Abstract

In organic devices, the morphology and the molecular/crystal orientation of the active molecular layer are key determinants for their (opto-)electronic performance. For instance, charge transport will strongly depend not only on the molecular orientation due to the highly anisotropic charge carrier mobility in organic crystals, but also on the film morphology via grain boundaries. The basis of controlled investigations of the inorganic-on-organic and organic-on-organic interfaces are epitaxial organic films with different unique orientations. Various measurements of highly oriented films of para-sexiphenyl (6P) and a-sexithiophene (6T) are presented here. The importance of the substrate and the balance between sticking and diffusion anisotropy on their growth is shown. Several UHV techniques have been applied to characterise the films and their surfaces. In valence band photoemission two distinct ionisation potentials are observed that are associated with the 6T and 6P having either planar or, unexpected for the solid state, twisted conformations. The former are shown to be associated with films with near perpendicular orientation (6P(001) and 6T(001)) while the latter are invariably observed when the molecules are oriented parallel to the substrate surface (whether in lying wetting monolayers or in thick 6T(010), 6P(203), 6P(213) films). Our present hypothesis is that the twisted molecules are a result of a form of reconstruction on the higher surface energy faces of organic crystals. Preliminary deliberations to independently confirm this involving comparison of experimental and calculated NEXAFS as a function of oligomer length and twist angle are presented. Calculations show that localisation arising from the core hole strongly affects the NEXAFS spectra. We are unable to distinguish between different conformations using NEXAFS .