Bridging the 'green gap' in LED technology

Colour mixing is the process of combining two or more colours. This process of mixing colours is the basis for the future of solid-state lighting. While currently white light is achieved by phosphor down-conversion, LED colour mixing has a higher theoretical maximum efficiency, which is needed to achieve energy efficiency goals. Despite the potential efficiency of colour-mixed LED sources, there exists one significant challenge: the “green gap”, described as the lack of suitable green LEDs. Current green LEDs are made from state-of-the-art hexagonal III-nitride but only reach one-third of the efficiency goals laid out in the 2035 US Department of Energy roadmap.

Researchers from the University of Illinois Urbana-Champaign have found a potential path to fill the green gap and have reported a green-emitting cubic III-nitride active layer with 32% internal quantum efficiency (IQE). Lead researcher Can Bayram said the ultimate goal is to triple the efficiency of today’s white light emitting diodes.

“To do that, we need to fill the green gap in the spectrum, which is no easy task. You need innovation. And we show the innovation from the materials side by using cubic nitrides,” Bayram said.

The most efficient white LEDs currently use blue light emitting diodes with a rare-earth phosphor coating that converts the blue light into yellow, green and/or red, which enables white lighting. This process is called phosphor down-conversion. The phosphors are luminescent materials that can absorb and convert high energy photons (like blue light) into lower energy/longer wavelength light (such as green, yellow and red, respectively).

However, phosphor down-conversion has limitations. The down-conversion process is inherently inefficient because the high energy photons must lose energy (in the form of heat) to be converted into photons of other energies. Currently, white LEDs used in SSL generate seven times more heat than light output. Further, phosphors are chemically unstable and add significant raw material and packaging costs to the LED device. Despite the increase in blue LED efficiency in recent years, SSL using phosphors only has a theoretical maximum luminous efficacy of 255 lumens/watt (lm/W), whereas LED colour mixing can achieve a theoretical maximum luminous efficacy of 408 lm/W.

However, many approaches for green LEDs are affected by “efficiency droop” at high current densities. Achieving high-efficiency green emission has been difficult with traditional hexagonal III-nitride even after increasing the indium content — a costly element required for green emission — which leads to higher defect densities and efficiency droop. This presents a challenge for the widespread adoption of SSL.

Graduate student Jaekwon Lee said the researchers found a way to synthesise low defect density, high-quality, single-phase cubic gallium nitride by using an aspect ratio phase trapping technique, invented by the Bayram group. In aspect ratio phase trapping, defects — as well as the undesirable hexagonal phase — are ‘trapped’ inside the grooves so that the surface of the active layer is a perfect cubic-phase material. The cubic and hexagonal phase refers to the way atoms in the materials organise themselves.

The researchers developed a cubic III-nitride system that can enable highly efficient, droop-free green LEDs with a 32% IQEE and 16% indium content. Bayram said that the green gap can be closed by using cubic III-nitride, as the advantages of these materials for SSL are well documented theoretically and experimentally. Actual efficiencies of cubic devices have been hampered by the quality and purity of the cubic phase, but the novel aspect ratio phase trapping technique used in this research enables high-quality, pure cubic III-nitride.

The results of this research were published in Applied Physics Letters.

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