An important metric of LED device operation is the control over the accuracy, quality, and stability of its color or peak emission wavelength. Three well-established approaches to generating white light using LEDs are shown in Figure 3.6. These include a blue LED with yellow phosphors; an ultraviolet (UV) LED with blue and yellow phosphors (or red, green, and blue phosphors); and a device that combines red, green, blue LEDs.
The color of emitted light from an LED depends on the structure and composition of the LED. Achieving a desired photon frequency (hence color) from an LED requires sensitive control over the thicknesses and material composition of the LED.
The active layers of the current blue LEDs used in SSL are extremely small, 3 nm thick, which classifies them as quantum wells. In other words, these nanostructures fall in the class of devices in which the light generation mechanism is controlled at the atomic level. Small changes in the indium composition and well thickness affect the emission wavelength and width of the emission. Currently, blue LEDs have a peak wavelength of 455 nm and a width of 15 nm. Any changes in the peak position or width can visibly affect the hue of white light obtained. The MOCVD deposition machines used in the manufacture of the LEDs have a huge influence on the uniformity of the wavelength and yield of white LEDs (see the section “Materials Growth”). One way to improve the color consistency and make wider, more reproducible quantum wells is to look at alternative substrates for the growth of the GaN LED structures.
FINDING: The color output of LEDs is extremely sensitive to the control of materials composition and thicknesses of the LED structure, which in turn are influenced by the control of the MOCVD growth process.
Another means of controlling the LED color output is through the use of phosphors. The phosphors absorb the (typically) blue light from the GaN LED and re-emit light at longer wavelengths. The phosphors are chosen so that the combination of the direct light from the LED and the light emitted from the phosphor will produce the desired white light. A selected few phosphors have garnered considerable attention, including for example rare-earth (RE) doped yttrium aluminum garnets (YAG:RE, Y3Al5O12(RE)). The cerium-doped YAG can absorb blue and UV light and emit it as yellow light with high efficiency. A critical aspect of this process is that the higher-energy light (e.g., UV or blue) is being converted into lower energy (e.g., yellow or red). As a consequence, LEDs emitting red light—the color having the lowest energy in the visible spectrum—cannot be used with phosphors to generate white light; instead, a short-wavelength UV, violet, or blue LED is required (Denbaars et al., 2013).
Phosphors are typically directly added on top of the LED in the encapsulation material, which is either silicone or epoxy-based. The uniformity of the phosphor coating and mixture selection can drastically affect the efficacy and quality
FIGURE 3.6 Three types of white LEDs for lighting: (a) blue LED plus yellow phosphors, (b) ultraviolet LED plus three phosphors, (c) three LEDs: red, green, blue connected in parallel. SOURCE: Pimputkar et al. (2009). Reprinted by permission from Macmillan Publishers Ltd.