Theory and Modeling for Organic Spintronics and Electronics
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Theory and Modeling for Organic Spintronics and Electronics
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2017-10
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Abstract
Organic semiconductor devices have attracted many scientists' interests due to their flexibility, low cost, ease of fabrication, and especially the significant advantage of long spin relaxation time, which open up realms of applications in spintronic devices. In addition, ionic liquids have drawn attention due to their suitability for high capacitance organic field effect transistors. Theoretical work aims at better understanding the physical mechanisms as well as providing guidance for future device design and development. In this dissertation, device theories, models, and calculations for various types of organic spintronic devices and organic field effect transistors are developed and presented. The first spintronic device we researched was a spin valve with a bulk layer of organic semiconductor sandwiched between ferromagnetic contacts. A device model for tunnel injection and extraction of spin-polarized charge carriers between ferromagnetic contacts and organic semiconductors with disordered molecular states is presented. Transition rates for tunneling are calculated based on a transfer Hamiltonian. Transport in the bulk semiconductor is described by macroscopic device equations. Tunneling predominantly involves organic molecular levels near the metal Fermi energy, and therefore typically in the tail of the band that supports carrier transport in the semiconductor. Disorder-induced broadening of the relevant band plays a critical role for the injection and extraction of charge carriers and for the resulting magneto-resistance of an organic semiconductor spin valve. Separate from the traditional spin valve, a large room-temperature magneto-resistance is also observed for devices composed of self-assembled monolayers of different oligophenylene thiols sandwiched between gold contacts. The transport mechanism through the organic molecules was determined to be nonresonant tunneling. To explain this kind of magneto-resistance, we develop an analytical model based on the interaction of the tunneling charge carrier with an unpaired charge carrier populating a contact-molecule interface state. The Coulomb interaction between carriers causes the transmission coefficients to depend on their relative spin orientation. Singlet and triplet pairing of the tunneling and the interface carriers thus correspond to separate conduction channels with different transmission probabilities. Spin relaxation enabling transitions between the different channels, and therefore tending to maximize the tunneling current for a given applied bias, can be suppressed by relatively small magnetic fields, leading to large magneto-resistance. Our model elucidates how the Coulomb interaction gives rise to transmission probabilities that depend on spin, and how an applied magnetic field can inhibit transitions between different spin configurations. Based on the above model of non-magnetic metal/molecule/non-magnetic metal (M/molecule/M), we further investigate a spintronic junction with ferromagnetic metal/molecule/ferromagnetic metal (FM/molecule/FM) structure. The transmission probability of an electron in the emitter contact depends on its spin, i.e. on its population of the four initial states formed with the unpaired interface state electron, and on the parallel or anti-parallel configuration of the two contacts. Four transport channels exist for both parallel and anti-parallel polarization of the contacts. However these channels do not correspond simply to singlet and triplet configurations, which makes the transport process more complicated. Spin relaxation enables transitions between the four different states and competes with the transmission rate to control the magnetoresistace. The FM/molecule/FM structure is compared with the simple Julliere model that does not include any interaction between electrons. When the incident and extracted electrons have the same spin direction and the transmission rates are the same for all channels, our new model reverts to a single particle model, which addresses the same physical mechanism as the Julliere model. Although ionic liquids (ILs) have been used extensively in recent years as high-capacitance "dielectrics" in electric double layer organic field effect transistors to substitute for the conventional gate dielectrics, the dynamics of the double layer formation have remained relatively unexplored. In this dissertation, we explore the dynamical characteristics of an IL in a metal/ionic liquid/metal (M/IL/M) capacitor. An equivalent circuit model is developed to explain the experimental results of impedance vs frequency data and the model is subsequently verified by calculating the current vs voltage characteristics for the applied potential profiles. The data analysis indicates that the dynamics of the structure are characterized by a wide distribution of relaxation times spanning the range of less than microseconds to longer than seconds. Possible causes for these time scales are discussed.
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University of Minnesota Ph.D. dissertation. October 2017. Major: Electrical Engineering. Advisor: P. Paul Ruden. 1 computer file (PDF); xv, 121 pages.
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Shi, Sha; Shi, Sha. (2017). Theory and Modeling for Organic Spintronics and Electronics. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/201511.
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