| The biggest challenge faced by the thin-film electroluminescence (EL) technology is achieving full color EL displays. The principal problem is the lack of an EL phosphor with efficient, stable, and saturated blue EL properties, characteristics which are need for full color sunlight applications. This work investigates the photoluminescent and electroluminescent properties of three thin-film blue EL phosphors: SrS:Cu, SrS:Ag and SrS:Ag,Cu. The optical and electrical properties of these materials were characterized by photoluminescence (PL), photoluminescence excitation (PLE), time decay and time-resolved spectroscopies and by luminance, efficiency, transferred charge and capacitance versus voltage techniques. This research addresses three main issues: (i) understanding the radiative mechanisms and local site symmetries of the activators in these materials, (ii) understanding the electroluminescence excitation mechanisms in these materials, and (iii) and utilizing sensitized electroluminescence to improve phosphor performance. Specifically, the luminescence mechanism of SrS:Cu is determined to be the 3E → 1A1 intra-ionic transition within the Cu+ ion. The energy level structure of SrS:Cu is examined, and the relationship between the crystalline environment (local symmetry) and the optically active ions is established in conjunction with the results of crystal field theory. The excitation mechanism in SrS:Cu is determined to be impact ionization, whereas that in SrS:Ag is shown to be impact excitation, and it is concluded that the excitation mechanism plays a crucial role in determining the electroluminescence efficiency. The improved EL properties of SrS:Ag,Cu are explained by energy transfer processes from Cu+ to Ag+ centers. From these results, a two-component phosphor model is proposed as a method for tailoring the electroluminescence properties of a material. The sensitizer is selected for its efficient excitation properties and the activator is selected for the desired emission characteristics. As examples of the viability of the two-component phosphor concept, this method is successfully applied to SrS:Eu,Cu and SrS:Mn,Cu red and green phosphor systems, respectively. |
