Duan, Zhongnan
(2024)
Conducting PEDOT: Carrageenan and Related Organic Electronic Materials.
PhD thesis, University of Nottingham.
Abstract
This thesis describes the synthesis of new materials for potential use as organic electronic semiconductors. After an introductory chapter, describing their underlying fundamental science and challenges for future development, Chapter 2 focuses on the preparation of PEDOT:Carr materials. These colloidal composites are analogues of the well-known PEDOT:PSS in which the poly(3,4-ethylenedioxythiophene) (PEDOT) p-type conductor is retained, but the polystyrene sulfonate anionic component is replaced by a seaweed-derived natural sulfonate polysaccharide from the Carrageenan (Carr) family. Either κ-carrageenan (possessing one ROSO3- unit per disaccharide, where R indicates the carbohydrate), λ-carrageenan (with three ROSO3- unit per disaccharide), or a mixture of these were used. The Carr used were commercial food gelling agent grades with molecular weights in the low molecular weight.
Polymer composites are attained when 3,4-ethylenedioxythiophene (EDOT) is polymerised by addition of FeCl3 in the presence of κ-Carr or λ-Carr in aqueous solution at 50-70 °C. The following stoichiometric ratios of EDOT:Carr:FeCl3 were investigated: 1:1:1, 4:1:1, 5:1:1, 7:1:1, 8:1:1 and 16:1:1. The incorporation of κ-Carr or λ-Carr during the polymerisation is confirmed in the final composite through infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS), as well as matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS). The resulting materials exhibit commendable conductivity (between 1 and 14 S cm-1) when processed into thin films (ca. 2.5 µm thick) from suspensions by drop-casting and additionally demonstrate thermoelectric properties (Seebeck coefficients of 5.5-11.5 µV K-1), rendering them suitable for diverse applications.
Chapter 3 focuses on the PEDOT-Carr+pDPP materials which incorporates a water-soluble diketopyrrolopyrrole polymer (pDPP) as an additive. Two approaches are presented: layer by layer approach (LBL) and mixing suspensions of composites. The latter is explored in detail. The following weight ratios of pDPP to PEDOT-Carr+pDPP composites are investigated: 2%, 4%, 6%, 9% and 50%. The conductivity of the composites is marginally lower than that of the pure PEDOT-Carr materials, despite enhanced thin film morphologies as demonstrated by SEM imaging. This is related to the decrease of PEDOT radicals which is evidenced by UV-Vis spectra. Conductivity of both pure PEDOT-Carr and PEDOT-Carr+pDPP produces a positive temperature coefficient. The thermoelectric properties are greatly improved approximately double that of pure PEDOT-Carr. 6% PEDOT-κ-Carr+pDPP (σ=3.65 S cm-1, Ⴝ = 17.6 μV K-1) and 4% PEDOT-λ-Carr+pDPP (σ= 10.96 S cm-1, Ⴝ = 20.0 μV K-1) materials strike a balance between conductivity and thermoelectric properties showing they are probably suited to thermoelectric device applications.
Chapter 4 begins by utilising PEDOT-Carr as p-type materials in the fabrication of two types of thermoelectric generators (TEGs). Fabricating PEDOT-λ-Carr Fraction E as p-legs and BBL:PEI as n-legs give a three np junction TEG with a Seebeck coefficient of 591.1 μV K-1 for the first configuration (Structure 1). The TEG with same p and n-legs results in a Seebeck coefficient to 1158 μV K-1 for three junctions of the second structure (Structure 2). This demonstrates that PEDOT-Carr is a promising candidate for utilizing in thermoelectric devices.
The research on Zolon blue chemistry is to explore the possibility using this material as the electrode in the organic electronic devices. Comprehensive crystallographic analysis and nuclear magnetic resonance (NMR) spectroscopy is used to investigate the chemical structure of both Zolon blue and Zolon red. These studies have provided corrections to existing literature, enhancing our understanding on their molecular configurations. This insight is crucial for optimizing the integration of Zolon blue into organic electronics, potentially leading to advancements in device performance and efficiency.
Electrical conductivity is a pivotal factor throughout this research. To address challenges related to contact resistance and the inhomogeneity of drop-casted films, we have employed four-point probe technology. This method significantly reduces the impact of contact resistance and provides a more accurate measurement of the intrinsic electrical conductivity of the materials under study, thereby ensuring the reliability of our findings. This thesis significantly advances the field of thermoelectric applications through an in-depth exploration of organic materials and their properties when formed into solid-state films. The investigation into surface morphology, electrical conductivity, and spectroscopic characteristics of these materials has been crucial in exploring their functional roles within innovative and sustainable organic electronic devices. Such insights are instrumental in harnessing the full potential of organic materials for future technological applications, marking a step forward in the development of high-performance, eco-friendly electronics.
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