In 1985, Ching Tang and Steven Van Slyke of Kodak’s R&D lab in Rochester, NY, demonstrated light-emitting devices based on thin films of fluorescent organic molecules. Although they might not have fully recognized it at the time, their invention carried the possibility of transforming display screens and, perhaps more importantly, interior lighting.
But the invention had a significant drawback that was imposed by quantum mechanics. Making these organic molecules emit light requires injecting electrons from electrical contacts on the film surfaces. But because of quantum-mechanical considerations, only one in four electrons injected will produce light emission. As a result, fluorescent organic light-emitting devices (OLEDs) had relatively low efficiency.
In 1998, my group at Princeton University, in collaboration with researchers at the University of Southern California under the direction of Mark Thompson, found that including a heavy-metal atom such as platinum or iridium in the organic molecule could overcome the quantum-mechanical limitations, allowing for 100 percent of the injected electrons to result in light emission via the process of phosphorescence. Phosphorescence is often associated with dim, long-lasting light, but with the addition of a heavy metal atom, organic molecules are capable of both rapid and exceedingly bright phosphorescence.
This new phenomenon, called electrophosphorescence, allows OLEDs to be used in high-efficiency, full-color displays. But perhaps more importantly, it allows for the emergence of a new generation of interior illumination sources. By combining the light emissions of red, green, and blue electrophosphorescent OLEDs, we can generate light that the eye perceives to be white – and do it very efficiently.
Current incandescent interior lighting, which has been in development for over 125 years, has an efficiency of approximately 15 lumens per watt. Electrophosphorescent white OLEDs have already demonstrated efficiencies of approximately 20 lumens per watt at levels bright enough for room illumination. We recently demonstrated in our labs that by combining phosphorescence and more conventional fluorescence, we can make a single OLED structure that produces nearly 30 lumens per watt, with the possibility of 50 to 60 lumens per watt in the near future. This device operates at lower voltage than a pure electrophosphorescent white OLED, resulting in improved efficiency.
Higher-efficiency lighting can reduce humankind’s ever increasing use of energy. OLEDs may play a vital role in the effort.
Stephen Forrest is vice president of research at the University of Michigan, Ann Arbor. He is also a professor in the Departments of Electrical Engineering and Computer Science, Physics, and Materials Science and Engineering.
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