When scientists at the National Renewable Energy Laboratory (NREL) tried to apply their expertise in solar cell technology to build a green LED light from the ground up, they surprisingly centered the objective at their very first try. In doing so they solved a long-standing technological problem and paved the way for the large-scale employment of white LEDs for public and domestic illumination over the course of the next few years.
What's wrong with your good ol' tungsten bulbs, you may ask? The problem is that they produce light by incandescence, which is about the least efficient way to produce light — it wastes the majority of energy to produce useless heat, which inevitably ends up inflating your electrical bill. To a lesser extent, compact-fluorescent lights also share this inefficiency problem, which has led the U.S. Department of Energy to predict that both kinds will be phased out in the space of only four and ten years respectively, leaving LEDs virtually the only player in the market.
LED lights are unanimously regarded as a vast improvement over previous light bulbs because of their much longer lifespan and higher efficiency, which ends up saving us money in the long run, even when the higher initial cost is taken into account.
But to create a white LED, red, blue and green light need to be combined. While the first two colors have been relatively easy to manufacture, researchers have struggled to produce a green LED. The LED-based lights available today circumvent the problem by aiming the blue light at a phosphor, which then emits green light. This does produce white light, but it is still wasteful compared to a white light that makes use of three distinct, all-LED components.
NREL researcher Angelo Mascarenhas, who holds patents in solar cell technology, realized that a LED can be thought of as the reverse of a solar panel, since one takes electricity and turns it into light, while the other takes (sun)light and turns it into electricity.
Mascarenhas used the knowledge gathered by NREL when they created a world-record inverted metamorphic solar cell by combining layers of different lattice sizes to optimally capture solar energy across the visible spectrum. The researchers had already tackled the problem of how to absorb sunlight in the green spectral region, and Mascarenhas built on this knowledge to reverse the process in order to manufacture a green LED.
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.
NREL is now trying to produce a fourth color to make the white light even whiter. They plan to arrange the four colors in a beehive structure, each cell being an LED of a specific color, so that the light will look white when seen from a distance.
The researchers also plan to make their LED light "intelligent," dynamically matching the percentage of the four colors to the time of the day - for instance, increasing the blue component during daylight, or the reddish-yellow during the night.
Further predictions from the Department of Energy estimate that the move toward LEDs for both public and domestic lighting could save the U.S. as much as US$120 billion over the course of the next 20 years, as well as 246 million metric tons of carbon (approx. 5 percent of U.S. total emissions during the period 1850 through 2000) that would otherwise be released into the Earth's atmosphere.
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