Freeing Trapped Light
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.
"Conjugated polymers are a terrible mess," Lupton says. "They now make only mediocre OLEDs, although people like to claim the opposite."
For one thing, three-quarters of the light energy is in a state that normally is inaccessible - a problem addressed by another recent
"Light is an oscillating field like a wave, and a wave moves in a certain direction," Lupton says. "We call this direction of oscillation a polarization."
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 fiber."
That, he adds, is why even with the latest OLED smart phones, "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 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."
Lupton compares the ability of the wagon-wheel molecules to emit unpolarized light in all directions to what happens when a pencil is balanced perfectly on its tip and falls in a different, random direction each time.
Cooking up a Wagon Wheel-Shaped Molecule
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."
Keywords for this news article include: Light Bulb, Electronics, Semiconductor,
Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2013, NewsRx LLC
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