Fermi-level pinning at conjugated polymer interfaces

Photoelectron spectroscopy has been used to map out energy level alignment of conjugated polymers at various organic-organic and hybrid interfaces. Specifically, we have investigated the hole-injection interface between the substrate and light-emitting polymer. Two different alignment regimes have been observed: (i) Vacuum-level alignment, which corresponds to the lack of vacuum-level offsets (Schottky-Mott limit) and (ii) Fermi-level pinning, where the substrate Fermi level and the positive polaronic level of the polymer align. The observation is rationalized in terms of spontaneous charge transfer whenever the substrate Fermi level exceeds the positive polaron/bipolaron formation energy per particle. The charge transfer leads to the formation of an interfacial dipole, as large as 2.1 eV. (c) 2006 American Institute of Physics.