Contact resistance and hole mobility in pentacene thin-film transistors.

摘要

Although, the class of materials known as organic semiconductors has been studied since the early 1900s, only recently has their performance (mobility) become sufficient for use in active devices. In the past few decades, organic semiconductors have garnered a great deal of attention due to their potential use in applications such as flexible circuitry, emissive displays, and solar cells. Many fundamental questions still remain, however, about the nature of charge injection and transport mechanisms along with limits of maximum performance. To address some of the remaining issues, this thesis employs field-effect transistor measurements to examine contact resistance properties of metal-organic interfaces and the causes of ultra-high mobilities observed under specific fabrication conditions.; This thesis describes field-effect transistor measurements in a gated four-probe configuration on the organic semiconductor pentacene in order to address questions of charge injection and transport as a function of pentacene film thickness, contact metal workfunction, electrode geometry, gate voltage, and temperature. The gated four-probe technique is a powerful tool to use for examining semiconductor materials because it can simultaneously extract information about the source resistance, drain resistance, and contact-corrected semiconductor mobility. Modeling was performed to study the accuracy of the gated four-probe technique. Pentacene thin-film transistors exhibited linear mobility as high as 1.75 cm2/V-s, contact resistance as low as 1.3 kO-cm, and activation energies for film mobility and contact resistances as low as 15 meV. Results indicate that contact barriers known to exist at metal-organic interfaces by ultraviolet photoelectron spectroscopy may not take part in charge injection.; In a standard three-terminal transistor configuration, process conditions leading to mobilities greater than 6 cm2/V-s were examined. Two thin-film transistor fabrication methods are presented and compared. Components of each fabrication process are examined for their role in production of the observed ultra-high mobilities. The ultra-high mobilities are only seen under certain pentacene purification and deposition conditions on polystyrene-coated substrates. The results presented in this thesis suggest careful formation of the first few pentacene layers on low surface energy substrates is critical to the production of ultra-high mobilities.