Growth and physical properties of crystalline rubrene

Organic compounds showing semiconducting properties are now moving from being a pure research interest to their industrial application in the fabrication of a wide range of electronic devices. Among small molecule organic semiconductors, one of the most promising is rubrene (5,6,11,12-tetraphenyltetracene), which in the single crystal form reaches the highest charge carrier mobility values among organic semiconductors and also an extremely high photoconductivity and large exciton diffusion length. While rubrene single crystals with a size up to few centimeters can be prepared quite easily by sublimation methods, the growth of crystalline thin films is not as straightforward. Moreover, rubrene molecules are also strongly affected by photo oxidation. Several theoretical and experimental studies have been carried out in order to understand the effects of oxidation over transport and optical properties of crystalline rubrene, but a clear picture is still missing and lots of contradictory results have been reported. In this work we first present the growth and structural/morphological characterization of rubrene thin films grown by means of organic molecular beam epitaxy on two different substrates, namely tetracene single crystals and α-quaterthiophene crystalline thin films grown on potassium hydrogen phthalate single crystals. We show that rubrene thin films grown on the (0 0 1) surface of tetracene single crystals are crystalline and, even more important, grow according to a unique epitaxial relationship with the substrate, which in turn leads to a unique crystalline orientation of the film in the growth plane. Rubrene thin films grown on α-quaterthiophene still grow according to a unique epitaxial relationship but, due to symmetry reasons, show four different in-plane orientations of their crystalline lattice. We then move to the study of rubrene crystalline thin films morphological and structural evolution upon oxidation. Thanks to a combination of various experimental techniques, we show that the exposition of rubrene thin films to ambient air leads to the formation of a stable crystalline layer of rubrene endoperoxide molecules, packed according to a specific crystalline structure, on top of the pristine rubrene film. This process somehow resembles what happens for silicon with the formation of a native oxide layer. Finally we present the results of dark and photo-conductivity measurements carried out on rubrene and rubrene derivatives single crystals, aimed at determining the effects of interaction with oxygen over rubrene transport properties. By these means we demonstrate that the interaction with oxygen plays an essential role in enhancing crystalline rubrene transport properties. In particular our experimental findings suggest that what is important for electrical transport in crystalline rubrene is the interstitial molecular oxygen included in the crystal and not the formation of rubrene endoperoxide molecules.