I’ve previously written about predictions for color-mixed white LED’s (CM-LEDs) to eventually surpass phosphor-converted LEDs (PC-LEDs) in both efficacy and spectral tuning capabilities. More on this is available here. But what is it going to take to bring about all of the advantages of CM-LEDs for general lighting? The primary technical barrier is the very low efficacy of green and amber direct emission LEDs. Together these are often referred to as the “green gap.” How will researchers close the green gap?

There are several promising technology paths to create more efficient green and amber LEDs, which are crucial for advancing CM-LED systems:

Cubic III-Nitride Materials

Breakthroughs have been achieved using cubic III-nitride materials:

• Researchers developed a green-emitting cubic III-nitride active layer with 32% internal quantum efficiency (IQE)
• This is more than six times higher efficiency than conventional cubic active layer.
• The system enables highly efficient, droop-free green LEDs with only 16% indium content

This approach addresses several challenges associated with traditional hexagonal III-nitride materials, including efficiency droop and high indium content requirements.

Aspect Ratio Phase Trapping Technique

A novel fabrication method shows promise:

• The “aspect ratio phase trapping” technique produces low defect density, high-quality, single-phase cubic gallium nitride
• This method traps defects and undesirable hexagonal phases inside grooves, resulting in a perfect cubic-phase material surface
• The technique enables high-quality, pure cubic III-nitride, which is crucial for closing the “green gap”

Phosphor-Converted Blue LEDs

An alternative approach to direct green emission:

• Lime and Mint LEDs, which are phosphor-converted blue LEDs, offer higher efficiency than traditional green LEDs
• These LEDs provide substantially higher flux and efficacy compared to direct green LEDs
• They also contribute to improved color rendering and system efficiency in color-mixing applications

Semiconductor Material Combinations

Exploring new material combinations shows potential:

• Researchers have combined aluminum gallium indium phosphide and gallium arsenide to produce yellow-green light.
• This approach aims to eliminate the efficiency loss associated with phosphor conversion in traditional methods.

Nitride-Based Materials for Blue-Green LEDs

While more challenging, developing efficient blue-green LEDs using nitride-based materials is an area of ongoing research:

• Improving the material science of nitrides for mass production is crucial
• Advancements in this area could lead to high-efficiency blue-green LEDs, complementing the progress made in yellow-green LEDs

Each of these technological paths aim to address the “green gap” in LED efficiency, which is essential for the development of more efficient color-mixed white LED systems. As research progresses, a combination of these approaches may lead to breakthroughs in green and amber LED efficiency, paving the way for the transition from phosphor-converted to color-mixed white LEDs in the general lighting industry.

Image:https://compoundsemiconductor.net/article/101325/Getting_To_Grips_With_The_Green_Gap

David Shiller

David Shiller is the Publisher of LightNOW, and President of Lighting Solution Development, a North American consulting firm providing business development services to advanced lighting manufacturers. The ALA awarded David the Pillar of the Industry Award. David has co-chaired ALA’s Engineering Committee since 2010. David established MaxLite’s OEM component sales into a multi-million dollar division. He invented GU24 lamps while leading ENERGY STAR lighting programs for the US EPA. David has been published in leading lighting publications, including LD+A, enLIGHTenment Magazine, LEDs Magazine, and more.

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