On the Characterization and Manipulation of Interfaces in Organic and Hybrid Electronic Devices
Martinez Hardigree, Josue F.
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Organic electronics comprises a field of study at the intersection of chemistry, physics, electrical engineering, and materials science focused on the development of electronic devices in which the active charge transporting materials are composed of organic conjugated molecules. This field has grown out of an interest in harnessing many attributes of organic materials not readily available to inorganic semiconductors, including: low synthesis temperatures for organic compounds; a nearly infinite combination of chemical moieties with similar conjugated character; and ease of fabricating thin films of organic compounds through both vacuum and solution processes. These properties make the fabrication of low-cost, highly-customizable electronics commercially viable, despite their inferior carrier transport to crystalline inorganic semiconductors. This key hurdle—understanding charge transport in organic molecules and thin films made from them—has become a primary research objective in the field. Understanding charge transport in organic electronic devices spans analysis across various size scales, each contributing to the observed behavior of an electronic device: * The chemical structure of the constituent conjugated molecules (Ås) * The arrangement of these molecules into ordered and disordered regions within a thin film (10s of Ås) * The configuration of the thin film within the working device (100s of Ås) At each of these scales, the concept of an interface acquires new meaning, scaling from van der Waals forces between molecules, to grain boundaries in polycrystalline materials, and incrementally to device-scale junctions between dissimilar materials. Because each of these interfaces can promote or inhibit carrier transport within an electronic device, a complete understanding of carrier transport in organic semiconductors (OSCs) demands comprehensive characterization of interfaces at each of these scales. The subject of this thesis is a critical examination of the insulator-OSC interface in the context of several electronic device architectures. The properties of this interface are of paramount importance in organic field-effect transistors (OFETs), where the low intrinsic carrier mobilities of OSCs renders them highly susceptible to even the most marginal deviations from an ideal interface. As a result, transistor switching characteristics quickly carry through to circuit-level reliability and power consumption. This dissertation aims to demonstrate the use of existing materials in new ways for exercising nanoscale control over this interface, with an eye towards understanding their individual and collective charge transport behavior. Chapter 1 reviews the state of the art in control over the threshold voltage of OFETs, of which two methods—dipolar self-assembled monolayers (SAMs) and electrostatic poling—are considered in the subsequent chapters. Chapter 2 details the use of SAMs of dipolar alkylsilanes as a surface treatment for tuning VT, reducing leakage currents, and improving switching efficiency. Increases in field-effect transconductance in SAM-treated OFETs are shown to be consistent with the presence of additional surface states. Chapter 3 details an approach to decouple the relative contributions of the insulator/SAM and SAM/OSC interfaces from the capacitive responses of the OFET multilayer, and is compared to recent theoretical predictions of increased energetic disorder in SAM-treated OSC layers. Increased mobility of equilibrium carriers as measured with charge extraction are compared to OFET measurements and are shown to further reinforce the notion that larger molecular dipoles contribute to enhanced carrier transport through changes in the energetic disorder at the insulator/OSC interface. In Chapter 4 electrostatic poling, or gate stressing, of lateral OFETs is explored. A Poisson’s equation model is applied to surface potential images of stressed lateral OFETs and shown to accurately predict the observed threshold voltage shift. Lastly, Chapter 5 presents future directions for the study of SAM-treated interfaces using charge extraction, with a focus on the use of SAMs as remedial layers for marginal quality OSCs. In addition, the potential of surface potential-derived charge densities for sensing applications is discussed.