MIT researchers have created an “over-unity” LED that absorbs ambient heat energy.
The LED produces 69 picowatts of light using 30 picowatts of power, giving it an efficiency of 230 percent. That means it operates above “unity efficiency” — putting it into a category normally occupied by perpetual motion machines.
However, while MIT’s diode puts out more than twice as much energy in photons as it’s fed in electrons, it doesn’t violate the conservation of energy because it appears to draw in heat energy from its surroundings instead. When it gets more than 100 percent electrically-efficient, it begins to cool down, stealing energy from its environment to convert into more photons.
The MIT Team
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Physicists have known for decades that, in principle, a semiconductor device can emit more light power than it consumes electrically. Experiments published in Physical Review Letters finally demonstrate this in practice, though at a small scale.
The energy absorbed by an electron as it traverses a light-emitting diode is equal to its charge times the applied voltage. But if the electron produces light, the emitted photon energy, which is determined by the semiconductor band gap, can be much larger.
As the researchers explain in their study, the key to achieving a power conversion efficiency above 100%, i.e., “unity efficiency,” is to greatly decrease the applied voltage. According to their calculations, as the voltage is halved, the input power is decreased by a factor of 4, while the emitted light power scales linearly with voltage so that it’s also only halved. In other words, an LED’s efficiency increases as its output power decreases. (The inverse of this relationship – that LED efficiency decreases as its output power increases – is one of the biggest hurdles in designing bright, efficient LED lights.)
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