Science News - July 31, 2010

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 passes through.

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

Si wires Back-reflector

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.

Sparse pile

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.

Antireflective
coating
Transparent
polymer
Light-
scattering
particles

Prospects

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 coordinated modules.

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?”