A phosphor is a solid material which emits visible light when exposed to radiation from a deep blue, ultra-violet, or electron beam source. Through careful tuning of the phosphor composition and structure, the spectral content of the emitted light can be tailored to meet certain performance criteria. For this reason, phosphors are widely used in the electronics and lighting industry to enable applications such as displays, fluorescent lighting, and white LEDs.
Most white LEDs consist of a LED chip, which emits blue light with a narrow spectrum between 440 - 470 nm, and a coating of yellow, green, and/or red phosphors. The phosphors are designed to absorb some of the blue light from the LED die. The light emitted by the phosphor, in combination with the remaining blue light leaking through the phosphor layer, result in a light which is perceived as white by the human eye.
The performance of a white LED, including its long-term reliability, is strongly dependent on the choice of phosphor materials as well as the method used to integrate those materials into the LED. Commercially available yellow phosphors typically offer good broadband emission in the visible spectral region (500 - 700 nm), efficient absorption of blue light (420 - 480 nm) and good chemical and thermal stability. However, the emission spectrum of these yellow phosphors lacks content in the red regime. Consequently, white LEDs, with yellow phosphors only, are often characterized by a bluish-white tinge and a CCT ranging between 4000K and 6500K. In addition, these LEDs often do not meet minimum CRI requirements, which is important for illumination-grade LEDs.
Recent advances in red phosphor materials have yielded warm white LEDs with CCT values ranging between 2700K and 4000K, and minimum CRI values of 80. These improvements in color point and color rendering properties have made white LEDs more competitive with other traditional light sources such as incandescent and halogen bulbs. However, warm white LEDs are still less efficient than cool white LEDs because a large portion of the red phosphor emission occurs above 700nm, which is beyond the sensitivity of the human eye. Minimum CRI values of 90 can be obtained by incorporating even redder phosphor materials at the expense of even less emission below 700nm, reducing the overall LED efficacy even further.
To maintain its leadership in white LEDs, Lumileds continues to invest in the research and development of new phosphor materials and manufacturing processes which result in higher quantum efficiencies and better lumen equivalent ratios. For example, recent research has focused on the synthesis of novel high-performance narrow-band red emitting phosphors, which can be efficiently excited by blue LEDs. Narrow band red phosphors enable even higher LED efficacies without compromising quality of light and color rendering (see Figure 1). The Nature article Narrow-band red-emitting Sr[LiAl3N4]:Eu2+ as a next-generation LED-phosphor material describes a particular class of narrow-band red phosphor materials which was recently developed by Lumileds. A prototype white LED, which included this particular class of red phosphors with a peak wavelength around 650nm and a FWHM of 50nm, resulted in an increase in luminous efficacy of 14% compared to commercially available high CRI LEDs.
In addition to developing novel phosphor materials, Lumileds maintains a dedicated team to develop new phosphor integration technologies, which can be adopted in a high-volume production environment to help drive down LED cost while maximizing production yields.