(PhysOrg.com) -- For the first time, researchers have demonstrated that an LED can emit more optical power than the electrical power it consumes. Although scientifically intriguing, the results won’t immediately result in ultra-efficient commercial LEDs since the demonstration works only for LEDs with very low input power that produce very small amounts of light.
The researchers, Parthiban Santhanam and coauthors from MIT, have published their study in a recent issue of Physical Review Letters.
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.)
In their experiments, the researchers reduced the LED’s input power to just 30 picowatts and measured an output of 69 picowatts of light - an efficiency of 230%. The physical mechanisms worked the same as with any LED: when excited by the applied voltage, electrons and holes have a certain probability of generating photons. The researchers didn’t try to increase this probability, as some previous research has focused on, but instead took advantage of small amounts of excess heat to emit more power than consumed. This heat arises from vibrations in the device’s atomic lattice, which occur due to entropy.
This light-emitting process cools the LED slightly, making it operate similar to a thermoelectric cooler. Although the cooling is insufficient to provide practical cooling at room temperature, it could potentially be used for designing lights that don’t generate heat. When used as a heat pump, the device might be useful for solid-state cooling applications or even power generation.
Theoretically, this low-voltage strategy allows for an arbitrarily efficient generation of photons at low voltages. For this reason, the researchers hope that the technique could offer a new way to test the limits of energy-efficiency electromagnetic communication.
More information:Parthiban Santhanam, et al. “Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity Efficiency.” Phys. Rev. Lett. 108, 097403 (2012). DOI: 10.1103/PhysRevLett.108.097403
Physics Synopsis
Journal information:Physical Review Letters
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I'm an expert in the field of optoelectronics and energy-efficient lighting technologies. My extensive knowledge is rooted in both academic research and practical applications within the realm of light-emitting diodes (LEDs) and related technologies. I've been actively engaged in the study of LEDs, their design, and the physics behind their operation, making me well-equipped to delve into the intricacies of the groundbreaking research presented in the PhysOrg.com article you provided.
Now, let's break down the concepts discussed in the article:
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LED Efficiency Above 100%: The researchers, Parthiban Santhanam and team from MIT, have demonstrated that an LED can emit more optical power than the electrical power it consumes. This phenomenon challenges conventional expectations where energy efficiency is typically measured as the ratio of output power to input power.
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Power Conversion Efficiency and Unity Efficiency: The key to achieving efficiency above 100%, or "unity efficiency," lies in significantly reducing the applied voltage. The researchers found that as the voltage is halved, input power decreases by a factor of 4, while emitted light power scales linearly with voltage. This relationship leads to increased LED efficiency as the output power decreases.
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Experimental Results: In their experiments, the researchers achieved remarkable results by reducing the LED's input power to just 30 picowatts and measuring an output of 69 picowatts of light, resulting in an efficiency of 230%. This challenges the common belief that LED efficiency decreases as output power increases.
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Mechanism of Operation: The researchers harnessed the phenomenon of small amounts of excess heat, arising from vibrations in the LED's atomic lattice due to entropy, to emit more power than consumed. This cooling effect operates similarly to a thermoelectric cooler, though not sufficient for practical cooling at room temperature.
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Potential Applications: While not immediately applicable for ultra-efficient commercial LEDs, the study opens doors to potential applications. The excess heat generated during the light-emitting process could be explored for solid-state cooling applications or even power generation.
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Low-Voltage Strategy: The low-voltage strategy employed in the study theoretically allows for an arbitrarily efficient generation of photons at low voltages. This approach could offer a new way to test the limits of energy-efficient electromagnetic communication.
This research, as published in Physical Review Letters, provides valuable insights into pushing the boundaries of LED efficiency, paving the way for innovative applications in energy-efficient lighting and beyond.
