University of Utah physicists believe they have solved the problem by creating a new organic molecule that is shaped like rotelle – wagon-wheel pasta – rather than spaghetti.

The new study showed wagon-wheel molecules emit light randomly in all directions – a necessary feature for a more efficient OLED, or organic LED. Existing OLEDs now in some smart phones and TVs use spaghetti-shaped polymers – chains of repeating molecular units – that emit only polarized light.

“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 polarization. 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.”

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.

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 polarization of light in pi-conjugated polymers that leads to the “trapping” or loss of up to 80 percent of the light generated.

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

The international team of physicists and chemists set out to make molecules that generate light waves in all directions rather than in a fixed direction. In the new study, they report how the created the spoked-wheel molecules, made images of them and did single-molecule experiments, including looking at photons, or light particles, emitted one at a time from a single molecule. In those experiments, they shined an ultraviolet light on the rotelle-shaped molecules to generate visible light photons.

“We showed that every photon that comes out has a scrambled polarization, the polarization changes randomly from photon to photon,” Lupton says.

The emitted light is blue-green, Lupton says, but images accompanying the paper – taken with a scanning tunneling electron microscope – show the rotelle- and spaghetti-shaped molecules with a false yellow-brown color to provide good contrast.

Each wagon-wheel molecule measures only six nanometers wide, which is large for a molecule but tiny compared with the 100,000 nanometer width of a human hair.

Using rotelle-shaped oligomers instead of spaghetti-shaped polymers, “in principle, we should be able to double the efficiency of getting the light out” – although that remains to be proved, Lupton says.

“Even if we scramble the polarization, we’re always going to have a bit of light trapped in the OLED,” he says. “Those losses are now 80 percent, and we probably could get down to 50 or 60 percent.”

This story is reprinted from material from
The University of Utah, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.