Organic light-emitting diode (OLED) displays are attractive because they are bright, efficient, and thin enough to be flexible. But they are currently limited to use in small displays, such as those in mobile phones. That’s in part due to the failings of one piece of the device, a transparent electrode used to light up the display. Now researchers at the University of Michigan have developed a new type of electrode that could help clear the way for large, flexible OLED displays.
OLEDs consist of organic semiconductor layers sandwiched between two electrodes, one of which must be transparent to allow light to escape. Today’s displays use a transparent film of indium tin oxide (ITO), but this material is expensive, fragile, and inflexible, which makes it unsuitable for large-area flexible displays. It can also degrade the organic light-emitting layers.
The new electrode is a grid of highly conductive metal wires so thin that they are essentially transparent. Electrical-engineering and computer-science professor L. Jay Guo says that the electrode should be more flexible and less expensive than ITO, while not degrading the organic materials. The researchers incorporated the grid into an OLED as the top electrode and observed no visible difference in brightness between their LED’s light emission and that of a conventional OLED made with an ITO electrode, although Guo says that he and his colleagues will need to do more-detailed optical measurements to see how the two compare. The work is described in an online paper in the journal Advanced Materials.
The researchers made grids of copper, gold, and silver, with wires 120 or 200 nanometers wide and separated by gaps of about 500 nanometers in one direction and by gaps of 10 micrometers in the perpendicular direction. The excellent conductivity of these metals results in a resistance as small as five ohms, which is less than the average ITO layer’s resistance.
The researchers use a technique called nanoimprint lithography, which allows them to make a grid of wires that can be transferred to any other surface, including a substrate for a flexible display. (See “10 Emerging Technologies That Will Change the World.”)
By changing the width and height of the wires, the researchers can change the transparency and conductivity. Making the wires thinner makes the electrode more transparent, but at the same time, the thinner wires have higher resistance. So the researchers double the wires’ height, which reduces the resistance by a factor of three but decreases the transparency by only 5 percent, Guo says. “There’s great potential [to] play around with these parameters,” he adds. “[There’s] a lot of room to optimize the structure.”
Jorma Peltola, who is a consultant with flat-panel display manufacturers, notes that while finding a robust, flexible alternative to ITO is a priority for the OLED-display industry, better organic materials and manufacturing methods will also be required before OLEDs can move into the marketplace for larger displays.
Also, the new technique faces a tough challenger: carbon nanotubes. Researchers are developing carbon-nanotube films that could replace ITO. Nanotube films presently have about three times higher resistance than the new metal grid for comparable transparency, but that difference is small and shrinking with new developments, says Andrew Rinzler, a physics professor at the University of Florida, who is studying carbon-nanotube films. Also, unlike the metal grid, nanotube layers contact every portion of the organic semiconductor layer that they are deposited on, which should increase device efficiency.
But as a first-time demonstration, the metal-grid idea is worth pursuing, Rinzler says. “The possible problems and competing technologies notwithstanding, this is a potentially viable technology that is well worth exploring.”