Distribution of Average Electric Field in tris-(8-hydroxyquinoline)aluminum and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl-based Double-Layer Light-Emitting Diodes

Abstract

We have determined the distribution of the average electric field in double-layer 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)aluminum (Alq) as electron-transport and emissive material, by means of electroabsorption spectroscopy. We found that the electric field in the α-NPD layer is considerably smaller than that in the Alq layer in the forward bias and that most of the dc bias is used for the formation of the electric field in the Alq layer, especially below the turn-on voltage, whereas in the reverse bias, the electric field in the Alq layer is identical to that in the α-NPD layer. This could be attributed to the injection of a relatively large amount of holes from the indium-tin-oxide electrode into the α-NPD and their transport, and the hole-blocking property of Alq. An abrupt change of the average electric field in both the Alq and α-NPD layers was observed in the vicinity of the turn-on voltage.