Energy level alignment and interface states at α-sexithiophene/Ag interfaces

We present a combined X-ray and ultraviolet photoemission study of the interface between the thiophene based organic semiconductor α-sexithiophene (6T) and silver. Thermally evaporated organic films were deposited in several steps without breaking the vacuum on polycrystalline silver foils. Subsequently, the films were characterized via photoemission spectroscopy. These series of spectra allowed the determination of the energy level alignment at the interface. Our results provide evidence that chemical interaction takes place between flat lying α-6T molecules and the silver surface resulting in interface electronic levels. From the observed well defined molecular orbitals we have analyzed the energy level alignment and have found an interface dipole of −0.7 eV and a hole injection barrier of about 1.8 eV.