Abstract
The electronic structure modifications of organic semiconductors by alkali metal doping have been investigated with photoemission and inverse photoemission spectroscopy. We have found that the doping induces energy level shift that 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. Furthermore, by adding an opposite dopant, such as Au or oxygen, the doping induced energy level shift can be partially recovered, which suggests that the doping process in organic semiconductors is mainly due to charge transfer.