Molecular Dynamics of PEDOT:PSS Treated with Ionic Liquids. Origin of Anion Dependence Leading to Cation Design Principles

Enhancing the Conductivity of PEDOT:PSS Films by the Confinement of Ice Crystals

Rapid developments in organic electronics demand highly conductive and freestanding (PEDOT:PSS) films. However, the synthesis of highly conductive PEDOT:PSS films requires toxic reagents, such as high-concentration acids and bases. Herein, an eco-friendly and cost-effective strategy is reported for improving the conductivity of PEDOT:PSS films through the confinement of ice crystals. The crystallization of water swelled by the film facilitated the phase separation of PEDOT and PSS, and the excess PSS in the skin layer is effectively removed. Moreover, under the confinement effect, the carrier mobility of the film is enhanced through the formation of a well-crystallized PEDOT molecular morphology. A detailed elucidation of aggregate structure evolution in PEDOT:PSS films during annealing, solvent post-treatment, and subsequent confined crystallization is presented herein. After multiple water crystallization cycles, the conductivity of the PEDOT:PSS film increased by over 85%, achieving a maximum of 2564 ± 142 S cm-1. Finally, compared to post-treatment with dimethyl sulfoxide (DMSO), the current strategy can improve the Seebeck coefficient by 5.6% and the power factor by 139%.

Ultrathin Mixed Ionic-Electronic Conducting Interlayer via the Solution Shearing Technique for High-Performance Lithium-Sulfur Batteries

The commercialization of lithium-sulfur (Li-S) batteries has been hampered by diverse challenges, including the shuttle phenomenon and low electrical/ionic conductivity of lithium sulfide and sulfur. To address these issues, extensive research has been devoted to developing multifunctional interlayers. However, interlayers capable of simultaneously suppressing the polysulfide (PS) shuttle and ensuring stable electrical and ionic conductivity are relatively uncommon. Moreover, the use of thick and heavy interlayers results in an unavoidable decline in the energy density of Li-S batteries. We developed an ultrathin (750 nm), lightweight (0.182 mg cm-2) interlayer that facilitates mixed ionic-electronic conduction using the solution shearing technique. The interlayer, composed of carbon nanotube (CNT)/Nafion/poly-3,4-ethylenedioxythiophene:tetracyanoborate (PEDOT:TCB), effectively suppresses the shuttle phenomenon through the synergistic segregation and adsorption effects on PSs by Nafion and CNT/PEDOT, respectively. Furthermore, the electrical/ionic conductivity of the interlayer can be improved via counterion exchange and homogeneous Li+ ion flux/good wettability from SO3- functional group of Nafion, respectively. Enhanced sulfur utilization and reaction kinetics through polysulfide shuttle inhibition and facilitated electron/ion transfer by interlayer enable a high discharge capacity of 1029 mA h g-1 in the Li-S pouch cell under a high sulfur loading of 5.3 mg cm-2 and low electrolyte/sulfur ratio of 5 μL mg-1.

Protic Ionic Liquids for Intrinsically Stretchable Conductive Polymers

Inspired by the classic hard-soft acid-base theory and intrigued by a theoretical prediction of spontaneous ion exchange between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and hard-cation-soft-anion ionic liquid (IL), we treat PEDOT:PSS with a new IL composed of a protic (i.e., extremely hard) cation (3-methylimidazolium, p-MIM⁺) and an extremely soft anion (tetracyanoborate, TCB^-). In fact, this protic IL (p-MIM:TCB) accomplishes the same levels of ion-exchange-mediated PEDOT-PSS separation, PEDOT-rich nanofibril formation, and electrical conductivity enhancement (∼2500 S/cm) as its aprotic counterpart (EMIM:TCB with 1-ethyl-3-methylimidazolium), the best IL used for this purpose so far. Furthermore, p-MIM:TCB significantly outperforms EMIM:TCB in terms of improving the stretchability (i.e., the highest tensile strain) of the PEDOT:PSS thin film. This enhancement is a result of the aromatic and protic cation p-MIM⁺, which acts as a molecular adhesive holding the exchanged ion pairs (PEDOT⁺:TCB^----p-MIM⁺:PSS^-) via ionic intercalation (at the surface of TCB^--decorated PEDOT⁺ clusters) and hydrogen bonding (to PSS^-), in which washing p-MIM⁺ out of the film degrades the stretchability while keeping the morphology. Our results offer molecular-level insight into the morphological, electrical, and mechanical properties of PEDOT:PSS and a molecular-interaction-based enhancement strategy that can be used for intrinsically stretchable conductive polymers.

