A Full-Color Screen That Bends
Flexible, full-color video displays could be closer to market because of a new advance by researchers at Arizona State University’s Flexible Display Center (FDC) and at Universal Display, in Ewing, NJ. The researchers have made bendy organic light-emitting diode (OLED) displays employing processes and tools that are used to make today’s flat-panel LCD screens. They demonstrated a new 4.1-inch video-quality display at the 2009 Society for Information Display conference last week.
OLED displays, which are lighter and less power hungry than LCDs, are used in cell phones and MP3 players. OLEDs can also be printed on plastic and offer the promise of bright color screens that can be rolled up and stowed in gadgets, worn on wrists, or plastered on clothes. Electronics makers Sony, LG, and Samsung Mobile Display have unveiled small flexible prototypes over the past two years. But these are very expensive, mainly because there’s no simple way to make high-performance flexible electronics that go behind OLED pixels.
Researchers at FDC have adapted the process used to make LCD electronics. A transistor acts as the switch behind each pixel, turning it on or off. Transistors for LCD pixels are made on glass screens at high temperatures–a tricky process for plastic substrates, says Nicholas Colaneri, director of FDC. “We’ve found a novel means of handling the plastic so it can be fed into conventional manufacturing equipment,” he says. This should make flexible displays almost as cheap as LCD displays, Colaneri notes.
The faster the transistors work, the better for video-quality displays. The transistor material’s mobility determines how much current it can carry and how fast it switches. Previous flexible display prototypes use either organic thin-film transistors that have low mobilities or exotic metal-oxide materials that are hard to work with.
In the new display, the FDC researchers use amorphous silicon, the material of choice for current LCD electronics. Amorphous silicon has a higher mobility than most organic semiconductors, and transistors made from the material are more reliable. But its mobility isn’t as high as that of polysilicon, which is used to make transistors for ultra-high-performance LCD screens, and OLEDs need high current to emit brightly. So the researchers use Universal Display’s phosphorescent OLEDs, which are four times as efficient as conventional OLEDs, converting nearly 100 percent of electricity into light.
The researchers use the same equipment that is employed to make electronics on glass screens for LCDs. They glue plastic on a piece of glass, make the thin-film transistors on the material, and then peel off the plastic. “We’ve only added two extra steps: gluing and peeling,” Colaneri says.
LCD electronics are processed at temperatures of above 300 °C, which can melt plastic. The FDC process works at a relatively low temperature of 180 °C. The process has required a lot of fine-tuning. The temperature is low enough that amorphous silicon transistors typically don’t perform well. “A lot of people have not been able to get good-quality amorphous silicon transistors at low temperatures,” says Mark Hartney, chief technical officer at the FlexTech Alliance, a display-industry consortium. “That’s really unique about what [FDC] have done.”
Yet at temperatures above 100 °C, “plastic tends to melt or stretch or wrinkle, so there’s distortion,” says Jennifer Colgrove, an analyst with the consulting firm DisplaySearch. “What’s significant here is that they can build amorphous silicon transistors on plastic with almost no distortion.”
Hartney says that this is a good starting point for manufacturing bendable OLED displays on a commercial scale. “Amorphous silicon is a mainstream tech for LCD manufacturing around the world today,” he says. “This opens the doors to being able to utilize any LCD fabrication facility. There has been billions of dollars of investment in LCD manufacturing capacity. You could go into any other LCD fab around the world and do the same process to get a flexible OLED product.”
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