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.

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)

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.