Organic light-emitting diode (OLED) displays are energy-efficient and crisp, but high manufacturing costs have kept them from being as widely available as liquid-crystal displays (LCDs), especially in larger devices such as TVs. A new type of OLED electronics could help bring down manufacturing costs and make the technology much more widely available.
“There is no good solution to make OLED electronics that can be scaled up inexpensively,” says Andrew Rinzler, professor of physics at the University of Florida. Rinzler led the work on developing a type of electronics for OLEDs that he hopes will provide such a solution. The work was funded in part by the venture capital firm Nano Holdings.
The pixels in OLED displays use transistors to stimulate organic molecules, which then emit different colors of light. OLED displays do not need the light-wasting filters that make LCDs such energy hogs. But LCDs dominate the market in large part because the amorphous-silicon transistor arrays used to drive LCDs can be made over areas as big as a single-car garage door and then sliced into smaller pieces to make displays for TVs and other devices. Manufacturing at this scale helps keep costs down.
OLED display makers can’t use the same electronics because switching the pixels in an OLED requires relatively high currents that rapidly burn out amorphous-silicon transistors. Instead, today’s OLED displays are built on more expensive polycrystalline-silicon transistor arrays. The largest OLED display on the market (in Europe, but not yet available in the U.S.) is a 15-inch model made by LG. It sells for just over $2,300; the same size LCD TV costs under $200.
Less-expensive OLED electronics could, in theory, be made by using organic materials for the electronics as well as the pixels. Transistors made using organic semiconductors provide the high currents needed to drive OLED pixels. But electrons move through conventional organic transistors slowly, which results in a display that doesn’t refresh the picture fast enough. To speed these transistors up, engineers have altered the design, shrinking components to bring the source and drain electrodes closer together and make the channel smaller. This makes the device faster because the electrons don’t have to travel as far through the organic material that makes up the channel, which can’t conduct the electrons very fast. But making such high-resolution devices requires expensive lithography techniques.
A less expensive method, developed by Rinzler and colleagues, is to bring the source and drain electrodes of a transistor closer together by stacking components on top of one another instead of side by side. Rinzler’s group, including graduate students Mitchell McCarthy and Bo Liu, made these transistors by depositing a film of aluminum on a glass substrate to act as the gate electrode, then oxidizing it to create a thin insulating layer on top. Next the researchers deposited an ultrathin, dilute layer of carbon nanotubes to act as a source electrode, followed by a layer of organic materials to act as the channel, and finally a top layer of gold as a drain electrode. Each of these films is very thin, enabling good performance without the need for high-resolution lithography techniques, says Rinzler.
Rinzler’s new electronic devices, described online in the journal Nano Letters, also operate at a tenth of the voltage of conventional OLED electronics, which saves power. The Florida researchers have not yet made large-area OLED displays driven by the vertical transistor arrays, but Rinzler says the transistors operate at suitable current and voltage to do so. While the researchers have so far been making these arrays on glass, the techniques used to make them are compatible with flexible substrates and could be used to make flexible OLED displays.
The vertical electronic structure was first proposed in 1994 by Yang Yang, professor of materials science and engineering at the University of California, Los Angeles, and Alan Heeger, professor of materials science and engineering at the University of California, Santa Barbara. Heeger shared the 2000 Nobel Prize in Chemistry for the discovery and development of conductive polymers like the ones used in the new device. In the mid-1990s, Yang and Heeger began developing these devices through a company called UNIAX that was subsequently acquired by DuPont. When the two did their original work, the performance of the available materials wasn’t as good as it is today.
“Carbon nanotubes weren’t available in 1994,” says Yang. In Rinzler’s device, he says, the thin nanotube layer allows very little current leakage, a problem that drained power from previous designs. The Florida devices also switch much faster than was possible in the past. “They did an excellent job of making the device work much better. I’m sure this paper will have an important impact on organic electronics,” he says.
Rinzler is now working to simplify the OLED display architecture in hopes of further reducing manufacturing costs and complexity. Instead of building a light-emitting pixel next to transistors, Rinzler wants to build low-power organic transistors that emit light themselves. His group has demonstrated that it’s possible to make light-emitting organic transistors if the active materials are electroluminescent, but these transistors only operate at high voltages, making them impractical. Rinzler believes that the vertical, nanotube-electrode-based architecture could greatly improve the efficiency of these devices.