Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene)

Backbone and Side Group Interchain Correlations Govern Wide-Angle X-ray Scattering of Poly(3-hexylthiophene)

Identifying the origin of scattering from polymer materials is crucial to infer structural features that can relate to functional properties. Here, we use our recently developed virtual-site coarse graining to accelerate atomistic simulations and show how various molecular features govern wide-angle X-ray scattering from a conjugated polymer, poly(3-hexylthiophene) (P3HT). The efficient molecular dynamics simulations can represent the structure and capture the emergence of crystalline order from amorphous melts upon cooling while retaining atomistic details of chain configurations. The scattering extracted from simulations shows good agreement with wide-angle X-ray scattering experiments. Amorphous P3HT exhibits broad scattering peaks: a high-q peak from interchain side-group correlations and a low-q peak from interchain backbone-backbone correlations. During amorphous to crystalline phase transitions, the distance between backbones along the side-group direction increases because of lack of interdigitation in the crystalline phase. Scattering from π-π stacking emerges only after crystallization takes place. Intrachain correlations contribute negligibly to the scattering from the amorphous and crystalline phases.

Meticulous Molecular Engineering of Crystal Orientation and Morphology in Conjugated Polymer Thin Films for Field-Effect Transistors

The ability to precisely tailor molecular packing and film morphology in conjugated polymers offers a robust means to control their optoelectronic properties. This, however, remains a grand challenge. Herein, we report the dependency of molecular packing of an important conjugated polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), on a set of intrinsic parameters and unveil the correlation between their crystalline structures and charge transport characteristics. Specifically, a family of PBTTT with varying side chains (i.e., hexyl, octyl, decyl, dodecyl, tetradecyl, and hexadecyl referred to as C6, C8, C10, C12, C14, and C16, respectively) and molecular weights (MWs) with a focus on C14 are judiciously designed and synthesized. Various crystalline structures are yielded by tuning the alkyl chain and MW of PBTTT together with thermal annealing. It reveals that extending the alkyl chain length of PBTTT to C14, along with a larger MW and heating at 180 °C, promotes the formation of edge-on crystallites with significantly improved orientation and ordering. Furthermore, these distinct crystalline structures greatly impact their charge mobilities. This study sheds light on the tailored design of crystalline structures in PBTTT through a synergetic approach, which paves the way for potential applications of PBTTT and other conjugated polymers in optoelectronic devices with enhanced performance.

Chain Length Dependence of Electron Transport in an n-Type Conjugated Polymer with a Rigid-Rod Chain Topology

Most currently known n-type conjugated polymers have a semiflexible chain topology, and their charge carrier mobilities are known to peak at modest chain lengths of below 40-60 repeat units. Herein, we show that the field-effect electron mobility of a model n-type conjugated polymer that has a rigid-rod chain topology grows continuously without saturation, even at a chain length exceeding 250 repeat units. We found the mechanism underlying the novel chain length-dependent electron transport to originate from the reduced structural disorder and energetic disorder with the increasing degree of polymerization inherent to the rigid-rod chain topology. Furthermore, we demonstrate a unique chain length-dependent decay of threshold voltage, which is rationalized by decreased trap densities and trap depths with respect to the degree of polymerization. Our findings provide new insights into the role of polymer chain topology in electron transport and demonstrate the promise of rigid-rod chain architectures for the design of future high-mobility conjugated polymers.

Blade Coating Poly(3-hexylthiophene): The Importance of Molecular Weight on Thin-Film Microstructures

