of light. For example, the use of just a yellow-emitting YAG phosphor with a single blue GaN LED results in high luminous efficacies but relative poor quality (color rendering index, CRI < 75) light. This has led to the perception that all LED lighting is blueish-white and cold. UV LEDs with phosphor mixtures provide a better CRI value, but at the expense of poorer efficacy. A combination of three (or more) LEDs having different wavelengths (red, green, and blue) may be used if one wishes to dynamically control white light. This approach may lead to higher efficacies than the UV-phosphor LED. In addition, moving the phosphor layer away from the chip and tailoring the optics between the LED chip and the remote phosphor layer has resulted in significant improvement in light output and luminous efficacy ( Narendran et al., 2005).
General Considerations: Mixed LEDs or Phosphors?
Using narrowband (colored) components to create white light, like the examples in Figure 1.9 in Chapter 1, allow manufacturers to manipulate the luminous efficacy of radiation (LER) and color qualities of a light source depending on their goals and priorities. At these relatively early stages of the technology, there are a number of technical difficulties with the use of multi-color LEDs (red, green, blue; red, green, blue, yellow) to produce a white source in SSL products. If one of the colored components ages differently from the others or responds to heat differently, the color properties of the light source will change. Furthermore, inadequate mixing of the light will result in colored shadows. Nonetheless, multi-color LED lighting has some advantages. The spectrum can be tuned to optimize LER. A wide range of chromaticities can be achieved by adjusting the relative intensities of the component colors. In fact, some SSL products currently on the market allow users to adjust the chromaticity—a heavily marketed feature (Philips Solid-State Lighting Solutions, 2012)—and some in the industry believe that eventually all consumers will expect their lighting products to offer such functionality (Thompson et al., 2011).
Even though entire portions of the visible spectrum are missing from such light sources, narrowband multi-color lights can achieve good (three components) or excellent (four or more components) color rendering. These types of lights can even exhibit some desirable color-rendering properties that broadband light sources cannot, such as inducing increases in the colorfulness/vividness of object colors beyond what the CRI deems to be “perfect.” Research on this effect suggests that people find these slightly enhanced object colors to appear more attractive (Jost-Boissard, 2009).
Phosphor white LEDs have a clear ease-of-use advantage and dominate the current SSL general illumination market. Although the resultant products do not offer the same flexibility in chromaticity as those from multi-color white LEDs, the phosphors themselves are available in a wide range of chromaticities. Color rendering varies depending on the particular phosphor involved, but can be disappointing, particularly compared to incandescent-based light sources. Over the past few years, manufacturers have found that adding some additional energy in the long (red) wavelengths, either with a red LED or remote phosphor, vastly improves the color quality of typical white phosphor LEDs (Hum, 2011). This solution is now widely used.
Quantum Dot “Phosphors”
There has recently emerged another interesting approach for the color control of both LEDs and OLEDs involving semiconductor nanocrystals or “quantum dots.” These materials are governed by the light emission mechanism described earlier, but these semiconductors, like the phosphors, can also absorb higher-energy photons and emit photons of lower energy or longer wavelength. What is distinctive about these chemically synthesized materials, with diameters of a few nanometers, is that the size of the nanocrystals will influence the wavelength of emission. Initial work on CdSe nanocrystals began in the late 1980s (Brus, 1991): the color tunability of these structures, their small size, the relative ease of production, and their optical robustness encouraged researchers to utilize these quantum dots in a variety of applications, such as selective tagging and in vivo imaging of features in cells (Michalet et al., 2005). Although much further assessment will be required to understand the full potential of this technology, initial results look promising.
FINDING: A number of approaches have successfully been used to achieve and modulate color rendition for LED lighting. Phosphor-converted and color-mixed LEDs show promise but face different challenges. The ultimate choice of approach will depend on a multiplicity of issues regarding sensitivity of color control, efficiency, reliability, manufacturability, and cost.
RECOMMENDATION 3-2: The Department of Energy has provided excellent guidance in its roadmap targets for both phosphor-converted and color-mixed light-emitting diodes. Core investment in these technologies should be continued, with consideration for promising new technologies (e.g., quantum dot layers replacing phosphors).
Materials issues in white LED technology affect the cost, yield, and reliability of the resulting luminaire at the most fundamental level. A particularly critical issue is that the current white LED materials substrate and growth technology do not produce LED devices that are uniform with respect to their color quality or efficiency. This in turn places an additional burden on the evaluation of individual devices: “Variability in lumen output, correlated color temperature (CCT), and forward voltage, is currently handled by testing