Electroabsorption spectroscopy on tris-(8-hydroxyquinoline) aluminum-based light emitting diodes

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Abstract
We determined the electric-field distribution in organic light emitting diode LED structures fabricated with 4,4′-bis [N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD) as hole-transport material and tris-(8-hydroxyquinoline) aluminum (Alq) as electron-transport and emissive material. The electric-field distribution was obtained from an investigation of the linear and the nonlinear Stark effect of the materials when employed in organic LEDs using electroabsorption spectroscopy. We measured the electric-field distribution as a function of the applied voltage in the forward and in the reverse direction. Whilst the average electric fields in the α-NPD and the Alq layers are equal in the reverse direction, the field in the Alq layer is considerably larger than that in the α-NPD layer in the forward direction, and the factor by which these fields differ changes with increasing voltage, in particular in the vicinity of the turn-on voltage. We discuss the electric-field distribution in terms of the charge injection and charge transport in devices as well as the possibility of charge accumulation at the α-NPD/Alq interface.