Regioregular Poly Research Articles

Aqueous deposition of a semiconducting polymer by electrocoating

Abstract An aqueous-based deposition of a semiconducting polymer using an electrolysis-based technique is demonstrated here. Regioregular poly (3-alkylthiophene) with a carboxylic acid group in the side chain was synthesized and dispersed in water with triethylamine to create a quaternary ammonium salt. The dispersion was then successfully electrocoated onto galvanized steel, forming films with good uniformity in just a few seconds. The optoelectronic properties of these new films were compared with that of organic solvent-casted regioregular poly (3-hexylthiophene). Optical and electronic properties of the electrocoated films were similar to that of the organic solvent processed films, indicating that the semiconducting polymer was stable to the voltages and aqueous environment used in the electrocoating process. These results suggest that electrodeposition is a promising deposition method for organic electronic applications including conformal deposition onto complex shaped surfaces for low-cost and environmentally friendly nanoscale film formation.

Water-soluble polythiophenes as efficient charge-transport layers for the improvement of photovoltaic performance in bulk heterojunction polymeric solar cells

Water-soluble regioregular poly{3-[(6-sodium sulfonate)hexyl]thiophene} (PT6S) and poly{3-[(6-trimethylammoniumbromide)hexyl]thiophene} (PT6N) have been synthesized and employed both as photoactive layers for the assembling of “green” bulk-heterojunction organic solar cells and as charge-collection layers in a cell with “classic” architecture. While the photovoltaic performances obtained with the two aforementioned polymers were lower than the reference cell, their latter use allowed to notably increase the inherent J-V properties, leading to a considerable enhancement in the overall photovoltaic output. The power conversion efficiency of the optimized multilayer BHJ solar cell reached 4.78%, revealing a higher efficiency than the reference cell (3.63%).

Supramolecular donor-acceptor structures via orienting predeveloped fibrillar poly(3-hexylthiophene) crystals on bared/functionalized/grafted reduced graphene oxide with novel thiophenic constituents

Abstract Reduced graphene oxide (rGO) and its derivatives functionalized with 2-thiophene acetic acid (rGO-f-TAA) and grafted with poly (3-dodecylthiophene) (rGO-g-P3DDT) and poly (3-thiophene ethanol) (rGO-g-PTEt) were prepared to design the donor-acceptor supramolecular structures based on the regioregular poly (3-hexylthiophene) (P3HT). The P3HT nanofibers, which were pre-grown in a separate system and possessed the π-stacked P3HT backbones in the length of fibrils, were perpendicular to the rGO surface in the solution and, consequently, they inclined onto the substrate during solvent evaporation. Through this inclination, the orientation of P3HT chains changed from face-on to edge-on. In rGO-f-TAA systems, for either short (80 nm) or long (420 nm) P3HT nanofibers, the P3HT nanofibers were attached from their sides onto the rGO. The appearance of (020) and (002) spots in selected area electron diffraction (SAED) patterns proved a fixed edge-on orientation even after solvent evaporation. Functionalization of rGO surface with 2-thiophene acetic acid altered the tethering tendency of P3HT chains from the thiophenic rings to the hexyl side chains. Via grafting the rGO with P3DDT and PTEt, the tethered P3HT nanofibers were perpendicularly fixed onto its surface with a face-on orientation having (002) and (100) spots even after solvent evaporation, resulting in the conductivity range of 9.9–10.9 S/cm. The pre-developed large single crystals were also employed to construct the donor-acceptor supramolecules with edge-on oriented P3HTs. Further tethering of P3HT fibrils onto the rGO based micron sheets reflected a red-shifting and more intensified A0–2, A0–1, and A0–0 peaks in the ultraviolet–visible spectra. A quenching in the photoluminescence spectra was observed in longer preparation time (18 h) based on the charge transferring from donor (P3HT) to acceptor (rGO derivatives).

Thiacalix[3]Triazine-centered regioregular poly(3-hexylthiophene) star: synthesis, structure and anion binding

The first synthesis of a well-defined star polymer consisting of three regioregular poly(3-hexylthiophene) (rr-P3HT) arms emanating from an electron acceptor thiacalix[3]wtriazine core via direct arylation polymerization is described. Its structural characteristics as well as optical and thermal properties were evaluated using Fourier transform infrared spectroscopy (FTIR), 1H and 13C nuclear magnetic resonance (NMR), X-ray diffraction (XRD), gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and atomic force microscopy (AFM) methods. The UV-vis and photoluminescence spectroscopy results evidenced the impact of the electron-withdrawing feature of the thiacalix[3]triazine core and the star structure on the optical behavior of the star-shaped material, as compared to the rr-P3HT linear analogue. The anion-binding property of the star-shaped P3HT resulting in fluorescence quenching was also studied.

