Assessing the Actual Efficiency of Blue LEDs: Comparing Absolute and Relative Efficiency

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 researchers in electrical and computer engineering finds that the ideal IQ assumption is always wrong: an LED’s IQ can be as high as 27.5%.

this New search It was recently published in Applied Physics Letters.

As ECE associate professor Kan Bayram said, LEDs are the ultimate light 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 by recombining electrons and holes (carriers), releasing energy in the form of photons. The color of the emitted light corresponds to the energy of the photon.

InGaN-based blue LEDs enable bright, energy-efficient white light. The transition to solid-state lighting sources has significantly reduced energy needs and greenhouse gas emissions, but continued efficiency improvements are necessary to achieve long-term energy saving goals. The US Department of Energy’s 2035 roadmap calls for an increase in the efficiency of blue LEDs from 70% to 90% and an increase in energy savings of 450 TWh (TWh) and CO2.₂ Emission savings of 150 million metric tons.

“The question is, how do we push this ultimate light source even further? The answer is by understanding its absolute efficiency, not relative efficiency,” says Perm. Relative efficiency measures a device against itself, while absolute efficiency allows comparison across different devices by measuring efficiency on a commonly shared scale.

IQE is defined as the ratio of photons generated to electrons injected into the active region of a semiconductor and is an important metric for measuring the performance of LEDs. The most widely used method for estimating intelligence is through temperature-dependent photoluminescence. In such analyses, it has been assumed that at lower temperatures (4, 10, or even 77 K), there is 100% radiative recombination—that is, photon production. At room temperature, because of the non-radiative mechanisms—that excess energy is emitted as heat, rather than as photons—the efficiency is much lower. The ratio of the two photoluminescence intensities gives the relative efficiency of the LED.

The original assumption was that at low temperatures, there is no non-radiative recombination – all loss mechanisms are ‘frozen’. Beram and graduate student Yu-chih Chiu contend, however, that this assumption may be wrong because non-radiative effects may not be completely frozen at lower temperatures.

In their paper, Bayram and Chiu demonstrated a different low-temperature absolute IQ detection method for InGaN-based LEDs. Using a ‘channel-based’ recombination model, they report surprising results: the absolute IQ of an LED on conventional sapphire and silicon substrates is 27.5% and 71.1%, respectively—significantly lower than the standard assumption.

To explain these unexpected results, Chiu says the channel-dependent recombination model is one way to think about what happens inside the active layer of an LED and how recombination in one channel affects another. A channel is a path a carrier may take to recombine, radially or non-radiologically.

“To determine the efficiency of a blue LED, usually only blue emission is taken into account,” says Chiu. “But this ignores the effects of everything else going on inside the device, specifically non-radiative channels and defective luminescence. Our approach is to get a more comprehensive 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 LED continues to progress, it is important to know absolute efficiency rather than relative efficiency. Perm stresses that “absolute competence is so important to the field that everyone can build on each other’s knowledge rather than each group improving their own competence. We need absolute measurements, not just relative ones.”

To meet the efficiency standards set by the Department of Energy, it will be increasingly important to correctly define the efficiency of LEDs. Even a 1% increase in efficiency will correspond to saving tons of carbon dioxide annually. Chiu says, “By understanding absolute efficiency, rather than relative efficiency, this will give us a more accurate picture and allow us to improve the devices further by being able to compare them against each other.”