.
Other possible applications for the technology include satellites and remote
sensing.
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.
The ability of light to produce electrons, and thus electricity, has been known
for more than a hundred years.
Posted December 22n