Blue Emitting Thin-Film Electroluminescence Devices Using a Valence Controlled Rare-Earth Luminescence Centers

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Thin-film Electroluminescent(EL)display panel is a solid state device and has many advantages of light emitting, high resolution, wide viewing angle and high operating speed, but color of display is limited to yellowish-orange. Therefore, color thin-film EL displays are becoming the next important target to be developed. For this purpose, a development of color thin film EL phosphors, especially, blue emitting EL phosphors, is strongly required. In this work, we studied blue emitting thin-fijm EL phosphors using a valence controlled rare-earth luminescence centers.1. We have studied the charging state of Eu luminescent centers in SrS : Eu, F thin films. At lower Eu concentration, Eu^<2+> center exist, but at higher concentration over 0.6 mol%, both Eu^<2+> and Eu^<3+> centers coexist. Both Eu^<2+> and Eu^<3+> centers showed red EL emission.2. Valence control of Eu luminescent centers in the ZnS lattice has been tested. It is well known that Eu^<3+> luminescent centers give rise to red … More EL emission due to f-f transitions of (4f)^6 electron configuration. On the contrary, Eu^<2+> in the ZnS lattice give rise to blue EL emission due to (4f)^6 (5d)-(4f)^7 transitions. To control valence of Eu ions, we have investigated several co-activators. However, Eu-doped ZnS thin-films prepared did not show blue emission due to Eu^<2+> centers.3. The EL and PL characteristics of blue emitting ZnS : Tm^<3+> phosphors have been studied. The PL excitation mechanism of T^<3+> centers is attributable to the non-radiative energy transfer from the ZnS host to the Tm^<3+> centers. On the contrary, the EL excitation mechanism is due to impact excitation by hot electrons rather than energy transfer. The cause for the relative inefficiency of blue emission in EL is that the exciation of the lower lying ^3F_4 state is dominant compared with that of the higher lying ^1G_4 state.4. Zinc- and alkaline-earth sulfides have some fundamental problems, such as low solubility of rare-earth ions. electric charge compensation etc. . To solve such problems, we took UP Y_2O_2S as a host materials for trivalent rare-earth ions. We discussed the properties of Y_2O_2S thin films prepared by multi-source-deposition (MSD) method. It has been found that rare-earth doped Y_2O_2S shows efficient EL emission Less