Dynamics of organic mixed ionic-electronic conductors
Keywords: Engineering.
Materials Science and Engineering.

Organic mixed ionic-electronic conductors (OMIECs) are a promising class of materials for numerous emerging technologies. The dual-conducting nature of OMIEC materials enables ionic signals to be transduced into electronic currents through the process of volumetric doping, where changes in ion concentration in the bulk of the material modulates the electronic conductivity of the semiconductor. The combination of this transduction and volumetric doping makes OMIECs attractive in a wide range of fields including energy storage, neuromorphic computing, and bioelectronics. Though OMIECs have been successfully utilized in electrochemical devices across many fields, the structure-property relations which govern material and device performance are still relatively immature. Critically, a robust understanding of the electrochemical doping processes which underpin the operation of OMIEC devices is necessary to inform chemical design. This dissertation encompasses my work studying state-of-the-art OMIEC devices and the fundamental mechanisms behind their operation. The first section of this dissertation will focus on OMIEC materials through the lens of devices for brain-like - or neuromorphic - computing. I will begin by explaining how OMIEC-based artificial synapses can be made solid-state and temperature-stable by leveraging a gel electrolyte based on ionic liquids. I will show how judicious selection of both OMIEC and ionic liquid leads to organic neuromorphic devices with switching speeds in the nanosecond regime. I will then demonstrate how these same ionic liquids can be encapsulated within a porous oxide to create a hybrid organic/inorganic electrolyte system which enables a vertically stacked device architecture compatible with integration into crossbar arrays. The next section of this dissertation seeks to better understand the interactions between the OMIEC materials studied in the first section and the molecular structure of the ionic liquid electrolyte. I show that PEDOT:PSS electrochemical devices uniquely require trace water in the electrolyte, whereas other single-phase OMIECs (e.g. p[g2T-TT]) do not. I show that that the trace water facilitates high speed switching in PEDOT:PSS neuromorphic devices through a proton hopping mechanism with PSS sites, and that the cation of certain protic ionic liquids can participate in this process. I then explore the origins of high-speed switching in other OMIEC materials such as p(g2T-TT) and reveal that the passive uptake of ion pairs by the OMIEC is essential for high-speed operation. Thus, the interactions between the semiconducting polymer and the electrolyte dictate not only the speed and performance of organic neuromorphic devices, but also the fundamental doping and transport mechanisms. The last section of this dissertation is concerned with investigating the structural changes which occur within OMIEC materials during operation. Electrochemical doping is a highly dynamic process, involving the motion of ionic charges, electronic charges, and lattice deformations of the semiconductor to accommodate these species. The structural transformations which occur in OMIECs during operation are not well characterized because of experimental limitations measuring the structure of these materials during electrochemical doping. The final section of my dissertation will present an operando grazing-incidence X-ray scattering technique which enables concurrent electrochemical and structural characterization of redox-active thin films at synchrotron beamlines. I will show how this technique can quantify structural changes during electrochemical doping and correlate the microstructural evolution to simultaneously measured electronic transport properties. The combination of a high-fidelity structural probe with simultaneous measurement of relevant electronic properties (e.g. carrier density and mobility) allows us to unambiguously determine how subtle changes to polymer chain conformation affect charge transport. I reveal a charge-induced ordering mechanism whereby the addition of charge carriers planarizes polymer chains within the crystallites which enhances intrachain order and increases the mobility of subsequent carriers. Then, in the final chapter, I will leverage this same operando X-ray scattering technique to investigate the microstructural stability of OMIEC materials during operation. I will reveal that high charge carrier densities degrade the crystalline structure of the materials, irrespective of molecular weight, producing irreversible disruptions to long-range order and charge transport.