PhD thesis - Evans Research Group - University of Wisconsin-Madison

141.3.2.2 RubreneRubrene (C 42 H 28 ) is a benchmark material for organic single-crystal FETs because thefield-effect mobility of carriers in rubrene single crystals is the highest reported for organicsemiconductors [54, 55]. Rubrene has four phenyl side groups connected to a tetracenebackbone. As with pentacene, rubrene can be crystallized in herringbone arrangement witha variety of crystal structures with monoclinic, triclinic and orthorhombic symmetry,depending on the conditions under which the crystals are grown [56-58].(a)(b)Fig. 1.7: Geometry of a rubrene molecule (a) in the gas phase (b) and in the crystallinephase [56].Rubrene thin films have been studied because of the ease of process and its potentialhigh mobility. Rubrene has different molecular conformations in gas phase and crystallinephase [56]. The conformation of rubrene deposited on SiO 2 substrate is close to the gasphase. In the gas phase, the tetracene backbone of rubrene is twisted as shown in Fig.1.7(a). The backbone of the bulk crystalline phase is planar (Fig. 1.7(b)). The molecularenergy difference between the two rubrene geometries is about 210 meV [56]. A free