Theory aided design and analysis of dielectric and semiconductor components for organic field -effect transistors (2009)

Perfluoroacyl/acyl-derivatized quaterthiophens are developed and synthesized. The frontier molecular orbital energies of these compounds are studied by optical spectroscopy and electrochemistry while solid-state/film properties are investigated by thermal analysis, x-ray diffraction, and scanning electron microscopy. Organic thin film transistors (OTFTs) performance parameters are discussed in terms of the interplay between semiconductor molecular energetics and film morphologies/microstructures. The majority charge carrier type and mobility exhibit a strong correlation with the regiochemistry of perfluoroarene incorporation. In quaterthiophene-based semiconductors, carbonyl-functionalization allows tuning of the majority carrier type from p-type to ambipolar and to n-type. In situ conversion of a p-type semiconducting film to n-type film is also demonstrated. The design of chemical and film microstructural alternative hybrid organic-inorganic gate dielectrics is described using the classic Clausius-Mossotti relation. The Maxwell-Wagner effective medium model is used to compute the effective dielectric permittivity of two types of dielectrics self-assembled nanodielectrics (SANDs) and crosslinked polymer blends (CPBs). In these calculations showing good agreement between theory and experiment, it is found that greater capacitances should be achievable with mixed composites than with layered composites. With this insight, a series of mixed metal oxide-polyolefin nanocomposites is synthesized via in-situ olefin polymerization using the single-site metallocene catalysts. By integrating organic and inorganic constituents, the resulting hybrid material exhibit high permittivity (from the inorganic inclusions) and high breakdown strength, mechanical flexibility, and facile processability (from the polymer matrices). In order to better optimize the capacitance and leakage current of hybrid organic-inorganic dielectrics, the capacitance, leakage current and OFET gate dielectric performance is investigated in two types of self-assembled nanodielectrics (SAND): solution deposited and vapor deposited (v-SANDs). The I(V,T) transport mechanism of SANDs types II (π-conjugated) and III (σ-saturated + π-conjugated) in Si/native SiO2/SAND/Au are investigated in metal-insulator-semiconductor devices over the temperature range of -60 – +100 °C. Hopping transport is observed over all temperatures for the π-conjugated system (II). For type III, the σ- and π-monolayers dominate transport over different temperature ranges: π-hopping at high temperatures and σ-tunneling at low temperatures. Type III has larger activation and barrier energies than type II. Furthermore, type III exhibits bulk dominated transport, whereas an ohmic contact is created by I - ions near the bottom Si electrode and yield electrode dominated transport in type II. The v-SAND films are fabricated in alternating and non-alternating microstructures. It is found that the alternating microstructure forces linear dielectric dipole order, and the largest capacitance, and current leakage is measured in these films. Low operating voltages, and enhanced pentacene OFET mobilities up to 2-3 cm2/Vs are achieved when these highly-polarazible organic molecular and inorganic building blocks are used as the gate dielectric.