Poly(3-hexylthiophene) is one of the most prevalent and promising conjugated polymers for use in organic electronics. However, the deposition of this material in thin films is highly dependent on the process, such as blade coating versus spin coating and material properties such as molecular weight. Typically, large polymer dispersity makes it difficult to isolate the effect of molecular weight without considering a distribution. In this study, we characterize oligothiophenes of exactly 8, 11, and 14 repeat units, which were deposited into thin films by varying blade coating conditions and postdeposition annealing. From synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS), scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS), Raman microscopy, optical microscopy, and X-ray diffraction (XRD), it was suggested that higher molecular weight polymers exhibit a fast-forming crystalline polymorph (form-1) while low molecular weight polymers exhibit a slow forming polymorph (form-2) with large domain boundaries. As molecular weight is gradually increased, the polymorph formed transitions from form-1 and form-2, where 11 repeat unit oligomers display both polymorphs. We also found that processing conditions can increase the formation of the form-2 polymorph. We also report improved organic thin film transistor (OTFT) performance when form-1 is present. Overall, oligothiophene polymorph formation is highly dependent on the molecular weight and processing conditions, providing critical insight into the importance of polymer weight control in the development of thin-film electronics based on conjugated polymers.

Photoinduced electron transfer in [10]CPP⊃C60 oligomers with stable and well-defined supramolecular structures

Recent synthesis of a new type of polymer containing conjugated cycloparaphenylene (CPP) macrocycles interconnected by a linear conjugated backbone opens up great potential of cyclic π-conjugated materials in organic photovoltaics. In this work, I report a theoretical study of the ground and excited state properties of such polymers and investigate an effect of inclusion of fullerene molecules into polymer chains. MD simulations reveal that oligomers ([10]CPP_Fused⊃C60)24 and ([10]CPP_Fused⊃C60)32 with π-extended CPPs tend to form stable, helix-like structures. I show that photoinduced electron transfer from the CPP-based polymer to C60 fullerene is favorable and occurs on a nanosecond time scale. The hole- and excess-electron transfer rates are found to be significantly higher than the corresponding charge recombination rates.

Microstructural and Thermal Transport Properties of Regioregular Poly(3-hexylthiophene-2,5-diyl) Thin Films

Polymeric thin films offer a wide range of exciting properties and applications, with several advantages compared to inorganic counterparts. The thermal conductivity of such thin films ranges typically between 0.1-1 W m-1 K-1. This low thermal conductivity can cause problems with heat dissipation in various applications. Detailed knowledge about thermal transport in polymeric thin films is desired to overcome these shortcomings, especially in light of the multitude of possible microstructures for semi-crystalline thin films. Therefore, poly(3-hexylthiophene-2,5-diyl) (P3HT) is chosen as a model system to analyze the microstructure and optoelectronic properties using X-ray scattering and absorption spectra along with the thermal transport properties using the photoacoustic technique. This combination of analysis methods allows for determining the optoelectronic and thermal transport properties on the same specimen, supplemented by structural information. The effect of different molecular weights and solvents during film preparation is systematically examined. A variation of the optoelectronic properties, mainly regarding molecular weight, is apparent, while no direct influence of the solvent during preparation is discernible. In contrast, the thermal conductivities of all films examined fall within a similar range. Therefore, the microstructural properties in the ordered regions do not significantly affect the resulting thermal properties in the sample space investigated in this work. We conclude that it is mainly the amorphous regions that determine the thermal transport properties, as these represent a bottleneck for thermal transport.

Design of Acrylate-Terminated Polyurethane for Nylon Seamless Bonding Fabric Part I: Design of the End-Capping Thermoplastic Polyurethane Adhesive with Acrylate Copolymer

