ABSTRACT
Organic electronic devices consist of undoped, insulating thin films of conjugated organic materials into which charged carriers are injected from metallic electrodes. The operation of the devices is determined by the charge injection properties of the metal-organic interface and the electrical transport properties of electron and hole polarons in the organic film. One of the most basic questions concerning the electronic structure of a metal-organic interface is the energy required to inject electrons and holes from the metal contact into the organic material, i.e., the difference between the Fermi energy of the metal contact and the energies of the electron and hole polaron states of the organic material. These energy differences are called the electron and hole Schottky energy barriers in analogy with the corresponding injection barriers at metal-semiconductor contacts. In inorganic semiconductors, such as Si and GaAs, Schottky energy barriers are weakly dependent on the type of metal contact for a given semiconductor, i.e., the Schottky energy barriers for various metals on Si are similar [1]. This is not the case at many metal-organic interfaces [2-5]; there is a qualitative difference in the observed behavior of Schottky energy barriers in conjugated organic materials compared with inorganic semiconductors. Indeed, the essential operating principle of organic diodes is based on the asymmetry in the Schottky energy barriers of the two contacts making up the structure.
