Design Rules for Finding New Oxide Phosphors for Solid-State Lighting

Design rules for finding new oxide phosphors for solid-state lighting

Nathan C. George,¹ Bradley F. Chmelka¹, Ram Seshadri²

¹Department of Chemical Engineering, ²Materials Department & Materials Research Laboratory, University of California, Santa Barbara, California, U.S.A. 93106

Solid-state lighting systems consisting of a yellow oxide phosphor, in conjunction with a blue InGaN source, are a promising way of creating efficient white light with devices that are long-lived and durable. In general, the oxide phosphor consists of a host crystal lattice, which is doped with small amounts of rare-earth or transition-metal ions.  These ions in the host crystal "activate" the lattice, enabling down-conversion of blue or UV light to yellow-orange light.  Many efforts have been directed towards finding new phosphor materials, but little has been done to understand the relationships between optical phenomena and the structure of phosphor materials. Structural and compositional reasons for the high efficiencies of the canonical yellow phosphor, Y_3-xCe_xAl₅O₁₂ (YAG:Ce, x<0.12) are established by using a synergistic combination of characterization techniques, including NMR, ESR, synchrotron X-ray and neutron scattering, and XANES/EXAFS.  In particular, an important feature of the YAG:Ce lattice is determined to be its high rigidity, which reduces the number of accessible phonon modes at typical operating temperatures, resulting in its high quantum efficiency. EXAFS and Reverse Monte Carlo (RMC) simulations of total neutron scattering show that the Ce³⁺–O polyhedra are furthermore highly compressed in YAG:Ce. The lattice rigidity and relatively small Y site into which Ce³⁺ ions substitute contribute to the yellow emission that is unusual for Ce-doped oxides. Both solid-state NMR and ESR enable quantification of the small amount of Ce³⁺ ions that substitute into nearest-neighbor Y³⁺ sites.  These Ce³⁺ sites begin to significantly decrease quantum efficiency above 2 mol% Ce in YAG, which suggests Ce substitution levels should be below this threshold. Though specific for YAG:Ce, these analyses provide general design criteria for the compositions and structures of  phosphor materials, including non-oxide systems, that are expected to lead to new materials with improved properties.