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by Staff Writers
Bonn, Germany (SPX) Nov 11, 2013
OLEDs are already used in the displays of smart phones or digital cameras today. They offer an especially bright image with high contrast, but come with a serious drawback: typically, only one quarter of the electrical energy invested in running the device 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. Making high-quality OLEDs is therefore a rather costly business.
This could change in the near future. The scientists from Bonn, Regensburg and the US have demonstrated a novel type of OLED, which shows potential for high conversion efficiencies without having to resort to noble metals. OLED displays could well get quite a bit cheaper soon.
OLEDs aren't really "organic"
This current is made up of positive and negative charges. When the charges meet, they annihilate, destroying each other in a flash of light.
Since positive and negative charges attract each other, generating light from electricity should be a pretty efficient business. The problem lies in the intricate quantum-mechanical nature of charges, which also posses a magnetic moment - scientists call this the "spin".
Charges with like spin repel each other, much as the north poles of two bar magnets do. This repulsion outweighs the attraction between positive and negative charges, so that different charges with like spin cannot generate light. Instead, they convert electrical energy into heat - a rather exotic and not overly useful way of electrical heating.
In conventional OLEDs this loss of energy occurs frequently: three quarters of all charges carry the same spin. Much like the needle of a compass, they point in the same direction but cannot touch each other, effectively lowering the yield of useful light.
OLED manufacturers have come up with a clever trick to raise the yield: they twirl the compass needles around with an even stronger magnet, allowing the charges to generate light after all.
To do this requires heavy metals such as platinum or iridium, which allow virtually all of the electrical energy to be converted into light. Strictly speaking, conventional materials in OLEDs are not organic compounds at all, but metal-organic substances. This distinction is more than semantic in nature, since noble metals are extremely expensive.
Useful spin flip flops
"It appears that, in our OLEDs, the molecules can store electrical energy for significantly longer than is conventionally assumed", notes chemists Professor Sigurd Hoger 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.
The study was supported by the Volkswagen Foundation and the German Science Foundation (DFG), with collaborators based at the University of Utah and the Massachusetts Institute of Technology (M.I.T.). Publication: Metal-free OLED triplet emitters by side-stepping Kasha's rule; D. Chaudhuri, E. Sigmund, A. Meyer, L. Rock, P. Klemm, S. Lautenschlager, A. Schmid, S. R. Yost, T. Van Voorhis, S. Bange, S. Hoger und J. M. Lupton; Angewandte Chemie (DOI: 10.1002/anie.201307601)
University of Bonn
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