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1. Impact of varying side chain structure on organic electrochemical transistor performance: a series of oligoethylene glycol-substituted polythiophenes

   https://doi.org/10.1039/D2TA00683A  

   Chen, Shinya E.; Flagg, Lucas Q.; Onorato, Jonathan W.; Richter, Lee J.; Guo, Jiajie; Luscombe, Christine K.; Ginger, David S. (May 2022, Journal of Materials Chemistry A)

   The electrochemical doping/dedoping kinetics, and the organic electrochemical transistor (OECT) performance of a series of polythiophene homopolymers with ethylene glycol units in their side chains using both kosmotropic and chaotropic anion solutions were studied. We compare their performance to a reference polymer, the polythiophene derivative with diethylene glycol side chains, poly(3-{[2-(2-methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl) (P3MEEMT). We find larger OECT material figure of merit, μC *, where μ is the carrier mobility and C * is the volumetric capacitance, and faster doping kinetics with more oxygen atoms on the side chains, and if the oxygen atom is farther from the polythiophene backbone. Replacing the oxygen atom close to the polythiophene backbone with an alkyl unit increases the film π-stacking crystallinity (higher electronic conductivity in the undoped film) but sacrifices the available doping sites (lower volumetric capacitance C * in OECT). We show that this variation in C * is the dominant factor in changing the μC * product for this family of polymers. With more oxygen atoms on the side chain, or with the oxygen atom farther from the polymer backbone, we observe both more passive swelling and higher C *. In addition, we show that, compared to the doping speed, the dedoping speed, as measured via spectroelectrochemistry, is both generally faster and less dependent on ion species or side chain oxygen content. Last, through OECT, electrochemical impedance spectroscopy (EIS) and spectroelectrochemistry measurements, we show that the chaotropic anion PF 6 − facilitates higher doping levels, faster doping kinetics, and lower doping thresholds compared to the kosmotropic anion Cl − , although the exact differences depend on the polymer side chains. Our results highlight the importance of balancing μ and C * when designing molecular structures for OECT active layers. 

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2. Understanding the Effect of Trace Solvent Content on Properties of Polymer Electrolytes through Molecular Dynamics Simulations

   https://doi.org/10.1149/MA2023-014862mtgabs  

   Asha, Aysha Siddika; Visayas, Benjoe_Rey B; Mayes, Maricris L; Shen, Caiwei (August 2023, ECS Meeting Abstracts)

   The rapid growth of mobile, portable, wearable and flexible electronics leads to the increasing demand for energy storage devices using solid-state polymer electrolytes (PEs), which outperform liquid electrolytes in terms of safety, mechanical properties, and simplicity of device fabrication and packaging. However, processing PEs will always introduce solvent molecules that greatly affect the ionic conductivity and mechanical properties. For example, PEs prepared through solution-casting methods always have solvent residues. A trace amount of water molecules absorbed from the air is also inevitable. Recently, we demonstrated the controlled introduction of solvent molecules to PEs to balance the ionic conductivity and mechanical stiffness for structural energy storage applications. To better understand how solvent molecules behave and interact with other components in PEs, here we present the molecular dynamics simulation of a representative polymer electrolyte system with various water content. We use simulation results to determine the effect of trace water content before forming a liquid phase on ionic conductivity and mechanical properties. The insights into the molecular interactions in the PE system will help us design and optimize Pes’ composition and processing for practical applications. The simulation model of polymer electrolyte is built with polyethylene oxide (PEO) and lithium perchlorate (LiClO₄) with various water contents, in which the water molecule to lithium-ion ratio ranges from 0 to 3. The electrolyte with each water content is simulated between two graphene electrodes to determine its ionic conductivity. Uniaxial deformation has been performed on the electrolyte to obtain the mechanical properties. All simulations were performed using the molecular dynamics simulation code LAMMPS with the CHARMM force field. The results show that the ionic conductivity of the polymer electrolyte system increases significantly (up to one order of magnitude) with the increase of water content (up to 3 water molecules per lithium ion), even when the added water does not form a continuous liquid phase. The change of ionic conductivity with water content is correlated to the degree of association between different types of ions or molecules in the system, as evidenced by the evaluation of the radial distribution functions. As the association between polymer molecules and lithium ions reduces with increasing water, it becomes easier for the lithium ions to diffuse and resulting in higher ionic conductivity. It is also observed that the perchlorate ions’ interactions with polymer molecules remain the same with different water contents, which shows different roles of lithium ions and perchlorate ions in ion conduction in this system. On the other hand, the modulus of elasticity of the polymer electrolyte does not change much with the increase of water, which agrees with the previous experimental work of our group. This means that the trace amount of water is strongly associated with other solid molecules or ions and is not affecting the stiffness of the system as long as no liquid phase is formed. The results will lead to novel strategies to design polymer electrolytes with both high ionic conductivity and good mechanical properties for flexible or multifunctional energy storage applications. 

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3. An overview of oxygen vacancy dynamics in (1 − x )(Bi _1/2 Na _1/2 )TiO ₃ – x BaTiO ₃ solid solution

   https://doi.org/10.1039/d1tc02668b  

   Fan, Zhongming; Randall, Clive A. (January 2021, Journal of Materials Chemistry C)

   null (Ed.)

