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New twist in electronics

A new silicon-based technology could light the way to a world where every schoolchild in Africa has a touchpad device to learn on.

July 31, 2013

In one of Uwe Kortshagen's dreams, cheap electronic inks result in touchpad devices that cost barely a dollar, and every schoolchild in Africa learns to read and do math with one.

In another dream, rooftop solar panels have given way to shingles embedded with electronic inks that help turn sunlight into electricity at hardly any extra cost.

Kortshagen, a professor of mechanical engineering at the University of Minnesota, graduate student Lance Wheeler, and their colleagues have come up with a technology that may make those dreams come true. They have found a new way to produce cheap but superior electronic inks from nanometer-sized crystals—nanocrystals—of silicon.

Printing with electronic inks holds promise as the basis for next-generation solar panels and electronic display devices like cell phones and touchpads. Silicon makes a better choice for electronic displays than the commonly used lead and cadmium because unlike them it's abundant, cheap, and nontoxic.

"It's a way of making electronic devices significantly cheaper and more accessible to a wide public," says Kortshagen. "For example, one could make a display device or touchpad computer with a few dollars instead of hundreds. We want to make electronics available to everybody."

The team, along with researchers from the National Renewable Energy Laboratory in Golden, Colorado, reports on the technology in the journal Nature Communications. The researchers have a U.S. patent pending on their work.

No doping necessary

Silicon has built its reputation—not to mention that valley in California—on its prowess as a semiconductor. That means its ability to conduct electric current falls between that of a metal like copper and an insulator like rubber or glass. A semiconductor can change its conductivity when prodded by various cues, which makes silicon an invaluable component of electrical on-off switching elements.

But electronic inks based on silicon nanoparticles have suffered from two big problems:

  • Soap-like organic molecules called ligands must be added to give the inks a good shelf life, but they lead to detrimental residues in films printed from the inks.
  • The nanoparticles must be "doped" with implanted impurities to enhance their electrical performance.

In a new approach, the researchers used an ionized gas, called nonthermal plasma, to create nanocrystals of silicon coated with a layer of chlorine atoms (see video).

The chlorine atoms set up an interaction between silicon and molecules of many commonly used solvents; this stabilizes the ink and gives it "an excellent shelf life," Kortshagen says, with no ligands needed.

"Our films are pure and stable, and don't crack like others," he notes. "[And] this mechanism dopes itself. The electrical conductivity of films printed from our inks is 1,000 times that of undoped silicon nanocrystal films."

Energy-saving inks

The researchers see great potential for the new inks to bring solar energy to a wider public. For that to happen, solar cells must be cheap enough to compete with fossil fuels.

"The biggest barrier to solar cells on the roof is cost," says Wheeler, who is first author on the journal article. "But inks can be printed. And different sizes of nanoparticles can absorb different wavelengths of light—that is, in a tunable way. This is important for solar cells."

Kortshagen says his group looks for materials that, while harder to make, have good electronic properties and are more stable and better for the environment.

"While many obstacles still remain, these … discoveries have brought the era of printed silicon electronic devices a step closer," he says.

Contact the writer at morri029@umn.edu

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