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.

Electrochemical Capacitors with Confined Redox Electrolytes and Porous Electrodes

Electrochemical capacitors (ECs), including electrical-double-layer capacitors and pseudocapacitors, feature high power densities but low energy densities. To improve the energy densities of ECs, redox electrolyte-enhanced ECs (R-ECs) or supercapbatteries are designed through employing confined soluble redox electrolytes and porous electrodes. In R-ECs the energy storage is based on diffusion-controlled faradaic processes of confined redox electrolytes at the surface of a porous electrode, which thus take the merits of high power densities of ECs and high energy densities of batteries. In the past few years, there has been great progress in the development of this energy storage technology, particularly in the design and synthesis of novel redox electrolytes and porous electrodes, as well as the configurations of new devices. Herein, a full-screen picture of the fundamentals and the state-of-art progress of R-ECs are given together with a discussion and outlines about the challenges and future perspectives of R-ECs. The strategies to improve the performance of R-ECs are highlighted from the aspects of their capacitances and capacitance retention, power densities, and energy densities. The insight into the philosophies behind these strategies will be favorable to promote the R-EC technology toward practical applications of supercapacitors in different fields.

Anion Size Effect of Ionic Liquids in Tuning the Thermoelectric and Mechanical Properties of PEDOT:PSS Films through a Counterion Exchange Strategy

Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) thermoelectric thin films have attracted significant interest due to their solution-processable manufacturing. However, molecular-level tuning or doping is still a challenge to synergistically boost their thermoelectric performance and mechanically stretchable capabilities. In this work, we report a counterion exchange between ionic liquid bis(x-fluorosulfonyl) amide lithium (Li:nFSI, n = 1, 3, 5) with different sizes of anions and a PEDOT:PSS-induced bipolaron network, which significantly boosted the thermoelectric power factor from 0.8 to 157 μW m K^-2 at 235 °C and the maximum tensile strain from 3% to over 30%. The π-π* stacking of the PEDOT polymer chains was fine-tuned by the hydrophobic anions of nFSI^-, providing a technical route for constructing a bipolaron network and inducing the transition from hopping transport to band-like transport. Furthermore, we found that the stretchable capabilities, that is, ε_max, were connected to the gelation time of the PEDOT:PSS-Li:nFSI aqueous solution. Thus, more fluorine-containing groups resulted in longer gelation times and higher ε_max values, which significantly improved the processability of the solution-derived films.

Hard-Cation-Soft-Anion Ionic Liquids for PEDOT:PSS Treatment

A promising conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) experiences significant conductivity enhancement when treated with proper ionic liquids (ILs). Based on the hard-soft-acid-base principle, we propose a combination of a hydrophilic hard cation A⁺ (instead of the commonly used 1-ethyl-3-methyl imidazolium, EMIM⁺) and a hydrophobic soft anion X^- (such as tetracyanoborate, TCB^-) as the best ILs for this purpose. Such ILs would decouple hydrophilic-but-insulating PSS^- from conducting-but-hydrophobic PEDOT⁺ most efficiently by strong interactions with hydrophilic A⁺ and hydrophobic X^-, respectively. Such a favorable ion exchange between PEDOT⁺:PSS^- and A⁺:X^- ILs would allow the growth of conducting PEDOT⁺ domains decorated by X^-, not disturbed by PSS^- or A⁺. Using density functional theory calculations and molecular dynamics simulations, we demonstrate that a protic cation- (aliphatic N-alkyl pyrrolidinium, in particular) combined with the hydrophobic anion TCB^- indeed outperforms EMIM⁺ by promptly leaving hydrophobic TCB^- and strongly binding to hydrophilic PSS^-.

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...