Role of the Charge Neutrality Level at Metal/Organic and Organic/Organic Interfaces

This paper describes how the concepts of Charge Neutrality Level (CNL) and Induced Density of Interface States (IDIS) can successfully explain the energy level alignment at metal-organic and organic-organic interfaces. We propose that the CNL acts as an effective Fermi level for the organic semiconductor: its partial alignment with the metal Fermi level (in the case of metal-organic interfaces) or with the CNL of the other organic material (at organic heterojunctions) determines the interface properties. We review results for several organic semiconductor interfaces. Non-reactive metal-organic interfaces, even in the absence of chemical reaction or defects, have an intrinsic density of states (DOS) induced in the organic energy gap. As the CNL tends to align with the metal Fermi level, the DOS pins the interface Fermi level near the CNL. Theoretical values for the S parameter and Fermi level are in good quantitative agreement with experiment. At organic heterojunctions, the tendency of the CNLs to align gives rise to a restoring dipole. Our intuitive, yet general, approach is able to reproduce the variety of interface dipoles observed experimentally. We conclude that the CNL and induced DOS are important physical quantities governing the energy level alignment at organic semiconductor interfaces.