Researchers at the National Renewable Energy Laboratory in the US have discovered a way to improve the efficiency of white LEDs – by producing a green one.
Making an LED that appears white requires, at the very least, the colours red, green and blue. The white light from the sun is really all the colours of the rainbow. Without at least red, blue and green from the spectrum, no lighting device will be practical for home or office use. Red proved easy to generate, and about 15 years ago, Japanese scientists found a way to generate blue, thus providing two of the key colours from the spectrum of white light. But green has been elusive. In fact, the white LEDs that are available now are made to look white by aiming the blue light at a phosphor, which then emits green. This workaround is effective, but the clunky process saps a big chunk of the efficiency from the light.
NREL scientist Angelo Mascarenhas, who holds patents in solar cell technology, realised that an LED is just the reverse of a solar cell: one takes electricity and turns it into light; the other takes sunlight and turns it into electricity. “We had been working with solar cells for 30 years,” Mascarenhas said. “Could we find some device where we could just reverse the process of making solar cells?”
Indeed, they found it. NREL had won major scientific awards with its inverted metamorphic solar cells, in which the cells are built by combining layers of different lattice sizes to optimally capture solar energy. In fact, an NREL-produced IMM cell set a world record by converting 40% of absorbed sunlight into electricity. Along the way, “We had already developed some of the know-how to capture sunlight in this green spectral region,” Mascarenhas said. They had not reached there, because solar cells do not need a green, but they had begun to understand the challenges of getting to a green.
For a decade, LED researchers had tried and failed to make a reliable efficient green light by putting indium into gallium nitride. Mascarenhas and his fellow solar cell researchers had dealt with the same problem trying to build a solar cell with gallium indium phosphide. When the lattices created by molecular gases do not match up with the lattices of the layer below, “It cannot grow well and the efficiency is very, very poor,” he said.
NREL’s solar cell experts found a way around that. They put in some extra layers that gradually bridge the gap between the mismatched lattices of the cell layers. “The approach is to grow a different material with an in-between lattice,” Mascarenhas said. The researchers deposited layers that had lattice patterns of atoms close to, but not exactly matching, the layers below. The tiny gap in size was at the so-called ‘elastic limit’ of the material – close enough that the lattices bonded to each other and impurities were deflected away.
Then, a third layer was added, this once again at the precise elastic limit of the one below. After about seven microns of layering, the result was a solar cell with a firm bond and almost no impurities. The research team wondered whether that same process, only in reverse, could be used to make a reliable deep-green LED using indium gallium phosphide.
Astonishingly, once the concept was understood, Mascarenhas’ team produced a radiant deep green on their very first try. The aim now is to provide a fourth colour to make that white light even whiter. NREL plans to use a slightly deeper red and a lemony green, which would then be combined with a blue and a very deep green made using the gallium nitride-based technology.
In three years, NREL expects to have a bi-coloured device that, when teamed with blue and deep green, can produce a sterling LED with a colour-rendering index well over 90. “It will give you one of the finest colour-rendering white lights” and the manufacturing costs should not increase, Mascarenhas said.
“We have a patent on a device that will provide these two colours, as one unit, to industry,” Mascarenhas said. “They will arrange them like the mosaic in a fly’s eye — our units side by side with the blue and deep green combination, alternating in a pattern. From afar, it will look like white. You will not be able to see the individual colours of the mosaic structure. We have full confidence that this is achievable.
The resulting white light LED will be intelligent. “We will be able to electronically control the hue of the lamp,” he said. “We can vary the combination of intensities of these four colours on an electronic circuit. By slightly increasing the blue, we can make it more suitable for daylight. By turning down the blue and increasing the reddish yellow, we can make it softer, more suitable for night. We can smoothly control the hue throughout the day like nobody has imagined.”
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