A structural study for highly efficient electroluminescence cells using perylene-doped organic materials

Highly efficient electroluminescence (EL) cells using perylene-doped organic materials have been studied. Three types of cell structures were used for this study, and the device properties were compared with each other. As the light emitting layer, 60 to 70 nm thick perylene-doped poly(N-vinylcarbazole) [PVCz] was used. 2-(4′-tert-butylphenyl)-5-(4′-biphenyl)-1,3,4,-oxadiazole [Bu-PBD], tris(8-hydroxyquinoline)aluminum [Alq3] were used as a hole blocking layer and an electron-transport layer, respectively. Copper phthalocyanine [CuPc] was used as a hole injection layer at the bottom of emitting layer. The wavelength at electroluminescence from the perylene-doped PVCz films shifted to a long wavelength as the perylene concentration was increased. The electroluminescence due to perylene was observed at the wavelengths of 455, 480, and 520 nm. Blue and yellow green luminescences were observed for the perylene concentrations of 0.76 and 12.7 mole percent (m/o), respectively. From the band diagram study, it has been revealed that the barrier height for electron injection from MgAg electrode is 1.9 eV, and the barrier height for hole injection from indium-tin oxide film is 0.9 eV. Although the activation energy obtained from current-temperature characteristics 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 activation energy for 12.7 m/o doped PVCz films is due to the carrier trap generation by the perylene molecules in the PVCz. The luminances and efficiencies for the EL cells with the carrier transport layers were over one order of magnitude improved. The maximum luminance for the EL cell with both electron and hole transport layers was 1900 cd/m² at the applied voltage of 15 V, and the efficiency was 0.77 lm/W at the current density of 10 mA/cm².