The Trouble With Touch Screens

Scientists search for a replacement for indium-tin oxide, a transparent conductor that's vanishing fast

Chances are that if you purchased a new handheld gadget this holiday season, it had some kind of touch screen. That’s good news for touch-screen makers, but they face a problem that is literally invisible. Indium-tin oxide (ITO), the transparent conductor used in touch displays, is in short supply. In fact, experts predict that we could run out of indium, a silvery metal produced as a byâ¿¿product of zinc mining, in the next 10 years. The price of the metal has shot up from around US $100 per kilogram to nearly $1000 in the past six years.

Fortunately, many companies and research groups are coming up with alternatives to conventional ITO technology. These include alternative ITO formulations that save on indium, organic polymers, and exotic options like carbon. By 2015, these alternatives will make up more than half of the market for transparent conductors, predicts market research firm NanoMarkets, in Glen Allen, Va.

The bulk of ITO goes to make transparent electrodes in flat-panel displays such as computer monitors and ever-expanding LCD TVs. But touch screens like those used for PDAs, supermarket kiosks, and ATMs will be the easiest market to chase for makers of ITO substitutes. These devices contain ITO-coated plastic films separated by a thin space; touching the panel brings these two layers together, connecting a circuit and indicating where the touch occurred. Alternatives to ITO might gain easier entry to the market for touch screens, because these displays typically have lower conductivity requirements than high-resolution color TV screens. And because some alternatives are flexible, they might have an advantage over brittle ITO coatings, which can crack with repeated pressing and bending.

Any replacement technology will have to match ITO’s transparency and come close to its conductivity while being more flexible and robust. Fujitsu has started selling touch screens made using the flexible polymer PEDOT. While this polymer is just as see-through as ITO, its conductivity (750 siemens per centimeter) is only one-tenth as much. Researchers at H.C. Starck, a German manufacturer of PEDOT, are getting higher and higher conductivities every year by doping it with different chemicals, says vice president Stephan Kirchmeyer, though environmental stability is an issue. Unlike ITO, PEDOT degrades over time when exposed to light or heat.

Carbon nanotubes look more promising. In addition to being strong and flexible, they are easier and cheaper than ITO to deposit on glass and plastic surfaces, because they can be formed into a solution. By comparison, ITO is ”sputtered” onto a surface in a vacuum, ”a clunky and expensive process,” says Peter Harrop, chairman at research and consulting firm IDTechEx in Cambridge, England.

A tangled mat of carbon nanotubes a few nanometers thick (and hence transparent) can match ITO’s conductivity, says Paul Drzaic, chief technology officer at Unidym, in Menlo Park, Calif. Unidym is working with Samsung on an electronic-paper device that uses such electrodes; Samsung demonstrated a prototype early in 2008. Unidym plans to start selling kilometers-long rolls of its nanotube-coated plastic films this spring. Drzaic claims that the cost of the films should compare favorably with that of ITO coatings.

Innovations are brewing in ITO laboratories too. Groups in Europe are working on ITO formulations that use less indium. These work well but are harder to coat on plastic, Harrop says. Others are making conductive inks by mixing ITO nanoparticles with carbon nanotubes and other nanomaterials. All of these approaches would save indium but are still in the lab. Meanwhile, researchers are cooking up indium-free transparent conducting metal oxides such as antimony tin oxide. Those are cheaper than ITO but just as brittle.

Indium-tin oxide won’t be completely dethroned anytime soon—too many devices rely on the material. But for anyone who can develop a viable alternative, says Harrop, there are lots of opportunities.

About the Author

PRACHI PATEL-PREDD, a frequent contributor to Spectrum , wrote two pieces for this month’s issue. For Careers, she wrote about oDesk [p. 23], a company that manages contract employees by monitoring their computer activity, ensuring that the employer won’t be billed for time spent on Facebook or YouTube. A freelancer herself, Patel-Predd thinks it’s not a bad idea. ”I would use it,” she says. Her story in Update, ”The Trouble With Touch Screens” [p. 11], explores the search for new transparent conductors, a problem that’s literally at your fingertips.