New Inks for OLED Screens

Why develop New Inks for OLED Screens?

Organic light emitting diode, or OLED, displays seem to have it all: energy efficiency and a beautiful, crisp picture that refreshes rapidly. But it's difficult to make them on a large scale, so OLED televisions remain very expensive. Using a custom-made printer from Japanese manufacturer Dainippon Screen, DuPont says it can print a 50 inch-television in under two minutes, and testing of the displays shows their performance is reliable--the displays should last 15 years.

OLEDs on the market today rely on an expensive, small-scale technique called shadow-mask evaporation to pattern the light-emitting organic molecules that make up the pixels in these displays. One of the most promising alternatives to shadow-mask deposition is printing, which is compatible with manufacturing large screens. But printing is difficult to do reliably, and the resulting devices have lagged behind the performance and durability of those made using conventional techniques. It's been a major materials challenge to develop inks that won't bleed during the printing process, that have the desired electrical and optical properties, and that don't deteriorate over time.

Related Reading: OLED: New Generation of TV's

OLED displays are made up of 12 to 15 layers of materials. In each pixel the red, green, and blue light-emitting materials are positioned side by side and sandwiched between materials that bring electrical current in and out of the device and that allow light to leave it. When these layers blend together during printing, device performance suffers.

DuPont has addressed this problem by using active molecules in the inks used to print each layer that are insoluble in the inks used to print the adjacent layers. This is more complex than it may sound. Each of these materials is a major development effort on its own, and having to tie it to the ones above and below it imposes a lot of constraints.

The company worked with Dainippon Screen to develop a multi-nozzle printer for the new inks. The Dainippon printer works like a garden hose. It generates a continuous stream of ink, rather than droplets, and moves over a surface at rates of four to five meters per second while patterning a display. DuPont says that its red, green, and blue OLED materials can make displays that will last 15 years.

OLED: A New Generation of Television's

OLED: A New Generation of Television's

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.

Transistor test: The glass slides in this image are patterned with transistors designed for driving OLED displays. The metal probes and wires are being used to test their performance. 

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 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, 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. These transistors are made 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.

New electronic devices, also operate at a tenth of the voltage of conventional OLED electronics, which saves power. The researchers have not yet made large-area OLED displays driven by the vertical transistor arrays, but 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. The thin nanotube layer allows very little current leakage, a problem that drained power from previous designs. The devices also switch much faster than was possible in the past. 

The OLED display architecture is being simplified in hopes of further reducing manufacturing costs and complexity. Instead of building a light-emitting pixel next to transistors, build low-power organic transistors that emit light themselves. 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. The vertical, nanotube-electrode-based architecture could greatly improve the efficiency of these devices.