A STRUCTURAL STUDY FOR HIGHLY EFFICIENT ELECTROLUMINESCENCE CELLS USING PERYLENE-DOPED ORGANIC MATERIALS

Highly efficient electroluminescence (EL) cells using perylene-doped o rganic materials have been studied. Three types of cell structures wer e used for this study, and the device properties were compared with ea ch other. As the light emitting layer, 60 to 70 nm thick perylene-dope d poly(N-vinylcarbazole) [PVCz] was used. ert-butylphenyl)-5-(4'-biphe nyl)-1,3,4,-oxadiazole [Bu-PBD], tris(8-hydroxyquinoline)aluminum [Alq 3] were used as a hole blocking layer and an electron-transport layer, respectively. Copper phthalocyanine [CuPc] was used as a hole injecti on layer at the bottom of emitting layer. The wavelength at electrolum inescence from the perylene-doped PVCz films shifted to a long wavelen gth as the perylene concentration was increased. The electroluminescen ce due to perylene was observed at the wavelengths of 455, 480, and 52 0 nm. Blue and yellow green luminescences were observed for the peryle ne concentrations of 0.76 and 12.7 mole percent (m/o), respectively. F rom the band diagram study, it has been revealed that the barrier heig ht for electron injection from MgAg electrode is 1.9 eV, and the barri er height for hole injection from indium-tin oxide film is 0.9 eV. Alt hough the activation energy obtained from current-temperature characte ristics was 0.36 eV for the low dopant concentration of 0.76 m/o, the activation energy for 12.7 m/o doped PVCz was 0.13 eV. This low activa tion energy for 12.7 m/o doped PVCz films is due to the carrier trap g eneration by the perylene molecules in the PVCz. The luminances and ef ficiencies for the EL cells with the carrier transport layers were ove r one order of magnitude improved. The maximum luminance for the EL ce ll with both electron and hole transport layers was 1900 cd/m(2) at th e applied voltage of 15 V, and the efficiency was 0.77 lm/W at the cur rent density of 10 mA/cm(2).