The Influence of Crystallinity and Charge Carrier Concentration on Electrical/Electrochemical Properties of Polymeric Mixed Ionic - Electronic Conductors

Abstract

Bioelectronics has recently been in the spotlight, which can be used for healthcare devices such as biosignal recording, biomolecule sensors, or neuromorphic transistors utilized for highly efficient computing by mimicking the computation mechanism of neural network. The core of these function is stemmed from the active layer, organic mixed electronic-ionic conductors (OMIECs) which has 'mixed conduction' characteristics conducting both electrons and ions simultaneously. Mixed conduction makes material be able to transduce electronic signal to ionic signal or oppoiste way, by employing reodx system of conjugated polymeric system. Therefore, the understanding of this unique feature of OMIECs would be robust foundation for designing new materials or advanced bioelectronics. To deeply understand the 'mixed conduction', it would be the first step that studyingabout the most fundamental elements of conduction, the charge carrier concentration and mobility of both electron and ion on the OMIECs and steady-state/transient behavior of electronic devices with OMIECs by using different OMIECs systems and characterization tools. The first part of this thesis mainly addresses the effect of crystallinity on electronic charge transport by investigating the hole mobility, ionic mobility, and ion-electron transduction efficiency of different OMIEC systems while controlling relative degree of crystallinity (crystalline volume) and crystallite size. There are two representative polymer system used for this study, which are poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polypyrrole. Chapter 1 is the study on the charge transport characteristics of PEDOT:PSS with varying crystallinity by adjusting the concentration of sulfuric acid treatment. The crystallographic and morphological analysis show that the molecular arrangement in less-crystallized regime is pivotal role as much as crystallinity parameters such as crystallite size, crystalline volume, and molecular orientation relative to the substrate. Chapter 2 is the investigation on the role of each crystallinity parameters of OMIECs system by using large-range crystallinity controllable polypyrrole system prepared by using two-monomer-connected precursor (TMCP). Varying equivalent of pyrrole monomer and connector, it is found that there are different range of crystallinity determining different crystallinity parameters, and each parameters affects different charge transport characteristics. The second part of the thesis focuses on the ion transport in OMIECs system, which attracts much attention due to the rising of biosignal recorders or neuromorphic transistors showing frequency-dependent characteristics. Chapter 3 is dealing with the ion-injection-direction-dependency of OMIECs, by controlling dominant molecular orientation and ion injection direction using synthetic chemistry and new device patterning methods. The results suggest that the ion injection direction relative to the polymer backbone face influences the ion mobiltiy and transient response of organic electrochemical transistors (OECTs). Chapter 4 is comprehensive analysis of PEDOT:PSS system neutralized by polyvinyl-alcohol-based crosslinking, which explore the effect of hole concentration on both steady-state and transient character of OMIEC system and OECT configuration. Furthermore, the way of neutralizing PEDOT:PSS is bio-compatible, which is practical way to finely tune the mixed conduction characteristics of bioelectronics. From those in-depth analysis of mixed conduction of OMIEC system with diversity, this work not only give insight for the synthetic chemistry researchers, but also device engineers who utilize various mixed conduction phenomenon observed from mixed conducting materials.