Cross-linked polymer electrolytes containing structurally dynamic disulfide bonds have been synthesized to investigate their combined ion transport and adhesive properties. Dynamic network polymers of varying cross-link densities are synthesized via thiol oxidation of a bisthiol monomer, 2,2′-(ethylenedioxy)diethanethiol, and tetrathiol cross-linker, pentaerythritol tetrakis(3-mercaptopropionate). At optimal loading of lithium bis(trifluoromethane-sulfonyl-imide) (LiTFSI) salt, the ionic conductivities (σ) at 90 °C are about 1 × 10 −4 and 1 × 10 −5 S/cm at the lowest and highest cross-linking, respectively. Notably, in comparison to the equivalent nondynamic network, the dynamic network shows a positive deviation in σ above 90 °C, which suggests the enhancement of ion transport occurs from the difference in structural relaxation on account of the dissociation of disulfide bonds. Lap shear adhesion and conductivity tests on ITO-coated glass substrates reveal the dynamic network exhibits a higher adhesive shear strength of 0.2 MPa (vs 0.03 MPa for the nondynamic network) and higher σ after the application of external stimulus (UV light or heat). The adhesive strength and σ are stable over multiple debonding/rebonding cycles and, thus, demonstrating the utility of these structurally dynamic networks as solid polymer electrolyte adhesives.

The ability to characterize bulk and interfacial transport properties of polymer electrolytes is critical to realizing their potential applications in electrochemical energy storage devices. In this study, we leverage custom microfabricated interdigitated electrode array (IDEs) as a platform to probe ion transport properties of polymer electrolytes films through electrochemical impedance spectroscopy (EIS) measurements. Using poly(ethylene oxide) (PEO) blended with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a model dry polymer electrolyte system, we investigate how geometric parameters of the IDEs influence the quality and analysis of EIS measurements. By focusing on films on the nanometer film thickness (ca. 50 nm), EIS measurements revealed diffusional processes near the electrode/polymer interface that may be difficult to observe with conventional thick films. Moreover, irreversible impedance spectra were observed at elevated temperatures when using IDEs with large electrode metal fractions. These irreversible processes were eliminated through passivation of the IDE with different oxides (SiO 2 , Al 2 O 3 , or TiO 2 ). Ultimately, the ionic conductivity of PEO-LiTFSI electrolytes is confidently determined when appropriate IDE geometries and equivalent circuits are used. Our work demonstrates the use of IDEs and nanothin polymer electrolytes films as a versatile platform for rapid and efficient interrogation of both bulk and interfacial electrochemical properties.

Molecularly doped conjugated polymers with polar side chains are an emerging class of conducting materials exhibiting enhanced and thermally stable conductivity. Here, we study the electronic conductivity (σ) and the corresponding thermal stability of two polythiophene derivatives comprising oligoethylene glycol side chains: one having oxygen attached to the thiophene ring (poly­(3-(methoxyethoxyethoxy)­thiophene) (P3MEET)) and the other having a methylene spacer between the oxygen and the thiophene ring (poly­(3-(methoxyethoxyethoxymethyl)­thiophene) (P3MEEMT)). Thin films were vapor-doped with fluorinated derivatives of tetracyanoquinodimethane (F n TCNQ, n = 4, 2, 1) to determine the role of dopant strength (electron affinity) in maximum achievable σ. Specifically, when vapor doping with F4TCNQ, P3MEET achieved a substantially higher σ of 37.1 ± 10.1 S/cm compared to a σ of 0.82 ± 0.06 S/cm for P3MEEMT. Structural characterization using a combination of X-ray and optical spectroscopy reveals that the higher degree of conformational order of polymer chains in the amorphous domain upon doping with F4TCNQ in P3MEET is a major contributing factor for the higher σ of P3MEET. Additionally, vapor-doped P3MEET exhibited superior thermal stability compared to P3MEEMT, highlighting that the presence of polar side chains alone does not ensure higher thermal stability. Molecular dynamics simulations indicate that the dopant–side-chain nonbond energy is lower in the P3MEET:F4TCNQ mixture, suggesting more favorable dopant–side-chain interaction, which is a factor in improving the thermal stability of a polymer/dopant pair. Our results reveal that additional factors such as polymer ionization energy and side-chain–dopant interaction should be taken into account for the design of thermally stable, highly conductive polymers.

ParagraphConducting organic materials, such as doped organic polymers, 1 molecular conductors, 2, 3 and emerging coordination polymers, 4 underpin technologies ranging from displays to flexible electronics. 5 Realizing high electrical conductivity in traditionally insulating organic materials necessitates tuning their electronic structure through chemical doping. 6 Furthermore, even materials that are intrinsically conductive, such as single-component molecular conductors, 7,8 require crystallinity for metallic behavior. However, commercial conducting polymers are often purposefully amorphous to aid in durability and processability. 9,10 Using molecular design to engender high conductivity in undoped amorphous materials would enable tunable and robust conductivity in many applications, but there are no intrinsically conducting organic materials which maintain high conductivity when disordered. Here we show that the completely amorphous coordination polymer Ni tetrathiafulvalene tetrathiolate (NiTTFtt) displays intrinsic metallic conductivity. Despite its disordered structure, NiTTFtt exhibits remarkably high electronic conductivity (1280 S/cm) and intrinsically glassy metallic behavior. Analysis with advanced theory shows that these properties are enabled by strong molecular overlap and correlation that are robust to structural perturbations. This unusual set of structural and electronic features results in remarkably stable organic conductivity which is maintained in air for weeks and at temperatures up to 140 °C. Our results demonstrate that molecular design can enable metallic conductivity even in heavily disordered materials. This both raises fundamental questions about how band-like transport can exist in the absence of periodic structure as well as suggests exciting new applications for these materials.

