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The performance of organic light-emitting diodes (OLEDs) and their interfacial charge density have been studied in tris-(8-hydroxyquinolate) aluminum (Alq3)-based OLEDs with four different hole transport layers (HTLs). The four HTLs have almost identical highest occupied molecular orbital (HOMO) energy levels. The performance of the four OLEDs is not greatly changed. The interfacial charge density is determined from the hole injection voltage in the capacitance—voltage characteristics of the OLEDs. Determination of the interfacial charge density is verified by device simulation. The interfacial charge density is dependent on the HTL materials in Alq3-based OLEDs, and is proportional to the difference in dipole moments between the HTLs and electron transport layers including Alq3. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]