Post-functionalization of poly(3-hexylthiophene) via anodic chlorination

Selective anodic chlorination of a poly(3-hexylthiophene) was successfully carried out by electrochemical polymer reaction. NMR and EDX analyses revealed the selective and sufficient substitution of chlorine atom at the 4-position of the repeating thiophene ring. GPC measurement of the polymer before and after electrolysis indicated that neither decomposition nor propagation of the polymer occurred even after passage of the excess charge. The optical and electrochemical properties of the obtained chlorinated polymer were investigated in detail.     

Thermochromic and solvatochromic effects in poly(3-hexylthiophene)

We report thermochromic and solvatochromic effects in the soluble conductive polymer poly(3-hexylthiophene). Thin solid films of the polymer change colour at higher temperatures. The colour of solutions of the polymer can be changed by varying the composition of the solvent. Optical absorption in good solvents is similar to that of thin solid films at higher temperatures, while in poor solvents, absorption features similar to those found in solid films at low temperatures are observed. We discuss these effects with reference to the existence of a hitherto disregarded type of conformational defect, conformons. These are rotational defects on the polymer chain, leading to loss of planarity. High vacuum studies of the thermochromic effect in thin films using X-ray photoemission spectroscopy support this interpretation. The standard series of defects in conductive polymers, solitons, polarons and bipolarons, may have to be extended to include conformons.     

Spectroscopic and electrical evaluation of poly(3-hexylthiophene) nanotubules made using template wetting nanofabrication

The unique properties of conjugated polymer nanostructures as compared to thin films of the same materials have fueled the investigation of a number of different polymer nanofabrication techniques. Here we examine the optoelectronic properties of poly(3-hexylthiophene)P3HT nanotubules made using one of the simplest of these techniques, template wetting nanofabrication. Comparison is made with results from our previous study in which in poly(2-methoxy-5-(2?-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) nanotubules prepared in this manner were shown to have dramatically different properties in comparison with thin films due to increased molecular order in the nanostructures. With semicrystalline P3HT, the differences are much more subtle, we believe due to the greater inherent order in this material. In P3HT nanotubules, a small bathochromic shift (～0.05 eV) in the onset of Uv–vis absorption was observed indicating increased long range order and a greater effective conjugation length in the polymer. Polarized FTIR spectroscopy indicated a small degree of preferential alignment of the inter-ring C–C bond with the axis of the nanotubules as the stretching vibration for this bond showed a dichroic ratio of 1.73. The degree of increased conjugation length and chain alignment in P3HT nanotubules indicated by both the UV–vis and polarized FTIR spectra were much smaller than that previously observed for MEH-PPV. No significant improvement in mobility as measured from the current voltage characteristics of simple single carrier devices based on the P3HT nanotubules was observed, in sharp contrast to previously studied MEH-PPV where more than an order of magnitude increase was seen. Unlike in amorphous MEH-PPV, the increase in molecular order suggested by spectral data was insufficient to significantly impact mobility of semicrystalline P3HT when using template wetting nanofabrication for either of the 100 or 200 nm nanotubules examined.     

Electrical characterization of diodes based on regioregular poly(3-hexylthiophene)

Conjugated polymer films were sandwiched between thin aluminium and gold electrodes in orderto determine current-voltagecharacteristics of the three-layer device. Poly (3-hexylthiophene) fractions different in their degrees of polymerization were used as the representatives of conjugated systems with different chain lengths. The analysis of the data showed that morphology of the polymer film and length of the chain influence the operation voltage of the device. Moreover, due to the position of the Fermi levels of the electrodes and HOMO (LUMO) levels, the device works in single-carrier current regime and space-charge limitation of current is present.     

