Non-covalent planarizing interactions yield highly ordered and thermotropic liquid crystalline conjugated polymers.

Controlling the multi-level assembly and morphological properties of conjugated polymers through structural manipulation has contributed significantly to the advancement of organic electronics. In this work, a redox active conjugated polymer, TPT-TT, composed of alternating 1,4-(2-thienyl)-2,5-dialkoxyphenylene (TPT) and thienothiophene (TT) units is reported with non-covalent intramolecular S⋯O and S⋯H-C interactions that induce controlled main-chain planarity and solid-state order. As confirmed by density functional theory (DFT) calculations, these intramolecular interactions influence the main chain conformation, promoting backbone planarization, while still allowing dihedral rotations at higher kinetic energies (higher temperature), and give rise to temperature-dependent aggregation properties. Thermotropic liquid crystalline (LC) behavior is confirmed by cross-polarized optical microscopy (CPOM) and closely correlated with multiple thermal transitions observed by differential scanning calorimetry (DSC). This LC behavior allows us to develop and utilize a thermal annealing treatment that results in thin films with notable long-range order, as shown by grazing-incidence X-ray diffraction (GIXD). Specifically, we identified a first LC phase, ranging from 218 °C to 107 °C, as a nematic phase featuring preferential face-on π-π stacking and edge-on lamellar stacking exhibiting a large extent of disorder and broad orientation distribution. A second LC phase is observed from 107 °C to 48 °C, as a smectic A phase featuring sharp, highly ordered out-of-plane lamellar stacking features and sharp tilted backbone stacking peaks, while the structure of a third LC phase with a transition at 48 °C remains unclear, but resembles that of the solid state at ambient temperature. Furthermore, the significance of thermal annealing is evident in the ∼3-fold enhancement of the electrical conductivity of ferric tosylate-doped annealed films reaching 55 S cm-1. More importantly, thermally annealed TPT-TT films exhibit both a narrow distribution of charge-carrier mobilities (1.4 ± 0.1) × 10-2 cm2 V-1 s-1 along with a remarkable device yield of 100% in an organic field-effect transistor (OFET) configuration. This molecular design approach to obtain highly ordered conjugated polymers in the solid state affords a deeper understanding of how intramolecular interactions and repeat-unit symmetry impact liquid crystallinity, solution aggregation, solution to solid-state transformation, solid-state morphology, and ultimately device applications.

Processability of Thermoelectric Ultrafine Fibers via Electrospinning for Wearable Electronics.

Polymer-based thermoelectric generators hold great appeal in the realm of wearable electronics as they enable the utilization of body heat for power generation. Fibers produced from conducting polymers for use in thermoelectric generators have high porosity and good flexibility, providing comfort-based performance advantages over thin films for wearable electronics. Some fiber processing techniques have been explored to produce textile-based thermoelectric generators; however, they fail to approach the conductivities of polymeric thin films. Ultrafine fibers solution processed through electrospinning yield fiber diameters on the nanoscale, allowing for high surface area to volume ratios and thus low thermal conductivity; however, a number of processing challenges in electrospinning conducting polymers limit the success of preparing high performing thermoelectric textiles. In this work, the specific processing challenges inherent to electrospinning conducting polymers are addressed for both n- and p-type materials. For the p-type polymer, 63 wt % PEDOT:PSS fibers are fabricated through solution formulation improvements yielding a conductivity of 3 S/cm and a power factor of 0.1 µW/mK2. The first of their kind n-type poly(NiETT)/PVA electrospun fibers were created yielding a conductivity of 0.11 S/cm and a power factor of 0.0036 µW/mK2. These nonwoven ultrafine fiber mats show progress toward achieving textile-based thermoelectric materials with equivalent performance of comparable polymeric thin films. This work shows the feasibility of creating ultrafine fibers for use in thermoelectric generators through electrospinning including the first demonstration of poly(NiETT)/PVA fibers.

Metal-like Charge Transport in PEDOT(OH) Films by Post-processing Side Chain Removal from a Soluble Precursor Polymer.

