Distinguishing between interface dipoles and band bending at metal/tris-(8-hydroxyquinoline) aluminum interfaces

Using a combination of photoelectron spectroscopies and ellipsometry to study the thin-film growth of Alq3 (tris-(8-hydroxyquinoline) aluminum) on Ag, we have been able to distinguish between molecular orbital energy shifts due to “band bending” and interface dipole formation. We have observed large binding energy shifts (≈0.5 eV) of the organic molecular levels at low coverages. We conclude that these shifts are consistent with the formation of a polarized molecular layer, or interface dipole, and are inconsistent with an electrostatic band-bending model.