Internal electric field in tris-(8-hydroxyquinoline)aluminium (Alq) based light emitting diode

We have determined the electric field distribution in organic light emitting diode structures fabricated with 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD) as hole-transport material and tris-(8-hydroxyquinoline)aluminium (Alq) as electron-transport and emissive material by means of electroabsorption (EA) spectroscopy. It was found that the integrated electric field in the α-NPD layer is considerably smaller than that in the Alq layer in the forward bias, whereas the integrated electric field in the Alq layer is identical to that in the α-NPD layer of the reverse bias. An abrupt change of the integrated electric field in both the Alq and the α-NPD layers was observed in the vicinity of the turn-on voltage. The factor by which these two electric fields differ was changed above the turn-on voltage.