Metal/Alq3 interactions in organic light emitting devices:: The different roles of Mg, Al, and Li atoms

The geometrical and electronic structures of tris(8-hydroxy-quinoline)aluminum (Alq(3)) molecule interacting with low work function metals of Mg, Al, and Li used in organic light emitting devices have been studied by first-principle calculations using density functional theory. We found that energetically the most favorable complexation for the interacting systems is the metal atom inserting into the core of the Alq(3) molecule with the metal atom bridging two oxygen atoms and being coplanar with one of the quinoline ligands. The related various core level Al(2p), O(1s), and N(1s) energy shifts and the characteristic vibrational modes determined consequently are in reasonable agreements with the available experimental data. The cohesion energies (E-c) of the metal-Alq(3) complexes increase in the order of E-c(Mg)<E-c(Al)<E-c(Li), among which the E-c(Mg) is considerably smaller, indicating the distinctive feature of Mg-Alq(3) interaction from those of Al and Li. The calculated electronic structures show that there are only slight changes in the Alq(3) frontier orbitals for Mg-Alq(3) and Li-Alq(3) complexations, while for the Al insertion into Alq(3) considerable electronic localizations are induced, indicating the significantly different roles they may play in metal-organic interface and thus in the device performance. (C) 2002 American Institute of Physics.