Computational Modelling of Organic Bioelectronic Devices and Materials

Abstract: Bioelectronics being the intersection field between electronics and biology, aims to investigate the transduction between electronic signals and ionic signals within a biological environment. Organic materials such as conducting polymers are extensively utilized in the fabrication and development of bioelectronic devices due to their ability to conduct both electrons and ions. In addition, organic materials offer advantages compared to their inorganic traditional counterparts, including being flexible, solution processable and printable as it is an easy strategy for the fabrication process. These unique properties make organic conductor materials a good match for a wide range of organic bioelectronic applications such as organic transistors and biosensors interacting with biological/physiological systems and pave the way for more developments in the in the state-of-the-art technology of organic bioelectronics. Many of the organic bioelectronic devices function in contact with a biological system, usually an electrolytic medium, where mostly ionic transport occurs. Therefore, understanding the structural, morphological, and electronic properties of materials and devices used for organic bioelectronics applications is the topic of strong current interest.  This thesis is focused on two levels of computational investigations: studying bioelectronic devices and studying materials used for bioelectronic applications. The former includes modelling of electrolyte-gated organic field effect transistors (EGOFET), whereas the latter provides theoretical insights into morphological changes, ion injection, water intake, and self-assembly of conducting polymers. In the part of the thesis addressing the device modelling we first proposed an EGOFET model based on the Nernst-Planck-Poisson equations to describe, on equal footing, both the polymer and the electrolyte regions within the device. Using the developed model, we modelled and analysed experimentally measured current–voltage characteristics of the device (the output and transfer curves), where a semi-qualitative agreement between the experimental and calculated results was achieved. In a follow-up study, we demonstrated that Nernst-Planck-Poisson modelling represents a powerful tool allowing quantitative device design, modelling and analysis enabling us to forecast the influence of geometrical parameters as well as the materials used as electrolyte and the organic semiconductor for the case of a printed EGOFET.  To explore the ion exchange phenomena at the interface of conducting polymers with aqueous electrolytes, we provided a detailed atomistic understanding of the water intake, swelling, and ion injection during cyclic voltammetry. By combining the molecular dynamics simulations with experimental measurements such as e-QCM (electrochemical quartz crystal microbalance), UV−VIS−NIR absorption spectroscopy, and XPS (X-ray photoelectron spectroscopy) we demonstrated that the PEDOT:Tos film underwent significant changes in morphology and mass during the redox processes. Finally, we studied the self-assembly of polythiophene based polymers with glycol and alkyl side chains deposited on the gold surface. Using molecular dynamics simulations, we investigated the diffusion of the molecules and analysed their conformations.  We explored how different side chains interact with each other and how they influence the conjugated polymers self-assembly.  We believe that the knowledge we acquired from our studies, combining experimental investigations with computational insights, provided an important understanding of the fundamental molecular processes at the material and device level that could help a practical enhancement in the field of organic bioelectronics.