Novel OLEDs pave the way for cheaper display technology

Novel OLEDs pave the way for cheaper display technology

Technology News |
By eeNews Europe

Lighting applications such as luminescent tiles are also conceivable.

OLEDs are already used in the displays of smart phones or digital cameras today and offer a bright image with high contrast.  However, OLEDs currently come with a serious drawback in that only one quarter of the electrical energy invested in running the device is actually converted into light. The 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 expensive. Making high-quality OLEDs is a costly business.

Now the scientists from Bonn University, Regensburg University and the University of Utah and MIT have demonstrated a novel type of OLED, which shows potential for high conversion efficiencies without having to resort to noble metals which means that OLED displays could become a cheaper soon.

Novel molecules for OLEDs. The synthetic design promotes random changes in orientation of the molecular ‘compass needle’, increasing overall brightness. Source: John Lupton

The operating principle of an OLED is simple: a thin film of the molecules is contacted by electrodes, which are connected to a battery so that an electrical current can flow. The 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 an efficient business. The problem lies in the intricate quantum-mechanical nature of charges, which also posses a magnetic moment or ‘spin’. Charges with like spin repel each other, much as the north poles of two bar magnets do. The 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 the 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 and noble metals are expensive.

“We can also raise the efficiency using a different mechanism,” explained Dr. 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.

“It appears that, in our OLEDs, the molecules can store electrical energy for significantly longer than is conventionally assumed”, said Prof. Dr. 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 hold the potential to minimize electrical generation of heat in OLEDs without having to resort to any ‘metal-organic tricks’, thereby converting the electrical energy 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. Röck, P. Klemm, S. Lautenschlager, A. Schmid, S. R. Yost, T. Van Voorhis, S. Bange, S. Höger und J. M. Lupton; Angewandte Chemie (DOI: 10.1002/anie.201307601)

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