Why the quest of new rare earth doped phosphors deserves to go on

Abstract

The scientific research on phosphors has a long history starting more than 100 years ago. But recently the appearance of new kinds of displays and lighting devices (plasma display, fluorescent lamp without mercury...) induced an increase of the research of new phosphors with better luminous efficiency than those available up to now. It has been shown that the behaviour of classical phosphors in a plasma display panel is quite different than in a cathode ray tube and that the vacuum ultraviolet (VUV) excitation process has to be studied with care in order to improve the phosphors efficiency. It is well established now that a good phosphor for electronic or ultraviolet excitation, is not necessarily a good choice for excitation in VUV. This is probably due to the fact that the excitation process is very different in that case. We will illustrate this difference on the well-known LaPO 4 :Ce 3 + , Tb 3 + phosphor. The penetration depth of the VUV photons is extremely small inducing a large contribution of the surface of the phosphor. We have shown that, for most phosphors, only a few tens of nanometers of the phosphor grain are really useful and we propose a way to realize phosphors powders using less than 20% of doped materials. However the traps of the material play a crucial role in the fluorescence properties due to the fact that autoionization process is likely when dopant ions absorb high energy photons. Fast aging process is one of the main drawbacks of VUV excitation. We demonstrate this effect on BaMgAl 1 0 O 1 7 :Eu 2 + , the blue emitting phosphor widely used up to now in Plasma Displays Panels and fluorescent lamps. Low energetic efficiency is another drawback of VUV excitation that can be solved only in the framework of fundamental studies. Quantum cutting emission may be a solution and calls new research to find good phosphors characterized by a high quantum efficiency, a high fluorescence efficiency and an adapted colorimetry.