New Strategy for the Design of Yellow, Red and IR-Emitting LED Phosphors

Abstract

Typical white LEDs composed of blue light excitable LED phosphors and blue LED chip. Recently, long wavelength (yellow and red)-emitting Eu2+ or Ce3+-activated nitride phosphors have been usually used in the white LEDs as commercial phosphors. The excitation and emission band of Eu2+ or Ce3+-activated phosphors are due to the energy transitions between the 4f ground state and 5d excited state of Eu2+. Strong crystal field splitting of the 5d band in the nitrides lead to decrease the energy gap between the 4f ground state and the lowest level of the 5d excited state. On the other hand, the Eu2+ or Ce3+-activated oxide phosphors are also expected to be obtained the excitation and emission band in the longer wavelength region (yellow, red and IR), if the oxide host materials have small site for substitution of activator ions. Therefore, we focused on oxide materials including a compact octahedral sites in the host lattices. As a result, we could successfully synthesize the novel long wavelength-emitting Eu2+ or Ce3+-activated phosphors such as NaMgPO4:Eu2+ and (Ba,Sr)3Sc4O9:Ce3+ [1,2]. The NaMgPO4:Eu2+ phosphor can be efficiently excited by blue light irradiation and presents red emission band centered at 628 nm due to the d - f transition of Eu2+. The internal quantum efficiency of NaMgPO4:2.5mol%Eu2+ at the excitation wavelength of 450 nm is estimated to be 81%. (Ba,Sr)3Sc4O9:Ce3+ shows orange to red emission peaking at 583(Ba) and 620(Sr) nm due to the 5d - 4f transition of Ce3+ under blue light excitation. Therefore, this new strategy (octahedrally coordinated sites for the Eu2+ or Ce3+) is a general and powerful tool for searching the long wavelength-emitting phosphors. This work was supported by a project from NEDO, New Energy and Industrial Technology Development Organization (Rare Metal Substitute Materials Development Project Development of Technology for Reducing Tb and Eu Usage in Phosphors for Fluorescent Lamp by High-speed Material Synthesis and Evaluation).