This series of studies aims to design acrylate-terminated polyurethanes for use in nylon seamless bonded fabrics. The first part used N,N-dimethylacrylamide (DMAA) and methyl methacrylate (MMA) to replace the chain extender in polyurethane synthesis as end-capping agent to synthesize thermoplastic polyurethane (TPU) adhesive. The molecular weight of the TPU is controlled to further influence the mechanical and processing properties of the polyurethane. Here, polytetramethylene ether glycol (PTMG) and 4,4-methylene diphenyl diisocyanate (MDI) were polymerized, and then a blocking agent was added thereto. The results show that the characteristic peaks of benzene ring and carbamate of TPU adhesive are at 1596 cm−1 and 1413 cm−1, respectively, while the characteristic peaks of DMAA are at 1644 cm−1 and 1642 cm−1 in the FT-IR spectrum. There is an absorption peak –N=C=O– which is not shown near 2268 cm−1, which proves that the structure of TPU contains the molecular structure of capping agent, PTMG and MDI. When the DMAA concentration in the capping agent was increased from 3.0 wt% to 10 wt%, the –C=O (H-bond) area percentage of hydrogen bonds formed at 1711 cm−1 increased from 41.7% to 57.6%, while the –NH (H bond) produced at 3330 cm−1 increased from 70% to 81%. These phenomena suggest that increasing the concentration of DMAA capping agent can effectively promote the formation of complex supramolecular network structures by hydrogen bonding in TPU. The content and concentration of the capping agent affects the molecular weight of the TPU. Chain growth is terminated when molecular weight growth can be effectively controlled and reduced. It was observed in thermal analysis that with increasing DMAA concentration in the molecular structure, the concentration of capping agent in TPU, hydrogen bonding force between hard segments, melting point (Tmh) and melting enthalpy (ΔH) all increased the capping agent. The pyrolysis temperature of TPU is increased by 10–20 °C.

Measuring the Pores’ Structure in P3HT Organic Polymeric Semiconductor Films Using Interface Electrolyte/Organic Semiconductor Redox Injection Reactions and Bulk Space-Charge

The article is another in a series of follow-up articles on the new spectroscopic method Energy Resolved–Electrochemical Impedance Spectroscopy (ER-EIS) and presents a continuation of the effort to explain the method for electronic structure elucidation and its possibilities in the study of organic polymeric semiconductors. In addition to the detailed information on the electronic structure of the investigated organic semiconductor, the paper deals with three of the hitherto not solved aspects of the method, (1) the pores structure, which has been embedded in the evaluation framework of the ER-EIS method and shown, how the basic quantities of the pores structure, the volume density of the pores’ density coefficient β = (0.038 ± 0.002) nm⁻¹ and the Brunauer-Emmet-Teller surface areas SABET SA == 34.5 m²g⁻¹ may be found by the method, here for the archetypal poly(3-hexylthiophene-2,5-diyl) (P3HT) films. It is next shown, why the pore’s existence needs not to endanger the spectroscopic results of the ER-EIS method, and a proper way of the ER-EIS data evaluation is presented to avoid it. It is highlighted (2), how may the measurements of the pore structure contribute to the determination of the, for the method ER-EIS important, real rate constant of the overall Marcus’ D-A charge-transfer process for the poreless material and found its value k_ctD-A = (2.2 ± 0.6) × 10⁻²⁵ cm⁴ s⁻¹ for P3HT films examined. It is also independently attempted (3) to evaluate the range of k_ctD-A, based on the knowledge of the individual reaction rates in a chain of reactions, forming the whole D-A process, where the slowest one (organic semiconductor hopping transport) determines the tentative total result k_ctD-A ≅ 10⁻²⁵ cm⁴ s⁻¹. The effect of injection of high current densities by redox interface reactions in the bulk of OS with built-in pores structure may be very interesting for the design of new devices of organic electronics.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Synthesis of Donor–Acceptor Copolymers Derived from Diketopyrrolopyrrole and Fluorene via Eco-Friendly Direct Arylation: Nonlinear Optical Properties, Transient Absorption Spectroscopy, and Theoretical Modeling

