Energy-level alignment at the tris-(8-hydroxyquinolate)-aluminum/Gd interface and Gd-electron-injection layer for organic electroluminescent device

The energy-level alignment for the tris-(8-hydroxyquinolate)-aluminum (Alq₃)/Gd interface is determined by ultraviolet photoemission spectroscopy. The energy difference between the Fermi level in Gd and the low-energy edge of the highest occupied molecular orbital in Alq₃ is 2.63 eV, and the vacuum level in the Alq₃ layer is moved upward to 0.35 eV with respect to its intrinsic level. Gd/Al, Al:Li (0.1%), and Al were employed as a cathode for the organic electroluminescent device. Among these devices, the device with the Gd-electron-injection layer has operated at the lowest voltage.