Asymmetry and rectification in the tunnel current of a nanometer-sized metal-conjugated polymer-metal junction

Electron transport processes of a nanometer metal-conjugated polymer–metal tunnel junction have been probed using a scanning tunneling microscope. The tunnel current of the junction shows two effects. The appearance of an asymmetry in the tunnel current indicates that the junction transport mechanism is different from that for which tunneling occurs directly between two metallic electrodes. Thus, understanding of the asymmetry and hence the transport mechanism demands a detailed description of the metal–polymer interface. By applying the theories of the metal–semiconductor interface to the tunnel junction, we show the presence of an asymmetric electrostatic potential-energy profile, which, together with the metal-induced gap states in the polymer, gives rise to the observed asymmetry in the tunnel current. In some cases, a threshold of anomalously large currents enhances the current asymmetry to give rise to rectification, indicating carrier excitations and carrier multiplication processes in the junction. Our results show that a detailed description of the interface electronic structure is essential to understanding electron transport in devices based on organic molecules.