Structure and transport in organic semiconductor thin films

Organic Semiconductors represent an exciting area of research due to their potential application in cheap and flexible electronics. In spite of the abundant interest in organic electronics the electronic transport mechanism remains poorly understood. Understanding the connection between molecular structure, crystal packing, intermolecular interactions and electronic delocalization is an important aspect of improving the transport properties of organics in thin film transistors (TFTs). In an organic thin film transistor, charge carrier transport is believed to occur within the first few monolayers of the organic material adjacent to the dielectric. It is therefore critical to understand the initial stages of film growth and molecular structure in these first few layers and relate this structure to electronic transport properties. The structure of organic films at the interface with an amorphous silicon dioxide ( a-SiO2) dielectric and how structure relates to transport in a TFT is the focus of this thesis.Pentacene films on a-SiO2 were extensively characterized with specular and in-plane X-ray diffraction, and CuKalpha1, and synchrotron radiation. The first layer of pentacene molecules adjacent to the a-SiO2 crystallized in a rectangular unit cell with the long axis of the molecules perpendicular to the substrate surface. Subsequent layers of pentacene crystallized in a slightly oblique in-plane unit cell that evolved as thickness was increased. The rectangular monolayer phase of pentacene did not persist when subsequent layers were deposited. Specular diffraction with Synchrotron radiation of a 160 A pentacene film (&sim 10 layers) revealed growth initiation of a bulk-like phase and persistence of the thin-film phase. Pentacene molecules were more tilted in the bulk-like phase and the in-plane unit cell was slightly more oblique. Pentacene grains began to grow randomly oriented with respect to the substrate surface (out-of-plane) in films near 650 A in thickness. The single crystal bulk phase of pentacene was observed from specular diffraction (CuKalpha1) of a 2.5 mum film. These results suggest that the thickness of pentacene films on a-SiO2 is an important aspect in the comparison of crystal structure and electronic transport.