Photons: Buy One, Get Three Free

In a bargain your coupon-clipping aunt would love, researchers have discovered a way to get four times as much light from state-of-the-art light emitting diodes (LEDs) for the same amount of energy. The advance, reported in the 17 February Nature, could slash the power needed to run the all-LED displays that may soon adorn car radios, cell phones, and laptop computers.

In so-called organic LEDs, organic molecules excited by electricity shed their extra energy by giving off photons, a process called fluorescence. But typically, only one excited molecule in four emits light: the other three give off their energy as heat. That's because in the excited state, two electrons waltz through the molecule, spinning like tops, and only when the electron spins point in opposite directions does the dance end with the release of a photon. When the spins have the same direction, as they do roughly three out of four times, the dance peters out as the energy seeps away in heat-generating vibrations of the molecule. Now, Princeton University electrical engineer Stephen Forrest and his team have found a way to reclaim this lost energy.

Forrest and colleagues had previously developed phosphorescent materials, in which same-spin pairs of electrons can metamorphose into opposite-spin pairs, which then produce light. However, simply adding some phosphorescent material to organic LEDs doesn't recover all the wasted energy, because same-spin pairs don't emit photons as fast as they pile up. So the researchers created a layer cake of organic diode material. They seasoned alternating layers with a phosphorescent material and a fluorescent dye. When they powered up the device, the same-spin excitation states generated in the organic material jumped onto the phosphorescent molecules; then they quickly transformed into opposite-spin states that jumped onto the dye molecules and produced light.

The researchers have tapped into "the wasted energy that everybody has been trying to capture," says Andrew Beeby, a chemist at the University of Durham, United Kingdom. There may be a catch, however. Researchers hope to combine red, green, and blue organic LEDs to create full-color displays. But in the new technique, the energy of the electron pairs decreases as the excitation hops from molecule to molecule, so Forrest and colleagues end up with lower energy red light. "Getting that shifted toward the green and blue is going to be harder," Beeby says.

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