Chemical Doping of Conjugated Polymers with the Strong Oxidant Magic Blue

Molecular doping of organic semiconductors is a powerful tool for the optimization of organic electronic devices and organic thermoelectric materials. However, there are few redox dopants that have a sufficiently high electron affinity to allow the doping of conjugated polymers with an ionization energy of more than 5.3\ua0eV. Here, p-doping of a broad palette of conjugated polymers with high ionization energies is achieved by using the strong oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue). In particular diketopyrrolopyrrole (DPP)-based copolymers reach a conductivity of up to 100 S cm−1 and a thermoelectric power factor of 10 \ub5W m−1 K−2. Further, both electron paramagnetic resonance (EPR) as well as a combination of spectroelectrochemistry and chronoamperometry is used to estimate the charge-carrier density of the polymer PDPP-3T doped with Magic Blue. A molar attenuation coefficient of 6.0\ua0\ub1\ua00.2 7 103 m2 mol−1 is obtained for the first polaronic sub-bandgap absorption of electrochemically oxidized PDPP-3T. Comparison with chemically doped PDPP-3T suggests a charge-carrier density on the order of 1026 m−3, which yields a charge-carrier mobility of up to 0.5 cm2 V−1 s−1 for the most heavily doped material

Electrically Conducting Elastomeric Fibers with High Stretchability and Stability

Stretchable conducting materials are appealing for the design of unobtrusive wearable electronic devices. Conjugated polymers with oligoethylene glycol side chains are excellent candidate materials owing to their low elastic modulus and good compatibility with polar stretchable polymers. Here, electrically conducting elastomeric blend fibers with high stretchability, wet spun from a blend of a doped polar polythiophene with tetraethylene glycol side chains and a polyurethane are reported. The wet-spinning process is versatile, reproducible, scalable, and produces continuous filaments with a diameter ranging from 30 to 70\ua0\ub5m. The fibers are stretchable up to 480% even after chemical doping with iron(III) p-toluenesulfonate hexahydrate and exhibit an electrical conductivity of up to 7.4 S cm−1, which represents a record combination of properties for conjugated polymer-based fibers. The fibers remain conductive during elongation until fiber fracture and display excellent long-term stability at ambient conditions. Cyclic stretching up to 50% strain for at least 400 strain cycles reveals that the doped fibers exhibit high cyclic stability and retain their electrical conductivity. Finally, a directional strain sensing device, which makes use of the linear increase in resistance of the fibers up to 120% strain is demonstrated

Mechanically Adaptive Mixed Ionic-Electronic Conductors Based on a Polar Polythiophene Reinforced with Cellulose Nanofibrils

Conjugated polymers with oligoether side chains are promising mixed ionic-electronic conductors, but they tend to feature a low glass transition temperature and hence a low elastic modulus, which prevents their use if mechanical robust materials are required. Carboxymethylated cellulose nanofibrils (CNF) are found to be a suitable reinforcing agent for a soft polythiophene with tetraethylene glycol side chains. Dry nanocomposites feature a Young’s modulus of more than 400 MPa, which reversibly decreases to 10 MPa or less upon passive swelling through water uptake. The presence of CNF results in a slight decrease in electronic mobility but enhances the ionic mobility and volumetric capacitance, with the latter increasing from 164 to 197 F cm-3 upon the addition of 20 vol % CNF. Overall, organic electrochemical transistors (OECTs) feature a higher switching speed and a transconductance that is independent of the CNF content up to at least 20 vol % CNF. Hence, CNF-reinforced conjugated polymers with oligoether side chains facilitate the design of mechanically adaptive mixed ionic-electronic conductors for wearable electronics and bioelectronics

Robust PEDOT:PSS Wet‐Spun Fibers for Thermoelectric Textiles

To realize thermoelectric textiles that can convert body heat to electricity, fibers with excellent mechanical and thermoelectric properties are needed. Although poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is among the most promising organic thermoelectric materials, reports that explore its use for thermoelectric fibers are all but absent. Herein, the mechanical and thermoelectric properties of wet‐spun PEDOT:PSS fibers are reported, and their use in energy‐harvesting textiles is discussed. Wet‐spinning into sulfuric acid results in water‐stable semicrystalline fibers with a Young\u27s modulus of up to 1.9 GPa, an electrical conductivity of 830 S cm−1, and a thermoelectric power factor of 30 μV m−1 K−2. Stretching beyond the yield point as well as repeated tensile deformation and bending leave the electrical properties of these fibers almost unaffected. The mechanical robustness/durability and excellent underwater stability of semicrystalline PEDOT:PSS fibers, combined with a promising thermoelectric performance, opens up their use in practical energy‐harvesting textiles, as illustrated by an embroidered thermoelectric fabric module

