Change of the depth profile of a light-emitting zone in organic EL devices with their degradation

The light-emitting zone in organic electroluminescent (EL) devices having the aluminum-hydroxyquinoline complex (ALQ)/diamine (TPD) junction was estimated from the analysis of the EL spectra. The zone distributed to the depth of about 20 nm from the interface at the initial performance. The zone however was found to shrink toward the ALQ/TPD interface as the devices degraded. From the results, it was concluded that the ALQ/TPD interface is responsible for the degradation, especially at the initial stage.