Thermoreversible Gelation of Regioregular Poly(3-hexylthiophene) in Xylene

Programmable Assembly of π-Conjugated Polymers

π-Conjugated polymers have numerous applications due to their advantageous optoelectronic and mechanical properties. These properties depend intrinsically on polymer ordering, including crystallinity, orientation, morphology, domain size, and π-π interactions. Programming, or deliberately controlling the composition and ordering of π-conjugated polymers by well-defined inputs, is a key facet in the development of organic electronics. Here, π-conjugated programming is described at each stage of material development, stressing the links between each programming mode. Covalent programming is performed during polymer synthesis such that complex architectures can be constructed, which direct polymer assembly by governing polymer orientation, π-π interactions, and morphological length-scales. Solution programming is performed in a solvated state as polymers dissolve, aggregate, crystallize, or react in solution. Solid-state programming occurs in the solid state and is governed by polymer crystallization, domain segregation, or gelation. Recent progress in programming across these stages is examined, highlighting order-dependent features and assembly techniques that are unique to π-conjugated polymers. This should serve as a guide for delineating the many ways of directing π-conjugated polymer assembly to control ordering, structure, and function, enabling the further development of organic electronics.

Dynamic Gelation of Conductive Polymer Nanocomposites Consisting of Poly(3-hexylthiophene) and ZnO Nanowires

The sol–gel transition of conductive nanocomposites consisting of poly(3-hexylthiophene) (P3HT) and ZnO nanowires in o-dichlorobenzene (o-DCB) has been investigated rheologically. The physical gelation of P3HT in o-DCB spontaneously occurs upon adding the small amount of ZnO nanowires. The rheological properties of the P3HT/ZnO nanocomposite gels have been systematically studied by varying factors such as polymer concentration, nanowire loading, and temperature. The nanocomposite gel exhibits shear-thinning in the low shear rate range and shear-thickening in the high shear rate range. The elastic storage modulus of the nanocomposite gel gradually increases with gelation time and is consistently independent of frequency at all investigated ranges. The isothermal gelation kinetics has been analyzed by monitoring the storage modulus with gelation time, and the data are well fitted with a first-order rate law. The structural analysis data reveal that the polymer forms the crystalline layer coated on ZnO nanowires. A fringed micelle model is proposed to explain the possible gelation mechanism.

Functionalized single-walled carbon-nanotube-blended P3HT-based high performance memory behavior thin-film transistor devices

We report on the fabrication and transport properties of single-walled carbon nanotubes (SWCNT) blended with P3HT (poly 3-hexyl thiophene-2, 5-diyl). The composite is used as a hybrid organic active channel transistor. The performances of the fabricated devices were investigated as a function of the SWCNTs' loads in the composite, and their response evaluated under white light illumination. Our results show that for SWCNT loads ≤1.5 wt%, all the devices behave as p-type transistors, exhibiting excellent performance, with an I _on /I _off ratio of 10⁴ and a maximum on-state current (I _on) exceeding 80 μA. Moreover, compared with pristine transistors with a P3HT channel, the Hall mobility of these hybrid TFTs was found to increase by more than one order of magnitude, i.e. increasing from 0.062 to 1.54 cm² V^-1 s^-1. Finally, under light illumination, the transfer characteristics (i.e. I _DS as a function of V _GS) were found to systematically undergo a typical shift together with a fully-reversible memory behavior. A fundamental understanding of this work can assist in providing new routes for the development of reliable efficient hybrid organic-based optoelectronic devices.