Impact of oxidation-induced ordering on the electrical and mechanical properties of a polythiophene co-processed with bistriflimidic acid

The interplay between the nanostructure of a doped polythiophene with oligoether side chains and its electrical as well as mechanical properties is investigated. The degree of order of the polymer is found to strongly vary when co-processed with bistriflimidic acid (H-TFSI). The neat polythiophene as well as strongly oxidized material are largely disordered while intermediate concentrations of H-TFSI give rise to a high degree of π-stacking. The structural disorder of strongly oxidized material correlates with a decrease in the kinetic fragility with H-TFSI concentration, suggesting that positive interactions between TFSI anions and the polymer reduce the ability to crystallize. The electrical conductivity as well as the Young\u27s modulus first increase upon the addition of 4-10 mol% of H-TFSI, while the loss of π-stacking observed for strongly oxidized material more significantly affects the latter. As a result, material comprising 25 mol% H-TFSI displays an electrical conductivity of 58 S cm−1 but features a relatively low Young\u27s modulus of only 80 MPa. Decoupling of the electrical and mechanical properties of doped conjugated polymers may allow the design of soft conductors that are in high demand for wearable electronics and bioelectronics

Tuning of the elastic modulus of a soft polythiophene through molecular doping

Molecular doping of a polythiophene with oligoethylene glycol side chains is found to strongly modulate not only the electrical but also the mechanical properties of the polymer. An oxidation level of up to 18% results in an electrical conductivity of more than 52 S cm(-1) and at the same time significantly enhances the elastic modulus from 8 to more than 200 MPa and toughness from 0.5 to 5.1 MJ m(-3). These changes arise because molecular doping strongly influences the glass transition temperature T-g and the degree of pi-stacking of the polymer, as indicated by both X-ray diffraction and molecular dynamics simulations. Surprisingly, a comparison of doped materials containing mono- or dianions reveals that - for a comparable oxidation level - the presence of multivalent counterions has little effect on the stiffness. Evidently, molecular doping is a powerful tool that can be used for the design of mechanically robust conducting materials, which may find use within the field of flexible and stretchable electronics

Impact of Oligoether Side-Chain Length on the Thermoelectric Properties of a Polar Polythiophene

This article is part of the Advanced Thermoelectric Materials and Devices special issue.Conjugated polymers with oligoether side chains make up a promising class of thermoelectric materials. In this work, the impact of the side-chain length on the thermoelectric and mechanical properties of polythiophenes is investigated. Polymers with tri-, tetra-, or hexaethylene glycol side chains are compared, and the shortest length is found to result in thin films with the highest degree of order upon doping with the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). As a result, a stiff material with an electrical conductivity of up to 830 ± 15 S cm–1 is obtained, resulting in a thermoelectric power factor of about 21 μW m–1 K–2 in the case of as-cast films. Aging at ambient conditions results in an initial decrease in thermoelectric properties but then yields a highly stable performance for at least 3 months, with values of about 200 S cm–1 and 5 μW m–1 K–2. Evidently, identification of the optimal side-chain length is an important criterion for the design of conjugated polymers for organic thermoelectrics.We acknowledge funding from the European Union’s Horizon 2020 research and innovation programme through the Marie Skłodowska-Curie grant agreement no. 955837 (HORATES) and the Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship Prolongation grant. We acknowledge financial support from the Spanish Ministerio de Ciencia e Innovacíon for its support through grant CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program, and grants PID2020-119777GBI00 (THERM2MAIN), and PDC2021-121814-I00 (COVEQ). K.X. acknowledges a fellowship (CSC201806950006) from China Scholarship Council. K.X. and J.G. thank the PhD programme in Materials Science from Universitat Autònoma de Barcelona in which they are enrolled. We thank Johanna Heimonen for help with SEC measurements and Anders Mårtensson for carrying out the AFM measurements. This project was in part performed at the Chalmers Materials Analysis Laboratory (CMAL).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Interplay of the Electrical and Mechanical Properties of Conjugated Polymers