   (Bi 1/2 Na 1/2 )TiO 3 (BNT) based ceramics have been the hot topic for a few years because of their multiple functions, from the piezoelectric properties to more recently the electrostatic energy storage performance. However, some basic issues are still unclear, preventing their wide application in real devices. One of them is the underlying conduction mechanism, the interplay of electronic and ionic carriers as a mixed ionic case and the subsequent quantification. This paper deals with the most basic compositions, which are the typical ones from the (1 − x )(Bi 1/2 Na 1/2 )TiO 3 – x BaTiO 3 (BNT– x BT) phase diagram. The conductivity is primarily investigated by impedance spectroscopy, while different equivalent circuits are applied to different conduction mechanisms. A transition from predominantly ionic to predominantly electronic conduction is revealed to occur with the increase in BaTiO 3 concentration. The mixed ionic–electronic conduction in the composition near the morphotropic phase boundary, namely BNT–7%BT, is identified and then quantified. To verify our interpretation of impedance results, dc degradation is, for the first time, conducted in this family of materials, from which the electronic and ionic conductions can be easily separated by accessing the mean time to failure. The successful combination of the two methods enables us to have an overview of how the oxygen vacancy dynamics in the BNT– x BT system depends upon the phase nature or the domain structure. 

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4. Accumulation and ordering of P3HT oligomers at the liquid–vapor interface with implications for thin-film morphology

   https://doi.org/10.1039/D3CP02718J  

   Sowa, Jakub K.; Allen, Thomas C.; Rossky, Peter J. (August 2023, Physical Chemistry Chemical Physics)

   The morphology of semiconducting polymer thin films is known to have a profound effect on their opto-electronic properties. Although considerable efforts have been made to control and understand the processes which influence the structures of these systems, it remains largely unclear what physical factors determine the arrangement of polymer chains in spin-cast films. Here, we investigate the role that the liquid–vapor interfaces in chlorobenzene solutions of poly(3-hexylthiophene) [P3HT] play in the conformational geometries adopted by the polymers. Using all-atom molecular dynamics (MD), and supported by toy-model simulations, we demonstrate that, with increasing concentration, P3HT oligomers in solution exhibit a strong propensity for the liquid–vapor interface. Due to the differential solubility of the backbone and side chains of the oligomers, in the vicinity of this interface, hexyl chains and the thiophene rings, have a clear orientational preference with respect to the liquid surface. At high concentrations, we additionally establish a substantial degree of inter-oligomer alignment and thiophene ring stacking near the interface. Our results broadly concur with the limited existing experimental evidence and we suggest that the interfacial structure can act as a template for film structure. We argue that the differences in solvent affinity of the side chain and backbone moieties are the driving force for the anisotropic orientations of the polymers near the interface. This finer grained description contrasts with the usual monolithic characterization of polymer units. Since this phenomenon can be controlled by concurrent chemical design and the choice of solvents, this work establishes a fabrication principle which may be useful to develop more highly functional polymer films. 

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5. Solution‐Doped Donor–Acceptor Copolymers Based on Diketopyrrolopyrrole and 3, 3′‐Bis (2‐(2‐(2‐Methoxyethoxy) Ethoxy) ethoxy)‐2, 2′‐Bithiophene Exhibiting Outstanding Thermoelectric Power Factors with p ‐Dopants

   https://doi.org/10.1002/adfm.202309646  

   Mukhopadhyaya, Tushita; Wagner, Justine; Lee, Taein D; Ganley, Connor; Tanwar, Swati; Raj, Piyush; Li, Lulin; Song, Yunjia; Melvin, Sophia J; Ji, Yuyang; et al (February 2024, Advanced Functional Materials)

   Katz, Howard (Ed.)

   Abstract The design of polymeric semiconductors exhibiting high electrical conductivity (σ) and thermoelectric power factor (PF) will be vital for flexible large‐area electronics. In this work, four polymers based on diketopyrrolopyrrole (DPP), 2,3‐dihydrothieno[3,4‐b][1,4]dioxine (EDOT), thieno[3,2‐b]thiophene (TT), and 3, 3′‐bis (2‐(2‐(2‐methoxyethoxy) ethoxy) ethoxy)‐2, 2′‐bithiophene (MEET) are investigated as side‐chains, with the MEET polymers newly synthesized for this study. These polymers are systematically doped with tetrafluorotetracyanoquinodimethane ( F₄TCNQ), CF3SO3H, and the synthesized dopant Cp(CN)₃‐(COOMe)₃, differing in geometry and electron affinity. The DPP‐EDOT‐based polymer containing MEET as side‐chains exhibits the highest conductivity (σ) ≈700 S cm−1 in this series with the acidic dopant (CF₃SO₃H). This polymer also shows the lowest oxidation potential by cyclic voltammetry (CV), the strongest intermolecular interactions evidenced by differential scanning calorimetry (DSC), and has the most oxygen‐based functionality for possible hydrogen bonding and ionic screening. Other polymers exhibit high σ ≈300–500 S cm−1 and power factor up to 300 µW m⁻¹K⁻². The mechanism of conductivity is predominantly electronic, as validated by time‐dependent conductance studies and transient thermo voltage monitoring over time, including for those doped with the acid. These materials maintain significant thermal stability and air stability over ≈6 weeks. Density functional theory calculations reveal molecular geometries and inform about frontier energy levels. Raman spectroscopy, in conjunction with scanning electron microscopy (SEM‐EDS) and x‐ray diffraction, provides insight into the solid‐state microstructure and degree of phase separation of the doped polymer films. Infrared spectroscopy enables this study to further quantify the degree of charge transfer from polymer to dopant. 

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