Making light work of LED droop

The drive to bring eco-friendly LED lighting into our homes is being stopped in its tracks by an embarrassing problem known as droop – the disappointing reduction in efficiency that happens when the light bulbs operate at the high power levels they need to shine at their brightest.

“Efficiency droop is one of the main obstacles to achieving cost-effective and high-efficiency LEDs,” says Seong-Ju Park at the Gwangju Institute of Science and Technology (GIST) in South Korea. “Droop becomes a very important issue as LEDs expand into applications like [indoor] lighting where they operate at high currents.”

For years, LED production has grown in tandem with the cellphone, providing the backlight for their displays. But manufacturers will have to tackle droop before high-power LEDs can hit the big time.

The cause of LED droop is disputed, making the solution to the problem far from clear – but now, Park and colleagues at GIST have teamed up with Samsung LED to prop up this flagging performance with an unconventional device design.

A standard LED has a surplus of electrons on one side and a dearth of electrons – or an abundance of electron “holes” – on the other. Plug the LED into a circuit, and the electrons and holes move towards each other, combine, and release energy as light.

Droop means that the proportion of the recombinations that produce light peaks at low electrical powers, with the record-holding prototype devices reaching about 250 lumens per watt. Raise the power to levels typically used for indoor lighting, though, and an increasing proportion of the electric current is lost as heat, so the efficiency drops below 100 lumens per watt.

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