BRIGHTER, flatter, less power-hungry display screens are on the way thanks to
a new breed of organic light-emitting diodes, researchers told a meeting of the
Materials Research Society in San Francisco last week.
For years, organic light-emitting materials have looked like a good bet for
making anything from flexible computer screens to colour displays for
cellphones. Screens based on these materials operate at low voltages, give good
colour contrast, and can be viewed from an oblique angle. However, they have
never been very efficient at converting electricity into light.
But now researchers have announced that they can make ultra-efficient organic
LEDs, including the coveted blue ones. The trick is to use phosphorescence in
addition to fluorescence, says Mark Thompson of the University of Southern
California.
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Organic LEDs are made by sandwiching a light-emitting material between layers
that allow charge to be transported between them. When a voltage is applied,
electrons from the layer on one side and holes from the other meet in the
organic light-emitting layer, where they couple together in an excited state
called an exciton. When excitons relax back to a ground energy state, the
electrons and holes combine, releasing photons of light.
This process is inefficient in organic devices because the only excitons that
emit light are the 25 per cent in which the exciton particles have opposite
spins鈥攃alled a 鈥渟inglet鈥� state. Emission from such singlets is called
fluorescence.
No photons are released from the majority of excitons. Their particle spins
are aligned in a different configuration, called a 鈥渢riplet鈥� state, in which
both spins are either up or down. So the USC team decided to make triplets do
some of the work. Triplet emission is called phosphorescence.
Last year, Thompson and his team coaxed photon emission from the triplets by
adding a layer of molecules containing a heavy metal, such as iridium. This
boosted the organic material鈥檚 ability to produce phosphorescent light. The
result is that both singlet and triplet excitons can be converted into photons.
鈥淲e realised we could produce light from both triplets and the singlets if we
use these phosphorescent materials, greatly enhancing our efficiency, perhaps
eventually to 100 per cent,鈥� says Thompson.
The group鈥檚 first go at harnessing phosphorescence produced a feeble red
colour (Nature, vol 403, p 750). But by trying out different iridium
compounds, the team has now made the first high-efficiency reds, yellows and
greens. They managed to create the blue hue by modifying the iridium compound to
increase its triplet exciton energy. Thompson is working with Universal Display
Corporation, based in Ewing, New Jersey, and with Sony to exploit the emerging
technology.