A comparative study on liquid-state photoelectrochemical cells based on poly(3-hexylthiophene) and a composite film of poly(3-hexylthiophene) and nanocrystalline titanium dioxide

A comparative study was done between liquid-state photoelectrochemical cells (PECs) consisting of photoactive electrodes of poly(3-hexylthiophene), P3HT, and a composite film of nanocrystalline titanium dioxide (nc-TiO₂) and P3HT. The nc-TiO₂/P3HT based device shows better performance with an open-circuit voltage of 0.51 V, a short-circuit current of 0.31 mA/cm², and a fill factor of 0.51 when illuminated with white light intensity of 100 mW/cm². The IPCE% obtained at 550 nm for P3HT based device was 0.18% while for that of the nc-TiO₂/P3HT based device was 4%. In P3HT based devices, P3HT showed its p-type behavior while in nc-TiO₂/P3HT based devices, P3HT acted as a sensitizer to nanocrystalline TiO₂.     

Thienylmanganese halides for the preparation of regioregular poly(3-hexylthiophene)

For the first time, thienylmanganese halides have been used for the synthesis of regioregular head-to-tail poly(3-hexylthiophene). A variety of conditions were examined, and the polymerization was successfully completed in the presence of Ni(dppe)Cl₂ at ambient temperature affording the title polymer in high yields with excellent regioregularity.     

Fabrication of regioregular poly(3-hexylthiophene) field-effect transistors by dip-coating

We report the influence of dip-coating speed and concentration of the polymer solution on the characteristics of field-effect transistors (FETs) fabricated in the bottom-contact structure with regioregular poly(3-hexylthiophene) (RR-P3HT) as the active semiconducting material. For each concentration of the polymer in solution, there is an optimum dip coating speed for film deposition with highest field effect mobility; for example, with chloroform as solvent the optimum speed is 0.5 mm/min for a solution containing 1.0 mg/ml and 1.0 mm/min for a solution containing 2.5 mg/ml. Based upon AFM studies of the resulting film morphology, we conclude that the formation of a “rod-like” morphology is the origin of the improved carrier transport in the FET channel.     

Improved device performance based on crosslinking of poly (3-hexylthiophene)

Diode devices (glass/ITO/polymer/Al) have been fabricated using poly (3-hexylthiophene) (P3HT) crosslinked with two different biaryl crosslinkers. Crosslinking was performed by exposing the thin films with different wt% of crosslinker to UV irradiation and progress of crosslinking was monitored by IR spectroscopy. An increase in hole mobility of two orders of magnitude has been observed after crosslinking.     

Photoelectrochemical response of poly(3-hexylthiophene) and poly(2,3-diethylquinoxaline-5,8-diyl) in aqueous media

The photoelectrochemical properties of poly(3-hexylthiophene) (P3HT) and poly(2,3-diethylquinoxaline-5,8-diyl) (PDEQx) thin films on ITO (indium-tin oxide) electrodes were investigated in aqueous media. Photoirradiation of the P3HT-coated electrode resulted in cathodic photocurrents in the presence of methyl benzoylformate, whereas the PDEQx-coated electrode generated stable anodic photocurrents in the presence of 2-mercaptoethanol. The action spectra indicate that photoexcitations of the polymers cause photoelectrochemical reactions. The films of P3HT and PDEQx exhibit a typical photochemical response of p- and n-type semiconducting materials, respectively.     

Anisotropic conductivity–temperature characteristic of solution-cast poly(3-hexylthiophene) films

