How Long are Polymer Chains in Poly(3,4-ethylenedioxythiophene):Tosylate Films? An Insight from Molecular Dynamics Simulations

Effective Model Reduction Scheme for the Electronic Structure of Highly Doped Semiconducting Polymers

Highly doped organic polymers have emerged as prominent candidates within novel technological disciplines, yet the fundamental correlation between structure and charge transport characteristics still remains missing. Toward this objective, an efficient model reduction scheme for highly doped polymer chains is developed considering the paradigmatic case of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS). The reduced model accounts for the chemical and structural details of the conducting polymer chain in addition to the long-range Coulombic interactions between charge carriers (holes) and dopant ions and the Coulombic repulsion between holes residing on the PEDOT chain. The model is shown to reproduce the intrachain hole-density profile of bulk polymer chains within a mean-field description. Furthermore, and critically, the model is adept at determining the energy distribution of doped PEDOT samples that in effect, influences the hole distribution among polymer chains. The hole distribution so obtained broadly upholds the approximation of a homogeneous charge-carrier distribution in doped polymers commonly found in the literature. In addition, it is observed that the spin configuration of the charge carriers dictates the energetics of the doped chains while it is a critical function of the chain length, carrier density, and disorder parameters.

Proton Conducting Neuromorphic Materials and Devices

Neuromorphic computing and artificial intelligence hardware generally aims to emulate features found in biological neural circuit components and to enable the development of energy-efficient machines. In the biological brain, ionic currents and temporal concentration gradients control information flow and storage. It is therefore of interest to examine materials and devices for neuromorphic computing wherein ionic and electronic currents can propagate. Protons being mobile under an external electric field offers a compelling avenue for facilitating biological functionalities in artificial synapses and neurons. In this review, we first highlight the interesting biological analog of protons as neurotransmitters in various animals. We then discuss the experimental approaches and mechanisms of proton doping in various classes of inorganic and organic proton-conducting materials for the advancement of neuromorphic architectures. Since hydrogen is among the lightest of elements, characterization in a solid matrix requires advanced techniques. We review powerful synchrotron-based spectroscopic techniques for characterizing hydrogen doping in various materials as well as complementary scattering techniques to detect hydrogen. First-principles calculations are then discussed as they help provide an understanding of proton migration and electronic structure modification. Outstanding scientific challenges to further our understanding of proton doping and its use in emerging neuromorphic electronics are pointed out.

An Electronic Structure Investigation of PEDOT with AlCl4- Anions-A Promising Redox Combination for Energy Storage Applications

In this work, we use density functional theory to investigate the electronic structure of poly(3,4-ethylenedioxythiophene) (PEDOT) oligomers with co-located AlCl4- anions, a promising combination for energy storage. The 1980s bipolaron model remains the dominant interpretation of the electronic structure of PEDOT despite recent theoretical progress that has provided new definitions of bipolarons and polarons. By considering the influence of oligomer length, oxidation or anion concentration and spin state, we find no evidence for many of the assertions of the 1980s bipolaron model and so further contribute to a new understanding. No self-localisation of positive charges in PEDOT is found, as predicted by the bipolaron model at the hybrid functional level. Instead, our results show distortions that exhibit a single or a double peak in bond length alternations and charge density. Either can occur at different oxidation or anion concentrations. Rather than representing bipolarons or polaron pairs in the original model, these are electron distributions driven by a range of factors. Distortions can span an arbitrary number of nearby anions. We also contribute a novel conductivity hypothesis. Conductivity in conducting polymers has been observed to reduce at anion concentrations above 0.5. We show that at high anion concentrations, the energy of the localised, non-bonding anionic orbitals approaches that of the system HOMO due to Coulombic repulsion between anions. We hypothesize that with nucleic motion in the macropolymer, these orbitals will interfere with the hopping of charge carriers between sites of similar energy, lowering conductivity.

