Internal electric field in tris-(8-hydroxyquinoline) aluminium (Alq) based light emitting diode Toshiki Yamada, Frank Rohlfing, Dechun Zou, Tetsuo Tsutsui Abstract 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.