Electronic structure of Cs-doped tris(8-hydroxyquinoline) aluminum

The evolution of the electronic structure of Cs-doped tris(8-hydroxyquinoline) aluminum (Alq) film has been investigated with photoemission spectroscopy. The results show that doping induces an energy level shift that can be divided into two stages. At the first stage, the Fermi level moves in the energy gap due to the charge transfer from Cs to Alq. Moreover, this energy level shift depends on the doping concentration in a semilogarithmic fashion. The second stage is characterized by a significant modification of the Alq electronic structure, manifested by the gap state and saturation of the energy level shift.