Rotelle-shaped molecules for better OLEDs

Physicists at the University of Utah have developed a new organic molecule shaped like rotelle, wagon-wheel pasta that emits light randomly in all directions - a necessary feature for a more efficient organic LED.

Existing OLEDs now in some smartphones and TVs use spaghetti-shaped polymers - chains of repeating molecular units - that emit only polarised light. “This work shows it is possible to scramble the polarisation of light from OLEDs and thereby build displays where light doesn’t get trapped inside the OLED,” says University of Utah physicist John Lupton, lead author of a study of the spoked-wheel-shaped molecules published in the journal Nature Chemistry.

“We made a molecule that is perfectly symmetrical, and that makes the light it generates perfectly random,” he adds. “It can generate light more efficiently because it is scrambling the polarisation. That holds promise for future OLEDs that would use less electricity and thus increase battery life for phones, and for OLED light bulbs that are more efficient and cheaper to operate.”

Lupton says smartphones could produce light more efficiently using molecules that don’t trap as much light. The large rotelle-shaped molecules also can ‘catch’ other molecules and thus would make effective biological sensors; they also have potential use in solar cells and switches, he adds.

While conventional LEDs use silicon semiconductors, OLEDs in some of the latest cell phones and TVs are made with ‘pi-conjugated polymers’, which are plastic-like, organic semiconductors made of a chain of repeating molecular units.

For one thing, three-quarters of the light energy is in a state that normally is inaccessible - a problem addressed by another recent University of Utah study of OLEDs. Lupton says his study deals with another problem, which exists even if the other problem is overcome: the polarisation of light in pi-conjugated polymers that leads to the ‘trapping’ or loss of up to 80% of the light generated.

Because polymers are long molecules like spaghetti, when an electrical current is applied to a polymer, “the electrons can only flow in one direction and that generates the light waves”, Lupton says. “Because those light waves only oscillate in one direction, the light can get trapped inside the OLED, which is a little bit like an optical fibre.” That, he adds, is why even with the latest OLED smartphones, “your battery is dead in two days because the display uses a lot of the electricity”.

“The rotelle - technically called oligomers - are basically wrapped-up polymers,” Lupton says. “They all have the same shape, but they do not emit polarised light because they are round. They generate waves that vibrate in all directions. The light doesn’t have a fixed polarisation; it doesn’t vibrate in a fixed direction. It always can get out.”

The study was funded by the Volkswagen Foundation, the German Chemical Industry Fund, the David and Lucille Packard Foundation and the European Research Council.