LED with Whiter Light Produced

An LED that generates pure white light could someday replace the conventional lighting sources that are found today in homes and offices around the world. But first the device must be built.

Researchers from the Indian Institute of Science in Bangalore and from the Centre for Advanced Materials at the Indian Association for the Cultivation of Science in Kolkata, both in India, are reporting a step in that direction with the development of a white-light phosphor that can be excited with a conventional UV source to construct a white LED that avoids many of the problems associated with other approaches.

One of the most common techniques for making white LEDs blends red, blue and green phosphors in the appropriate ratio. Besides the complexity associated with this process, the biggest problem is with self-absorption. Blue, for instance, is absorbed by the reds and greens. This mars the purity of the emitted white light.

Researchers Angshuman Nag and D.D. Sarma avoided this altogether and instead used a novel approach involving cadmium sulfide nanocrystals doped with manganese. This setup emitted clean white light with chromaticity coordinates of (0.30, 0.40) and (0.35, 0.40).

As reported in the Sept. 20 issue of The Journal of Physical Chemistry C, they proved that their approach does not suffer from self-absorption problems and that the chromaticity of the light can be tuned significantly. The nanocrystals also can be excited over a wide range of wavelengths without disturbing emission chromaticity.

They did, however, measure a quantum efficiency of 2 percent, which is very low for most practical applications requiring devices with high efficiency. The researchers said that they hope to improve this in the near future after further experimentation.