Formation and Structure of Lyotropic Liquid Crystalline Mesophases in Donor–Acceptor Semiconducting Polymers

Controlling crystallinity and molecular packing at nano- and macroscopic length scales in conjugated polymer thin films is vital for improving the performance of polymer-based electronic devices. Herein, the inherent amphiphilicity of rigid donor–acceptor copolymers used in high performance polymer electronics is leveraged to allow the formation of highly ordered lyotropic mesophases. By increasing the length and branching of solubilizing chains on cyclopenta­dithiophene-<i>alt</i>-thiadiazolo­pyridine-based alternating copolymers, amphiphilicity can be increased, and lyotropic liquid crystalline mesophases are observed in selective solvents. These lyotropic mesophases consist of chain extended polymers exhibiting close, ordered π-stacking. This is evidenced by birefringent solutions and red-shifted absorbance spectra displaying pronounced excitonic coupling. Crystallinity developed in solution can be transferred to the solid state, and thin films of donor–acceptor copolymers cast from lyotropic solutions exhibit improved crystalline order in both the alkyl and π-stacking directions. Because of this improved crystallinity, transistors with active layers cast from lyotropic solutions exhibit a significant improvement in carrier mobility compared to those cast from isotropic solution, reaching a maximum value of 0.61 cm² V^–1 s^–1. This approach of rational side chain design bridges the gap from solution structure to solid state structure and is a promising and general approach to allow the expression of lyotropic mesophases in rigid conjugated polymers

Structure–Conductivity Relationships of Block Copolymer Membranes Based on Hydrated Protic Polymerized Ionic Liquids: Effect of Domain Spacing

Elucidating the relationship between chemical structure, morphology, and ionic conductivity is essential for designing novel high-performance materials for electrochemical applications. In this work, the effect of lamellar domain spacing (<i>d</i>) on ionic conductivity (σ) is investigated for a model system of hydrated diblock copolymer based on a protic polymerized ionic liquid. We present a strategy that allows for the synthesis of a well-defined series of narrowly dispersed PS-<i>b</i>-PIL with constant volume fraction of ionic liquid moieties (<i>f</i>_IL ≈ 0.39) and with two types of mobile charge carriers: trifluoroacetate anions and protons. These materials self-assemble into ordered lamellar morphologies with variable domain spacing (ca. 20–70 nm) as demonstrated by small-angle X-ray scattering. PS-<i>b-</i>PIL membranes exhibit ionic conductivities above 10^–4 S/cm at room temperature, which are independent of domain spacing consistent with their nearly identical water content. The conductivity scaling relationship demonstrated in this paper suggests that a mechanically robust membrane can be designed without compromising its ability to transport ions. In addition, PIL-based membranes exhibit low water uptake (λ ≈ 10) in comparison with many proton-conducting systems reported elsewhere. The low water content of the materials described herein makes them promising candidates for electrochemical devices operating in aqueous electrolytes at low current densities where moderate ion conduction and low product crossover are required

Mussel-Inspired Strategy for Stabilizing Ultrathin Polymer Films and Its Application to Spin-On Doping of Semiconductors

Stabilizing ultrathin films, in particular avoiding dewetting, is critical to the application of polymer thin films from biology to electronics. To address this issue, a wide range of approaches have been developed, including self-assembled monolayers to modify surface energy, and covalent attachment methods, such as surface-initiated polymerization and grafting of end-functionalized polymers. However, most of these approaches either require postprocessing of the substrates or are applicable only to the specific combination of polymers and substrates. Herein, we introduce a mussel-inspired universal adhesive moiety, dopamine, as an end group for any polymer to promote film stability, and demonstrate its application to spin-on doping on silicon, in particular. Leveraging the versatility of reversible addition–fragmentation chain transfer (RAFT) polymerization, the dopamine moiety is incorporated as an end group. Dopamine functionalized 15 nm thick films are more thermally stable at 230 °C on a variety of semiconductor-relevant surfaces (Si–OH, SiO_<i>x</i>, TiN, and Si₃N₄), while control polymer films with a carboxyl end group severely dewet. The dopamine end group also ensures successful sub-10 nm thick conformal coatings on three-dimensional surfaces, confirmed by cross-sectional scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS). Furthermore, as a polymeric spin-on doping material, dosage of dopant with the dopamine-functionalized polymer is comparable or higher than that with the control end group, demonstrating one of the promising applications of such conformal coatings

Harvesting Waste Heat in Unipolar Ion Conducting Polymers

The Seebeck effect in unipolar ion-conducting, solid-state polymers is characterized. The high Seebeck coefficient and sign in polymer ion conductors is explained via analysis of thermogalvanic multicomponent transport. A solid-state, water-processeable, flexible device based on these materials is demonstrated, showcasing the promise of polymers as thermogalvanic materials. Thermogalvanic materials based on ion-conducting polymer membranes show great promise in the harvesting of waste heat

Electrochemical Effects in Thermoelectric Polymers

Conductive polymers such as PEDOT:PSS hold great promise as flexible thermoelectric devices. The thermoelectric power factor of PEDOT:PSS is small relative to inorganic materials because the Seebeck coefficient is small. Ion conducting materials have previously been demonstrated to have very large Seebeck coefficients, and a major advantage of polymers over inorganics is the high room temperature ionic conductivity. Notably, PEDOT:PSS demonstrates a significant but short-term increase in Seebeck coefficient which is attributed to a large ionic Seebeck contribution. By controlling whether electrochemistry occurs at the PEDOT:PSS/electrode interface, the duration of the ionic Seebeck enhancement can be controlled, and a material can be designed with long-lived ionic Seebeck enhancements