Thin Displays as Wristbands
The U.S. Army is testing a prototype “watch” that’s lightweight and thin and has a full-color display. This display is built on flexible materials encased in a rugged plastic case and can be worn on a wristband to display streaming video and other information. It uses newly developed phosphorescent materials that are efficient at converting electricity into red, blue, and green light, which means the display needs less power to work.
Most phones, laptops, and TVs today use liquid-crystal displays (LCDs) controlled by electronics built on glass. To make more energy-efficient displays that are controlled by flexible electronics, which are lightweight and won’t shatter like glass, many companies are turning to organic light-emitting diodes (OLEDs). The pixels in OLED displays replace the layers of electronics and filters in LCDs with organic dye molecules that emit light in response to electrical current.
For consumers, flexible OLEDs promise portable electronics with beautiful screens that don’t drain battery life and won’t shatter when dropped. But so far, no companies have developed economically viable manufacturing methods for producing flexible OLEDs with long enough lifetimes and consistent quality. The U.S. military has been funding development with the aim of providing soldiers with rugged, thin communications devices that can display maps and video without adding too much weight to their load.
The new display prototypes use efficient OLED materials developed by Universal Display of Ewing, New Jersey, and are built on foil-backed electronic controls developed by LG Display, headquartered in Seoul, South Korea. The devices were designed by L-3 Display Systems of Alpharetta, Georgia. The display is 4.3 inches. As part of military demonstration tests, the device has been used to stream real-time video from unmanned air vehicles.
“These prototypes represent not so much one major advance but continued progress on many fronts,” says Janice Mahon, vice president of technology development at Universal Display. Those fronts include the OLED materials themselves, the electronics that control them, and the integration and packaging of the device.
The first generation of OLED materials, used today in glass-backed cell-phone displays and some small TVs, can convert only 25 percent of electrical current into light; the rest is lost as heat. Universal Display is designing and developing materials that work by a different mechanism and that have a theoretical efficiency of 100 percent. The prototypes for the Army use a full set of phosphorescent materials; the companies have not released specifications about power consumption, but Mahon says displays made with these materials use one-fourth the power of a conventional OLED.
Samsung Mobile Display, the biggest maker of OLED displays, currently uses Universal Display’s red phosphorescent materials in its products; Samsung and other companies are currently evaluating green materials. Phosphorescent materials that work with higher energy light such as blue tend to be less stable over time and have been slower in coming. The companies have not disclosed information on the expected lifetime of the all-phosphorescent displays.
Universal Display applied the light-emitting layer to electronic controls made by LG Displays. The electronics are an array of amorphous-silicon transistors built on stainless steel foil instead of glass. Other companies, including Hewlett-Packard and Samsung, are developing flexible amorphous-silicon transistor arrays, mostly on sheets of plastic. Working with metal poses some challenges because the surface is rough, which can disrupt the structure of the transistors, but metal can withstand higher processing temperatures than plastic can. That’s an important trait when it comes to laying down the silicon. High-temperature processing results in silicon crystal that’s not only higher quality but also more stable over time.
“The broader story is that we’re starting to see some good-looking demos of flexible OLED displays,” says Nicholas Colaneri, who heads the Flexible Display Center at Arizona State University. Sony and Samsung Mobile Display have both demonstrated flexible displays built on sheets of plastic; both companies have been tight-lipped about these technologies. But, Colaneri notes, “just because you can do it doesn’t mean you can afford to do it.”
A major hurdle remains before displays like the prototype made for the Army will arrive on store shelves. Amorphous-silicon transistor arrays can be made at temperatures suitable for flexible electronics, and the LCD industry has created a lot of infrastructure for making them. But over time, they’re not the best electronics for controlling OLEDs. The electrical currents required to switch OLED pixels burn out these transistors; the pixels that are on most frequently start to malfunction.
Canadian startup Ignis Innovation is developing software and other controls to extend the lifetime of the transistor arrays by ensuring that no single pixel is on too often. Colaneri says its initial prototypes are promising. In the meantime, Colaneri and other researchers are developing alternative transistor materials such as metal oxides to make OLED electronics that won’t burn out.
The companies that made the Army prototype are not disclosing the metal-silicon electronics used to run it, but say they have met the Army’s specifications.
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