On validity of the Schottky-Mott rule in organic semiconductors: Sexithiophene on various substrates

The energy level alignment of sexithiophene monolayer and multilayer films grown on a variety of well-defined substrates with different work functions is presented in this study. The ionization potential of the films as measured by valence band photoemission is found to have two distinct values that differ by similar to 0.8 eV depending on substrate and/or growth conditions of the films. The results suggest the necessity of considering the variance of the ionization potential for the correct interpretation of energy level alignment obtained by photoemission; we demonstrate that the commonly reported absence of the relation between the band alignment and the substrate work function is partly determined by extrinsic effects. These are induced by varying electronic properties of solid films in addition to the intrinsic effects of the interfaces. (c) 2007 American Institute of Physics.