Mobile Point-of-Care Device Using Molecularly Imprinted Polymer-Based Chemosensors Targeting Interleukin-1β Biomarker

Molecularly imprinted polymers (MIPs) have garnered significant attention as a promising material for engineering specific biological receptors with superior chemical complementarity to target molecules. In this study, we present an electrochemical biosensing platform incorporating MIP films for the selective detection of the interleukin-1β (IL-1β) biomarker, particularly suitable for mobile point-of-care testing (POCT) applications. The IL-1β-imprinted biosensors were composed of poly(eriochrome black T (EBT)), including an interlayer of poly(3,4-ethylene dioxythiophene) and a 4-aminothiophenol monolayer, which were electrochemically polymerized simultaneously with template proteins (i.e., IL-1β) on custom flexible screen-printed carbon electrodes (SPCEs). The architecture of the MIP films was designed to enhance the sensor sensitivity and signal stability. This approach involved a straightforward sequential-electropolymerization process and extraction for leaving behind cavities (i.e., rebinding sites), resulting in the efficient production of MIP-based biosensors capable of molecular recognition for selective IL-1β detection. The electrochemical behaviors were comprehensively investigated using cyclic voltammograms and electrochemical impedance spectroscopy responses to assess the imprinting effect on the MIP films formed on the SPCEs. In line with the current trend in in vitro diagnostic medical devices, our simple and effective MIP-based analytical system integrated with mobile POCT devices offers a promising route to the rapid detection of biomarkers, with particular potential for periodontitis screening.

Charge carrier dynamics in conducting polymer PEDOT using ab initio molecular dynamics simulations

As conducting polymers become increasingly important in electronic devices, understanding their charge transport is essential for material and device development. Various semi-empirical approaches have been used to describe temporal charge carrier dynamics in these materials, but there have yet to be any theoretical approaches utilizing ab initio molecular dynamics. In this work, we develop a computational technique based on ab initio Car-Parrinello molecular dynamics to trace charge carrier temporal motion in archetypical conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). Particularly, we analyze charge dynamics in a single PEDOT chain and in two coupled chains with different degrees of coupling and study the effect of temperature. In our model we first initiate a positively charged polaron (compensated by a negative counterion) at one end of the chain, and subsequently displace the counterion to the other end of the chain and trace polaron dynamics in the system by monitoring bond length alternation in the PEDOT backbone and charge density distribution. We find that at low temperature (T = 1 K) the polaron distortion gradually disappears from its initial location and reappears near the new position of the counterion. At the room temperature (T = 300 K), we find that the distortions induced by polaron, and atomic vibrations are of the same magnitude, which makes tracking the polaron distortion challenging because it is hidden behind the temperature-induced vibrations. The novel approach developed in this work can be used to study polaron mobility along and between the chains, investigate charge transport in highly doped polymers, and explore other flexible polymers, including n-doped ones.

Methods of poly(3,4)-ethylenedioxithiophene (PEDOT) electrodeposition on metal electrodes for neural stimulation and recording

Conductive polymers are of great interest in the field of neural electrodes because of their potential to improve the interfacial properties of electrodes. In particular, the conductive polymer poly (3,4)-ethylenedioxithiophene (PEDOT) has been widely studied for neural applications.Objective:This review compares methods for electrodeposition of PEDOT on metal neural electrodes, and analyses the effects of deposition methods on morphology and electrochemical performance.Approach:Electrochemical performances were analysed against several deposition method choices, including deposition charge density and co-ion, and correlations were explained to morphological and structural arguments as well as characterisation methods choices.Main results:Coating thickness and charge storage capacity are positively correlated with PEDOT electrodeposition charge density. We also show that PEDOT coated electrode impedance at 1 kHz, the only consistently reported impedance quantity, is strongly dependent upon electrode radius across a wide range of studies, because PEDOT coatings reduces the reactance of the complex impedance, conferring a more resistive behaviour to electrodes (at 1 kHz) dominated by the solution resistance and electrode geometry. This review also summarises how PEDOT co-ion choice affects coating structure and morphology and shows that co-ions notably influence the charge injection limit but have a limited influence on charge storage capacity and impedance. Finally we discuss the possible influence of characterisation methods to assess the robustness of comparisons between published results using different methods of characterisation.Significance:This review aims to serve as a common basis for researchers working with PEDOT by showing the effects of deposition methods on electrochemical performance, and aims to set a standard for accurate and uniform reporting of methods.

