Crystalline-silicon micro-photovoltaic cells measure from 14 to 20 micrometers thick and 0.25 to 1 millimeter across, offering a diverse array of potential uses.
Photo by Murat Okandan
The following news release was provided Dec.
21, 2009, by Sandia National Laboratories.
Sandia National Laboratories’ tiny solar PV cells could turn a person into a
walking solar battery charger if the cells were fastened to flexible substrates
molded around unusual shapes, such as clothing.
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 micro-electronic and micro-electromechanical 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 micro-photovoltaic cells. These include new applications, improved
performance, potential for reduced costs and higher efficiencies.
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 micro-engineered 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
engineer Vipin Gupta said, “Photovoltaic modules made from these micro-sized
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.”
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-by-6-inch brick-sized cells, yet perform at about the same
Micro Solar Power Saves
“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.”
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-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.
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 micro-scale PV cells would be a dramatic change from conventional silicon PV
modules composed of arrays of 6-by-6-inch wafers. However, by bringing in
techniques normally used in MEMS, electronics and the light-emitting diode
(LED) industries, 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
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
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 pre-established 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.
concentrators — low-cost, prefabricated, optically efficient micro-lens 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
micro-lens arrays can be fabricated for this purpose.
Sandia National Laboratories is a multi-program
laboratory operated by Sandia Corporation, a wholly owned subsidiary of
Lockheed Martin, for the U.S.
Department of Energy’s National Nuclear Security Administration.