Graphene-Based Hole-Selective Layers for High-Efficiency, Solution-Processed, Large-Area, Flexible, Hydrogen-Evolving Organic Photocathodes

Regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the workhorse of organic photovoltaics, has been recently exploited in bulk heterojunction (BHJ) configuration with phenyl-C61-butyric acid methyl ester (PCBM) for solution-processed hydrogen-evolving photocathodes, reaching cathodic photocurrents at 0 V vs RHE (J0 V vs RHE) of up to 8 mA cm–2. The photoelectrochemical performance of these photocathodes strongly depends on the presence of the electron- (ESL) and hole- (HSL) selective layers. While TiO2 and its substoichiometric phases are consolidated ESL materials, the currently used HSLs (e.g., MoO3, CuI, PEDOTT:PSS, WO3) suffer electrochemical degradation under hydrogen evolution reaction (HER) working conditions. In this work, we use solution-processed graphene derivatives as HSL to boost the efficiency and durability of rr-P3HT:PCBM-based photocathodes, demonstrating record-high performance. In fact, our devices show cathodic J0 V vs RHE of 6.01 mA cm–2, onset potential (Vo) of 0.6 V vs RHE, ratiometric power-saved efficiency (φsaved) of 1.11%, and operational activity of 20 h in 0.5 M H2SO4 solution. Moreover, the designed photocathodes are effectively working in different pH environments ranging from acidic to basic. This is pivotal for their exploitation in tandem configurations, where photoanodes operate only in restricted electrochemical conditions. Furthermore, we demonstrate the scalability of our all-solution-processed approach by fabricating a large-area (∼9 cm2) photocathode on flexible substrate, achieving a remarkable cathodic J0 V vs RHE of 2.8 mA cm–2, Vo of 0.45 V vs RHE, and φsaved of 0.31%. This is the first demonstration of highly efficient rr-P3HT:PCBM flexible photocathodes based on low-cost and solution-processed manufacturing techniques.

Quantitative Analysis of the Molecular Dynamics of P3HT:PCBM Bulk Heterojunction.

The optoelectronic properties of blends of conjugated polymers and small molecules are likely to be affected by the molecular dynamics of the active layer components. We study the dynamics of regioregular poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular dynamics (MD) simulation on time scales up to 50 ns and in a temperature range of 250-360 K. First, we compare the MD results with quasi-elastic neutron-scattering (QENS) measurements. Experiment and simulation give evidence of the vitrification of P3HT upon blending and the plasticization of PCBM by P3HT. Second, we reconstruct the QENS signal based on the independent simulations of the three phases constituting the complex microstructure of such blends. Finally, we found that P3HT chains tend to wrap around PCBM molecules in the amorphous mixture of P3HT and PCBM; this molecular interaction between P3HT and PCBM is likely to be responsible for the observed frustration of P3HT, the plasticization of PCBM, and the partial miscibility of P3HT and PCBM.

Light-trapping in polymer solar cells by processing with nanostructured diatomaceous earth

Abstract We demonstrate the use of fossilized diatoms (diatomaceous earth) as light traps in regioregular poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) solar cells. Diatoms, the most common type of phytoplankton found in nature, are optimized for light absorption through millions of years of adaptive evolution. They are also an earth-abundant source of silica that can be incorporated into polymer solar cells without the need for complicated processing. Here we establish protocols dispersing the diatomaceous earth throughout the P3HT:PCBM active layer with characterization by optical and current-voltage measurements. We show that through the addition of diatomaceous earth, we can achieve the same power conversion efficiencies as standard thickness cells while using 36% thinner active layers. We find that adding the diatomaceous earth acts as a scattering center and textures the silver back contact, contributing to increases in the optical path length within devices. Results from this study open up pathways for incorporating hierarchical materials from nature into energy conversion devices.