Morphology of conducting polymer blends at the interface of conducting and insulating phases: insight from PEDOT:PSS atomistic simulations

Having phase-separated conductive and less-conductive domains is a common morphology in semiconducting polymer blends as it exists in the case of PEDOT:PSS, which is a representative example with a wide range of applications. In this paper, we constructed atomistic models for the interface between the PEDOT-rich (conductive) grains and the PSS-rich (less-conductive) phase through molecular dynamics simulations. Our models are obtained from experimentally relevant compositions, based on precise force field parameters, and through a robust relaxation procedure. We show that both PEDOT-rich and PSS-rich phases consist of PEDOT lamellae embedded in PSS chains. The size of these lamellae depends on the PEDOT concentration in each phase and our model predictions are in quantitative agreement with the experimental data. Furthermore, our models suggest that neither the phases nor the interfaces are entirely connected by π-π stacking. Thus, inter-lamellae tunnelling is essential for both intra- and inter-grain charge transport. We also show that a small increase (≈8 wt%) in the PEDOT concentration results in rather larger lamellae sizes, considerably more oriented lamellae, and slightly better inter-lamellae connectivity, which result in enhanced intra-grain conductivity. Moreover, we show how enhancing phase separation between PEDOT-rich and PSS-rich domains (similar to the effect of polar co-solvents), i.e., pulling out PEDOT from the PSS-rich phase and adding it in the PEDOT-rich phase, highly enhances the intra-grain connectivity but decreases the inter-grain conduction paths through the interface. Our results explain how the marginal extra degree of phase separation (based on experimentally obtained values) could result in a great enhancement in the overall film conductivity.

Gaining control over conjugated polymer morphology to improve the performance of organic electronics

Conjugated polymers (CPs) are widely used in various domains of organic electronics. However, the performance of organic electronic devices can be variable due to the lack of precise predictive control over the polymer microstructure. While the chemical structure of CPs is important, CP microstructure also plays an important role in determining the charge-transport, optical and mechanical properties suitable for a target device. Understanding the interplay between CP microstructure and the resulting properties, as well as predicting and targeting specific polymer morphologies, would allow current comprehension of organic electronic device performance to be improved and potentially enable more facile device optimization and fabrication. In this Feature Article, we highlight the importance of investigating CP microstructure, discuss previous developments in the field, and provide an overview of the key aspects of the CP microstructure-property relationship, carried out in our group over recent years.

Understanding PEDOT doped with tosylate

Now in their 5th decade of research and development, conducting polymers represent an interesting class of materials to underpin new wearable or conformable electronic devices. Of particular interest over the years has been poly(3,4-ethylenedioxythiophene), commonly known as PEDOT, owing to its ease of fabrication and relative stability under typical ambient conditions. Understanding PEDOT from a variety of fundamental and applied perspectives is important for how it can be enhanced, modified, functionalised, and/or processed for use in commercial products. This feature article highlights the contribution of the research team at the University of South Australia led by Professor Evans, and their collaborators, putting their work into the broader context of conducting polymer research and application. This review focuses on the vapour synthesis of PEDOT doped with the tosylate anion, the benefits of controlling its morphology/structure during synthesis, and its application as an active material interacting with secondary anions in sensors, energy devices and drug delivery.

Why does solvent treatment increase conductivity of PEDOT:PSS? Insight from molecular dynamics simulations

Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is one of the most important conducting polymers. In its pristine form its electrical conductivity is low, but it can be enhanced by several orders of magnitude by...