Efficiency of organic light-emitting diodes depending on Al based cathodes

We have investigated the effects of cathode in a device structure of ITO/TPD/Alq/sub 3//cathode with cathodes of Al, LiF/Al, LiAl, and Ca/Al by measuring the current-voltage-luminance characteristics at room temperature. The device with LiF/Al cathode shows an improvement of external quantum efficiency approximately by a factor of ten compared with Al cathode only device. Our observation shows that the energy barrier-height in cathode side is important in improving the efficiency of the organic light-emitting diodes.