title: Well-Controlled Synthesis and Applications of Polythiophenebased Mixed Ion-Electron Conductors
Creators: name: Schmode, Philip Armin
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Well-Controlled Synthesis
subject: Polythiophenebased Mixed Ion-Electron Conductors
subject: 500
subject: 530
subject: 540

description: The majority of organic mixed ion-electron conductors (OMIEC) are polar/ionic semiconducting polymers, which are able to conduct both ions and electronic charges. This class of materials is highly attractive for bioelectronic applications in aqueous media, where such a mixed conduction is required. The aim of this thesis was on the one hand the controlled-synthesis and characterization of a new generation of watercompatible OMIECs based on the parent conjugated polyelectrolyte poly[6-(thiophen- 3-yl) hexane-1-sulfonate tetraalkyl ammonium] (PTHS-M+), which is water soluble. On the other hand, the relevant properties like electronic charge carrier transport, volumetric capacitance, swelling and stability in aqueous solution were studied in detail and the interplay of these properties was elucidated. The first two chapters of this thesis are dealing with the analysis of the crystal structure, the aggregation and the hole mobility of the precursor polymer, poly(3-(6-bromohexyl)- thiophene) P3BrHT, as well as copolymers consisting of both P3HT and P3BrHT. P3BrHT is the starting material for all ω-functionalized polythiophenes; eg. PTHS-M+. Therefore, the copolymers P3HT-co-P3BrHT 1,2 and 3 with different copolymer compositions were studied using SAXS/WAXS, absorption spectroscopy and by testing as active materials in OFETs. In this study, we elucidate the tremendous impact of side chain functionalization of conjugated p-type polymers on aggregation, crystallinity and charge transport; all these properties increasing with the increasing content of 3HT. Additionally, we investigated crystal orientation of P3BrHT and P3HT on different substrates. Molecular orientation is an important aspect of improving the charge transport within organic electronic devices. The results evidence that P3HT films of various thicknesses on graphene show mixed edge-on and face-one crystal orientation with edge-on crystals formed. In contrast, P3BrHT on graphene has solely face-on oriented crystals in thin films up to a particular film thickness. In order to design new OMIECs, we used different strategies. The first concept involves copolymerization of functional comonomers, to improve the properties and the performance of PTHS-M+ in bioelectronic devices. We incorporated 3-hexylthiophene (3HT) and THS-TMA+ as comonomers, because of the excellent hole transport properties of P3HT and water compatibility as well as good ion transport of PTHSTMA+. For this, we converted the above-described copolymers, P3HT-co-P3BrHT to PTHS-TMA+-co-P3HT using a post polymerization method. Upon testing the copolymers PTHS-TMA+-co-P3HT in OECTs, we observed several improvements like reduced threshold voltage, higher stability in contact with water, increased hole mobility and high ion transport properties, especially for the copolymer with 51:49 mol% ratio for 3HT: THS-TMA+. In a second strategy, polar polythiophenes carrying polar groups as side chains, were synthesized in a well-controlled manner and investigated. Here, different diethylene glycol functionalized polythiophene derivatives were prepared, which differ only in the nature of side-chain linkage on 3-position of thiophene moiety. We investigated three such polythiophenes, equipped with diethylene glycol side chains, with no spacer (P3MEET), with a methyl (P3MEEMT) and an ethyl spacer (P3MEEET). Among the three, P3MEEET showed the highest crystallinity (~58 %), best hole transport mobility (0.005 cm2/Vs) and best performance in OECTs. Also, P3MEEET shows the highest reported water uptake (84 % swelling) in the oxidized state in p-type polymers, as determined with E-QCMD experiments. In depletion-mode OECT devices, the most used p-type semiconductor is still PEDOT:PSS, due to the commercial availability and the extraordinary conductivity of these doped systems. But PEDOT is insoluble in organic solvents and therefore the PSS-doped system is used as a dispersion in aqueous/alcohol medium. However, there is a need for a highly soluble, highly ion conducting, and easily oxidizable PEDOT polymer which can be used in accumulation mode devices. So, the eighth chapter of this thesis deals with the design and synthesis of a highly soluble EDOT monomer with a swallow-tail substituent and its controlled polymerization using KCTP. With this newly designed monomer, we achieved highly soluble PEDOT polymers up to a molecular weight of 10 kg/mol and low polydispersity. Furthermore, the polymer can be oxidized in presence of an aqueous electrolyte, which makes the polymer highly attractive for bioelectronic applications. In summary, my research work gives fundamental inputs towards the understanding of what is necessary to design and tune new organic mixed ion-electron conductors. This necessitates the understanding of a complex interplay of different and often conflicting parameters in conjugated polymers which determine the mixed conductivity in general. Especially, this knowledge offers tools towards the application-related demands to realize high performance materials. Thus, these materials should show an optimum balance between high ionic and electronic charge transport. A compromise is needed since the ion conduction requires hydration and swelling of the polymer in aqueous media, whereas efficient electron transport is generally observed in highly crystalline hydrophobic conjugated polymers. Additionally, polymers for OECT applications should exhibit low ionization potentials and undergo oxidation in contact with the aqueous analyte at low applied voltages, because for the application as biosensors one must avoid the electrolysis of water at higher gate voltages (&gt; 1 V). Also, an optimum swelling of the oxidized polymer in contact with the aqueous analyte solutions is required. This is important, because a too poor water and ion uptake inhibit ion transport through the polymer bulk, whereas a too high degree of swelling disrupts the intermolecular electronic hopping mechanism, thus lowering the effective electronic transport properties. I demonstrated that this complex demand can be fulfilled by the tool of copolymerization, the diligent choice of polar side chains and the copolymer composition. Thus, this work extends and improves the existing use of conjugated polyelectrolytes and polar polymers in OECTs.