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Natural look for white-light LEDs

16 Jul 2007

An LED chip coated with quantum dots has produced light emission that mimics natural daylight.

Quantum dot (QD) enhanced LEDs could soon be grabbing a large chunk of the daylighting market if the work of Korean scientists takes off. Soenghoon Lee and his team from Seoul National University have come up with a “superior” natural-looking white-light source by coating the surface of an LED chip with a mixture of different-sized (CdSe)ZnSe or (CdSe)ZnS nanophosphors.

"We use the LED chip as the excitation source and the QDs as colour-conversion centres," Lee told nanotechweb.org. "So far we've made white-light devices by covering a blue LED chip with green and red QDs and by covering a near-UV LED chip with red, green and blue QDs to show that the idea works."

By selecting the right combination of photoluminescent particles, the researchers were able to come up with a white-light source that mimics natural light and possesses the advantages of an LED. The QDs range from 2 to 6 nm in diameter and emit colours from blue through to red depending on their size. "QDs have advantages over conventional phosphors," added Lee. "Each colour of a QD has a broad absorption feature, which means that different QD colours can be simultaneously excited by a single excitation source."

The particles are embedded in a matrix of Si resin that also contains the bare LED chip. According to Lee, one of the major challenges is dispersing the QDs so that the matrix is free of clumps or aggregates. Another hurdle is being able to maximize light extraction from the device and the researchers are busy developing an efficient white LED design to solve this part of the puzzle.

As Lee explains, momentum for the idea is gathering pace. "Some well known firms in the electronics sector have shown an interest in our solid-state lighting – the overall goal is to commercialize our design by collaborating with industry," he said. "We've filed several patents worldwide and plan to license the technology."

Ultimately, the success of QD-enhanced LEDs will come down to a combination of manufacturing cost, efficiency, brightness and light quality. Lee thinks that some of the early adopters will be galleries and retail outlets. In other words, customers who are willing to pay a premium for a light source with natural qualities. At the moment, energy-inefficient halogen lamps dominate this market.

The researchers reported their work in Nanotechnology.

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