DEVELOPMENT OF FLAT PANEL DISPLAYS BASED ON ELECTROLUMINESCENCE FROM SEMICONDUCTOR NANOCRYSTALS

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Thin-film devices utilizing electroluminescence (EL) from ZnS nanocrystals (NCs) doped with activator-ions have been studied for flat panel displays with an emphasis on the blue emission as well as on the improvement of luminance. The device structure was a conventional double-insulating structure consisting of glass substrate/indium tin oxide/insulating layer/ZnS NC phosphor layer/insulating layer/Al electrode. ZnS NCs were formed in the multilayers consisting of (ZnS/interlayer)_m with a period m of 30. The interlayer is for breaking ZnS continuum growth, or for controlling the crystal size of ZnS NCs. Si_3N_4,AIN, and Ta_2O_5 were investigated as the insulating layer, and AlN is the most effective in increasing the luminance, which agrees with the result of 100-K photoluminescence. Furthermore, charge transport analyses revealed that the thickness of the ZnS NC layer, which is almost equivalent with the crystal size of ZnS NCs, influences on the EL efficiency and 8 nm is optimum in … More this experimental series. The material properties of the double-insulating layers are also important, and Si_3N_4 is the most effective in improving the maximum luminance. As for the multi-color emission, blue emission was achieved from ZnS:Tm NCs. The EL spectra of ZnS:Tm NCs consist of blue, quite week red, and infrared bands as found in other Tm-doped phosphors. Nevertheless, being different from other materials, the blue band is the strongest, and the intensity ratio of blue/infrared increases as the applied voltage increases. The voltage evolution of the blue/infrared intensity ratio strongly suggests that the energy distribution of the hot electron shifts to higher with increasing the electric fields in ZnS NCs, i.e., the major EL mechanism of ZnS NCs should be hot-electron-induced impact ionization. Finally, the thin-film device with sub-pixels corresponding to the primary colors has been demonstrated utilizing red emission from ZnS:Mn NCs, green emission from ZnS:Tb NCs, and blue emission from ZnS:Tm NCs. Less