Design, Fabrication, and Characterization of Conjugated Polymeric Electrochemical Memristors as Neuromorphic/Integrated Circuits
Date of Award
Doctor of Philosophy (PhD)
Materials Science and Engineering
Stephen H. Foulger
Organic materials are promising candidates for future electronic devices compared to the complementing inorganic materials due to their ease of processability, use, and disposal, low cost of fabrication, energy efficiency, and flexible nature toward implementation as flexible and non-conformal devices.With that in mind, electrochemical materials have been widely demonstrated with commercial use as sensors, displays, and a variety of other electronic devices. As Moore's law predicts the increase in the density of transistors on a chip, the requirement to create either smaller transistors or the replacement of the transistor device entirely is apparent. Memory resistors, coined ``memristor", are variable resistive tuning devices that are capable of information processing and data storage in one device. This work focuses on the embodiment of a non-volatile conjugated polymeric electrochemical memristor. Three-terminal memristive systems are fabricated and studied using various electrochemicals (a self-doped PEDOT derivative, a polypyrrole, and a dithienopyrrole derivative) and are tested for their electronic properties and biomimicking capabilities. Optical absorbance properties are studied in order to verify the electrochemical material's redox tuning potential for their respective oxidized and reduced chemical forms. The three-terminal device employed a post-synaptic ``read'' channel where conductivity of the electrochemical material was equated to synaptic weight and was electronically decoupled from the pre-synaptic programming electrode by means of a polymeric gel electrolyte. Basic electronic characteristics are exhibited for these three devices such as state stability and retention, non-volatile voltage-driven conductivity tuning, input parameter characteristic trends, and power consumption per input program. Biological synapses consume, on the order of, 1 - 100 fJ of energy per synaptic energy. The electrochemical materials used in this study, at their most optimized input parameters, were capable of demonstrating a 4.16 fJ/mm2 power consumption per input pulse and lead to a promising candidate for implementation as future artificial neural networks. Biological mimicry was displayed for these devices in the form of paired-pulse facilitation and paired-pulse depression, both a form of short term memory which observes the effect the timescale between two incoming inputs has on the change in the final output signal. Toward the indication for the replacement of transistors with three-terminal memristors, basic circuit operations are achieved and demonstrated for these devices. These operations include both Boolean and elementary algebra, key features that demonstrate data processing and storage in-memory where the physical states of the conjugated polymer film represent either logical statements or arithmetic counting variables. The Boolean algebra demonstrated the use of a single memristive device equal to a variety of single logic gates (AND, NAND, OR and NOR) where, by wiring several devices in series, more advanced combinational logic gates can be achieved. Furthermore, each device was capable of displaying elementary algebra for the basic arithmetic functions of addition, subtraction, multiplication, and division. In regards to thin film deposition techniques, the self-doped PEDOT device employed roll-to-roll gravure printing, a high speed and high resolution commercially used deposition technique. The polypyrrole device was fabricated implementing an in-situ polymerization technique, referred to as vapor phase polymerization, and demonstrated the use of this technique toward non-conformal devices. The dithienopyrrole derivative was polymerized through the same vapor phase polymerization technique as the polypyrrole and used in tandem with screen printing in order to construct the final device, including the oxidant film, the silver electrodes, and the polymeric gel electrolyte.
Grant, Benjamin, "Design, Fabrication, and Characterization of Conjugated Polymeric Electrochemical Memristors as Neuromorphic/Integrated Circuits" (2022). All Dissertations. 3214.
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