Energy Transfer in Down Conversion Rare Earth Phosphors

Abstract

For co-doped and tri-doped rare earth (RE) down conversion phosphors, the energy transfer between different rare earth ions plays an important role because they determine the luminous efficiency of the phosphors. In this chapter, some popular energy transfer mechanisms are presented at first, including cooperative energy transfer (CET), cross relaxation and charge transfer state (CTS). Then, the energy transfers in the co-doped and tri-doped Y3Al5O12 phosphors are discussed. It is shown that the efficiency of the energy transfer between the rare earth ions varies with different ion couples, which depends on the energy levels of these ions. In the down conversion phosphors with two or more rare earth elements, the energy transfer is an important process because they have significant effects on the luminous efficiency of the phosphors [ ]. In principle, the energy transfer seems a simple process in which the sensitizers (donors) in the phosphors absorb the energy of incident photons at first, then the part of the energy is emitted luminescently and the rest is transferred to neighbouring activators (acceptors). After that, the activators emit the photons whose wavelength is different from that emitted from the sensitizers. However, due to the complexity of host lattices and the abundant energy levels of rare earth ions, clarifying the mechanism of the energy transfer is a challenge. In this chapter, some proposed mechanisms of the energy transfer are presented, including cooperative energy transfer（CET), cross relaxation, charge transfer state (CTS), etc. Then, the energy transfer processes in co-doped and tri doped Y3Al5O12 (YAG) phosphors are discussed.