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OLED efficiency enhanced by wagon-wheel shaped molecule

Posted: 01 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:pi-conjugated spoked-wheel macrocycle? OLED? smartphone?

A team of researchers from the University of Utah has developed a rotelle-shaped molecule that acts the opposite of polarizing sunglasses, which screen out glare reflected off water and other surfaces and allow only direct sunlight to enter the eyes. According to them, the "pi-conjugated spoked-wheel macrocycle" addresses one of the problems in developing more efficient organic LED light bulbs and displays for TVs and phones: much of the light is polarized in one direction and thus trapped within the LED.

The study showed that wagon-wheel molecules emit light randomly in all directionsa necessary feature for a more efficient OLED, or organic LED. Existing OLEDs in some smartphones and TVs use spaghetti-shaped polymerschains of repeating molecular unitsthat emit only polarized 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," noted University of Utah physicist John Lupton, lead author of a study.

"We made a molecule that is perfectly symmetrical, and that makes the light it generates perfectly random," he added. "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 emphasizes the study is basic science, and new OLEDs based on the rotelle-shaped molecules are "quite a way down the road."

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Figure 1: Electron microscope image of spaghetti-shaped organic polymers (upper left), current standard for OLEDs, and new molecules shaped like rotelle pasta (lower left) that emit light more efficiently.

"OLEDs in smartphones have caught on because they are somewhat more efficient than conventional LCDs like those used in the iPhone," he continued. "That means longer battery life. Samsung has already demonstrated flexible, full-colour OLED displays for future roll-up smartphones." Lupton said 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 added.

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 stated. "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 inaccessiblea problem addressed by another recent University of Utah study of OLEDs. Lupton said 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 per cent of the light generated.

"Light is an oscillating field like a wave, and a wave moves in a certain direction," Lupton explained. "We call this direction of oscillation a polarisation."

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