Morphology and Charge Transport in Polymer Organic Semiconductor Field-Effect Transistors

Abstract

pi-conjugated organic semiconductors is a new class of materials that have applications in a variety of opto(electronic) devices. Despite significant recent advances in the development of high-performance materials, a number of fundamental aspects of their semiconductor physics and device chemistry that determine for example the organic field-effect transistor (OFET) characteristics, such as charge-carrier mobility and ambient stability remain to be clarified. In this thesis using thiophene-based polymers as model organic semiconductors (OSCs), I have uncovered two new mechanisms that are fundamental to the understanding of these properties. In the first part of the work, I describe a reversible photo-induced doping mechanism involving sorbed moisture that is responsible for the degradation of the on-off ratio when devices are exposed to ambient air. This new mechanism was established through a correlation between the off-current, polaron band intensity and hydroxide ion species using sensitive in-situ Fourier-transformed infrared spectroscopy measurements on devices inside an ?environmental? chamber together with electrical characterisation. This mechanism reconciles conflicting reports in the literature and reveals the synergistic role of both light and moisture in causing unintentional ?doping? the OSCs. In the second part of the work, I show that the strong dependence of carrier mobility of the prototype high mobility polymer, regioregular poly(3-hexylthiophene), on the dielectric interface is in fact caused by the presence of a polymer transition layer that is absorbed at the interface. The formation of this ultrathin ?wetting? layer of a lying-down polymer phase is decisively controlled by van der Waals interaction with the surface, and strongly detrimental to charge transport at the interface. This new mechanism established by a combination of atomic force microscopy, near-edge absorption fine structure spectroscopy, mobility activation measurements and charge-modulation spectroscopy, resolves a long-standing mystery of how the interface may exert a pronounced influence on FET characteristics.