A series of PFDPP copolymers based on fluorene (F) and diketopyrrolopyrrole (DPP) monomers were synthesized via direct arylation polycondensation using Fagnou conditions which involved palladium acetate as catalyst (a gradual catalyst addition of three different percentages were used), potassium carbonate as the base, and neodecanoic acid in N, N-dimethylacetamide. This synthesis provides a low cost compared with traditional methods of transition-metal-catalyzed polymerization. Among the different amounts of catalyst used in the present work, 12% was optimal because it gave the highest reaction yield (81.5%) and one of the highest molecular weights (Mn = 13.8 KDa). Copolymers’ chemical structures, molecular weight distributions, and optical and thermal properties were analyzed. The linear optical properties of PFDPP copolymers resulted very similarly independently to the catalyst amounts used in the synthesis of the PFDPP copolymers: two absorptions bands distinctive of donor–acceptor copolymers, Stokes shifts of 41 nm, a good quantum yield of fluorescence around 47%, and an optical bandgap of 1.7 eV were determined. Electronic nonlinearities were observed in these copolymers with a relatively high two-photon absorption cross-section of 621 GM at 950 nm. The dynamics of excited states and aggregation effects were studied in solutions, nanoparticles, and films of PFDPP. Theoretical calculations modeled the ground-state structures of the (PFDPP)n copolymers with n = 1 to 4 units, determining the charge distribution by the electrostatic potential and modeling the absorption spectra determining the orbital transitions responsible for the experimentally observed leading bands. Experimental and theoretical structure–properties analysis of these donor–acceptor copolymers allowed finding their best synthesis conditions to use them in optoelectronic applications.

O-ATRP synthesized poly(β-pinene) blended with chitosan for antimicrobial and antioxidant bio-based films production

Antioxidant and antimicrobial activities are important characteristics of active film packaging designed to extend food preservation. In this study, functional bio-based films were produced using different concentrations of antioxidant poly(β-pinene) bio-oligomer synthesized via organocatalyzed atom transfer radical polymerization (O-ATRP) and blended with chitosan of different molecular weights. The structural, mechanical, thermal, solubility, antioxidant, and antimicrobial properties of the films were investigated. The poly(β-pinene)-chitosan blends presented significant pores and irregularities with the increase of poly(β-pinene) concentration over 30%. Chitosan molecular weight did not show any important influence in the physical properties of the blends. Poly(β-pinene) load decreased the materials' tensile strength and melting temperature, exhibiting a plasticizing effect on chitosan chains. The antioxidant and antimicrobial activities of the films were improved by poly(β-pinene) incorporation and mainly depended on its concentration. Therefore, the incorporation of poly(β-pinene) in chitosan films can be an alternative for active packaging production.

Molecular Weight-Dependent Physical and Photovoltaic Properties of Poly(3-alkylthiophene)s with Butyl, Hexyl, and Octyl Side-Chains

A series of poly-3-alkylthiophenes (P3ATs) with butyl (P3BT), hexyl (P3HT), and octyl (P3OT) side-chains and well-defined molecular weights (MWs) were synthesized using Grignard metathesis polymerization. The MWs of P3HTs and P3OTs obtained via gel permeation chromatography agreed well with the calculated MWs ranging from approximately 10 to 70 kDa. Differential scanning calorimetry results showed that the crystalline melting temperature increased with increasing MWs and decreasing alkyl side-chain length, whereas the crystallinity of the P3ATs increased with the growth of MWs. An MW-dependent red shift was observed in the UV-Vis and photoluminiscence spectra of the P3ATs in solution, which might be a strong evidence for the extended effective conjugation occurring in polymers with longer chain lengths. The photoluminescence quantum yields of pristine films in all polymers were lower than those of the diluted solutions, whereas they were higher than those of the phenyl-C61-butyric acid methyl ester-blended films. The UV-Vis spectra of the films showed fine structures with pronounced red shifts, and the interchain interaction-induced features were weakly dependent on the MW but significantly dependent on the alkyl side-chain length. The photovoltaic device performances of the P3BT and P3HT samples significantly improved upon blending with a fullerene derivative and subsequent annealing, whereas those of P3OTs mostly degraded, particularly after annealing. The optimal power conversion efficiencies of P3BT, P3HT, and P3OT were 2.4%, 3.6%, and 1.5%, respectively, after annealing with MWs of ~11, ~39, and ~38 kDa, respectively.

