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 ==========================================================================
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

========================================================================== 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.


========================================================================== 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 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230526183206.htm

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