Herein, a route to produce highly electrically conductive doped hydroxymethyl functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) films, termed PEDOT(OH) with metal-like charge transport properties using a fully solution processable precursor polymer is reported. This is achieved via an ester-functionalized PEDOT derivative [PEDOT(EHE)] that is soluble in a range of solvents with excellent film-forming ability. PEDOT(EHE) demonstrates moderate electrical conductivities of 20-60 S cm-1 and hopping-like (i.e., thermally activated) transport when doped with ferric tosylate (FeTos3 ). Upon basic hydrolysis of PEDOT(EHE) films, the electrically insulative side chains are cleaved and washed from the polymer film, leaving a densified film of PEDOT(OH). These films, when optimally doped, reach electrical conductivities of ≈1200 S cm-1 and demonstrate metal-like (i.e., thermally deactivated and band-like) transport properties and high stability at comparable doping levels.

Iron(III) Dopant Counterions Affect the Charge-Transport Properties of Poly(Thiophene) and Poly(Dialkoxythiophene) Derivatives.

This study investigates the charge-transport properties of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(ProDOT-alt-biEDOT) (PE2) films doped with a set of iron(III)-based dopants and as a function of dopant concentration. X-ray photoelectron spectroscopy measurements show that doping P3HT with 12 mM iron(III) solutions leads to similar extents of oxidation, independent of the dopant anion; however, the electrical conductivities and Seebeck coefficients vary significantly (5 S cm-1 and + 82 µV K-1 with tosylate and 56 S cm-1 and +31 µV K-1 with perchlorate). In contrast, PE2 thermoelectric transport properties vary less with respect to the iron(III) anion chemistry, which is attributed to PE2 having a lower onset of oxidation than P3HT. Consequentially, PE2 doped with 12 mM iron(III) perchlorate obtained an electrical conductivity of 315 S cm-1 and a Seebeck coefficient of + 7 µV K-1. Modeling these thermoelectric properties with the semilocalized transport (SLoT) model suggests that tosylate-doped P3HT remains mostly in the localized transport regime, attributed to more disorder in the microstructure. In contrast perchlorate-doped P3HT and PE2 films exhibited thermally deactivated electrical conductivities and metal-like transport at high doping levels over limited temperature ranges. Finally, the SLoT model suggests that PE2 has the potential to be more electrically conductive than P3HT due to PE2's ability to achieve higher extents of oxidation and larger shifts in the reduced Fermi energy levels.

Zwitterions Raise the Dielectric Constant of Soft Materials.

Materials exhibiting high dielectric constants (ϵ_{s}) are critical for energy storage and actuators. A successful approach to increase ϵ_{s} is to incorporate polar additives (with high ϵ_{s}) but controlling the resulting dispersion state is difficult. Here, we show that significant ϵ_{s} increases are realized by adding zwitterions, which are small molecules with a cation and an anion separated by covalent bonds. The increase in ϵ_{s} with zwitterion addition is attributed to the large molecular dipole of zwitterions, ranging from 35 to 41 D, as experimentally quantified and confirmed using density functional theory. At elevated zwitterion concentration in an ethylene glycol medium, there is a nonlinear increase of ϵ_{s} that eventually saturates due to the strong Coulombic interactions between zwitterions. The presented work provides a fundamental molecular understanding of why zwitterions are effective additives in boosting ϵ_{s} in soft materials.

Quantifying charge carrier localization in chemically doped semiconducting polymers.

Charge transport in semiconducting polymers ranges from localized (hopping-like) to delocalized (metal-like), yet no quantitative model exists to fully capture this transport spectrum and its dependency on charge carrier density. In this study, using an archetypal polymer-dopant system, we measure the temperature-dependent electrical conductivity, Seebeck coefficient and extent of oxidation. We then use these measurements to develop a semi-localized transport (SLoT) model, which captures both localized and delocalized transport contributions. By applying the SLoT model to published data, we demonstrate its broad utility. We are able to determine system-dependent parameters such as the maximum localization energy of the system, how this localization energy changes with doping, the amount of dopant required to achieve metal-like conductivity and the conductivity a system could have in the absence of localization effects. This proposed SLoT model improves our ability to predict and tailor electronic properties of doped semiconducting polymers.