Highly Stable Pickering Emulsions with Xylan Hydrate Nanocrystals

Xylan is a highly abundant plant-based biopolymer. Original xylans in plants are in an amorphous state, but deacetylated and low-branched xylan can form a crystalline structure with water molecules. The utilizations of xylan have been limited to bulk applications either with inconsistency and uncertainty or with extensive chemical derivatization due to the insufficient studies on its crystallization. The applications of xylan could be greatly broadened in advanced green materials if xylan crystals are effectively utilized. In this paper, we show a completely green production of nano-sized xylan crystals and propose their application in forming Pickering emulsions. The branches of xylan were regulated during the separation step to controllably induce the formation of xylan hydrate crystals. Xylan hydrate nanocrystals (XNCs) with a uniform size were successfully produced solely by a mild ultrasonic treatment. XNCs can be adsorbed onto oil-water interfaces at a high density to form highly stable Pickering emulsions. The emulsifying properties of XNCs were comparable to some synthetic emulsifiers and better than some other common biopolymer nanocrystals, demonstrating that XNCs have great potential in industrial emulsification.

Role of Postdeposition Thermal Annealing on Intracrystallite and Intercrystallite Structuring and Charge Transport in Poly(3-hexylthiophene)

The performance of electronic devices comprising conjugated polymers as the active layer depends not only on the intrinsic characteristics of the materials but also on the details of the extrinsic processing conditions. In this study, we examine the effect of postdeposition thermal treatments on the microstructure of poly(3-hexylthiophene) (P3HT) thin films and its impact on their electrical properties. Unsurprisingly, we find thermal annealing of P3HT thin films to generally increase their crystallinity and crystallite coherence length while retaining the same crystal structure. Despite such favorable structural improvements of the polymer active layers, however, thermal annealing at high temperatures can lead to a net reduction in the mobility of transistors, implicating structural changes in the intercrystallite amorphous regions of these semicrystalline active layers take place on annealing, and the simplistic picture that crystallinity governs charge transport is not always valid. Our results instead suggest tie-chain pullout, which occurs during crystal growth and perfection upon thermal annealing to govern charge transport, particularly in low-molecular-weight systems in which the tie-chain fraction is low. By demonstrating the interplay between intracrystallite and intercrystallite structuring in determining the macroscopic charge transport, we shed light on how structural evolution and charge-transport properties of nominally the same polymer can vary depending on the details of processing.

Multiscale modelling of charge transport in P3HT:DIPBI bulk heterojunction organic solar cells

Charge transport properties of a P3HT:DIPBI bulk heterojunction solar cell are modelled by kinetic Monte Carlo simulations based on a morphology obtained from coarse-grained molecular dynamics. Different methods for calculating the hopping integrals entering the charge transfer rates are compared and calibrated for hole transport in amorphous P3HT. The influence of intermolecular and intramolecular charge transfer on the total charge carrier mobility and hence the power conversion efficiency is investigated in detail. An analysis of the most probable pathways with low resistance for hole transport is performed, establishing a connection between charge mobility and morphology.

Use of diisocyanate to enhance the flame-retardant, mechanical and crystalline properties of poly (butylene succinate-co-butylene 3-hydroxyphenylphosphinyl-propionate) (PBSH)

Poly (butylene succinate-co-butylene-3-hydroxyphenylphosphinyl-propionate) (PBSH) was synthesized through polycondensation. Then, 4,4′-methylene diphenyl diisocyanate (MDI) was introduced into PBSH matrix by reactive blending. The chemical structure and properties of the blending products were investigated using Fourier Transform Infrared Spectroscopy (FT-IR), differential scanning calorimetry (DSC), limiting oxygen index (LOI) tests, thermogravimetric analysis (TGA), and vertical burning tests. The results proved that MDI can improve the mechanical properties and flame retardancy of PBSH. In addition, it was found that the crosslinking structure can reduce the hydrolysis rate of PBSH and effectively eliminate the melt-dripping of PBSH during combustion.

Synthesis and characterization of donor–acceptor semiconducting polymers containing 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole for organic solar cells

D–A polymers containing 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione were successfully synthesized and applied for organic solar cells.

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.

