We measured the reliability of two different organic light-emitting diodes (OLEDs) composed of indium tin oxide (ITO)/4,4 0bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (-NPD)/tris(8-hydroxyquinoline)aluminum (Alq 3 )/MgAg (two layer) or ITO/ -NPD/-NPD:Alq 3 (mixed layer)/Alq 3 /MgAg. Changes in the carrier trap distribution were estimated by thermally stimulated current (TSC) measurements. Improved device reliability was observed in the mixed-layer device. In the case of the twolayer device, the TSC peaks gradually changed depending on the duration of continuous operation, while the mixed device showed no change in its TSC spectra. These spectral differences indicate that the mixing of -NPD and Alq 3 occurs during the continuous operation in the two-layer device.

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