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Experts Make Significant Advances In Flat-Screen Light Sources

May 15, 2009

Researchers may have found the future flat-screen light sources of choice by demonstrating that white, organic light-emitting diode (OLED) sources have the same efficiency as fluorescent light bulbs, BBC News reported.

The team suggests that new blue-emitting materials currently on the horizon would be able to outperform the limited lifetime of the blue-emitting part of the devices that can only survive for a few hours.

The last few years have yielded significant advances in OLEDs, so that small displays and even televisions based on the technology are beginning to emerge.

Until now, OLEDs had not passed the efficiency benchmark set by fluorescent bulbs, but advances in the technology are showing promise.

Both phosphorescent and fluorescent organic polymers can be used in the devices.

However, the fluorescent materials used in OLED displays and televisions are only one-fourth as efficient despite having significantly longer lives.

Researchers are currently working toward optimizing the efficiency and lifetime of devices based on phosphorescent materials.

As of now, the first devices to outperform fluorescent bulbs in the efficiency stakes are being produced by Karl Leo and colleagues of the Institute for Applied Photophysics in Dresden.

The team was able to reduce the sources of loss by optimizing the design in the emitter layer.

The losses, or stages in which electrical energy goes in but does not exit in the form of usable light, occur because charge carriers recombine rather than dumping their energy into the polymers that give rise to colored light.

The edge of the diode structure where the light is actually produced created another significant source of loss because if it is not extracted efficiently, photons can bounce around inside it or be re-absorbed.

But a specially designed efficient nano-structured interface was created to suck out more light than previous efforts.

Leo told BBC News the combined result achieves an efficiency that is for the first time higher than a fluorescent tube and that unlike previous white OLEDs, that efficiency does not decrease as the devices are turned up to produce higher-intensity light.

However, much like prior white OLEDs, the significant problem is that the devices degrade within an hour or two, because the polymers that produce the blue part of the light are unstable.

But Leo believes promising first results on stable, phosphorescent blue polymers are starting to emerge.

He said that while it may take a few years, chemists would eventually solve this problem by finding materials that are stable enough.

“I think if you went back five or 10 years and said this is where we’re going to end up, there would’ve been all-round skepticism,” said John de Mello, an optoelectronics expert at Imperial College London who described the work as “impressive”.

He said the team has shown that by taking existing materials and known methods, tweaking them a little bit, and addressing several issues in parallel, they have brought efficiencies up to parity with fluorescent tube lighting.

Leo said that with more improvement in the design part of the OLEDs that whisks the light out, they could be twice as efficient as fluorescent bulbs.

But the devices are comparatively expensive because of the manufacturing methods the group employs.

With the proper materials and designs, OLEDs can be produced in so-called “roll-to-roll” manufacturing in which vast sheets are made, making them economical on a commercial scale.

Leo believes there is a real commercial opportunity in the new OLED developments.

“I’m pretty convinced that in a few years OLEDs will be a standard in buildings,” he added.

The results are published in the journal Nature.

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Imperial College London

Institute for Applied Photophysics

Nature




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