Implication of Molecular Weight on Optical and Charge Transport Anisotropy in PQT-C12 Films Fabricated by Dynamic FTM

Large area (>20 cm × 2 cm)-oriented thin films of PQT-C12 with varying molecular weight and polydispersity index (PDI) were fabricated by the ribbon-shaped floating film transfer method aiming toward their application as an active semiconductor element of organic field effect transistors (OFETs). Investigation on the influence of the molecular weight and PDI upon the extent of molecular alignment and anisotropic charge transport was systematically carried out. It has been demonstrated that high molecular weight in combination with low PDI not only leads to a very high optical anisotropy >10 but also high charge carrier anisotropy with a hole mobility of about 0.07 cm²/V·s for OFETs using parallel-oriented PQT-C12 thin films. Such a structure-property correlation is highly beneficial for the development of high performance organic electronic devices by synergistic and amicable tuning of the optoelectronic anisotropies and polymer synthetic variables.

Poly(3-alkylthiophene)- block-poly(3-alkylselenophene)s: Conjugated Diblock Co-polymers with Atypical Self-Assembly Behavior

Understanding self-assembly behavior and resulting morphologies in block co-polymer films is an essential aspect of chemistry and materials science. Although the self-assembly of amorphous coil-coil block co-polymers is relatively well understood, that of semicrystalline block co-polymers where each block has distinct crystallization properties remains unclear. Here, we report a detailed study to elucidate the rich self-assembly behavior of conjugated thiophene-selenophene (P3AT- b-P3AS) block co-polymers. Using a combination of microscopy and synchrotron-based X-ray techniques, we show that three different film morphologies, denoted as lamellae, co-crystallized fibers, and patchy fibers, arise from the self-assembly of these block co-polymers over a relatively narrow range of overall degrees of polymerization (30 < N < 90). Crystallization-driven phase separation occurs at a very low N (<35), and lamellar films are formed. Conversely, at medium N (50-60) and high N (>80), the thiophene and selenophene blocks co-crystallize into nanofibers, where medium N leads to much more mixing than high N. The overall tendency for phase separation in these systems follows rather different trends than phase separation in amorphous polymers in that we observe the greatest degree of phase separation at the lowest N. Finally, we demonstrate how each morphology influences transport properties in organic thin-film transistors comprised of these conjugated polymers.

Self-ordering promoted by the nanoconfinement of poly(3-hexylthiophene) and its nanocomposite with single-walled carbon nanotubes

Nanostructuration and self-ordering of semiconducting organic materials are required to fabricate highly efficient photovoltaic and photoemissive devices. In this work, we investigated the combined effect of melt-assisted template processing and self-ordering of high purity regio-regular poly (3-hexylthiophene) (P3HT) to obtain nanofibers of P3HT and of P3HT-single-walled carbon nanotubes (SWNT) nanocomposites. An original ordering of the polymer and the carbon nanotubes within the nanofibers, as well as their surprising anisotropic photoluminescent properties were determined by vibrational and optical spectroscopy. It was attributed to the combined effect of the melt-assisted wetting confined within alumina nanopores, altogether with the self-organization of both P3HT chains on the one hand, and of the P3HT charged with SWNT on the other hand. It is proposed that the well-ordered regio-regular P3HT matrix orientation is promoted by the interaction with the alumina pore surface and the 1D confinement. For the composite case, the P3HT matrix imposes additionally a preferential orientation of the SWNT transversal to the nanofiber axis. This original organization is responsible for the unexpected polarization of the composite nanofibers photoluminescence. This work opens the way to alternative methods for tackling challenges of nanofabrication to obtain more efficient optoelectronic nanodevices.

