Cheap, Transparent, and Flexible Displays

New high-performance transistors could lead to windows and helmet visors that double as high-quality displays.
By Kevin Bullis

The transparent film over this penny has 70 transistors on it. They are made of invisible materials developed at Northwestern University. (Credit: Lian Wang and Myung-Han Yoon, Northwestern University)

By developing a low-cost method for making high-performance transparent transistors, researchers at Northwestern University have taken an important step toward creating sharp, bright displays that could be laminated to windshields, computer monitors, and televisions but would blend into the background when not in use.
For years, researchers have attempted to make flexible electronics based on electrically conducting plastics that can be manufactured inexpensively. There has been some success in making ones are nearly transparent. But these organic materials have produced transistors with disappointing performance, falling well short of the capabilities of transistors made with inorganic materials such as silicon. The Northwestern researchers, led by chemistry and materials-science professor Tobin Marks, combined the best of both worlds by making hybrid organic-inorganic devices that have high performance but could be manufactured inexpensively. The transistors are transparent, so they could be used in see-through displays.
Most of the transistor is composed of indium oxide, an inorganic semiconductor that can be produced at low cost because it can be deposited over large areas at room temperature. The process Marks employs to make them is a standard technique that uses ion beams to control the crystallization and adhesion of the oxide as it is deposited onto a surface. The method can also be used to adjust the conductivity of the final material, which makes it possible to use indium oxide as a semiconductor in one part of the device and as a conductor in other parts.
The organic material in the device is made of molecules that, once applied to a surface, self-assemble into a well-ordered structure that gives it superior insulating properties. Combined with the indium-oxide semiconductor, it makes possible transistors that perform better than the amorphous silicon transistors often used in LCD screens today. Indeed, the transistors are nearly as good as the much more expensive polysilicon transistors used in high-end displays. Marks says this high performance includes low operating voltages and good switching behavior that should make the transistors easy to integrate into devices, and could lead to energy-saving, crisp-looking displays.
Since both the thin films of indium oxide and the self-assembling organic material are transparent and can be assembled on glass, as demonstrated in an article appearing online in the journal Nature Materials, they could be embedded without a trace in windows. And because the processes used are low temperature, the electronics could be deposited on a plastic substrate, allowing flexible, transparent displays.

"There are a lot of interesting things you could do if you had truly transparent electronics," Marks says. "You could almost envision a display floating in space." The displays could also be applied to glasses or helmet visors. "You could imagine an assembly-line worker, a race-car driver, or some military application where you might want a map or something like that on your visor."
The new transistors' ability to challenge silicon in performance suggests that they could be used not just as pixel switches, but also as transparent processors and memory--all of which could be incorporated into a thin, flexible sheet, saving manufacturing costs and introducing a new form of electronics. Such applications are still a long way off and require improving the performance of the transistors. But prototype displays based on the new transistors could be ready in as little as 12 to 18 months, Marks says. Polyera, a startup in Evanston, IL, has been founded to help bring the novel materials to market.

The Northwestern researchers are not the first to combine organic and inorganic materials into transistors: as early as 1999, IBM researchers produced such devices (see "Flexible Transistors"). However, these were not transparent and did not perform as well as the Northwestern transistors. Others are now working toward transparent electronics using materials such as zinc oxide and carbon nanotubes, says John Rogers, professor of materials science and engineering at the University of Illinois, Urbana-Champagne. While the carbon nanotubes could theoretically lead to significantly better-performing devices, manufacturing arrays of nanotube-based devices will be much more difficult than those made with the Northwestern techniques. The new work is also distinct from some prototype flexible displays, which still rely on visible wires.
"This is some very nice work from one of the leading groups in low-temperature materials for electronics," Rogers says. "The performance that they achieve is very impressive. This paper represents a valuable contribution to the emerging field of transparent electronics."
Copyright Technology Review 2006.