PBTTT-C16 sol-gel transition by rod associations and networking

A low-molecular-weight poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (designated as Lw-pBTTT-C₁₆) in a fair solvent (chlorobenzene, CB) displays peculiar structural, mechanical, and electronic features during sol-gel transition. Using comprehensive (multiscale) dynamic/static analysis schemes, the Lw-pBTTT-C₁₆/CB solution (10 mg mL^-1) is shown to capitalize on rod associations and networking to form a gel, in stark contrast with its high-molecular-weight companion previously reported to form gels through hierarchical colloidal bridging. The present study reveals, however, that the molecular weight of pBTTT-C₁₆ has a subtle impact on the gelation behaviors through the rarely recognized, contrasting supramolecular conformations (rod-like vs. wormlike) of the aggregate clusters fostered in the pristine solution. The ac conductivity nearly doubles as a result of improved (mesoscale) packing of cylindrical aggregates near the gel state as well as enhanced backbone rigidity of the constituting chains. Other distinguishing features include: (1) there is no real crossover of the dynamic moduli (G' and G'') upon increasing the temperature from gel (T = 15 °C) to solution (T = 80 °C) states. (2) The gel is about a hundredfold softer in dynamic modulus, yet more resilient with a fivefold increase in the yield strain. Both viscoelastic features are expected to greatly benefit the gel processability. (3) The coexistent microgels and cylinder (aggregate) bundles form a peculiar gel network that has not been reported previously with polymer or colloidal gels. The overall findings provide new mechanistic insight into the phenomenological effects of molecular weight for the pBTTT-C_n series in solution, sol, gel, and thin film.

Unconventional Nanofabrication for Supramolecular Electronics

The scientific effort toward achieving a full control over the correlation between structure and function in organic and polymer electronics has prompted the use of supramolecular interactions to drive the formation of highly ordered functional assemblies, which have been integrated into real devices. In the resulting field of supramolecular electronics, self-assembly of organic semiconducting materials constitutes a powerful tool to generate low-dimensional and crystalline functional architectures. These include 1D nanostructures (nanoribbons, nanotubes, and nanowires) and 2D molecular crystals with tuneable and unique optical, electronic, and mechanical properties. Optimizing the (opto)electronic properties of organic semiconducting materials is imperative to harness such supramolecular structures as active components for supramolecular electronics. However, their integration in real devices currently represents a significant challenge to the advancement of (opto)electronics. Here, an overview of the unconventional nanofabrication techniques and device configurations to enable supramolecular electronics to become a real technology is provided. A particular focus is put on how single and multiple supramolecular fibers and gels as well as supramolecularly engineered 2D materials can be integrated into novel vertical or horizontal junctions to realize flexible and high-density multifunctional transistors, photodetectors, and memristors, exhibiting a set of new properties and excelling in their performances.

Dynamic Evolution from Chain Disorder to Order of PTB7 Condensed State Structures under External Fields

In this research, the effect of external fields (solvent, temperature, solution concentration, and external force) on dynamic evolution from chain disorder to order of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5- b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophenediyl]] (PTB7) condensed state structures was explored by UV-vis absorption spectra, atomic force microscope, and transmission electron microscopy (TEM). It was found that PTB7 main chains presented amorphous conformations induced by the poor solvent 1,2-dichloroethane. However, the local ordered aggregation appeared in amorphous conformations when the solubility of the poor solvent was again lowered by reducing temperature. It is worth noting that the size of ordered aggregation was further increased with the decrease of solution concentration or increase of external force. It was found that there were two main PTB7 absorption peaks in the UV-vis absorption spectra; we denoted A_0 -0 for the intensity of the lower energy absorption peak and A_0-1 for the intensity of the higher energy absorption peak. The ratio R = A_0-0/ A_0-1 was used to characterize the dynamic evolution from disorder to order of the PTB7 condensed state structures in absorption spectra. It increased from 0.94 for PTB7 amorphous state to 1.25 for PTB7 large-size ordered aggregation. The dynamic evolution from chain disorder to order could also be distinctly observed by TEM. It was inferred that PTB7 condensed state structures (amorphous state, local ordered aggregation, and large-scale ordered aggregation) might exist simultaneously because of the complexity of copolymer conformations. This research is meaningful to establish physical basis for the molecule design and the synthesis of materials to enhance photoelectronic device efficiency based on condensed matter physics of conjugated polymer.

