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 by 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 for InGaN-based LEDs,” was recently published in Applied physics letters.
As ECE lecturer Can Bayram puts it, LED is 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 is passed 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 sharp and energy-saving white lighting. The transition to solid-state lighting sources has significantly reduced energy demand and greenhouse gas emissions, but continuous efficiency improvements are necessary to achieve energy saving goals in the long term. The US Department of Energy’s 2035 roadmap calls for blue LED efficiency to increase from 70% to 90% and increase energy savings by 450 terawatt hours (TWh) and CO2 emission savings of 150 million tonnes.
Says Bayram, “The question is, how can we push this ultimate light source further? The answer is by understanding its absolute efficiency, not relative efficiency.” Relative efficiency measures a unit against itself, while absolute efficiency enables comparison across different units by measuring efficiency on a common 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 for quantifying the performance of LEDs. The most widely used method to quantify IQE is 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, which means producing a photon. At room temperature, due to non-radiative mechanisms – emitting excess energy as heat, rather than photons – the efficiency is significantly lower. The ratio between the two photoluminescence intensities gives a relative efficiency of the LED.
The original assumption has been that at low temperatures there is no non-radiative recombination – all the loss mechanisms are “frozen”. However, Bayram and graduate student Yu-Chieh Chiu argue that this assumption may be wrong because non-radiative effects are not actually completely frozen out at low temperatures.
In their paper, Bayram and Chiu demonstrate another method to reveal the low-temperature absolute IQE of InGaN-based LEDs. Using a “channel-based” recombination model, they report surprising results: the absolute IQE of LEDs 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 way 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 path that a carrier can take to recombine radiatively or non-radiatively.
“To determine the efficiency of the blue LED, usually only the blue emission is considered,” says Chiu. “But that ignores the effect of everything else going on inside the device, especially the non-radiative and defective luminescence channels. Our approach is to get a more holistic view of the device and find out if there is recombination in the blue channel, how is that affected of the second and third channel(s)?”
As research on LEDs continues to develop, it is important to know an absolute efficiency rather than a relative efficiency. Bayram emphasizes 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 its own efficiency. We need absolute measurements, not just relative measurements.”
To meet the efficiency standards set by the DOE, it will become increasingly important to properly quantify the efficiency of LEDs. Even a 1% increase in efficiency will equate to tons of carbon dioxide savings annually. Chiu says, “By understanding the absolute efficiency, rather than the relative efficiency, it will give us a more accurate picture and allow us to further improve the devices by being able to compare them to each other.”