Spectroelectrochemistry of poly(3-hexylthiophenes) in solution

The first comprehensive spectroelectrochemical account of the behaviour of regioregular (RR-P3HT) and statistical (ST-P3HT) poly(3-hexylthiophenes) in solution is presented, in contrast to the many reports dealing with P3HT films merely deposited from solution. The conducted experiments revealed that the two types of P3HTs behave in sharply different ways upon the application of electrochemical stimuli: ST-P3HT readily precipitates at mildly oxidative potentials, while the precipitation of the RR-P3HT takes place to a much lesser extent, even at higher potentials. The two polymers, studied via UV–Vis–NIR–EPR spectroelectrochemistry, exhibited properties mostly in line with earlier reports. Further study revealed that RR-P3HT largely remains in solution, even in its doped form, whereas only traces of the doped ST-P3HT are observed in solution in identical conditions. The high concentration of the doped RR-P3HT in solution can be explained by its ability to form soluble polymer agglomerates, in which the positive charge of the p-doped chains is stabilised by and delocalised over neighbouring, interacting undoped chains. These conclusions are consistent with SEM micrographs, which show that after cycling the potential of the electrode in a solution of ST-P3HT, a uniform layer is formed, covering most of the surface of the electrode, whereas in the case of RR-P3HT surface coverage is marginal and formed layer has the appearance of veined blotches.Graphical abstract

“Secondary electron spectra of semi-crystalline polymers – A novel polymer characterisation tool?”

The nano-scale dispersion of ordered/disordered phases in semi-crystalline polymers can strongly influence their performance e.g. in terms of mechanical properties and/or electronic properties. However, to reveal the latter in scanning electron microscopy (SEM) often requires invasive sample preparation (etching of amorphous phase), because SEM usually exploits topographical contrast or yield differences between different materials. However, for pure carbon materials the secondary spectra were shown to differ substantially with increased order/disorder. The aims here is to gain an understanding of the shape of secondary electron spectrum (SES) of a widely used semi-crystalline polymer regioregular poly(3-hexylthiophene-2,5-diyl), commonly known as P3HT, and its links to the underlying secondary electron emission mechanisms so SES can be exploited for the mapping the nano-morphology. The comparison of simulated and experimental SES shows an excellent agreement, revealing a peak (at about 0.8eV) followed by a broad shoulder (between 2eV and 4.5eV) with respective relative intensities reflecting order/disorder.

Long Lived Photoexcitation Dynamics in π-Conjugated Polymer/PbS Quantum Dot Blended Films for Photovoltaic Application.

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in a blend of PbS quantum dot and regio-regular poly (3-hexylthiophene) (RR-P3HT). The charge transfer from RR-P3HT to PbS as well as from PbS to RR-P3HT were observed after changing the capping ligand of PbS from a long chain molecular to a short one. Therefore, PbS could be used to extend the working spectral range in hybrid solar cells with a proper capping ligand. However, we found that the recombination mechanism in the millisecond time region is dominated by the trap/defects in blended films, while it improves to a bimolecular recombination partially after ligand exchange. Our results suggest that passivating traps of nanocrystals by improving surface ligands will be crucial for relevant solar cell applications.

Self-assembly of regioregular poly [2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene], pBTTT-C14 in solvent-mixture and study of its junction behaviour

The molecular ordering in organic polymer chains through π-π interactions is emerged to improve semiconductor performance. In the present work, long-range ordering of poly [2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene], pBTTT-C14 fiber is optimized in mixture of good and poor solvent (chloroform+toluene) blend using the concept of solvent driven self-assembly. Here, self-assembly is introduced by ageing followed by complete dissolution at elevated temperature in solvent-mixture. UV–visible spectroscopy demonstrated that the evidence of long range pBTTT-C14 fiber when experimentation is carried out in mixture of chloroform and toluene (9:1 v/v). However, in chloroform only, aggregation is slow while in toluene, aggregation is very fast under similar conditions. The fiber growth and its size are further verified by atomic force microscopy and HRTEM exemplified that the resultant aggregation of polymer chain in nanometric (diameter∼11 nm) range. Thereafter, the change in surface potential of fiber is studied by KPFM. This fibrous polymer is further sandwiched between ITO and Al-metal in order to study its junction behaviour. Fibrous pBTTT-C14 exhibits unsymmetrical charge transport properties over its isolated analogue with ∼9 times enhancement in mobility and ∼6.5 times enhancement in forward current density under the applied potential (−1.0 V to +1.0 V). This is owing to existence of a new charge transition path along the aggregation that consists of trapped charge between π-π stacking.

Effect of blending polymer insulators on the improvement of the performance of poly(3-hexylthiophene) transistors

This study investigated the correlations between the microstructure and electrical properties of regioregular poly(3-hexylthiophene) (rr-P3HT) blended with polystyrene, poly(vinyl phenol), or poly(methyl methacrylate) (PMMA) thin-film transistors (TFTs). Compared with pure rr-P3HT TFT, the blended rr-P3HT TFTs exhibited superior characteristics such as higher on/off current ratio of approximately 104, lower leakage current of <10−11A, smaller sub-threshold swing of approximately 2.09V/dec, and higher mobility of approximately 8.18×10−3cm2/Vs. The suppression of the leakage current of the sub-threshold and off regimes may be attributed to the enhanced oxygen/humidity resistance of blended rr-P3HT TFTs. The evaluation of the physical properties showed that polymer insulators positively contributed to the morphology, molecular orientation, and effective conjugated length of the rr-P3HT film, thereby enhancing the device characteristics. The hybrid Al2O3/PMMA gate insulator was also utilized to apply low-voltage TFTs with rr-P3HT/polymer blends and yielded a low operation voltage of −2V.

