Electronic structure of 8-hydroxyquinoline aluminum/LiF/Al interface for organic electroluminescent device studied by ultraviolet photoelectron spectroscopy

Electronic structures of the 8-hydroxyquinoline aluminum ${\mathrm{(Alq}}_{\mathrm{3}}\mathrm{)/LiF/Al}$ and ${\mathrm{Alq}}_{\mathrm{3}}\mathrm{/Al}$ interfaces were measured by ultraviolet photoelectron spectroscopy. Shifts of the highest occupied molecular orbital level and the vacuum level of the ${\mathrm{Alq}}_{\mathrm{3}}$ layer due to insertion of a thin LiF layer were observed. This result indicates that the thin LiF layer at the ${\mathrm{Alq}}_{\mathrm{3}}\mathrm{/Al}$ interface reduces barrier height for electron injection from the Al to ${\mathrm{Alq}}_{\mathrm{3}}.$ We, therefore, conclude that lowering of the driving voltage in an organic electroluminescent device with a thin LiF layer is attributable to the reduction of the barrier height.