Red light emitting diodes were easy, and have been around for a while. Blue LEDs were harder; Shuji Nakamura won the million euro Millenium Technology Prize for figuring them out.
Green LEDs have been tougher still. To get a good white light you need to mix all three, so designers have relied on a work-around, firing blue LEDs at a phosphor that emits green light, much like the ultraviolet light from mercury vapour excites the phosphors in a fluorescent light. (Mike has shown a couple of wild and crazy ways they get white light out of blue LEDs)
Now Angelo Mascarenhas of the National Renewable Energy Laboratory (NREL) has figured out how to go green, by building a solar cell in reverse.
Angelo Mascarenhas, right, talks over how to do a table-top laser experiment on LEDs. Credit: Bill Scanlon
When they build solar cells, scientists had to figure out how to capture energy across the spectrum and had already solved the green problem. Gismag explains:
Absorbing green light is technically challenging because of the way the different layers of lattice that should absorb it are manufactured: if the layers don't match up with the layer below, leaving too big a gap, the efficiency plummets to next to zero. NREL's solution was essentially to insert extra layers of lattice that gradually bridge the gap, improving the cell's efficiency.
Mascarenhas's idea was to reverse the process -- that is, making a current flow between appropriately spaced layers of lattice to obtain green light - and reportedly managed to produce a radiant deep green light on the very first try.
The phosphor based LEDs have a lower efficiency and run hotter; the broad band representing the phosphors in the graph above is a lot less intense than the peak of the blue LED. Three separate coloured LEDs can be tuned to any colour a designer wants, will run cooler and will be a lot brighter. Mascarenhas is quoted in physorg:
"We'll be able to electronically control the hue of the lamp," he said. "We can vary the combination of intensities of these four colors 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. "
Something to look forward to. More at NREL
More on LEDs:
Big LED Breakthrough at Purdue University Could Change the World
Luxim Plasma Light Bulb Kicks Some Serious LED Butt
LED Street-Lights are Greenest Choice, Life-Cycle Study Shows
Nanocrystal Coating = White LED Big Breakthrough?
Researchers Make White LED Light Bulbs using Salmon DNA