Structural Properties of Picene-Perfluoropentacene and Picene-Pentacene Blends: Superlattice Formation versus Limited Intermixing

The structure and morphology of mixed thin films of picene (C<inf>22</inf>H<inf>14</inf>, PIC) and perfluoropentacene (C<inf>22</inf>F<inf>14</inf>, PFP) as well as mixed thin films of PIC and pentacene (C<inf>22</inf>H<inf>14</inf>, PEN) grown by simultaneous coevaporation is investigated using X-ray diffraction, atomic force microscopy, and near-edge X-ray absorption spectroscopy. For both systems we find mixing on the molecular level and the formation of mixed structures. However, due to the strongly different interactions in both mixtures the ordering is fundamentally different. For the equimolar PFP:PIC mixtures, we observe the formation of two different mixed polymorphs with unit cells containing 2 PIC and 2 PFP molecules depending on the growth temperature. One of these polymorphs is a superlattice with in-plane compound segregation. The other polymorph is less symmetric and results only in a very short ranged in-plane ordering. In contrast, the PEN:PIC mixtures form crystals with unit cell parameters continuously changing with the molar concentrations between those of the pure compounds. The position of molecular species within the crystal lattice is statistical. Surprisingly, for higher concentrations of PIC we observe phase separation of surplus PIC molecules which corresponds to a limited intermixing of the two compounds. Finally, the results are discussed in the context of other organic semiconductor binary mixtures showing that besides chemical composition and steric compatibility the intramolecular arrangement of the atoms important for intermolecular interactions significantly influences the structure formation in organic semiconductor blends.