Knowledge about organic semiconductors has drastically developed in the past decades. They have a myriad of applications in areas such as energy harvesting and storage, bioelectronics and wearable electronics. For most of these applications, mechanical flexibility is desirable. Conjugated polymers, a class of organic semiconductors, tend to be brittle and rigid. The latter is a consequence of their planar‑aromatic backbones that endow them with a high glass transition temperature and a tendency to strongly aggregate. Polythiophenes with oligoethylene glycol side chains, on the contrary, tend to be soft materials with a low glass transition temperature and low degree of crystallinity, which limit their use as a bulk free‑standing material. At the same time, they can feature high ionic and electrical conductivity. This thesis explores different strategies to modulate the mechanical properties of polythiophenes with polyethylene glycol side chains without unduly affecting their electrical properties. This thesis will compare the mechanical and electrical properties of a soft polythiophene and a copolymer of the same material with hard urethane blocks, which enable the formation of a reversible network. Then, blending of a doped soft conjugated polymer with melt‑processable insulating polymers such as polycaprolactone is explored with the goal to prepare thermally stable blends for melt-processing. Conducting stretchable fibers of a doped conjugated polymer and a polyurethane elastomer are demonstrated that feature a high degree of electrical and mechanical stability. Further, the properties of composites with cellulose nanomaterials are described. The nanocomposites feature a high elastic modulus, and the presence of cellulose nanofibrils does not affect the electrical conductivity. Finally, the impact of molecular doping, which is an essential step for rendering the conjugated polymers conductive, on the nanostructure and thermomechanical properties of polythiophenes with oligoethylene glycol side chains is explored. In particular, doping is found to strongly increase the elastic modulus of the polymer. Evidently, a wide range of methods such as copolymerization, blending, the use of a reinforcing agent as well as molecular doping itself can be used for the which may facilitate the design of mechanically robust electrical conductors

Improved mechanical properties of a polar polythiophene through copolymerization or blending with urethanes

Polar polythiophenes with oligoethylene glycol side chains are a developing class of materials that are great candidates for energy harvesting and storage as well as bioelectronics. The polar side chains enhance the compatibility with molecular dopants and improve the electrical conductivity and thermal stability of the doped material. However, polar polythiophenes are extremely soft at room temperature due to a sub-zero glass transition temperature. To enhance the usefulness of these polymers in electronic devices, strategies are needed for tuning their mechanical properties, particularly increasing their robustness. In this thesis we report our efforts to copolymerize a polar polythiophene with urethane blocks to increase the toughness of the material without unduly reducing its electrical and electrochemical properties. Further, we describe wet spinning blend of a polar polythiophene with polyurethane into fibers. The resulting fibers with low hysteresis exhibit elasticity and stretchability, yet higher stiffness

Chemical Doping of Conjugated Polymers with the Strong Oxidant Magic Blue

Molecular doping of organic semiconductors is a powerful tool for the optimization of organic electronic devices and organic thermoelectric materials. However, there are few redox dopants that have a sufficiently high electron affinity to allow the doping of conjugated polymers with an ionization energy of more than 5.3\ua0eV. Here, p-doping of a broad palette of conjugated polymers with high ionization energies is achieved by using the strong oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue). In particular diketopyrrolopyrrole (DPP)-based copolymers reach a conductivity of up to 100 S cm−1 and a thermoelectric power factor of 10 \ub5W m−1 K−2. Further, both electron paramagnetic resonance (EPR) as well as a combination of spectroelectrochemistry and chronoamperometry is used to estimate the charge-carrier density of the polymer PDPP-3T doped with Magic Blue. A molar attenuation coefficient of 6.0\ua0\ub1\ua00.2 7 103 m2 mol−1 is obtained for the first polaronic sub-bandgap absorption of electrochemically oxidized PDPP-3T. Comparison with chemically doped PDPP-3T suggests a charge-carrier density on the order of 1026 m−3, which yields a charge-carrier mobility of up to 0.5 cm2 V−1 s−1 for the most heavily doped material