Investigating the Impact of Crystallinity on the Molecular Doping of Rubrene Thin Films

Abstract

Due to their high mechanical flexibility, low density, low production cost, and abundant availability, organic semiconductors may enable electronic device applications that are inaccessible to conventional inorganic semiconductors. Because of their weak intermolecular bonding forces, organic semiconductors are more disordered compared to inorganic semiconductors, such as crystalline silicon. This higher disorder lowers the efficiency of the transport of charge carriers, potentially decreasing the conductivity of this class of materials and limiting their applicability in electronic devices. To address this general weakness, there are efforts to use doping, or the addition of extra hole or electron charge carriers, to increase the conductivity of disordered organic semiconductors. However, there is still a lack of clear understanding of the doping mechanisms and dopability of organic semiconductors. Therefore, in this thesis, we study the impact of crystallinity (order) on the molecular doping of the organic semiconductor rubrene (C42H28). We specifically study rubrene because we are able to prepare amorphous to highly polycrystalline rubrene films by a process previously developed in the group. To p-dope rubrene films, we use molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd)3), chosen because of its appropriate energy levels for charge transfer with rubrene. Importantly, using electrical, optical, and materials characterization techniques, we find that while amorphous rubrene films can be successfully doped with the dopant Mo(tfd)3, we do not observe evidence of doping in crystalline rubrene films with Mo(tfd)3. We discuss potential origins for the difficulty in molecular doping crystalline rubrene and future directions to better understand the doping mechanism.