19.2: Distinguished Paper: Characterization of Electron‐Hole Pair Migration and Trapping in Rare Earth Doped YBO₃ under Vacuum Ultraviolet Excitation

Abstract

We have measured host-to-activator energy transfer efficiencies in rare-earth doped YBO 3 under vacuum ultraviolet excitation. From these data, relative electron-hole (e-h) pair trapping rate constants can be estimated. Our analysis suggests a correlation between e-h trapping efficiency and the energy of activator electronic states relative to the host conduction band. IntroductionDeveloping a detailed understanding of electron transport processes in solids is important to a wide variety of fields within materials science, including the study of scintillators, two-photon phosphors, dye-sensitized solar cells and transparent conductors. Functioning devices based on these technologies often require the transport of electrons through an oxide insulator. Although a fairly large number of useful materials have been developed for these applications, relatively little is known about the fundamental factors governing these processes. Our research focuses on the electron transport and trapping processes associated with vacuum ultraviolet (VUV) excitation of luminescent materials, such as are found in plasma display panels (PDP) and xenon-based mercuryfree lamps. Our goal is to establish the basic structure/property relationships that govern efficient VUV excitation.Absorption of a VUV photon by a doped luminescent material results in the formation of an electron-hole (e-h) pair in the host. For luminescence to occur, this e-h pair must be trapped by the activator. Thus, a critical factor dictating the overall VUV efficiency of a phosphor will be the efficiency of e-h pair migration and trapping, which we refer to here as the energy transfer efficiency,  t . Over the last several years we have developed spectroscopic methods for evaluating host-to-activator transfer efficiency in doped luminescent materials under vacuum ultraviolet (VUV) excitation. Previously we applied these tools to the study of [3,4]. By evaluating our data using accepted kinetic models [5,6] we are able to characterize e-h migration, capture and surface trapping in these materials. In our study of (Y,Gd)BO 3 :Eu 3+ we quantified the effect of energy migration along the Gd sublattice as the Gd concentration was increased. We observed a three-fold increase in the rate constant of capture in going from 0 -10% Gd, while at 30% Gd the migration and trapping was so efficient that we were unable to quantify it using our methods [2]. In our work on nano-crystalline materials we found that the decrease in efficiency in YBO 3 :Eu 3+ is due primarily to surface losses, with about 40% of e-h pairs being lost to the surface at particle sizes < 100 nm [3]. In nano-Y 2 O 3 :Eu 3+ prepared by a combustion process, the loss of efficiency appears to correlate more with poor bulk crystallinity than with surface losses [4].We are now pursuing a comprehensive study of e-h migration and trapping by doping YBO 3 with elements across the entire lanthanide series. We seek to identify how or if the activator capture cross section is related to the relative energies o...