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January 15, 2009

Graphene OLED Thin Film Displays


Big and bendy: A transparent graphene film, two centimeters on each side, stretches and flexes when transferred to a rubber stamp. The stamp can be used to deposit the film on any substrate. Credit: Ji Hye Hong

MIT Technology Review reports that Korean researchers have found a way to make large graphene films that are both strong and stretchy and have the best electrical properties yet. These atom-thick sheets of carbon are a promising material for making flexible, see-through electrodes and transistors for flat-panel displays. Graphene could also lead to foldable organic light-emitting diode (OLED) displays and organic solar cells.

Columbia University physics professor Philip Kim, who is a coauthor of the new paper, says that chemical vapor deposition is one of the cheapest ways to make quality graphene on a large scale and should be compatible with existing semiconductor fabrication technologies. Right now, the researchers can make four-inch pieces, but Hong says that they could easily scale up the process. "The conductivity is sufficient for some entry-level applications in small LCD displays and touch-panel displays," says Yang Yang, a materials-science and engineering professor at the University of California, Los Angeles. The conductivity would still need to be 10 times better in order to replace ITO in organic solar cells and OLEDs.


Researchers from the Sungkyunkwan University and the Samsung Advanced Institute of Technology, in Suwon, Korea, have made centimeters-wide graphene films that are 80 percent transparent and can be bent and stretched without breaking or losing their electrical properties. Others have made large graphene films using simpler techniques, but the new films are 30 times more conductive. In addition, it is easy to transfer the new films onto different substrates. "We have demonstrated that graphene is one of the best materials for stretchable transparent electronics," says Byung Hee Hong, who led the work, which is published in Nature.

Graphene is an excellent conductor, and it transports electrons tens of times faster than silicon does. It could replace the brittle indium tin oxide (ITO) electrodes that are currently used in displays, organic solar cells, and touch screens. Graphene transistors could also replace silicon thin-film transistors, which are not transparent and are hard to fabricate on plastic.





The Korean researchers use a method called chemical vapor deposition. First, they deposit a 300-nanometer-thick layer of nickel on top of a silicon substrate. Next, they heat this substrate to 1,000 Cº in the presence of methane, and then cool it quickly down to room temperature. This leaves behind graphene films containing six to ten graphene layers on top of the nickel. By patterning the nickel layer, the researchers can create patterned graphene films.

Others, such as MIT electrical-engineering professor Jing Kong, are working on similar approaches to making large graphene pieces. But the Korean researchers have taken the work a step further, transferring the films to flexible substrates while maintaining high quality. The transfer is done in one of two ways. One is to etch away the nickel in a solution so that the graphene film floats on its surface, ready to be deposited on any substrate. A simpler trick is to use a rubber stamp to transfer the film.

FURTHER READING
Unidym is making transparent displays from carbon nanotubes

Perfect Successor to ITO
Unidym will supply standalone CNT-on-plastic films to current purchasers of ITO-on-plastic films, and partner with companies that use ITO in more integrated applications. Unidym is working with customers and partners in the touch screen, LCD display, OLED and solar industries.

Problems with ITO
Although ITO is currently the most commonly used transparent electrode material, it can be an inadequate solution for many device applications due to its brittle nature and correspondingly inferior flexibility and abrasion resistance. In addition, the indium component of ITO is rapidly becoming a scarce, and therefore increasingly-expensive, commodity, which has fueled demand for lower-cost solutions in recent years. Moreover, integration of ITO components into products such as LCD displays requires an expensive process that is complicated by ITO's incompatibility with many chemicals used in the display manufacturing process.

Benefits of Unidym Films
Unidym's CNT-based films have been demonstrated as substantially more mechanically robust than ITO, and can be deposited using a variety of low-cost and low-impact methods. Further, such films are chemically resistant and are manufactured from carbon, which is one of the most abundant elements on Earth. CNT prices continue to decrease every year, and Unidym expects to expand its existing film production to a commercial scale by 2008.

Anti-static Films
New designs and increasing size have made static a significant problem in LCD manufacturing. Unidym is developing high transparency anti-static films to eliminate the yield loss associated with ESD and particle accumulation.



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