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17 Jun 2002
Researchers in the US have devised a way to greatly improve the efficiency of fluorescent organic light-emitting devices (OLEDs).
The luminous efficiency of an organic molecule depends on the way that excited electrons relax and release their energy. Most organic materials emit light by fluorescence but this is not very efficient because the electrons will only lose energy in a way that maintains the same spin symmetry. Phosphorescent materials release light energy about four times more efficiently than fluorescent species but phosphorescence is very rare in organic molecules at room temperature.
In a paper in the 17 February edition of the journal Nature, M.A. Baldo and S.R. Forrest of the University of Southern Carolina and M.E. Thompson of Princeton University described a way to allow fluorescent molecules to take advantage of the efficiency of phosphorescent molecules and to emit more light. This method involves adding an additional 'phosphorescent sensitizer' molecule to the material. This molecule disrupts the symmetry of the energy levels in the fluorescent molecule and allows the electrons to lose energy in ways that are usually forbidden.
The researchers demonstrated this technique with DCM2 - an organic dye that absorbs green light and fluoresces in the red region between 570 and 650 nanometres. The phosphorescent sensitizer material was an iridium complex with a green emitting compound as a ligand.
This system gives luminous efficiencies similar to those of phosphorescent materials and much better than systems without the phosphorescent sensitizer.
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