Fabrication and characterization of organic single crystal and printed polymer transistors.

Abstract

The key challenges in the development of organic electronics lie in the understanding of the charge transport physics and the realization of low cost device fabrication. Innovative studies on both aspects have been demonstrated in this thesis. On the fundamental side, first, charge transport and localization processes in various organic single crystal transistors have been investigated using a novel "air-gap"device geometry. Second, comparison of mobility - carrier density relation in polymer and single crystal transistors has been made by the utilization of different liquid gate dielectrics with extremely wide capacitance range, and fundamentally different charge transport mechanisms have been proposed. Third, direct measurement of the electrochemical potential at organic semiconductor/gate dielectric interfaces in electrolyte gated transistors has been achieved with the assistance of an embedded reference electrode. The correlation between the referenced turn-on voltages and the organic semiconductor ionization potentials has been discovered. Finally, an unusual negative differential transconductance behavior in electrolyte gated transistors upon inducing high gate carrier densities has been extensively investigated. On the application side, high performance polymer transistors and circuits were fabricated by a commercial aerosol jet printing technique. Printing not only saves the device manufacturing cost through its simple procedure, high throughput and low waste of materials, but also enables the fabrication of electronic devices over large area and on flexible substrates. All-printed transistors with exceptionally large transconductance of 10 mS/mm under 1 V of operating voltage have been realized with the application of specially designed printable high capacitance (>10 μF/cm2) ion gel as the gate dielectric material. Various device configurations and parameters have been investigated to further reduce the fabrication cost and improve the operating speed. Based on these transistors, high performance, low voltage operation logic and analog circuits such as inverters, NAND logic gates, D Flip-flop circuits and ring oscillators have been demonstrated.