Energy level alignment regimes at hybrid organic-organic and inorganic-organic interfaces

Ultraviolet photoelectron spectroscopy has been used to determine the energy level alignment at interfaces of molecular hole-transporting materials and various conductive substrates. Depending on the work function of the substrate, phi(s), a transition between two different energy level alignment regimes has been observed: namely vacuum level alignment and Fermi level pinning. The transition is associated with spontaneous positive charge transfer across the interface to the organic semiconductors above a certain material-specific threshold value of phi(s). The charge transfer results in formation of an interfacial dipole of a magnitude that scales with phi(s). In the vacuum level alignment regime, the hole-injection barriers scale linearly with phi(s), while in the Fermi level pinning regime, these barriers are constant and independent of phi(s). (C) 2006 Elsevier B.V. All rights reserved.