Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley–Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission.

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In a combined theoretical and experimental investigation the optical excitations of three polymorphs of crystalline pentacene are characterized in detail.

Using atomic-force microscopy and x-ray diffraction we show that perfluoropentacene (C 22 F 14 , PFP) forms long-range ordered, epitaxial films on KCl(100) and NaF(100) cleavage planes. On both substrates the films adopt the same crystalline bulk phase, but surprisingly exhibit quite different molecular orientations, being upright oriented on NaF and recumbent oriented on KCl. Accompanied thermal desorption spectroscopy measurements indicate the absence of a stabilized seed layer, like on metals, hence suggesting that in both cases the PFP films are stabilized by an electrostatic point-in-line relationship between the outermost fluorine atoms and the alkali cations of the alkali halide surfaces. Furthermore, the transparency of both substrates was utilized to perform detailed transmission UV/Vis spectroscopy and polarized optical microscopy measurements along well-defined crystallographic directions. From these data the orientation of transition dipole moments of the various optical excitations were experimentally determined and a directional anisotropic exciton coupling was observed, which is attributed to the asymmetric molecular packing motif within the (100) plane of the PFP crystal lattice.