Photovoltaic device performance of highly regioregular fluorinated poly(3-hexylthiophene)

Abstract Highly regioregular poly(3-hexylthiophene) derivatives with varying degrees of fluorine substitution on the thiophene moieties have been demonstrated in photovoltaic devices and characterized using ultraviolet and inverse photoelectron spectroscopy. As fluorine content is increased, an increase in ionization energy of 0.3 eV is observed for 50% fluorination compared to non-fluorinated poly(3-hexylthiophene). The electron affinity is observed to increase to a lesser extent with increased fluorination, consistent with a systematic increase in the optical bandgaps of up to 0.12 eV. Bulk heterojunction photovoltaic devices made from polymer:PC 61 BM blends achieve power conversion efficiencies of 3%, however film morphologies measured using atomic force microscopy indicate that strong phase separation with increasing fluorination limits device performance. UV–vis spectra of thin films of the fluorinated materials exhibit a long tail in the red, extending to longer wavelengths than non-fluorinated poly(3-hexylthiophene). Photovoltaic devices similarly exhibit non-zero quantum efficiency in this region. This behavior has been attributed to a low energy, interchain charge transfer state.

CO2‐Sourced α‐Alkylidene Cyclic Carbonates: A Step Forward in the Quest for Functional Regioregular Poly(urethane)s and Poly(carbonate)s

AbstractDescribed is a robust platform for the synthesis of a large diversity of novel functional CO2‐sourced polymers by exploiting the regiocontrolled ring‐opening of α‐alkylidene carbonates by various nucleophiles. The reactivity of α‐alkylidene carbonates is dictated by the exocyclic olefinic group. The polyaddition of CO2‐sourced bis(α‐alkylidene carbonate)s (bis‐αCCs) with primary and secondary diamines provides novel regioregular functional poly(urethane)s. The reactivity of bis‐αCCs is also exploited for producing new poly(β‐oxo‐carbonate)s by organocatalyzed polyaddition with a diol. This synthesis platform provides new functional variants of world‐class leading polymer families (polyurethanes, polycarbonates) and valorizes CO2 as a chemical feedstock.

CO2 -Sourced α-Alkylidene Cyclic Carbonates: A Step Forward in the Quest for Functional Regioregular Poly(urethane)s and Poly(carbonate)s.

Described is a robust platform for the synthesis of a large diversity of novel functional CO2 -sourced polymers by exploiting the regiocontrolled ring-opening of α-alkylidene carbonates by various nucleophiles. The reactivity of α-alkylidene carbonates is dictated by the exocyclic olefinic group. The polyaddition of CO2 -sourced bis(α-alkylidene carbonate)s (bis-αCCs) with primary and secondary diamines provides novel regioregular functional poly(urethane)s. The reactivity of bis-αCCs is also exploited for producing new poly(β-oxo-carbonate)s by organocatalyzed polyaddition with a diol. This synthesis platform provides new functional variants of world-class leading polymer families (polyurethanes, polycarbonates) and valorizes CO2 as a chemical feedstock.

A Generalized Packing Model for Bulk Crystalline Regioregular Poly(3‐alkylthiophenes) with Extended Side Chains

AbstractPoly(3‐alkylthiophenes) are one of the most frequently used conjugated polymer classes in organic photovoltaics. Here, a generalized packing model for the polythiophene main chains in the crystalline form I of high molecular weight regioregular poly(3‐alkylthiophenes) with extended side chains (pentyl through octyl) is reported. The model is based on structural constraints from solid‐state NMR: short internuclear distances of less than 4.0 Å of neighboring thiophene protons parallel to the stacking direction and the isotropic chemical shift for the thiophene protons is high‐field shifted by 0.9 ± 0.1 ppm. Nucleus‐independent chemical shift calculations show that only the most recent structure for P3HT (space group P21/c) is compatible with these structural constraints. On this basis, a generalized packing model is developed, showing that slipping parallel to the stacked polymer chains of up to 1.5 Å is allowed, while out‐of‐plane tilts perpendicular to the stacked chains are only tolerated up to 20°.

