The molecular structure is key to the efficiency gains —

Efficient OLED gets rid of heavy metals

The structure of the light-emitting molecule is key to the efficiency gains.

Organic LEDs are thin and flexible.

A new molecule increases the efficiency of fluorescent organic light emitting diodes without using heavy metals. Early OLEDs were made from fluorescent molecules that transform added electrical energy into light. However, rules of quantum mechanics make this transformation rather inefficient.

Here’s why. Pumping electricity through a fluorescent OLED excites charge carriers—electrons and positively charged “holes”—in the molecules. These charge carriers meet to form a bound state called an exciton. This exciton can be one of two types: a singlet exciton or a triplet exciton. That energy has to be released, but only singlet excitons can release it as light, and singlets only occur 25 percent of the time. The other 75 percent of excitons are triplets that relax by releasing heat, not light.

To increase the efficiency of these OLEDs, scientists add heavy metal atoms to help the previously heat-producing states emit light—with nearly 100 percent efficiency—through phosphorescence.

Now scientists have accomplished similar efficiencies without the metal. Hiroki Uoyama, of Kyushu University in Japan, and colleagues designed a molecule that has a small energy difference between the singlet and triplet excitons. That way, triplet excitons can cross over to an excited singlet state and then relax to emit light.

In this carefully designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

The molecule can be easily synthesized in one step, and its shape and structure are key to its increased efficiency. Tweaking the structure allows the researchers to build OLEDs that emit blue, green, yellow and orange light.

The lack of heavy atoms in the molecule means it might be cheaper to produce than phosphorescent OLEDs containing iridium or platinum, says Z. Valy Vardeny, at the University of Utah.

OLEDs are already finding their way into thin, high-def screens of some commercial devices. But building a molecule doesn’t necessarily mean it will find its way into device production, writes Brian D’Andrade of Exponent, Inc., in an accompanying commentary. To be useful in production, these molecules need to be optimized to emit colors of light needed in displays, they need to be able to be manufactured into devices, and the lifetime of those devices needs to be comparable—or better than—the state-of the art, he writes.

Nature, 2012. DOI: 10.1038/nature11687  (About DOIs).

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