News in Science

Pinch of platinum gives white organic LEDs

Bright light A team of scientists have developed a plastic-like polymer that emits white light more efficiently than current organic LEDs.

In recent years household lighting has moved from the incandescent light bulb to the compact fluorescent, and more recently to LEDs.

But to create white light manufacturers cluster red, green and blue LEDs, or use blue LED light, some of which is converted to yellow, and then mixing the two colours to create white light.

Organic light emitting diodes (OLEDs) use polymer chains that glow when they are stimulated with an electrical current or light. However, current OLED displays also combine red, blue and green to create white light.

Professor Valy Vardeny of the University of Utah and colleagues have combined polymer chains with heavy atoms to create a new type of OLED that they believe could create white light much more simply.

Their results appear online in the Nature journal Scientific Reports.

"This new polymer has all those colours simultaneously, so [there is] no need for small pixels and complicated engineering to create them," says Vardeny.

Singlet and triplet states

Polymers have two kinds of electronic states - single and triplet. When the polymer is stimulated, the singlet state emits blue light. The triplet state, which can emit lower energy red light, is much harder to stimulate.

If a heavy atom such as iridium, platinum or palladium is incorporated into the polymer chain, this triplet state is easier to stimulate.

"The idea is to incorporate the heavy atom into the chain so that you don't have to rely on mixtures or energy transfers. The compound itself contains all the ingredients," says Vardeny.

Varying how often platinum appears in the chain allows the researchers to 'tune' the light it emits. For example, a platinum atom after each 'unit' in the chain emits violet and yellow light, while a platinum atom after every third unit results in blue and orange light.

"The idea ultimately is to mix this polymer with different platinum units so we can cover the whole spectrum easily and produce white light," says Vardeny.

Currently, the polymers rely on light to become stimulated and produce light. The researchers believe electrically stimulated polymers will be possible within the next two years.

Highly versatile

Vardeny says one of the advantages of using OLEDs is that they produce light across a wider band in the light spectrum compared to LEDs. They can also be made into various shapes.

"The organics are also very flexible," he says. "You can do all kinds of fixtures which you cannot do with LEDs, because polymers are flexible while LEDs are based on crystals."

The new polymer could also lead to the development of a new type of solar cell that converts light into electrical current more efficiently.

"In current solar cells, for each photon that is absorbed it generates one electron and hole pair," says Vardeny. But by utilising the platinum-rich polymer's triplet state, "photons with a sufficient amount of energy when absorbed by the polymer chain may produce two triplets that can dissociate into two electrons and two holes, and thus double the amount of photocurrent."