I-03 Effects of various types of doping on the electronic structure of organic interfaces

Recently much interest has been focused on the electronic structure of the organic/metal interfaces. The subjects can be classified into two: (a) phenomena within a few molecular from the contacts. e.g. energy level alignment, and (b) phenomena extending into thicker region, such as the Fermi level alignment associated with band bending. The major subject in (a) is dipole layer formation, which was extensively studied by various groups. We now have some sound insight into the origin of dipole layer, although the details still needs further studies [1-4]. In contrast, the thick layer problems (b) still remain mostly unexploited. Here we report our recent efforts for establishing the whole picture of this kind of problems, with special attention to the effects of various types of electrical doping. In most organic electronic devices, the organic materials are used without electrical dopants. This is in contrast to inorganic semiconductors, which are almost always doped with controlled amount of impurities. These impurities play the role of (a) fixing the Fermi level (EF) of the doped layer at some fixed position within the bandgap tuned by the species and the amount of the dopant(s), and (b) causing band bending leading to Fermi level alignment by forming space charge localized in the impurity atoms accompanied by the ionization of the dopant atoms. Thus we have essential questions about organics such as (1) Can we expect band bending leading to EF alignment for "undoped " organic layer of limited thickness