Enhancing the Thermoelectric Performance through the Mutual Interaction between Conjugated Polyelectrolytes and Single-walled Carbon Nanotubes

We present a method of constructing composites composed of conjugated polyelectrolytes (CPEs) and single-walled carbon nanotubes (SWCNTs) to obtain a high-performing flexible thermoelectric generator. In this approach, three kinds of polymers, namely, poly[(1,4-(2,5-didodecyloxybenzene)-alt-2,5-thiophene] (P1), poly[(1,4-(2,5-bis-sodium butoxysulfonate-phenylene)-alt-2,5-thiophene] (P2), and poly[(1,4-(2,5-bis-acid butoxysulfonic-phenylene)-alt-2,5-thiophene] (P3) are designed, synthesized and complexed with SWCNTs as thermoelectric composites. The electrical conductivities of CPEs/SWCNTs (P2/SWCNTs, and P3/SWCNTs) nanocomposites are much higher than those of non-CPEs/SWCNTs (P1/SWCNTs) nanocomposites. Among them, the electrical conductivity of P2/SWCNTs with a ratio of 1:4 reaches 3686 S cm-1, which is 12.4 times that of P1/SWCNTs at the same SWCNT mass ratio. Moreover, CPEs/SWCNTs composites (P2/SWCNTs) display remarkably improved thermoelectric properties with the highest power factor (PF) of 163 μW m-1 K-2. In addition, a thermoelectric generator is fabricated with P2/SWCNTs composite films, and the output power and power density of this generator reach 1.37 μW and 1.4 W m-2 (cross-section) at ΔT=70 K. This result is over three times that of the thermoelectric generator composed of non-CPEs/SWCNTs composite films (P1/SWCNTs, 0.37 μW). The remarkably improved electrical conductivities and thermoelectric properties of the CPEs/SWCNTs composites (P2/SWCNTs) are attributed to the enhanced interaction. This method for constructing CPEs/SWCNTs composites can be applied to produce thermoelectric materials and devices.

Modeling the Interplay of Conformational and Electronic Structure in Conjugated Polyelectrolytes

Conjugated polyelectrolytes (CPEs) combine ionic, electronic, and optical functionality with the mechanical and thermodynamic properties of semiflexible, amphiphilic polyelectrolytes. Critical to CPE design is the coupling between macromolecular conformations, ionic interactions, and electronic transport, the combination of which spans electronic to mesoscopic length scales, rendering coherent theoretical analysis challenging. Here, we utilize a recently developed anisotropic CG model in combination with a phenomenological tight-binding Hamiltonian to explore the interplay of single-chain conformational and electronic structure in CPEs. Accessible single chain conformations are explored as a function of solvent conditions and chain stiffness, reproducing a rich landscape of rod-like, racquet, pearl necklace, and helical conformations observed in previous works. The electronic structure of each conformational archtype is further analyzed, incorporating through-bond coupling, through-space coupling, and electrostatic contributions to the Hamiltonian. Electrostatics is observed to influence electronic structure primarily by modifying the accessible conformational space, and only minimally by direct modulation of on-site energies. Electron transport in CPEs is most efficient in helical and racquet conformations, which is attributed to the flattening of dihedrals and through-space coupling within collapsed conformations. Relatedly, kink formation within racquets does not significantly deteriorate electronic conjugation within CPEs - an insight critical to understanding transport within locally ordered aggregates. These conclusions provide unprecedented computational insight into structure function relationships defining emerging classes of CPEs.

Control of the Optical and Physical Characteristics of Conjugated Polyelectrolytes in the Solution and Solid Phase

<p>Conjugated Polyelectrolytes (CPEs) are a branch of conducting polymers that combine the electronic and solution processability of conjugated polymers (CPs) with the ionic and self-assembling nature of polyelectrolytes. These systems have been shown to exhibit high sensitivity with changes in aggregation state and optical character dependant on the local environment. The ionic character of the CPEs can be used as scaffolds for post-synthetic alterations allowing for control of the optical and physical characteristics. In this thesis, the control of the optical and physical characteristics of the conjugated polyelectrolytes (CPEs) sodium poly[2-(3-thienyl)ethoxyl-4-butylsulfonate] (PTEBS) and poly(9,9-bis[6-(N,N,N-trimethylammonium)hexyl]fluorine-co-altphenylene] (FPQ-X, where X denotes the various counter-ions of the polymers) is investigated though the addition of various extrinsic ions to dilute solutions and concentrated solutions used for film casting, with the main focus being in the solution phase behavior. The CPE characteristics were studied primarily through UV/Vis absorption and fluorescence spectroscopy coupled with dynamic light scattering and surface tension techniques. Controlling the solution phase characteristics of the CPE was investigated through a study of through of solvent composition effects, monovalent and divalent ion addition, organic salt addition, and surfactant additions to dilute aqueous solutions of the CPEs. Solvent composition effects were shown to result in fluorescence enhancement with changes in the polarity of the solvent, while the addition of monovalent and divalent ions was shown to induce fluorescence quenching through ionic strength, ion condensation, and cross-linking of CPE molecules dependant on the concentration and valency of the metal ion. Organic salt additions of a range of concentrations were shown to result in both concentration and alkyl chain length dependant fluorescence intensity enhancements with little changes in the particle size of aggregates in solution. The lack of change in particle size suggested that the effects were localized to the aggregate surface with the size of the organic salt inducing a steric prying effect on the CPE aggregate. A proposed model of this was created to this effect. Large changes in the optical and physical characteristics of the CPEs were found with addition of surfactants to the CPE solutions. Fluorescence quenching and enhancements, particle size increases and decreases, and absorption hypsochromic shifts have been noted, with surfactant structure and concentration dependence. The resulting effects are shown to be hydrophobically, electrostatically, and self-assembly driven. Concentration control of the CPE aggregate size and optical characteristics is completed with surfactant micelles being noted at pre-CMC concentrations within the solutions. A model of interactions at the various concentration levels of surfactant has been developed explaining these results. Transferring this system to the solid state has been shown to exhibit both bathochromic and hypsochromic shifts in absorption and have two optically active phases. The dual phase absorption and emission was attributed to a CPE-surfactant complex where the CPE backbone and surfactant self assemblies result in lamellar type structures within the cast films. The optical overlap of the emission and absorption of the CPEs used was also shown to be favorable for FRET based transfer from FPQ-X to PTEBS. Films created by the layer-by-layer technique showed FRET based signal of PTEBS via excitation of FPQ-Br showing effective FRET based energy transfer between the two species. The absorption signatures of the films with multiple layer-by-layer processes showed that the films do not result in unique layers but rather interdigitated mixtures within the film. Proof of concept P3HT with DOD addition OFET devices were then created in the attempt to alter the electrode potentials using mobile ions. The devices were found to be less efficient than that of the controls due to the disruption of self assembled structures within the devices hampering electron movement.</p>