PBTTT-C16 sol-gel transition by hierarchical colloidal bridging

A versatile conjugated polymer, poly(2,5-bis(3-hexadecyllthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C₁₆, with M_w = 61 309 g mol^-1), in a relatively good solvent (chlorobenzene, CB) medium is shown to produce gels through hierarchical colloidal bridging. Multiscale static/dynamic light and X-ray scattering analysis schemes along with complementary microscopy imaging techniques clearly reveal that upon cooling from the solution state at 80 °C to various gelation temperatures (5, 10, and 15 °C), rod-like colloidal pBTTT-C₁₆ aggregates morph into spherical ones, triggering hierarchical colloid formation and bridging that eventually turn the solution into a gel after about one-day aging. A certain fraction of primal packing units-spherical gelators (∼1 nm in mean radius)-constitute the spherical building particles (∼10 nm) noted above, which in turn constitute loose-packing aggregate clusters (∼300 nm) in the sol state. As gelation proceeds, the aggregate cluster interiors tighten substantially, and micrometer-sized clusters (∼3 μm) formed by them begin to take shape and further interconnect to form the gel network (mean porosity size ∼240 nm and spatial inhomogeneity length ∼20 μm). Rheological measurements and kinetic analysis reveal that the gelation temperature can also have a notable impact on gel microstructure, gelation rate, and mechanical strength, resulting in, for instance, a prominently nonergodic and porous structure for the soft gel incubated at a higher temperature T = 15 °C. The ac conductivity exhibits a notable upturn near pBTTT-C₁₆/CB gelation, well above those achieved by the counterpart pBTTT-C₁₄ solutions, which, in interesting contrast, cannot be brought to the gel phase under similar experimental conditions.

The influence of branching on the Kumada catalyst transfer condensative polymerization of 3-alkylthiophenes

The influence of branching of the substituent in polythiophenes on the rate and the livingness of the polymerization is discussed.

Photoactive composite films prepared from mixtures of polystyrene microgel dispersions and poly(3-hexylthiophene) solutions

Whilst polystyrene microgels belong to the oldest family of microgel particles, their behaviours when deposited onto substrates or prepared as composites have received little attention. Because polystyrene microgels are solvent-swellable, and inherently colloidally stable, they are well suited to form composites with conjugated polymers. Here, we investigate the morphology and light absorption properties of spin coated composite films prepared from mixed dispersions of polystyrene microgels and poly(3-hexylthiophene) (P3HT) for the first time. We compare the morphologies of the composite films to spin coated microgel films. The films were studied using optical microscopy, SEM, AFM, wide-angle X-ray diffraction and UV-visible spectroscopy. The films contained flattened microgel particles with an aspect ratio of ∼10. Microgel islands containing hexagonally close packed particles were evident for both the pure microgel and microgel/P3HT composite films. The latter were electrically conducting. The composite film morphology was dependent on the microgel and P3HT concentration used for film preparation and a morphology phase diagram was constructed. The P3HT phase acted as an electrically conducting cement and increased the robustness of the films to solvent washing. The composite films were photoactive due to the P3HT component. The absorbance for the films was tuneable and increased linearly with both microgel and P3HT concentration. The results of the study should apply to other organic swellable microgel/conjugated polymer combinations and may lead to new colloidal composites for future optoelectronic applications.

Colloidal aggregate and gel incubated by amorphous conjugated polymer in hybrid-solvent medium

