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

We determined the electric-field distribution in organic light emitting diode LED structures fabricated with 4,4'-bis [N-(1-naphthyl)-N-phenylamino]-biphenyl (alpha-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 alpha-NPD and the Alq layers are equal in the reverse direction, the field in the Alq layer is considerably larger than that in the alpha-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 alpha-NPD/Alq interface. (C) 1999 American Institute of Physics. [S0021-8979(99)06821-8].