Dependence of electroluminescent properties on cathode metalsin PVCz double-layered organic EL devices

We have investigated cathode metal dependence of electroluminescence (EL) characteristics of PVCz based EL devices by using Sr, Mg and Ag. The following has been clarified. First, we have obtained luminance as high as 1000 cd/m 2 for Poly(Nvinylcarbazole) (PVCz) EL devices without electron transport molecules but only emissive dye by using Mg and Sr metals cathode; while the device with Ag cathode showed a brightness about half as high. Secondly, threshold voltage for electroluminescence and barrier height for charge carrier injection both showed clear dependence on metal work function. Thirdly, chemical shift of core electron binding energy by X-ray photoelectron spectroscopy showed oxidation of Sr metal electrode near PVCz surface even if deposited under background pressure of 10 −10 Torr, while that of Mg showed negligible shift.