LEDs shine all white

17 Jun 2002

White LEDs could soon reach our screens now that scientists can make molecule blends emit white light.

Organic light-emitting diodes (LEDs) hold the key to manufacturing cheap displays, but today's LEDs typically produce only single-wavelength light, such as red, green or blue. However, scientists at the University of Leece, Italy, are now one step closer to producing white LEDs for displays after coaxing white light from two blue-light-emitting molecules.

Thompson and colleagues selected four blue-emitting aromatic organic molecules and blended them together in pairs. Thin-films of the blends were spin-coated onto glass substrates and stimulated with an ultraviolet light. The team noticed that once excited, each blend emitted a broad-band spectrum of wavelengths that appeared white.

In the past, researchers have tried to produce white light by using emitters with widely different wavelengths. These attempts have often failed because the light from the short-wavelength emitter can be absorbed by the longer wavelength source and re-emitted at the longer wavelength. However, by using two blue emitters Thompson and colleagues have overcome this problem.

The scientists believe that the formation of an excited complex, or an "exciplex" is responsible for the white photoluminescence. Here the two different excited molecules exchange electrons, which creates a new light-emitting state with a range of wavelengths in the visible spectrum. Thomson's team is also certain that this phenomenon is likely to include all blended blue-emitting organics.

"Our results show that the occurrence of exciplexes is likely to be a general phenomenon in blended blue-emitting organics," said Thompson. "These blends represent a simple, cheap and general method to obtain white emission from organic LEDs."