Atomistic modelling of entropy driven phase transitions between different crystal modifications in polymers: the case of poly(3-alkylthiophenes)

Polymorphism and related solid-state phase transitions affect the structure and morphology and hence the properties of materials, but they are not-so-well understood. Atomistic computational methods can provide molecular-level insights, but they have rarely proven successful for transitions between polymorphic forms of crystalline polymers. In this work, we report atomistic molecular dynamics (MD) simulations of poly(3-alkylthiophenes) (P3ATs), widely used organic semiconductors to explore the experimentally observed, entropy-driven transition from form II to more common form I type polymorphs, or, more precisely, to form I mesophases. The transition is followed continuously, also considering X-ray diffraction evidence, for poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT), evidencing three main steps: (i) loss of side chain interdigitation, (ii) partial disruption of the original stacking order and (iii) reorganization of polymer chains into new, tighter, main-chain stacks and new layers with characteristic form I periodicities, substantially larger than those in the original form II. The described approach, likely applicable to other important transitions in polymers, provides previously inaccessible insight into the structural organization and disorder features of form I structures of P3ATs, not only in their development from form II structures but also from melts or solutions.

Effect of External Electric Field on the Ordered Structure of Molecular Chains and Hole Mobility in Regioregular Poly(3-hexylthiophene) with Different Molecular Weights

This research investigated the effect of a high-voltage external electric field on the ordered structure of molecular chains and hole mobility in regioregular poly(3-hexylthiophene) (P3HT) with different molecular weights through X-ray diffraction, atomic force microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, micro-Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, and organic field-effect transistors. The optimal magnitude of the external electric field was 5000 V/cm. With the optimized electric field applied to a series of P3HT films, the carrier mobility of all P3HT films increased, and the increase rate changed from 105% to 56%, closely depending on the increase in molecular weight from 33 kg/mol to 100 kg/mol. The results indicated that the increase in carrier mobility was attributed to the P3HT conformation order, which was controlled by the external electric field. Molecular weight was a critical factor in determining the P3HT conformation response to the external electric field. The external electric field orientated lower-molecular-weight (33 kg/mol) P3HT into ordered structures more obviously than higher-molecular-weight (100 kg/mol) P3HT. This research contributes to the understanding of the effect of an external electric field on the ordered structure of the chains and carrier mobility in P3HT with different molecular weights. This research also reveals the regularity and mechanism of the formation of ordered structures and essentially enhances the carrier mobility of P3HT films with different molecular weights, to fabricate photovoltaic devices with high efficiency, based on polymer physics.

A Polymer Blend Approach for Creation of Effective Conjugated Polymer Charge Transport Pathways

Understanding the role of the distribution of polymer chain lengths on process-structure-property relationships in semiconducting organic electronics has remained elusive due to challenges in synthesizing targeted molecular weights ( M_w) and polydispersity indices. Here, a facile blending approach of various poly(3-hexylthiophene) (P3HT) molecular weights is used to investigate the impact of the distribution of polymer chain lengths on self-assembly into aggregates and associated charge transport properties. Low and high M_w samples were blended to form a highly polydisperse sample which was compared to a similar, medium M_w control. Self-assembly was induced by preprocessing the polymer solution with UV irradiation and subsequent solution aging before deposition via blade-coating. Superior charge carrier (hole) mobilities were observed for the blend and control samples. Furthermore, their solution lifetimes exceeded 14 days. UV-vis spectral analysis suggests that low M_w P3HT lacks the mesoscale crystallinity required for percolative charge transport. In contrast, when the M_w is too high, the polymer rapidly aggregates, leading to paracrystalline disorder and structural inhomogeneity that interrupts charge-transfer pathways. The role of grain boundaries, fibrillar order, and macroscale alignment is characterized via grazing-incidence wide-angle X-ray scattering, atomic force microscopic, and optical microscopic techniques. The results presented here provide additional guidance on the interplay between polymer solubility, self-assembly, network interconnectivity, and charge transport to enable robust polymer ink formulations with reliable and reproducible performance attributes.