Mechanical, Thermal, and Electrical Properties of Flexible Polythiophene with Disiloxane Side Chains

Poly(3‐hexylthiophene) (P3HT) is widely shown to have considerable advantages in photovoltaic cells, including excellent electrical performance, but its fragility remains a challenge for material applications and processing of stretchable devices. Herein, high‐molecular‐weight and highly regioregular poly(3‐substituted thiophene) with disiloxane moieties in the side chains (P3SiT) is synthesized. An investigation of the molecular structure and physical properties of self‐supported cast films of P3SiT reveals excellent flexible mechanical properties derived from the disiloxane groups in the side chains. The larger fracture strain reaches over 200% compared with that of poly(3‐hexylthiophene) (14%). In addition, its Young's modulus (43 MPa) and low glass transition temperature (−10 °C) result in a typical elastic mechanical property. For the electrical property, the sheet resistivity of the drawn thiophene polymer shows the value almost equal to that of poly(3‐hexylthiophene). The anisotropy of electrical resistivity is observed due to orientation of the drawn polymer.

Glass Transition Temperature of Conjugated Polymers by Oscillatory Shear Rheometry

The stiff backbones of conjugated polymers can lead to a rich phase behavior that includes both crystalline and liquid crystalline phases, making measurements of the glass transition challenging. In this work, the glass transitions of regioregular poly(3-hexylthiophene-2,5-diyl) (RR P3HT), regiorandom (RRa) P3HT, and poly((9,9-bis(2-octyl)-fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (PFTBT) are probed by linear viscoelastic measurements as a function of molecular weight. We find two glass transition temperatures (Tg’s) for both RR and RRa P3HT and one for PFTBT. The higher Tg, Tα, is associated with the backbone segmental motion and depends on the molecular weight, such that the Flory–Fox model yields Tα = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a different molecular weight dependence of Tα is seen below Mn = 14 kg/mol, suggesting this is the typical molecular weight of intercrystal tie chains. The lower Tg (TαPE ≈ −100 °C)...

Uniform electroactive fibre-like micelle nanowires for organic electronics

Micelles formed by the self-assembly of block copolymers in selective solvents have attracted widespread attention and have uses in a wide variety of fields, whereas applications based on their electronic properties are virtually unexplored. Herein we describe studies of solution-processable, low-dispersity, electroactive fibre-like micelles of controlled length from π-conjugated diblock copolymers containing a crystalline regioregular poly(3-hexylthiophene) core and a solubilizing, amorphous regiosymmetric poly(3-hexylthiophene) or polystyrene corona. Tunnelling atomic force microscopy measurements demonstrate that the individual fibres exhibit appreciable conductivity. The fibres were subsequently incorporated as the active layer in field-effect transistors. The resulting charge carrier mobility strongly depends on both the degree of polymerization of the core-forming block and the fibre length, and is independent of corona composition. The use of uniform, colloidally stable electroactive fibre-like micelles based on common π-conjugated block copolymers highlights their significant potential to provide fundamental insight into charge carrier processes in devices, and to enable future electronic applications.

Organization of polythiophenes at ultrathin films mixed with stearic acid investigated with polarization-modulation infrared reflection–absorption spectroscopy

In this present work, the polymers poly(3-alkylthiophene)s, regioregular poly(3-butylthiophene), poly(3-hexylthiophene) and poly(3-octylthiophene) were spread at the air-water interface to form Langmuir monolayers. They formed stable films by mixing them with Stearic Acid (SA), as confirmed with surface pressure-area isotherms and polarization-modulation infrared reflection–absorption spectroscopy (PM-IRRAS). The floating monolayers were transferred from the liquid interface to solid supports by using the Langmuir-Blodgett (LB) and Langmuir-Schaeffer (LS) techniques, and characterized with PM-IRRAS. Surface pressure-area isotherms showed that SA monolayers are expanded upon polymer incorporation altering the rheological properties of the monolayer since the monolayer became less elastic at higher pressures and presented first-order transitions at lower pressures. PM-IRRAS showed that the organization of the films is affected at the molecular level depending on the chemical structure of the polymer chosen. Among the polymers analysed, the poly(3-butylthiophene) shows a more irregular conformational behaviour depending on the amount of stearic acid. The conformation of the films when transferred to solid support changes and is dependent on the type of deposition (LB or LS). The importance of this paper relies on the fact that SA can be employed to help the formation of stable floating monolayers of these polythiophenes at the air-water interface, and the successful transfer to solid supports may provide nanostructured films with a high control of their molecular properties, which may enable a molecular architecture to be applied as electronic devices of interest.