Interface in Organic Semiconductor Devices: Dipole, Doping, Band Bending, and Growth
As proven in over fi ve decades of research, the understanding of interfaces has a tremendous impact on semiconductor device technology. In organic semiconductor (OSC) devices, the thickness of the active organic layer is typically only a few hundred angstroms, which further blurs the distinction between the bulk and the interface. At this point, an important distinction must be made between inorganic semiconductors and OSCs. Inorganic semiconductors have occupied and unoccupied energy levels, valence and conduction bands, respectively, that can extend over many unit cells. The semiconductor can be appropriately doped n-type or p-type. The interaction between charge carriers and the lattice is generally weak, and the transport of the charge carriers can be adequately described as delocalized Bloch
waves in the bands. In OSCs, the occupied and unoccupied energy levels for OSCs are formed from planar structures of sp2 bonds as well as π-bonds, and the π-bonds between carbon atoms in organic molecules usually form the highest occupied molecular orbital (HOMO) and lowest unoccupied molecule orbital (LUMO) in most OSCs. The interactions between the molecules are van der Waals in nature and the electron wave function overlap between the molecules is small. The charge carriers are localized and surrounded by signifi cant nuclear relaxation, and are better described together with the surrounding nuclear deformation as polarons instead of electrons or holes. As a result, the transport from one molecule to another is typically described by the hopping of polarons.