A practical valuable amorphous conjugated polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV), has been revealed to foster an abundance of micrometer-sized colloidal aggregates at relatively low concentration (below 1 wt %) in a hybrid-solvent medium that contains a nonsolvent, and the solution turned into gel by colloidal bridging after one-day aging at 30 °C. In contrast with typical polymer gels fostered by (anisotropic) chain cross-linking or planar packing on selective interacting sites, the MEH-PPV gel has been revealed (via dynamic light scattering, small-angle light scattering, time-sweep dynamic modulus and optical microscope) to first develop featureless aggregate clusters in solution and, as the solvent quality worsens with reduced system temperature, bridge themselves to form gel through a one-dimensional (1-D) to three-dimensional (3-D) kinetic pathway. Combined dynamic/static light scattering analyses, along with supporting scanning electron microscope image and molecular dynamics simulation, indicated a concomitant structural reorganization within the colloidal aggregates, where spontaneous chain packing was perceived to form local fiber-like materials that are elastic by nature (i.e., a q-independent decay rate). The near coincidence of the above-mentioned microscopic and macroscopic phase alterations led us to contend that similar fibrous materials have served as the exterior bridging agent to fabricate colloidal strands upon gelation. The present findings clarify previously enigmatic, much speculative, gelation phenomena of MEH-PPV, and shed light on the prospect of capitalizing on specific polymer-solvent interactions to incubate desirable colloidal aggregates and gels in room-temperature processing of practical valuable conjugated polymers.

Controlled gelation of poly(3-alkylthiophene)s in bulk and in thin-films using low volatility solvent/poor-solvent mixtures

Low volatility solvent/non-solvent mixtures were utilized to engineer the structure of poly(3-alkylthiophene) (P3AT) organogels. The ratio of solvent to poor-solvent was manipulated to systematically control the kinetics of self-assembly. Specific solvent mixtures were utilized to produce similar gelation kinetics with polymers of differing alkyl chain length. Simultaneous small angle neutron scattering (SANS) and dielectric spectroscopy was used to probe the structure and the electronic properties of isothermally self-assembled P3ATs. It was determined that alkyl chain length has a dominant effect on the relationship between fiber development and conductivity. Furthermore, using transmission electron microscopy (TEM) and SANS we demonstrate that, for P3HT organogels, lower poor-solvent content leads to fewer fibers with larger nucleation centers while higher poor-solvent amounts results in more fibers and nucleation centers that are smaller and more abundant. This leads to opposing effects in terms of the contribution of the solvent quality to the electrical conductivity. An optimum in conductivity is found at different amounts of poor-solvent for each type of polymer. It is also shown that low volatility solvent mixtures allow for the formation of thermoreversible conjugated polymer organogels in thin-films at room temperature. This work highlights the potential to engineer the structure of conjugated polymer organogels while enabling the use of interconnected gel structures in thin-film organic electronic devices.

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.

Elucidation of the structure of poly(γ-benzyl-l-glutamate) nanofibers and gel networks in a helicogenic solvent

The synthesis, characterization, self-assembly, and gel formation of poly(γ-benzyl-l-glutamate) (PBLG) in a molecular weight range from ca. 7,000–100,000 g/mol and with narrow molecular weight distribution are described. The PBLG is synthesized by the nickel-mediated ring-opening polymerization and is characterized by size-exclusion chromatography coupled with multiple-angle laser light scattering, NMR, and Fourier transform infrared spectroscopy. The self-assembly and thermoreversible gel formation in the helicogenic solvent toluene is investigated by transmission electron microscopy, atomic force microscopy, small-angle X-ray scattering, and synchrotron powder X-ray diffraction. At concentrations significantly below the minimum gelation concentration, spherical aggregates are observed. At higher concentrations, gels are formed, which show a 3D network structure composed of nanofibers. The proposed self-assembly mechanism is based on a distorted hexagonal packing of PBLG helices parallel to the axis of the nanofiber. The gel network forms due to branching and rejoining of bundles of PBLG nanofibers. The network exhibits uniform domains with a length of 200 ± 42 nm composed of densely packed PBLG helices.

Moving through the phase diagram: morphology formation in solution cast polymer-fullerene blend films for organic solar cells

The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.

