Web Date: April 30, 2014
Transparent Electrodes Made On A Roll
A simple method prints continuous sheets of flexible, transparent electrodes—hundreds of meters at a time—for use with organic light-emitting diodes (ACS Appl. Mat. Interfaces 2014, DOI: 10.1021/am5011354). The electrodes combine silver nanowires and transparent conductive oxides to form a hybrid material that could lead to inexpensive flexible displays.
Korean display-makers Samsung and LG have each unveiled prototypes of large-area, flexible displays that use organic light-emitting diodes (OLEDs). The flexible, transparent electrodes in these one-off devices are made using expensive, complex processing techniques. But for this technology to become accessible to most consumers, companies need to make the electrodes inexpensively and on a large scale, says Jin-Woo Park, a materials scientist at Yonsei University, in Seoul.
Some materials scientists think silver nanowires could be the solution. Webs of the nanowires are bendable and are good conductors. But they have two stumbling blocks. The webs have rough surfaces that degrade the performance of organic light-emitting materials coated on them. They also tend to get electrical shorts where individual wires meet.
Park and others have come up with ways to overcome these hurdles, but none have been amenable to continuously printing large-area electrodes, Park says. With the help of Korea-based printing company InkTec Corp., Park and her group tweaked these previous solutions to produce their high-throughput method.
They run a roll of the plastic polyethylene terephthalate through a roll-to-roll machine analogous to the printing presses used to print newspapers. As the plastic moves through the rollers, a rod coats the surface with a silver-nanowire ink. The plastic then passes under a heater to evaporate the solvent in the ink. Next it travels through a sputtering chamber where indium tin oxide (ITO) is deposited on top of the nanowires. The coating is thick enough to smooth the wires’ surfaces and seal them together, but thin enough to make sure the oxide doesn’t become brittle. With this method, the group has made sheets of electrodes 500 to 1,000 m long and 35 cm wide, at a rate of about 20 or 30 m per hour, Park says.
In electrical tests of the hybrid material, the best electrode had a sheet resistance of 30 ohms per square, which is two times higher than the industry standard of 15 ohms per square for a display electrode. But the sheet resistance did not significantly change even after the researchers put the electrode through 10,000 bending cycles.
Finally, to test the quality of the hybrid electrodes in action, they built a single flexible OLED, equivalent to one display pixel, on top of it, and demonstrated that the device was functional. Park points out that although other groups have developed hybrid electrode materials made of nanowires and conductive oxides, they weren’t able to use roll-to-roll methods or make working displays.
Zhenan Bao, a materials scientist at Stanford University whose work on flexible, transparent carbon-nanotube electrodes has been licensed by C3Nano, a Hayward, Calif., startup company, says the use of the roll-to-roll method is promising. However, she says, to make practical flexible OLED displays, the group needs to demonstrate electrodes with lower sheet resistance.
Park says her group is continuing to improve the hybrid electrode materials, including experimenting with different transparent oxides that don’t contain indium, which is expensive. Their next goal is to demonstrate a large-area flexible display that goes beyond the 20 mm2 OLED made in this work.
- Chemical & Engineering News
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