A Look At The Color-Mixed White LED Future

In February 2022, the US DOE released its 2022 SSL Research & Development Opportunities report. While the report covers many promising directions for SSL R&D, one of the most significant evolutions predicted for LED light sources is the shift from phosphor-converted white LEDs (PC-LED) to color-mixed white LEDs (CM-LED):

1. 

   Image: US DOE 2022 SSL R&D Opportunities

   the PC-LED is based on a blue LED pumping yellow and red wavelength optical down-converters (typically phosphors) to produce white light, and

2. the CM-LED approach uses primary colors that compose a red, green, blue, and amber (RGBA) LED combined to produce white light.

The PC-LED architecture is the dominant white light architecture used for LED lighting today. It has three major advantages: simplicity (only one LED type), temperature robustness (the InGaN blue LED and YAG phosphor down-converters can operate at relatively high temperatures), and color stability (the fractions of red, green, and blue source colors are determined during manufacture by the phosphor optical density and are relatively stable over time).

Over the past decade, luminous efficacies have more than doubled, from ~85 lm/W to approximately 185 lm/W. The principal reason has been improvement in blue LED efficiency, although progress has also been made in phosphors (efficiency and wavelengths to maximize spectral efficiency) and package efficiency (optical scattering/absorption). Despite these improvements, there is significant remaining potential for improved efficacy. Luminous efficacies of approximately 250 lm/W at the prescribed operating conditions are believed to be practically possible for PC-LEDs.

For the color-mixed architectures, an upper limit of 325 lm/W is considered achievable with greater breakthroughs in the technology. While the performance potential is high, today’s efficacies are much lower than the PC-LED approach due to the inefficient green and amber direct emission LEDs (known as the ‘green gap’).

While LED emitter materials have improved rapidly over the past decade, there are still key technological challenges that are limiting further improvement. The low efficiency of green and amber direct emission LEDs constrains the performance of color-mixed LED systems.

The full 2022 SSL R&D Opportunities report can be downloaded here.

Image: US DOE 2022 SSL R&D Opportunities

Image: US DOE 2022 SSL R&D Opportunities