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Hybrid WLEDs get brighter

16 Nov 2007

Luminescent nanocrystals and highly fluorescent polymers give controlled white-light generation with a CRI of more than 80.

Researchers in Turkey and Germany have combined luminescent nanocrystals with fluorescent polymers to fabricate hybrid LEDs that for the first time allow controlled white-light generation with a colour rendering index (CRI) of more than 80. The LEDs could be used in any of the situations where light bulbs and fluorescent strips are used today, and in future could provide low-cost, safe lighting in areas that have no access to electricity.

Elisabeth Holder of the University of Wuppertal and Hilmi Volkan Demir of Bilkent University in Ankara and colleagues generated white light in devices made from a blue conjugated polymer as an organic host for coloured inorganic nanocrystals. "Together these components generate white light of a very high quality when pumped by a very efficient inorganic LED," Holder told nanotechweb.org.

The white-light output is tuned using a layer-by-layer assembly of closely packed nanocrystals with a core of cadmium selenide and a shell of zinc sulphide and polyfluorene-conjugated polymer hybridized on near-UV emitting nitride-based LEDs. The devices are the first white-emitting LEDs to have CRIs of greater than 80, the figure required for future solid-state lighting applications.

"Today, most commercial WLEDs typically have colour rendering indices of around 70, but this is not as high as desired," explained Demir. "This is the basic motivation for our work to use a combination of nanocrystals in polymers."

Holder continued: "LEDs have become very popular in lighting applications due to the fact that they are able to save about 80% of the energy costs compared to standard light bulbs. LED lighting could be important in all areas, but mainly in indoor lighting where soft white light is preferred."

The team now plans to use more efficient conjugated polymers to allow "triplet emitting channels". It will also optimize the morphology of the devices by employing polymers that allow the nanocrystals and polymers to be closer together in the finished LEDs.

The work was published in New Journal of Physics.

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