which sport efficiencies somewhat more
than 25 percent, Okandan adds.
The small print
At the University of Illinois at Urbana-Champaign, John Rogers works with
even thinner silicon — 10 to 15 micrometers thick — because when it’s slim
enough it flexes like a strand of hair.
Although he’s testing silicon even thinner than that, the material presents special challenges, he notes, “because even
at 10 to 15 micrometers the silicon won’t
absorb all of the incident light.” Much
By backing the cells with a reflective
material, however, photons that initially
evaded the silicon will bounce back for a
second chance at collection. “We found
that 15 micrometers is just about the
right thickness for that kind of double-pass configuration,” Rogers says. “It will
collect about 90 percent of the light.”
And the efficiency of these cells is already
good, he says, on the order of 12 percent.
The Illinois microcells also rely on concentrators to focus sunlight. Another key
to keeping cell costs low, Rogers contends,
will be avoiding a need to “pick and place”
each cell individually within a module of
perhaps legions of others, which is what
the integrated circuit industry does today.
In the February Energy & Environmental
Science, Rogers’ team describes a way to
simultaneously lift and transfer thousands of microcells.
After building a block of pure crystalline silicon, the researchers etch out
thousands of tiny cells from its surface
by cutting around the sides of each one
and even underneath. After the etching
process is finished, the only thing holding the cells to the starting silicon are
tiny anchors of material left at either
end of each cell.
The scientists then place a soft piece
of slightly tacky rubber onto the batch of
cells and press down just hard enough to
fracture the anchors. When they lift this
rubber pad up, the freed cells come with it.
“We can lift up thousands of these
cells at a time and then simply rubber-stamp them down onto a surface” coated
with a thin-film adhesive, Rogers says.
“Our throughputs correspond to millions of devices per hour — much, much
higher than can be achieved with even
the most sophisticated tools for doing
that [by] pick-and-place.”
University of Illinois solar microcells
can be stamped onto a flexible surface.
Caltech scientists have upended the
silicon elements in their microcells
and jettisoned the concentrator. In the
April Nature Materials, the team
describes a prototype that resembles a
sparse carpet of tiny fibersthat stretch
up toward the light. In the latest designs
the fibers are 100 micrometers long and
1 or 2 percent as wide.
Some photons entering the carpet will
immediately hit a semiconductor fiber.
Many more will miss the wires, which
cover only 1 to 5 percent of the carpet’s footprint. But by making the wires
effectively long and the carpet’s bottom
reflective, photons not initially collected
will ricochet repeatedly within the carpet
until the silicon collects most of them,
explains team leader Harry Atwater.
To protect and hold the fibers, the
Caltech scientists pour a liquid akin
to clear bathroom caulk
(a polymer that solidifies
into a pliable plastic) to fill
space separating the carpet’s sparse pile.
“We can now peel this
composite array of wires and
polymer off the starting sub-
strate just as if it were a piece
of Scotch tape,” Atwater says.
The solar cell—this wire-
studded polymer — “has the
mechanical properties of a
plastic bag,” he notes. “So you can roll it
or bend it and the wires won’t break.”
By maximizing photon ricochets
within the carpet, the applied physicist
explains, “you’re getting the same light
absorption as you would from a sheet
that’s 100 percent silicon,” but using only
1 percent as much of the pricey material.
Outdoor carpet caltech scientists have created a solar
collector that bounces sunlight through a forest of silicon
semiconductor wires until most of the energy is absorbed.
Although none of the emerging designs
are quite ready for prime time, several
groups think that products based on
their innovations could enter the marketplace in as little as three to five years.
“Right now the solar industry is kind
of in a race to bring costs down to $1
per watt,” Nielson says. “From our cost
models, it looks like we can get well
below that with high-volume production.” But that’s a ways off, he concedes,
since his team has only just begun networking individual glitter cells to make
Atwater has conducted all of his experiments with silicon carpets a few square
centimeters in size. “The technology
looks promising,” he says, “but you have
to ask: Will everything translate when
you scale up to very large areas?”
s U.s. doe solar program photovoltaics
july 31, 2010 | science news | 29