Biotechnology

How efficient are blue LEDs really?

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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 an always-perfect IQE is wrong: the IQE of an LED can be as low as 27.5%.

Credit: Chiu et al.

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 an 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 an InGaN-based light-emitting diode”, recently published in Applied Physics Letters.

As ECE associate professor Can Bayram says, LEDs are the main source of lighting. Since their invention, they have become increasingly popular due to their energy efficiency and cost effectiveness.

LEDs are semiconductors that emit 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 emitted light corresponds to the energy of the photon.

InGaN-based blue LEDs enable bright, energy-efficient white lighting. The switch to solid-state lighting sources has significantly reduced energy requirements and greenhouse gas emissions, but continued efficiency improvements are needed to achieve energy-saving goals over the long term. US Department of Energy 2035 roadmap calls for increasing blue LED efficiency from 70% to 90% and continuing energy savings of 450 kWh (TWh) and CO22 emission savings of 150 million metric tons.

Bayram said, “The question is, how can we push this primary lighting source further? The answer is to understand absolute efficiency, not relative efficiency.” Relative efficiency compares a device to itself, while absolute efficiency allows comparisons between different devices by measuring efficiency on a commonly shared scale.

IQE is defined as the ratio of generated photons to injected electrons in the active region of a semiconductor and is an important metric for measuring LED performance. The most widely used method of measuring IQE is by temperature-dependent photoluminescence. In such an analysis, it is assumed that at low temperatures (4, 10, or even 77 Kelvin), there is 100% radiative recombination- meaning it produces photons. At room temperature, due to non-radiative mechanisms – which emit excess energy as heat instead of photons – the efficiency is much lower. The ratio of the two photoluminescence intensities gives the relative efficiency of the LED.

The initial assumption was that at low temperatures, there is no non-radiative recombination – all loss mechanisms are “frozen”. Bayram and graduate student Yu-Chieh Chiu suggest, however, that this assumption may be wrong because non-radiative effects may not actually completely freeze at low temperatures.

In their paper, Bayram and Chiu demonstrate a different method to reveal the low-temperature absolute IQE of InGaN-based LEDs. Using a “channel-based” recombination model, they reported surprising results: The absolute IQE of the LEDs on traditional sapphire and silicon substrates were 27.5% and 71.1%, respectively – much lower than standard assumptions.

To explain this unexpected result, Chiu says that a channel-based recombination model is one way to think about what’s going on inside the active layer of an LED and how recombination in one channel affects the other. Channels are the paths that can be taken by carriers to recombine radiatively or nonradiatively.

“To determine the efficiency of a blue LED, usually only the blue emission is taken into account,” says Chiu. “But that ignores the effects of everything that goes on inside the device, specifically the non-radiation channels and defects. 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 channels?”

As research on LEDs continues to evolve, it is important to know absolute efficiency rather than relative efficiency. Bayram emphasized that “absolute efficiency is very important for the field so that everyone can build on each other’s knowledge rather than each group increasing its own efficiency. We need absolute measurements, not just relative measurements.”

In order to meet the efficiency standards set by the DOE, it is increasingly important to properly measure the efficiency of LEDs. Even a 1% increase in efficiency would match tons of carbon dioxide savings each year. Chiu said, “Understanding absolute efficiency instead of relative efficiency will give us a more accurate picture and allow us to further improve devices by being able to compare them with each other.”


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