IF YOU have decorated your Christmas tree with coloured LEDs instead of traditional bulbs this year, you’re not the only one switching to this emerging form of solid-state lighting.
The 20-year lifespan and impressive energy efficiency of LEDs (light-emitting diodes) means they have already seen off competition from fragile, power-hungry incandescent bulbs in applications such as traffic lights, railway signals, airport runway lights, bicycle lamps, flashlights, brake lights, TV backlights and all manner of architectural spotlights.
So, why aren’t we using LEDs to light our homes? The snag is they cannot efficiently produce the warm white light we get from conventional sources, at least not yet. “There is no doubt this will happen, despite technical hurdles,” predicts LED engineer Fred Schubert of Rensselaer Polytechnic Institute in Troy, New York.
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LEDs have numerous advantages over incandescent lights already. An incandescent filament must reach 2000 °C before it emits light. In a traffic light, for example, a bulb with a red filter uses between 100 and 150 watts, whereas the array of 12 red LEDs now used in 70 per cent of traffic lights in California uses just 12 watts. Also the low running temperature of LEDs means they can last for decades, as there is no hot filament to burn out. And they do not need to be housed in a delicate glass bulb.
But LEDs still can’t outperform fluorescent lights. Fluorescents are about five times as efficient as incandescent bulbs. They produce light by exciting vaporised mercury atoms in a glass tube, causing them to release ultraviolet light. That then strikes a phosphor coating inside the tube, which absorbs one wavelength of light and emits another, fluorescing white light. The process draws very little power: an 11-watt fluorescent tube is as bright as a 60-watt incandescent light.
“With LEDs, we have surpassed incandescence, but the holy grail is beating fluorescence, which requires more research,” says Jeff Tsao, a researcher at the Sandia National Laboratories in Albuquerque, New Mexico.
White LEDs could do better if a handful of challenges could be overcome. LEDs comprise a sandwich of two types of semiconductor, one rich in electrons and one depleted of them. When a voltage is applied across an LED, electrons from the electron-rich side move to the depleted side and fall into lower energy levels, surrendering their energy as light. Adjusting the diode’s chemistry changes the size of the energy gap, and thus the colour of the light emitted.
“New materials could slash the power used by white LEDs to a tenth of today’s levels making them a compelling option”
Progress towards making white LEDs began in earnest in 1993, when Japanese firm Nichia used gallium nitride to make the first LED to emit visible blue light. This meant that for the first time, LED light of the right wavelength could be produced and used to bombard a phosphor to produce white light.
Right now, the cheapest, most common way to produce a white-light LED is to coat a blue LED in phosphor grains that absorb near-UV blue light and emit a combination of red, yellow and green wavelengths that together look white. LEDs such as these are used in pocket flashlights and bicycle lamps.
But this two-stage light production mechanism means some energy is wasted. The phosphor grains convert some of the energy to heat and for LEDs to beat the efficiency of domestic white fluorescent lighting, this waste has to be eliminated.
So researchers are looking for a way to produce white light from LEDs by placing blue, green and red LEDs together on a chip, without using phosphors. But there’s a problem: green LEDs are inefficient.
“Green sticks out like a sore thumb, it’s the missing piece,” Tsao says. Blue and green LEDs use the same semiconductor, indium-doped gallium nitride, but green LEDs require more indium. The more indium is added, the slower the electrons fall into the holes, which lowers the light output.
Researchers are trying to overcome these problems by finding new green emitters. Schubert predicts that while today’s white LEDs use half as much power as incandescent bulbs to produce the same brightness, the new materials could slash that to one-tenth in about two years time. This will make white LEDs a compelling option for use in the home.
