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Developing a Better Phosphor for Solid-State White Lighting
Achievement/Results
Advances in solid-state white lighting have led to widespread use of white LEDs in devices including as laptops, cell phones, light bulbs, flashlights, and car headlights. These white LEDs are advantageous to traditional white light sources such as incandescent and fluorescent lights given that they consume less energy, have longer lifetimes, and an environmentally friendly design that does not use mercury. White LEDs are made using phosphors that convert the blue or ultraviolet light emitted from the LED into the longer wavelengths of blue, green, yellow, or red. These colors of the visible spectrum then mix to create white light. The phosphor material that is used greatly affects the overall device, including the luminous efficacy, color rendition, thermal and chemical stability, and the color quality of the light. Specific luminescence properties are needed to produce an optimal white light with a high color-rendering index (Ra). These properties include strong absorption in the region of the LED emission and a broad emission profile across all regions of the visible spectrum. The thermal stability of the phosphor is also important because thermal quenching of phosphor luminescence leads to decreased efficiency and color instability at the operating temperatures of modern devices. In addition, the phosphors must be highly efficient if they are to convert the LED emission with little or no energy loss.
The most commonly used phosphor for solid-state white lighting is yellow-emitting cerium-doped yttrium aluminum garnet (YAG). Although efficient, the quality of the white light YAG produces is not perfect. Emission in the red region of the visible spectrum is inadequate, and temperature stability is poor. In our research, we work to understand the structure-composition-property relationships that exist in materials systems, and then use this knowledge to engineer better materials. Recently, we have developed a new oxyfluoride phosphor composition, Sr2Ba(AlO4F)1-x(SiO5)x:Ce3+. This solid-solution phosphor has high thermal stability and high quantum efficiency with a green-to-yellow emission color is easily tunable through chemical substitutions. It also has a broad emission spectrum that covers a wide range of colors in the visible spectrum, including some red. This makes for devices that render better color and a warmer white light. We explore these structure-composition-property relationships using a combination of techniques including density functional theory, synchrotron x-ray and neutron total scattering, electron paramagnetic resonance, and optical and thermal characterization.
Through understanding the structures of phosphor compositions with excellent properties for solid-state white lighting, we gain knowledge as to “what makes a good phosphor,” and then use this knowledge to develop new and improved phosphor compositions.
Kristin A. Denault, Nathan C. George, Sara R. Paden, Stuart Brinkley, Alexander A. Mikhailovsky, Jörg Neuefeind, Steven P. DenBaars, and Ram Seshadri, A green-yellow emitting oxyfluoride solid solution phosphor Sr2Ba(AlO4F)1-x(SiO5)x:Ce3+ for thermally stable, high color rendition solid state white lighting, J. Mater. Chem. 22, 18204-18213 (2012); DOI: 10.1039/c2jm33620k.
Address Goals
Understanding the structures of phosphor compositions with excellent properties for solid-state white lighting, is important to the development of new and improved phosphor compositions. The work, a collaboration between two IGERT fellows, an IGERT faculty member, and researchers at a national laboratory, is also highly interdisciplinary, bringing skills of engineering, chemistry, and materials science to this important problem.
