First-principles theoretical study of Alq3Al interfaces: origin of the interfacial dipole

We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq(3))Al interfaces by using density functional theoretical calculations, and clarified the origin of the interfacial dipole moment. We have examined various possible adsorption geometries of Alq(3) on Al surfaces and calculated the work function change induced by adsorption of Alq(3) on Al surfaces. We found that the stability depends crucially on the number of O-Al bonds formed at the interface, and Alq(3) tends to expose its O atoms to the Al substrate side and its N atoms to the vacuum side. Although the binding energies are influenced by the poor description of the van der Waals interaction by the density functionals used, the resulting bonding configurations are found to give correct binding energies when the van der Waals interaction is taken into account based on the recently proposed van der Waals density functional [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. This bonding configuration arranges molecular permanent dipoles of Alq(3) directed towards the vacuum, leading to the decrease of the surface work function. The calculated interface dipoles agree reasonably well with the experimental results and the origin of the interface dipole formation mainly comes from the alignment of the permanent dipoles of Alq(3). The HOMO levels of the Alq(3) molecules significantly depend on the orientation of the molecular permanent dipoles and the interfacial gap state observed by experiments is ascribed to the coexistence of the two orientations of the molecular dipole moments.