Competition between fullerene aggregation and poly(3-hexylthiophene) crystallization upon annealing of bulk heterojunction solar cells

Concomitant development of [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) aggregation and poly(3-hexylthiophene) (P3HT) crystallization in bulk heterojunction (BHJ) thin-film (ca. 85 nm) solar cells has been revealed using simultaneous grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS). With enhanced time and spatial resolutions (5 s/frame; minimum q ≈ 0.004 Å(-1)), synchrotron GISAXS has captured in detail the fast growth in size of PCBM aggregates from 7 to 18 nm within 100 s of annealing at 150 °C. Simultaneously observed is the enhanced crystallization of P3HT into lamellae oriented mainly perpendicular but also parallel to the substrate. An Avrami analysis of the observed structural evolution indicates that the faster PCBM aggregation follows a diffusion-controlled growth process (confined by P3HT segmental motion), whereas the slower development of crystalline P3HT nanograins is characterized by constant nucleation rate (determined by the degree of supercooling and PCBM demixing). These two competing kinetics result in local phase separation with space-filling PCBM and P3HT nanodomains less than 20 nm in size when annealing temperature is kept below 180 °C. Accompanying the morphological development is the synchronized increase in electron and hole mobilities of the BHJ thin-film solar cells, revealing the sensitivity of the carrier transport of the device on the structural features of PCBM and P3HT nanodomains. Optimized structural parameters, including the aggregate size and mean spacing of the PCBM aggregates, are quantitatively correlated to the device performance; a comprehensive network structure of the optimized BHJ thin film is presented.

Oriented poly(3-hexylthiophene) nanofibril with the π-π stacking growth direction by solvent directional evaporation

In this article, the uniaxial alignment of poly(3-hexylthiophene) (P3HT) nanofibrils with a π-π stacking growth direction in which P3HT chains adopt a flat-on conformation was obtained by solvent directional evaporation using a glass cover slide and a poly(dimethylsiloxane) (PDMS) sheet to press the P3HT film in a carbon disulfide (CS(2)) atmosphere. By controlling the CS(2) vapor pressure during the film-forming process, we got a well-oriented P3HT film whose order parameter reached as high as 0.97. The orientation of the film was induced by the crystallization nucleation of P3HT and the directional evaporation of the solvent. Under a CS(2) vapor atmosphere, P3HT crystals preferred to adopt the form II modification, which started by nucleation. Owing to the solvent directional evaporation from the center to the margin, P3HT at the center of the sample would precipitate first to induce nucleation. Then the peripheral P3HT would directly diffuse, precipitate, and then adhere to the nucleus to form the uniaxial alignment of P3HT nanofibrils along the direction of solvent evaporation. Furthermore, in the P3HT nanofibrils, the π-π stacking direction of P3HT lamellae was parallel to the crystal growth direction, which would provide an effective path for charge transport.

A new method to improve poly(3-hexyl thiophene) (P3HT) crystalline behavior: decreasing chains entanglement to promote order-disorder transformation in solution

We promoted order-disorder transformation of poly(3-hexylthiophene) (P3HT) in solution by ultrasonic oscillation which substantially improved crystallinity in its pure film. P3HT with low molecular weight (M(w)) dispersed very well in p-xylene solvent and few aggregates generated in the solution. For P3HT with high M(w), the results suggested the coexistence of two phases: disordered coils in solution and ordered microcrystals in suspension. Upon ultrasonic oscillating, more ordered precursors generated in solution due to increased self-assembly from disordered to ordered configuration, which resulting from decreased extent of chains entanglement existing in high molecular system, and red shift at absorption maximum and growing intensity of the pi-pi* absorption at ca. 604 nm were observed in solution. The films prepared from the oscillated solution then showed increased degree of crystallinity, pi-pi interactions and homogeneously distributed nanofibrils, which should be attributed to the ordered precursors constructed in solution. Furthermore, the best crystallinity of the film was obtained at the oscillating time of 4 min, showing the equilibrium state between the increased content of crystalline molecules and the shortened crystalline length. This simple method paves the way for the decreasing chains entanglement during crystallizing process of conjugated polymer in solution, and it enriches the ways to improve crystalline order in thin films comprising crystallizable polymers.