Bad news for organic LEDs from spin experiment

University of Utah physicists have claimed to have successfully controlled the electrical current passing through an organic material using electron spin rather than charge and uncovered bad news for those hoping for high-efficiency light-emitting diodes.

The findings hint organic LEDs would convert no more than 25 per cent of electricity into light rather than heat, contrary to earlier estimates of up to 63 per cent.

In the study, published in Nature Materials, the researchers showed that information can be carried by spins in an organic polymer, and that a spin transistor is possible because “we can convert the spin information into a current, and manipulate it and change it”, said John Lupton, associate professor of physics at the University of Utah. “We are manipulating this information and reading it out again. We are writing it and reading it.”

Christoph Boehme, assistant professor at the university, said spin transistors and other spin electronics could make possible much smaller computer chips, and computers that are orders of magnitude faster than today's. “Even the smallest transistor today consists of hundreds of thousands of atoms. The ultimate goal of miniaturisation is to implement electronics on the scale of atoms and electrons.”

LED semiconductors using compounds of gallium, arsenic, indium and other inorganic materials have made their way into traffic signals, vehicle brake lights and home electronics in recent years. Some inorganic LEDs can convert 47 per cent to 64 per cent of incoming electricity into white light rather than waste heat. But efforts to replace incandescent and even compact fluorescent light bulbs with LEDs have been hindered by costs exceeding $100 per LED bulb.

LEDs made of electrically conducting organic materials are cheaper and easier to manufacture, but their efficiency long was thought to have an upper limit of 25 per cent. A 2001 Nature paper by other University of Utah physicists suggested it might be possible to make organic LEDs that converted 41 per cent to 63 per cent of incoming electricity into light. But the new study suggests 25 per cent efficiency may be correct – at least for the organic polymer studied – pure MEH-PPV – and possibly for others.

Doping organic semiconductors with other chemicals someday might lead to organic LED efficiencies above 25 per cent, but Boehme said he is sceptical. Even if organic LEDs are less efficient and have a shorter lifespan than inorganic LEDs, they still may be more economical because their cost is so much less, he added.

Boehme said organic LEDs' greatest promise is not in lighting, but to replace the liquid crystal display (LCD) 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 said.

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 form either a singlet, with a net spin of zero, or a triplet with a net spin of one.

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 per cent – something the new study supports.

Lupton, Boehme used a plastic semiconductor LED in the form of a piece of the polymer MEH-PPV. Electrodes were attached, and the apparatus was bombarded by a microwave pulse for a few nanoseconds 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,” said Lupton. “That was unexpected.”

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

Instead, the study showed electron spin quantum states can rhythmically and predictably oscillate between triplets and singlets and back again for 0.5µs 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 said.

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