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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Absolute vs. relative efficiency: How efficient are blue LEDs, actually?   
    LEDs are the ultimate lighting source    
      
     Date:   
         May 26, 2023   
     Source:   
         Beckman Institute for Advanced Science and Technology   
     Summary:   
         The absolute internal quantum efficiency (IQE) of indium gallium   
         nitride (InGaN) based blue light-emitting diodes (LEDs) at low   
         temperatures is often assumed to be 100%. However, a new study   
         has found that the assumption of always perfect IQE is wrong:   
         the IQE of an LED can be as low as 27.5%.   
      
      
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   FULL STORY   
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   The absolute internal quantum efficiency (IQE) of indium gallium nitride   
   (InGaN) based blue light-emitting diodes (LEDs) at low temperatures is   
   often assumed to be 100%. However, a new study from University of Illinois   
   Urbana- Champaign Electrical and Computer Engineering researchers has   
   found that the assumption of always perfect IQE is wrong: the IQE of an   
   LED can be as low as 27.5%.   
      
   This new research, "Low temperature absolute internal quantum efficiency   
   of InGaN-based light-emitting diodes," was recently published in Applied   
   Physics Letters.   
      
   As ECE associate professor Can Bayram puts it, LEDs are the ultimate   
   lighting source. Since their invention, they have become increasingly   
   popular due to their energy efficiency and cost-effectiveness.   
      
   An LED is a semiconductor that emits light when current flows through   
   the device. It generates photons through the recombination of electrons   
   and holes (carriers), releasing energy in the form of photons. The color   
   of the light emitted corresponds to the energy of the photon.   
      
   InGaN-based blue LEDs enable bright and energy-saving white lighting. The   
   transition to solid-state lighting sources has significantly reduced   
   energy needs and greenhouse gas emissions, but continual efficiency   
   improvements are necessary to hit energy savings goals in the long   
   term. The U.S. Department of Energy's 2035 roadmap calls for blue LED   
   efficiency to increase from 70% to 90% and furthering energy savings   
   by 450 terawatt hours (TWh) and CO2emission savings by 150 million   
   metric tons.   
      
   Bayram says, "The question is, how can we push this ultimate lighting   
   source further? The answer is by understanding its absolute efficiency,   
   not relative efficiency." Relative efficiency benchmarks a device with   
   itself, while absolute efficiency allows for comparison across different   
   devices by measuring the efficiency on a commonly shared scale.   
      
   IQE is defined as the ratio of the generated photons to the injected   
   electrons in the active region of the semiconductor and is an important   
   metric to quantify the performance of LEDs. The most widely used method   
   to quantify IQE is by temperature-dependent photoluminescence. In such   
   analyses, it has been assumed that at low temperatures (4, 10, or even   
   77 Kelvin), there is 100% radiative recombination- meaning producing   
   a photon. At room temperature, because of non-radiative mechanisms-   
   which emit excess energy as heat, rather than photons- the efficiency is   
   significantly lower. The ratio of the two photoluminescence intensities   
   gives a relative efficiency of the LED.   
      
   The original assumption has been that at low temperatures, there are no   
   non- radiative recombination- all the loss mechanisms are "frozen." Bayram   
   and graduate student Yu-Chieh Chiu assert, however, that this assumption   
   may be wrong because non-radiative effects might not in fact be completely   
   frozen out at low temperatures.   
      
   In their paper, Bayram and Chiu demonstrate a different method for   
   revealing low temperature absolute IQE of InGaN-based LEDs. Using a   
   "channel-based" recombination model, they report surprising results: the   
   absolute IQE of the LED on traditional sapphire and silicon substrates   
   is 27.5% and 71.1%, respectively- drastically lower than the standard   
   assumption.   
      
   To explain these unexpected results, Chiu says that the channel-based   
   recombination model is one of the ways to think about what happens   
   inside the active layer of the LED and how recombination in one channel   
   affects another channel. A channel is a pathway that a carrier may take   
   to recombine radiatively or nonradiatively.   
      
   "To determine the efficiency of the blue LED, usually only the blue   
   emission is considered," Chiu says. "But that ignores the effects   
   of everything else happening inside the device, specifically the   
   non-radiative and defect luminescence channels. Our approach is to get a   
   more holistic view of the device and determine, if there is recombination   
   in the blue channel, how is that affected by the second and third   
   channel(s)?"  As research on the LED continues to advance, it is important   
   to know an absolute efficiency rather than a relative efficiency. Bayram   
   stresses that "the absolute efficiency is very important to the field   
   so that everyone can build on each other's knowledge rather than each   
   group improving their own efficiency. We need absolute measurements,   
   not just relative measurements."  To meet the efficiency standards laid   
   out by the DOE, it will be increasingly important to properly quantify   
   the efficiency of LEDs. Even a 1% increase in efficiency will correspond   
   to tons of carbon dioxide savings annually. Chiu says, "By understanding   
   the absolute efficiency, instead of the relative efficiency, that will   
   give us a more accurate picture and allow us to improve devices further   
   by being able to compare them to each other."   
       * RELATED_TOPICS   
             o Matter_&_Energy   
                   # Petroleum # Optics # Energy_Technology # Physics #   
                   Spintronics # Thermodynamics # Electronics # Solar_Energy   
       * RELATED_TERMS   
             o Gallium o Indium o Bose-Einstein_condensate o   
             Absolute_zero o Combustion o Fluid_mechanics o Quantum_dot   
             o Quantum_entanglement   
      
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   Story Source: Materials provided by   
   Beckman_Institute_for_Advanced_Science_and_Technology.   
      
   Original written by Amber Rose. Note: Content may be edited for style   
   and length.   
      
      
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   Journal Reference:   
      1. Y. C. Chiu, C. Bayram. Low temperature absolute internal quantum   
         efficiency of InGaN-based light-emitting diodes. Applied Physics   
         Letters, 2023; 122 (9): 091101 DOI: 10.1063/5.0142701   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/05/230526183206.htm   
      
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