Researchers in Germany in collaboration with researchers in the U.S. have demonstrated a novel type of organic light emitting diode which shows potential for high conversion efficiencies without having to resort to noble metals, thus leading to less costly OLED displays.
OLEDs used in today's displays of smart phones or digital cameras have a bright image with high contrast, but typically only one quarter of the electrical energy is actually converted into light. This ratio can be raised by adding traces of noble metals such as platinum or iridium to the active material, but these elements are rare and very expensive.
OLEDs are made up of organic molecules consisting solely of carbon and hydrogen. An OLED's thin film of the molecules generates light from electricity from the intricate quantum-mechanical nature of charges, which also posses a magnetic moment called "spin." Charges with like spin repel each other and different charges with like spin cannot generate light. Instead, they convert electrical energy into heat.
In conventional OLEDs this loss of energy occurs frequently: three quarters of all charges carry the same spin, effectively lowering the yield of useful light. OLED manufacturers, in order to raise the yield, use expensive noble metals such as platinum or iridium, which allow virtually all of the electrical energy to be converted into light.
"We can also raise the efficiency using a different mechanism," said John Lupton, professor of physics at the University of Regensburg. "Charges can flip the orientation of their spins spontaneously—you just have to wait for long enough for this to occur."
In conventional OLEDs, however, there is not enough time to do this since the electrical energy is not stored for long enough in the molecular architecture. Instead, the molecules give up and simply convert the energy to heat.
In the OLEDs developed for this study, "the molecules can store electrical energy for significantly longer than is conventionally assumed," noted chemistry professor Sigurd Höger of the University of Bonn. "Our molecules can therefore exploit the spontaneous jumps in spin orientation in order to generate light."
The new compounds therefore hold potential to minimize electrical generation of heat in OLEDs without having to resort to any "metal-organic tricks," thereby converting the electrical energy very effectively into light, according to the researchers.
The study was supported by the Volkswagen Foundation, and the German Science Foundation (DFG), with collaborators at the University of Utah and the Massachusetts Institute of Technology. It has been published in Angewandte Chemie.
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