Control of the Optical and Physical Characteristics of Conjugated Polyelectrolytes in the Solution and Solid Phase

<p>Conjugated Polyelectrolytes (CPEs) are a branch of conducting polymers that combine the electronic and solution processability of conjugated polymers (CPs) with the ionic and self-assembling nature of polyelectrolytes. These systems have been shown to exhibit high sensitivity with changes in aggregation state and optical character dependant on the local environment. The ionic character of the CPEs can be used as scaffolds for post-synthetic alterations allowing for control of the optical and physical characteristics. In this thesis, the control of the optical and physical characteristics of the conjugated polyelectrolytes (CPEs) sodium poly[2-(3-thienyl)ethoxyl-4-butylsulfonate] (PTEBS) and poly(9,9-bis[6-(N,N,N-trimethylammonium)hexyl]fluorine-co-altphenylene] (FPQ-X, where X denotes the various counter-ions of the polymers) is investigated though the addition of various extrinsic ions to dilute solutions and concentrated solutions used for film casting, with the main focus being in the solution phase behavior. The CPE characteristics were studied primarily through UV/Vis absorption and fluorescence spectroscopy coupled with dynamic light scattering and surface tension techniques. Controlling the solution phase characteristics of the CPE was investigated through a study of through of solvent composition effects, monovalent and divalent ion addition, organic salt addition, and surfactant additions to dilute aqueous solutions of the CPEs. Solvent composition effects were shown to result in fluorescence enhancement with changes in the polarity of the solvent, while the addition of monovalent and divalent ions was shown to induce fluorescence quenching through ionic strength, ion condensation, and cross-linking of CPE molecules dependant on the concentration and valency of the metal ion. Organic salt additions of a range of concentrations were shown to result in both concentration and alkyl chain length dependant fluorescence intensity enhancements with little changes in the particle size of aggregates in solution. The lack of change in particle size suggested that the effects were localized to the aggregate surface with the size of the organic salt inducing a steric prying effect on the CPE aggregate. A proposed model of this was created to this effect. Large changes in the optical and physical characteristics of the CPEs were found with addition of surfactants to the CPE solutions. Fluorescence quenching and enhancements, particle size increases and decreases, and absorption hypsochromic shifts have been noted, with surfactant structure and concentration dependence. The resulting effects are shown to be hydrophobically, electrostatically, and self-assembly driven. Concentration control of the CPE aggregate size and optical characteristics is completed with surfactant micelles being noted at pre-CMC concentrations within the solutions. A model of interactions at the various concentration levels of surfactant has been developed explaining these results. Transferring this system to the solid state has been shown to exhibit both bathochromic and hypsochromic shifts in absorption and have two optically active phases. The dual phase absorption and emission was attributed to a CPE-surfactant complex where the CPE backbone and surfactant self assemblies result in lamellar type structures within the cast films. The optical overlap of the emission and absorption of the CPEs used was also shown to be favorable for FRET based transfer from FPQ-X to PTEBS. Films created by the layer-by-layer technique showed FRET based signal of PTEBS via excitation of FPQ-Br showing effective FRET based energy transfer between the two species. The absorption signatures of the films with multiple layer-by-layer processes showed that the films do not result in unique layers but rather interdigitated mixtures within the film. Proof of concept P3HT with DOD addition OFET devices were then created in the attempt to alter the electrode potentials using mobile ions. The devices were found to be less efficient than that of the controls due to the disruption of self assembled structures within the devices hampering electron movement.</p>

Data driven discovery of conjugated polyelectrolytes for optoelectronic and photocatalytic applications

Conjugated polyelectrolytes (CPEs), comprised of conjugated backbones and pendant ionic functionalities, are versatile organic materials with diverse applications. However, the myriad of possible molecular structures of CPEs render traditional, trial-and-error materials discovery strategy impractical. Here, we tackle this problem using a data-centric approach by incorporating machine learning with high-throughput first-principles calculations. We systematically examine how key materials properties depend on individual structural components of CPEs and from which the structure–property relationships are established. By means of machine learning, we uncover structural features crucial to the CPE properties, and these features are then used as descriptors in the machine learning to predict the properties of unknown CPEs. Lastly, we discover promising CPEs as hole transport materials in halide perovskite-based optoelectronic devices and as photocatalysts for water splitting. Our work could accelerate the discovery of CPEs for optoelectronic and photocatalytic applications.