A relationship between driving voltage and the highest occupied molecular orbital level of hole-transporting metallophthalocyanine layer for organic electroluminescence devices

Effects of metallophthalocyanines (MPcs) are systematically investigated on electroluminescence (EL) characteristics of ITO/ MPc/TPD/Alq(3)/Mg-Ag devi ces. A linear relationship is found between driving voltages of the devices and energies of the highest occupied molecular orbital (HOMO) level of MPc s, showing that the driving voltage of the device is substantially determin ed by the energy barrier for hole injection at the ITO/MPc interface. When an MPc having a higher HOMO level, such as CuPc or ZnPc, is selected as a h ole transport material, the driving voltage of the device at a luminance of 100 cd m(-2) significantly reaches as low as 5.5 V compared with 7.8 V for the device without an MPc layer. The insertion of MPc layer leads to incre asing of the EL output, but does not affect the EL efficiency of the device s. (C) 2001 Elsevier Science B.V. All rights reserved.