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

Toshiki Yamada, Frank Rohlfing, Dechun Zou, Tetsuo Tsutsui

We have determined the electric field distribution in organic light emitting diode structures fabricated with 4,4'-bis[ N-(1-naphthyl)-N-phenylamino]-biphenyl (a-NPD) as hole-transport material and Iris-(8-hydroxyquinoline) aluminium (Alq) as electron-transport and emissive material by mews of electroabsorption (EA) spectroscopy. It was found that the integrated electric field in the a-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 a-NPD layer of the reverse bias. An abrupt change of the integrated electric field in both the Alq and the a-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.