With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are at the forefront in enabling organic thermoelectric devices for thermal energy management. In contrast to uniform thermoelectric material properties, an alternative route focuses on functionally graded materials (FGMs) where one spatially controls and optimizes transport properties across the length of a thermoelectric material. While primarily studied in the context of inorganic materials, the concept of FGMs for organic thermoelectrics has not been explored. Herein, we introduce how molecular doping of semiconducting polymers enables spatial compositional control of thin-film FGMs. Specifically, we use sequential vapor doping of poly­[2,5-bis­(3-tetradecyl­thiophen-2-yl)­thieno­[3,2-b]­thiophene] (PBTTT) with the small molecule acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano­quinodimethane (F4TCNQ) to fabricate the simplest form of FGMsdouble-segmented thin films. The two thin-film segments are of equal length (7.5 mm) but each set to different doping levels. Our study focuses on understanding the thermoelectric properties (Seebeck coefficient, α, and electronic conductivity, σ) and structural properties (through X-ray scattering, UV–vis–NIR spectroscopy, and Raman spectroscopy) within and across the two segments. We observe the presence of a small diffuse interfacial region of 0.5–1 mm between the two segments where the doping level and transport properties vary continuously. Despite the diffuse interface, the measured α across the two segments is simply the average of α within each segment. Importantly, this experimental result is consistent with reported mathematical models describing the spatial average of α  in graded thermoelectric materials. Our results demonstrate the facile fabrication and characterization of functionally graded organic thermoelectric materials, providing guidelines for further development on more complex FGMs.

Two-dimensional (2D) inorganic materials have emerged as exciting platforms for (opto)electronic, thermoelectric, magnetic, and energy storage applications. However, electronic redox tuning of these materials can be difficult. Instead, 2D metal−organic frameworks (MOFs) offer the possibility of electronic tuning through stoichiometric redox changes, with several examples featuring one to two redox events per formula unit. Here, we demonstrate that this principle can be extended over a far greater span with the isolation of four discrete redox states in the 2D MOFs Li x Fe 3 (THT) 2 (x = 0−3, THT = triphenylenehexathiol). This redox modulation results in 10,000-fold greater conductivity, p-to n-type carrier switching, and modulation of antiferromagnetic coupling. Physical characterization suggests that changes in carrier density drive these trends with relatively constant charge transport activation energies and mobilities. This series illustrates that 2D MOFs are uniquely redox flexible, making them an ideal materials platform for tunable and switchable applications.

The emergence of conductive 2D and less commonly 3D coordination polymers (CPs) and metalorganic frameworks (MOFs) promises novel applications in many fields. However, the synthetic parameters for these electronically complex materials are not thoroughly understood. Here we report a new 3D semiconducting CP Fe 5 (C 6 O 6 ) 3 , which is a fusion of 2D Fe-semiquinoid materials and 3D cubic Fe x (C 6 O 6 ) y materials, by using a different initial redox-state of the C 6 O 6 linker. The material displays high electrical conductivity (0.02 S cm À 1 ), broad electronic transitions, promising thermoelectric behavior (S 2 σ = 7.0 × 10 À 9 W m À 1 K À 2 ), and strong antiferromagnetic interactions at room temperature. This material illustrates how controlling the oxidation states of redox-active components in conducting CPs/MOFs can be a "pre-synthetic" strategy to carefully tune material topologies and properties in contrast to more commonly encountered post-synthetic modifications.

Polymeric mixed ionic-electronic conductors (MIECs) are of broad interest in the field of energy storage and conversion, optoelectronics, and bioelectronics. A class of polymeric MIECs are conjugated polyelectrolytes (CPEs), which possess a π-conjugated backbone imparting electronic transport characteristics along with side chains composed of a pendant ionic group to allow for ionic transport. Here, our study focuses on the humidity-dependent structure–transport properties of poly­[3-(potassium-n-alkanoate) thiophene-2,5-diyl] (P3KnT) CPEs with varied side-chain lengths of n = 4–7. UV–vis spectroscopy along with electronic paramagnetic resonance (EPR) spectroscopy reveals that the infiltration of water leads to a hydrated, self-doped state that allows for electronic transport. The resulting humidity-dependent ionic conductivity (σi) of the thin films shows a monotonic increase with relative humidity (RH) while electronic conductivity (σe) follows a non-monotonic profile. The values of σe continue to rise with increasing RH reaching a local maximum after which σe begins to decrease. P3KnTs with higher n values demonstrate greater resiliency to increasing RH before suffering a decrease in σe. This drop in σe is attributed to two factors. First, disruption of the locally ordered π-stacked domains observed through in situ humidity-dependent grazing incidence wide-angle X-ray scattering (GIWAXS) experiments can account for some of the decrease in σe. A second and more dominant factor is attributed to the swelling of the amorphous domains where electronic transport pathways connecting ordered domains are impeded. P3K7T is most resilient to swelling (based on ellipsometry and water uptake measurements) where sufficient hydration allows for high σi (1.0 × 10–1 S/cm at 95% RH) while not substantially disrupting σe (1.7 × 10–2 S/cm at 85% RH and 8.0 × 10–3 S/cm at 95% RH). Overall, our study highlights the complexity of balancing electronic and ionic transport in hydrated CPEs.