29 April 2011—A device that combines the advantages of organic light-emitting diodes (OLEDs) with carbon nanotubes could lead to better, longer-lasting high-definition televisions and other large displays, say the researchers who invented it.
The device, a carbon-nanotube-enabled vertical organic light-emitting transistor, or CN-VOLET, was described in this week’s Science. "It basically compares very favorably to what’s on the market now for small handheld OLED displays," says Andrew Rinzler, a physicist at the University of Florida, in Gainesville, who led the work.
Many large-screen televisions use liquid crystal displays, but OLEDs require less power and offer brighter pixels, faster switching, and better viewing angles. The problem with building large OLED screens is that the transistor that drives them, amorphous silicon, requires unacceptably high voltages.
Small active-matrix OLED displays, popular in smartphones, use polycrystalline silicon, but variations in the size and orientation of the crystals make it difficult to build an array of pixels that all have the same brightness. "Right now they have to throw away 30 percent of the small displays that are manufactured because one side of the display is brighter than the other side," says Rinzler. The problem only gets worse at larger sizes.
So Rinzler and his team came up with a new design that turns the standard transistor—which is usually built horizontally in the plane of the glass—vertical. On a glass substrate, which will be the surface of the pixel, they deposit layers of indium tin oxide, aluminum oxide, and an organic polymer to form the gate and its insulator. Atop that is a stack that includes the transistor channel and the light-emitting element in one part and the source contact in the other. The source electrode connects the contact with the organic semiconductor transistor channel and light emitter by means of a "dilute network" of carbon nanotubes. Nanotubes naturally come in a mix of metal and semiconductor versions, but because of the way the team uses the nanotubes, they don’t have to separate the two, nor do they have to worry about how the nanotubes are aligned. On top of the electrode they place a stack of what postdoctoral student Mitchell McCarthy, lead author of the paper, calls "pretty typical OLED materials." Finally, they place a drain on top of the OLED.
Other researchers have developed vertical organic light-emitting transistors. But Rinzler says his device has eight times the power efficiency of the best of those, because his design does not cover part of the pixel’s aperture with an insulating layer. Instead, the light shines through the nanotube network. Michele Muccini, head of a research unit at the Institute for Nanostructured Materials, in Italy agrees. "It is in principle a promising approach for high-resolution large-area displays," he says. He adds that the design seems to be effective at "lowering the parasitic power consumption and…increasing the pixel illumination ratio."
However, Muccini, who is working with flexible circuits firm Polyera on organic light-emitting transistors, sees some obstacles to scaling up the technique to mass production. "The fabrications process is clearly rather complex," he says.
But scaling up the process is just what Rinzler has in mind. A venture capital company that provided funding for Rinzler’s research, Nanoholdings, has formed a company called nVerPix to try to commercialize the technology. It wants to show it can make a display with enough homogeneity among the pixels to meet the needs of large-scale manufacturers. "We have an 18-month program to develop this technology to its fullest extent to try to get big display manufacturers interested," he says. He thinks that displays based on his technology could roll off assembly lines in two to five years.
About the Author
Neil Savage writes about nanotech, optoelectronics, and other technology from Lowell, Mass. In April 2011 he reported on a technique for building diodes inside optical fibers.