Charge Transport in Field-Effect and Ion-Gel Gated Conjugated Polymers
Organic field effect transistors (OFETs) have recently made remarkable advancements, which have mostly been driven by the design, characterisation, and comprehension of new conjugated polymers. Significant effects can arise from making minor alterations to the monomers and building blocks, which impacts charge transport; however, some of mechanisms responsible for the effects have yet to be fully understood. There have been simultaneous interests in devising various arrangements of co-monomeric donor−acceptor (DA) copolymers. To create the next generation of high-performance organic semiconductors, it is essential that the relationship between structures and properties of conjugated systems are fully understood. To investigate diverse, customised conjugated polymers with a low degree of disorder, 15 derivatives of the benchmark low disorder polymer semiconductor IDT-BT were synthesised. To create the derivatives, a number of modifications were made to the acceptor unit, donor unit, molecular weight or side chains. The modifications resulted in two polymers, IDT-BS and TIF-BT, which when properly optimised, have the potential to perform better than IDT-BT. Thus, these novel polymers are suitable for use in flexible electronic devices. Moreover, the properties of semiconducting organic materials can be altered by chalcogen substitution. In this thesis, we not only explore the effects of chalcogens on the field-effect charge transport properties of such materials but also examine the effect of chalcogens upon doping. It is considered that interchain interactions are enhanced by introducing a single chalcogen atom into the backbone of the polymer. As a result, the doped samples adopt better electrical properties. In our opinion, the enhanced properties are the product of the heteroatom stimulating the formation of unique microstructures. We also investigate the impact on conductivity caused by three different doping methods (molecular doping/ ion exchange doping and electrochemical doping). We discovered that electrochemical doping based on organic electrochemical transistor devices is one of the most effective ways to continually modify the conductivities by applied voltage with incredibly high doping levels (one charge per monomer). The potential for future double-gating investigations is provided by this reversible and stable procedure. Furthermore, this thesis not only discusses the performance but also the charge transport mechanism of conjugated polymers. We provide a novel experimental method for directly probing the density of states close to the Fermi level without changing the shape of the density of states (DOS) based on dual-gated organic electrochemical transistors with an electrolyte top-gate and a dielectric bottom gate. This new technique suggests a strategy for modifying carrier concentration while simultaneously employing electrochemical and field-effect gating to calculate mobility in semiconducting polymers. This is due to the fact that, in contrast to electrochemical gating, field-effect gating is manageable and sufficient to enable the modulation of a defined population of carriers, leading to an accurate measurement of field-effect mobility. According to our knowledge, this double-gating research is the first to use this technique to investigate field-effect charge transport at various strongly doped states and link field-effect transport to the Seebeck coefficient's sign changes. Last but not least, we studied five low band-gap open-shell co-polymers. This project is evident that high temperature annealing has an impact on both mobility and microstructure, proving that interchain ordering is crucial for charge transport in open-shell polymers. Therefore, it can be assumed that increasing charge transfer at both the intrachain and interchain levels will improve both the hole and electron mobilities of open-shell polymers. As a result of our research, we think it may be possible to create more stable open-shell polymers with higher mobility that may provide new possibilities for investigating the physical phenomena associated with spin in polymer semiconductors.
Organic Field Effect Transistor, Conjugated Polymer
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