Dalsu Choi

Solvent Based Hydrogen Bonding: Impact on Poly(3-hexylthiophene) Nanoscale Morphology and Charge Transport Characteristics

We demonstrate that supramolecular assembly and subsequent enhancement of charge transport characteristics of conjugated polymers can be facilitated simply by adding small amounts of a more volatile poor solvent, which can hydrogen bond with the majority solvent. Addition of up to 2 vol % acetone to a precursor solution of poly(3-hexylthiophene) (P3HT) in chloroform leads to approximately a 4-fold increase in P3HT field-effect mobility. The improvement is associated with hydrogen bonding interactions between acetone and chloroform which decrease the evaporation rate of the mixed solvent. P3HT is less soluble in the binary solvent than in the more readily vaporized chloroform component, and this characteristic enables the supramolecular assembly of P3HT chains at the nanoscale. Two-dimensional molecular ordering of the polymer film was controlled by varying the quantity of poor solvent added to the precursor solution, and the correlation between field-effect mobility and molecular ordering was investigated. Hansen solubility parameters were used to systematically understand how the solvent mixture enhances the alignment and assembly of polymer chains and influences subsequent thin film properties. The value of the relative energy difference (RED) of the solvent with respect to P3HT increased from less than 1 to more than 1 during film formation, which indicates that the solvent characteristics are initially those of a good solvent but transform into those of a poor dissolution medium. A mechanistic illustration of the molecular ordering process during film formation is postulated.

Ultrasound-induced ordering in poly(3-hexylthiophene): role of molecular and process parameters on morphology and charge transport.

Facile methods for controlling the microstructure of polymeric semiconductors are critical to the success of large area flexible electronics. Here we explore ultrasonic irradiation of solutions of poly(3-hexylthiophene) (P3HT) as a simple route to creating ordered molecular aggregates that result in a one to two order of magnitude improvement in field effect mobility. A detailed investigation of the ultrasound induced phenomenon, including the role of solvent, polymer regioregularity (RR) and film deposition method, is conducted. Absorption spectroscopy reveals that the development of low energy vibronic features is dependent on both the regioregularity as well as the solvent, with the latter especially influential on the intensity and shape of the band. Use of either higher regioregular polymer or ultrasonic irradiation of lower regioregular polymer solutions results in high field effect mobilities that are nearly independent of the dynamics of the film formation process. Surprisingly, no distinct correlation between thin-film morphology and macroscopic charge transport could be ascertained. The relationships between molecular and process parameters are very subtle: modulation of one effects changes in the others, which in turn impact charge transport on the macroscale. Our results provide insight into the degree of control that is required for the development of reproducible, robust materials and processes for advanced flexible electronics based on polymeric materials.

Anisotropic Assembly of Conjugated Polymer Nanocrystallites for Enhanced Charge Transport

The anisotropic assembly of P3HT nanocrystallites into longer nanofibrillar structures was demonstrated via sequential UV irradiation after ultrasonication to the pristine polymer solutions. The morphology of resultant films was studied by atomic force microscopy (AFM), and quantitative analysis of intra- and intermolecular ordering of polymer chains was performed by means of static absorption spectroscopy and quantitative modeling. Consequently, the approach to treat the precursor solution enhanced intra- and intermolecular ordering and reduced the incidence of grain boundaries within P3HT films, which contributed to the excellent charge carrier transport characteristics of the corresponding films (μ ≈ 12.0 × 10(-2) cm(2) V(-1) s(-1) for 96% RR P3HT).

Toward Uniformly Dispersed Battery Electrode Composite Materials: Characteristics and Performance

Battery electrodes are complex mesoscale systems comprised of electroactive components, conductive additives, and binders. In this report, methods for processing electrodes with dispersion of the components are described. To investigate the degree of material dispersion, a spin-coating technique was adopted to provide a thin, uniform layer that enabled observation of the morphology. Distinct differences in the distribution profile of the electrode components arising from individual materials physical affinities were readily identified. Hansen solubility parameter (HSP) analysis revealed pertinent surface interactions associated with materials dispersivity. Further studies demonstrated that HSPs can provide an effective strategy to identify surface modification approaches for improved dispersions of battery electrode materials. Specifically, introduction of surfactantlike functionality such as oleic acid (OA) capping and P3HT-conjugated polymer wrapping on the surface of nanomaterials significantly enhanced material dispersity over the composite electrode. The approach to the surface treatment on the basis of HSP study can facilitate design of composite electrodes with uniformly dispersed morphology and may contribute to enhancing their electrical and electrochemical behaviors. The conductivity of the composites and their electrochemical performance was also characterized. The study illustrates the importance of considering electronic conductivity, electron transfer, and ion transport in the design of environments incorporating active nanomaterials.

Photoinduced Anisotropic Assembly of Conjugated Polymers in Insulating Polymer Blends

Low-dose UV irradiation of poly(3-hexylthiophene) (P3HT)-insulating polymer (polystyrene (PS) or polyisobutylene (PIB)) blend solutions led to the formation of highly ordered P3HT nanofibrillar structures in solidified thin films. The P3HT nanofibers were effectively interconnected through P3HT islands phase-separated from insulating polymer regions in blend films comprising a relatively low fraction of P3HT. Films prepared with a P3HT content as low as 5 wt % exhibited excellent macroscopic charge transport characteristics. The impact of PS on P3HT intramolecular and intermolecular interactions was systematically investigated. The presence of PS chains appeared to assist in the UV irradiation process of the blend solutions to facilitate molecular interactions of the semiconductor component, and to enhance P3HT chain interactions during spin coating because of relatively unfavorable P3HT-PS chain interactions. However, P3HT lamellar packing was hindered in the presence of PS chains, because of favorable hydrophobic interactions between the P3HT hexyl substituents and the PS chains. As a result, the lamellar packing d-spacing increased, and the coherence length corresponding to the lamellar packing decreased, as the amount of PS in the blend films increased.

Macroscopic Alignment of One-Dimensional Conjugated Polymer Nanocrystallites for High-Mobility Organic Field-Effect Transistors

Controlling the morphology of polymer semiconductors remains a fundamental challenge that hinders their widespread applications in electronic and optoelectronic devices and commercial feasibility. Although conjugated polymer nanowires (NWs) are envisioned to afford high charge-carrier mobility, the alignment of preformed conjugated polymer NWs has not been reported. Here, we demonstrate an extremely simple and effective strategy to generate well-aligned arrays of one-dimensional (1D) polymer semiconductors that exhibit remarkable enhancement in charge transport using a solution shear-coating technique. We show that solution shear coating of poly(alkylthiophene) NWs induces extension or coplanarization of the polymer backbone and highly aligned network films, which results in enhanced intra- and intermolecular ordering and reduced grain boundaries. Consequently, highly aligned poly(3-hexylthiophene) NWs exhibited over 33-fold enhancement in the average carrier mobility, with the highest mobility of 0.32 cm(2) V(-1) s(-1) compared to pristine films. The presented platform is a promising strategy and general approach for achieving well-aligned 1D nanostructures of polymer semiconductors and could enable the next generation of high-performance flexible electronic devices for a wide range of applications.