Phosphors for White-Light LEDs Through the Principle of Energy Transfer

Abstract

This chapter is an attempt to investigate and develop single-phased white light-emitting phosphors essentially for potential applications in white light-emitting diodes (WLEDs) based on the principle of resonant-type energy transfer. We first review the fundamentals of energy-transfer processes and propose a set of semi-empirical protocols for the design of white-emitting phosphors on the basis of bonding nature of hosts and the resonant-type energy transfer. The luminescent properties and energy-transfer processes for four categories of single-phased white light-emitting phosphors and a multi-composition phosphor are used as examples and discussed in detail. In summary, white-light emission can be realized by adopting single-composition two-complementary systems such as blue/yellow-emitting (CaAl2Si2O8:Eu2+, Mn2+), cyan/red-emitting (BaGa4S7:Eu2+, Mn2+), or green-yellow/red-emitting phosphor (Ba2ZnS3:Ce3+, Eu2+) under blue or ultraviolet (UV) excitation. On the other hand, white-light emission can also be realized in three-primary phosphors such as blue/green/red-emitting SrZn2(PO4)2:Eu2+, Mn2+ under blue excitation. Furthermore, the most intriguing examples of white-light generation with a high color-rendering index through resonant energy transfer are trichromatic inorganic phosphors such as Ca3Y(GaO)3(BO3)4:Ce3+, Mn2+, Tb3+, Mg2Y8(SiO4)6O2:Ce3+, Mn2+, Tb3+, NaCaBO3:Ce3+, Mn2+, Tb3+, and BaMg2Al6Si9O30:Eu2+, Tb3+, Mn2+, which can effectively convert UV, near-UV, or blue light into a combination of RGB. To support and confirm the occurrence of energy transfer from a sensitizer (energy donor) to an activator (energy acceptor), one must provide two evidences, namely, spectral overlapping between a sensitizer and activator from steady-state spectra and shortening of luminescence decay rate of a sensitizer with increasing content of an activator. With transient spectra of the white-emitting phosphors, the mechanism of electrical multipolar resonance-type energy transfer can also be deduced.