Organic n-Channel Transistors Based on [1]Benzothieno[3,2- b]benzothiophene-Rylene Diimide Donor-Acceptor Conjugated Polymers

Improving the charge-carrier mobility of conjugated polymers is important for developing high-performance, solution-processed optoelectronic devices. Although [1]benzothieno[3,2- b]benzothiophene (BTBT) has been frequently used as a high-performance p-type small molecular semiconductor and employed a few times as a building block for p-type conjugated polymers, it has never been explored as a donor moiety for high-performance n-type conjugated polymers. Here, BTBT has been conjugated with either n-type perylene diimide (PDI) or naphthalene diimide (NDI) units to generate a donor-acceptor copolymer backbone, for the first time. Charge-transport measurements of organic field-effect transistors show n-type dominant behaviors, with the electron mobility reaching ∼0.11 cm² V^-1 s^-1 for PDI-BTBT and ∼0.050 cm² V^-1 s^-1 for NDI-BTBT. The PDI-BTBT mobility value is one of the highest among the PDI-containing polymers. The high π-π stacking propensity of BTBT significantly improves the charge-carrier mobility in these polymers, as supported by atomic force microscopy and grazing incidence X-ray diffraction analyses. Phototransistor applications of these polymers in the n-type mode show highly sensitive photoresponses. Our findings demonstrate that incorporation of the BTBT donor unit within the rylene diimide acceptor-based conjugated polymers can improve the molecular ordering and electron mobility.

N-Benzoyl dithieno[3,2-b:2',3'-d]pyrrole-based hyperbranched polymers by direct arylation polymerization

BACKGROUND

Although poly(N-acyl dithieno[3,2-b:2',3'-d]pyrrole)s have attracted great attention as a new class of conducting polymers with highly stabilized energy levels, hyperbranched polymers based on this monomer type have not yet been studied. Thus, this work aims at the synthesis of novel hyperbranched polymers containing N-benzoyl dithieno[3,23,2-b:2',3'-d]pyrrole acceptor unit and 3-hexylthiophene donor moiety via the direct arylation polymerization method. Their structures, molecular weights and thermal properties were characterized via 1H NMR and FTIR spectroscopies, GPC, TGA, DSC and XRD measurements, and the optical properties were investigated by UV-vis and fluorescence spectroscopies.

RESULTS

Hyperbranched conjugated polymers containing N-benzoyl dithieno[3,23,2-b:2',3'-d]pyrrole acceptor unit and 3-hexylthiophene donor moiety, linked with either triphenylamine or triphenylbenzene as branching unit, were obtained via direct arylation polymerization of the N-benzoyl dithieno[3,23,2-b:2',3'-d]pyrrole, 2,5-dibromo 3-hexylthiophene and tris(4-bromophenyl)amine (or 1,3,5-tris(4-bromophenyl)benzene) monomers. Organic solvent-soluble polymers with number-average molecular weights of around 18,000 g mol-1 were obtained in 80-92% yields. The DSC and XRD results suggested that the branching structure hindered the stacking of polymer chains, leading to crystalline domains with less ordered packing in comparison with the linear analogous polymers. The results revealed that the hyperbranched polymer with triphenylbenzene as the branching unit exhibited a strong red-shift of the maximum absorption wavelength, attributed to a higher polymer stacking order as a result of the planar structure of triphenylbenzene.

CONCLUSION

Both hyperbranched polymers with triphenylamine/triphenylbenzene as branching moieties exhibited high structural order in thin films, which can be promising for organic solar cell applications. The UV-vis absorption of the hyperbranched polymer containing triphenylbenzene as branching unit was red-shifted as compared with the triphenylamine-containing polymer, as a result of a higher chain packing degree.

Photoluminescence Properties of Polymorphic Modifications of Low Molecular Weight Poly(3-hexylthiophene)

The structural and photoluminescence (PL) properties of thin films of poly(3-hexylthophene) (P3HT) with molecular weights (MWs) of 3000 and 13,300 have been investigated. Although high MW P3HT always self-organizes into one packing structure (form I), low MW P3HT forms two different packing structures (forms I and II) depending on the fabrication conditions. In this work, several fabrication techniques have been examined to obtain form II samples with little inclusion of a form I component. It is found that drop-cast thin films of low MW P3HT (form II) exhibit a PL spectrum that is different from that of form I and does not contain the form I component. The PL spectrum can thus be attributed to form II. The differences in PL properties between forms I and II can be understood in terms of weakened interchain interactions due to the longer interchain distance in form II.