Negative capacitance of bilayer organic light-emitting diodes-its correlation with current efficiency and device lifetime

The organic LED (OLED) performance of standard bilayer OLEDs with three different cathode materials has been studied. The device structure was indium tin oxide/1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile/N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine/tris-(8-hydroxyquinolate) aluminum/cathode of aluminum, silver-doped magnesium or lithium fluoride/aluminum. Negative capacitance is observed in the OLEDs with the three different cathodes, and the value of the negative capacitance is closely correlated to the current efficiency of the OLEDs and to the device lifetime. The experimental results are examined with the aid of device simulation. It is shown that measurements of negative capacitance in the bilayer OLEDs can offer useful information concerning device operation and degradation.