Theoretical investigation of the electronic structure of the Alq3/Mg interface

We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq(3))/Mg interface by using density functional theoretical calculations. We have found that the chemical bond is formed between the O atoms of Alq(3) and the substrate Mg atoms, and the stability of the interface structures depends on the number of O-Mg bonds. In the up configurations, where two or three O-Mg chemical bonds are formed and the Alq(3) molecular dipoles are oriented up to the vacuum side, the work function is decreased by as much as 1.1 eV or more. The interface dipole is dominated by the orientation of the molecular dipoles of Alq(3). The interface gap state reported from experiments is ascribed to the highest occupied molecular orbital (HOMO) levels of the down configurations, which may coexist with the dominant up configurations.