Toward plastic spin transistors

Boehme says organic LEDs' greatest promise is not in lighting, but to replace the LCD (liquid crystal display) technology in modern televisions and computer screens. Organic LEDs will be much cheaper, can be made on flexible materials, have a wider viewing angle and color range and will be more energy efficient than LCDs, he says.

Flip-Flopping on Flipping and Flopping

LEDs produce light when incoming negative and positive electrical charges – electrons and "holes" – combine. The spins of each electron-hole pair can combine in four quantum states, which in turn can combine in two different ways to form:

• A "singlet," with a net spin of zero (up-down minus down-up).

• A "triplet," with net spin one (up-up, down-down or up-down plus down-up).

In some organic materials, singlets emit light when they decay, and triplets do not. So the efficiency of an organic LED depends on the relative production of singlets and triplets. The fact that a singlet is only one of four quantum states suggests the maximum efficiency of an organic LED can be 25 percent – something the new study supports.

Lupton, Boehme used a plastic semiconductor LED in the form of a piece of the polymer MEH-PPV measuring about one-twelfth-inch long by one-eighth-inch wide. It was mounted on a piece of glass about 2 inches long and one-sixth inch wide.

Electrodes were attached, and the apparatus was bombarded by a microwave pulse for a few nanoseconds (billionths of a second) to turn and align the spins of electron-hole pairs in the LED. The electrodes also were used to measure the strength of the electrical current from the device.

"Just like a mass on a spring, the pulse produces an oscillation of the spins [of singlets and triplets] in the organic LED," Lupton says. "That was unexpected."

The 2001 study indicated that some triplets randomly, unpredictably "lose their memory," changing spin orientation or "flipping" to become singlets, boosting possible organic LED efficiencies as high as 63 percent. The new study, however, found triplets "flip" into singlets too slowly to produce much light, Boehme says.

Instead, the study showed electron spin quantum states can rhythmically and predictably oscillate or "flop" between triplets and singlets and back again for one-half microsecond (millionths of a second) when excited by microwaves.

Because the combination of electrons and holes that produces light happens faster than that, "flipping likely isn't involved in producing light" from the LED, and thus it will be difficult to make organic LEDs with efficiencies above 25 percent, Lupton says.

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