We investigated the electronic structure of organic thin films doped with alkali metal using photoemission and inverse
photoemission spectroscopy (UPS, XPS and IPES). We found that doping induces energy level shift, which can be seen
as 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 the significant
modification of organic energy levels such as the introduction of a new gap state, new core level components, and
change of binding energies with respect to the frontier orbital. In addition, we observed that the energy level shift in the
first stage depended approximately in a semi-logarithmic fashion on the doping concentration, whose slope could not be
explained by the conventional model used in inorganic semiconductors. The lowest unoccupied molecular orbital
(LUMO) is observed to diminish as doping progresses. Furthermore, we observed that the doping induced modification
can be compensated by depositing Au or O2 on alkali metal doped organic films. The modification of the electronic
structure by other inorganic or organic dopants will also be discussed.
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