Innovative LEDs light the path toward cheaper displays

A team of scientists from Bonn University, Regensburg University, the University of Utah and MIT has demonstrated a novel type of OLED that shows potential for high conversion efficiencies without having to resort to noble metals. This could lead to less costly OLED displays that will benefit OEMs requiring this particular type of technology.

OLEDs are widely used in the displays of smartphones or digital cameras. 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.

These are novel molecules for OLEDs that can store electrical energy for significantly longer than is conventionally assumed.

OLEDs are called so because, ideally, they are made up of organic molecules, which consist solely of carbon and hydrogen. The operating principle of an OLED is rather simple: a thin film of the molecules is contacted by electrodes, which are connected to a battery so that an electrical current can flow. 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 possess 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.

"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.

"It appears that, in our OLEDs, the molecules can store electrical energy for significantly longer than is conventionally assumed", noted 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.