Organic transistor targets displays
May 2, 2011
Tiny carbon nanotubes could have a big impact on the size and performance of television screens based on organic light-emitting diodes (OLEDs). That's the claim of a team of researchers in the US, which has created red, green and blue pixels from transistors incorporating nanotubes and light emitting organic materials.
Displays based on OLEDs have shown great commercial promise because they offer high pixel brightness, wide viewing angle, a very high contrast ratio, fast response times and low power consumption. But display size is limited – today’s OLED TVs made by the likes of Sony and LG have screens just 11–15 inches across.
Now, Andrew Rinzler and colleagues at the University of Florida have built display pixels from light-emitting transistors using a technique that could be scaled to form large arrays of organic emitters.
Not suitable for large screens
Their technology could replace the polycrystalline silicon transistors that are used today to deliver the high drive currents needed to turn on the light emission from the OLED pixels of a display. Rinzler claims that these silicon transistors are unsuitable for deployment in larger screens because they suffer from large variations in electrical characteristics, which impact commercial production yields.
"Even for a 3-inch diagonal hand-held display, the yield off the assembly line is only about 70%," Rinzler told physicsworld.com.
It is impractical to turn to amorphous silicon transistors that control pixels in liquid crystal displays, due to their low electron mobility. To deliver enough current from the transistor to turn on the OLED, drive voltages have to be cranked up, leading to very high power consumption. Amorphous silicon transistors have an unacceptably high degree of instability when driven in this manner, making them unsuitable for displays.
New power benchmark
The Florida researchers are not the first to make a light-emitting transistor – this accolade goes to a team from Darmstadt University of Technology, Germany, who reported success in 2003. However, the device from the University of Florida sets a new benchmark for power management.
At a display brightness of 500 cd/m2 – broadly comparable to that of a TV screen – the transistor element of green pixels made by Rinzler’s team accounts for just 6% of total power consumption, with the remainder used for light generation. The closest contender, which employs the same organic material to make the OLED, devours more than half its power in the transistor section.
The researchers describe their device as a carbon nanotube enabled, vertical field-effect transistor. It is assembled on a glass substrate coated in indium tin oxide, a transparent material employed for the transistor’s gate.
A lithium fluoride and aluminum alloy forms the drain contact that injects electrons into the OLED. Positive charge carriers, known as holes, are provided by single-wall carbon nanotubes, and when these two types of charge carriers come together in the organic layer they interact to produce light.
Controlling the flow
The new device features a "contact barrier", formed between the nanotubes and an organic, highly conductive layer that sits on top of them. Adjusting the gate voltage alters the contact barrier, which determines whether holes flow or not and ultimately governs when the OLED emits light.
Red, green and blue pixels made by the Florida team produce 500 cd/m2 when driven by single-digit voltages. The transistors driving red and blue pixels take a bigger share of the power consumption budget than those powering green pixels, accounting for 19% and 15% of the total power, respectively.
When the pixels made by Rinzler and team are turned off they consume very little power. The researchers calculate that a 50-inch screen made from their devices would consume just 67 mW if every pixel were in its "off" state. In comparison, LCD displays of the same size consume 100–200 W, whether the pixels are on or off.
Venture capital support
The work of the team has been supported by venture capital firm Nanoholdings LLC. "They recognize that development of high technology takes effort, money, and most importantly, time," says Rinzler. "We have taken the attitude that if we build it, the OLED developers will come [to us]."
Peter Ho from the National University of Singapore is impressed by the work of the Florida team: "I think this is an exciting step forward for organic electronics". He added, "[This work] allows the typically high drive current required in the OLED to be controlled by a separate voltage control circuit, which reduces the demand on its transistor".
The researchers report their work in Science 332 570.
About the author
Richard Stevenson is a science and technology journalist based in Chepstow, Wales