Changes in electronic structure of 8-hydroxyquinoline aluminum/Al interface by insertion of thin electron injection layers

Abstract

The electronic structures of 8-hydroxyquinoline aluminum (Alq<SUB>3</SUB>)/electron injection layer/Al interfaces, used in organic electroluminescent devices, were measured by ultraviolet photoelectron spectroscopy (UPS). LiF and alkaline earth fluorides (CaF<SUB>2</SUB>, SrF<SUB>2</SUB> and BaF<SUB>2</SUB>) were used as an electron injection layer. Shifts of the highest occupied molecular orbital (HOMO) level and the vacuum level of Alq<SUB>3</SUB> layer due to the insertion of the fluorides were observed. These shifts indicate that the alkaline earth fluoride layers as well as the LiF layer at the Alq<SUB>3</SUB>/Al interface reduce the barrier height for electron injection from the Al to Alq<SUB>3</SUB>. The reduction of the barrier height is consistent with the driving voltage in the organic EL device in which these fluorides are used as the electron injection layers. We believe that lowering in the driving voltage in organic EL devices with the thin insulator layers, such as LiF and alkaline earth fluorides, is attributable to the reduction of the barrier height.