Alkali metal doping and energy level shift in organic semiconductors

We have investigated Cs and Na doping in copper phthalocyanine (CuPc) and tris(8-hydroxyquinoline) aluminum (Alq) using photoemission spectroscopy. We observed valence and core level spectra changes at different doping levels, and found that the doping induces an energy level shift that can be seen in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of the doping of electrons from the alkaline metal to the organic, and the second stage is characterized by a significant modification of organic energy levels, such as the introduction of a new gap state, new core level components and a change of binding energies. Furthermore, we observed that the energy level shift in the first stage depends in a semi-logarithmic fashion on the doping concentration, whose slope cannot be explained by the conventional model used in inorganic semiconductors. These results indicate that the molecular nature and strong correlation must be considered for doping in organic semiconductors.