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.