Organic semiconductor interfaces: Electronic structure and transport properties

Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) have been used to investigate a wide range of metal/organic and organic/organic semiconductor interfaces. UPS was used to determine the binding energies of the highest occupied molecular orbitals and vacuum level positions, while XPS was used to find evidence of chemical interactions at these heterointerfaces. It was found that, with a few exceptions, the vacuum levels align at most organic/organic interfaces, while strong interface dipoles, which abruptly offset the vacuum level, exist at virtually all metal/organic semiconductor interfaces. Furthermore, strong dipoles exist at metal/organic semiconductor interfaces at which the Fermi level is completely unpinned within the semiconductor gap implying that the dipoles are not the result of populating or emptying Fermi level-pinning gap states.