Development of High-Performance/Multifunctional Bioelectronic Devices via Optimization of Organic Mixed Ionic – Electronic Conductors Morphology and Device Structure

Abstract

Organic mixed ionic-electronic conductors (OMIECs), capable of transporting both ions and electrons, have garnered significant attention as active layers in bioelectronic devices, neuromorphic systems, and energy storage applications. The unique composition of OMIECs structure, which combines conjugated polymer backbones with water-swellable polymer side chains or additives, enables the transport of both electronic (electrons and holes for n- and p-type materials) and ionic charges (anions and cations) within the material structure. However, despite the numerous advantages of OMIECs in advanced applications, their operation in electrolyte environments requires careful consideration of swelling behavior and ionic properties. This imposes limitations on the applicability of strategies commonly employed for conventional electronic materials and devices. Additionally, while these unique properties can potentially influence device architecture, there have been relatively few attempts to explore this aspect comprehensively. In this respect, this thesis presents a comprehensive strategy for the development of high-performance and multifunctional devices based on OMIECs, encompassing rational molecular design to device aspects that utilizing OMIECs unique properties. Chapter 1 provides an overall understanding of organic-based bioelectronics, highlighting the fundamental characteristics of OMIECs and derived applications. Additionally, it delves into the parameters affecting the structure and performance of organic electrochemical transistors (OECTs) from both the perspective of the active layer and device structure. Lastly, it explains the basic theory of OMIECs' ion trapping characteristics and their utilization in neuromorphic devices, along with existing strategies. PART 1 (Chapter 2 and Chapter 3) focus on enhancing OECTs through material innovation and improved charge transport. Chapter 2 introduces advantages of oligo(ethylene glycol) (OEG) side chains for improved solubility and polymer packing based on naphthalene diimide (NDI) polymers OMIECs with asymmetrically branched OEG side chains. The amphiphilic nature of these polymers solvated selectively by ethanol/water mixtures, results in highly ordered polymer packing with an edge-on orientation, enhancing lateral charge transport. These findings provide valuable insights for designing n-type OMIECs and highlight the potential of aqueous processing in OECTs. While Chapter 3 focuses on improving the steady-state performance of OECTs by integrating molecular design and structural alignment approaches. This study introduces a method for controlling crystal orientation of OMIEC film which poly(diketopyrrolopyrrole) (PDPP) polymer with aliphatic-glycol hybrid side chains of various lengths. The floating film transfer method, which has never been used before in OMIEC research, further enhances structural ordering, significantly boosting the figure-of-merit to over 800 F V−1 cm−1 s−1. These findings underscore the critical role of side chain engineering and anisotropic microstructural optimization in maximizing OECT performance. Part 1 emphasize enhancing electrochemical stability and achieving better charge transport properties through ordered polymer packing and reducing energetic disorder. PART 2 (Chapter 4 and Chapter 5) focus on advancing the practical applications of OECTs through innovative design and optimization. In Chapter 4, we propose a vertical type OECT device incorporating ion-permeable electrodes, which can be fabricated through simple metal thermal evaporation, provide both the rapid operational speed due to vertical ion injection found in traditional planar type OECTs and the high steady-state performance characteristic of vertical OECTs. We have determined the thickness at which the ion-permeable electrodes are formed through surface analysis and confirmed their ion permeability through electrochemical analysis. The fabricated vertical type OECTs demonstrate high electrical performance as well as transient response comparable to that of planar type devices. Chapter 5 introduces a vertically structured hybrid active layer to develop solution-processed CMOS-like electronic circuits operating at low voltages. The active layer consists of an ion-permeable p-type OMIEC polymer and a n-type solution-processed metal-oxide semiconductor serving as an ambipolar active layer. The vertically structured organic-inorganic hybrid layer leverages the ion permeability of OMIECs to facilitate ion access to the metal-oxide semiconductor layer, enabling ambipolar operation through electron and hole transport mechanisms. The optimized hybrid inverters demonstrate high performance with a voltage gain exceeding 20 under a driving voltage of only 0.7 VDD and an inverted point around 0.4 Vinv, highlighting their potential for low-voltage electronic circuits. Part 3 (Chapter 6) introduces a novel OMIEC-based neuromorphic device fabricated through simple channel patterning, capable of exhibiting synaptic properties with controllable potentiation and depression characteristics via drain voltage sequence adjustments. The study demonstrates the significant influence of electrolyte selection, particularly anion size, on the long-term potentiation (LTP) and short-term potentiation (STP) properties. By optimizing bias sequences and electrolyte types, stable and repeatable synaptic cycles were achieved, essential for reliable neuromorphic device performance. This study underscores the versatility of OMIEC-based synaptic devices, demonstrating their potential for tailored LTP and STP characteristics through simple structural and process adjustments, paving the way for advanced neuromorphic computing systems and complex electronic applications. In summary, this study presents a comprehensive range of strategies across six chapters to enhance the performance and multifunctionality of next-generation OMIEC-based bioelectronics, from the molecular structure of the active layer to device architecture. From a molecular structure perspective, improving the crystal structure and controlling the crystal orientation of OMIECs are introduced as effective methods for enhancing electrical properties. From a device configuration perspective, the unique ion transport properties of OMIECs are leveraged to develop multifunctional bioelectronic devices. These strategies provide inspiration for the future development of next-generation OMIEC-based bioelectronic devices.