For the purpose of developing a positive temperature coefficient (PTC) device, thin films of poly(3-hexylthiophene) (P3HT) were prepared by solution-cast from chloroform. In the present work, the P3HTs of high molecular weight (M_w) with various regioregularities were employed, and the structure anisotropy and the temperature dependence of conductivities in directions parallel (σ_∥) and perpendicular (σ_⊥) to the film surface were investigated. For highly orientated P3HT films, the temperature dependence of σ_⊥ was found to reflect an amorphous character of charge hopping along this direction. However, the σ_∥ decreased greatly above 50 °C, which was attributed to a decrease in the in-plane π-stacking caused both by the melting of crystallized side chains and the enhanced side-chain disturbance with increasing temperature. Different charge transport mechanisms were proposed to explain the anisotropic conductivity–temperature characteristics observed in the two directions. The high conductivity in the parallel direction was deteriorated significantly by thermal recycles, probably due to a reduction of the orientation degree of P3HT crystallites in the bulk film, rather than a change of conjugated length of polymer backbone. A reversible PTC effect was observed for the P3HT film with a high M_w and medium regioregularity in the perpendicular direction, which suggested that the high M_w was more important than the high regioregularity to design the thin film PTC devices based on the soluble P3HT films.     

Enhanced stability of poly(3-hexylthiophene) transistors with optimally cured poly(methyl methacrylate) dielectric layers

Poly(3-hexylthiophene)-based organic field-effect transistors (OFETs) have been fabricated on poly(methyl methacrylate) (PMMA) gate dielectric layers under different process conditions, resulting in very different device stability in ambient air. The dielectric layers were prepared by spin coating and subsequently curing at various temperatures (120, 150, and 180 °C) or by ultraviolet light (UV) exposure. With respect to the variations of the on/off current ratio and the threshold voltage, dramatically enhanced stability of the OFETs with the PMMA layer cured at 150 °C has been demonstrated when compared to those cured at different temperatures. The devices cured by UV exposure showed even more superior stability, with reliable performance in ambient air for more than 10 days. The differences in the film surface morphology were analyzed and possible mechanisms for the enhanced stability are discussed.     

Time-dependent Langevin-type bimolecular charge carrier recombination in regiorandom poly(3-hexylthiophene)

We have studied transport and recombination of photogenerated charge carriers in thin films of regiorandom poly(3-hexylthiophene) (RRaPHT). Time-of-Flight studies show space charge perturbed current transients with a saturation of current level at high light intensities as a result of diffusion controlled Langevin-type bimolecular charge carrier recombination. We have been able to measure the time-dependent charge carrier mobility by using the carrier extraction by linearly increasing voltage technique. We found that the hole mobility follows a power law dependence over a few orders of magnitude in time as μ = μ₀t^−0.42. By using the carrier extraction by pulsed voltage technique we measured the photogenerated charge carrier concentration decay as a function of delay time and found that the decay can be fitted using a dispersive time-dependent bimolecular recombination coefficient governed by the time-dependent hole mobility.     

Performance recovery and optimization of poly(3-hexylthiophene) transistors by thermal cycling

We present the results of studies on the electrical and physical modifications to poly(3-hexylthiophene), upon thermal annealing. Thermally induced performance enhancements and thermal stability of polythiophene thin film transistors (TFTs) are explored. We observe substantial mobility improvements in devices annealed at low temperatures (<80 °C), as well as increases in on/off ratios by more than two orders of magnitude at moderate anneal temperatures (∼120 °C). We document changes in conductivity, mobility, on current, and on/off ratio with anneal temperature and total thermal budget. We are able to show the expulsion of environmental contaminants and increases in film density as means to performance recovery and enhancement. This study provides a comprehensive analysis of the effect of thermal cycling of polythiophene TFTs on various device performance metrics, and identifies relevant thermal limits and failure mechanisms.     

Effect of wetting solvent on poly(3-hexylthiophene) (P3HT) nanotubles fabricated via template wetting

Template wetting is a simple, solution based nanofabrication method that has been shown effective for a wide range of polymers. Like other solution based polymer processing methods, it is reasonable to expect that the choice of solvent will have a significant impact on the chain orientation in the final solid structure. Here we examine the impact of wetting solvent on the properties of 100 nm diameter poly(3-hexylthiophene) (P3HT) nanotubules made via template wetting. The degree of alignment of the P3HT backbone with the nanotubule axis as observed through dichroism in the FTIR spectrum was observed to depend on the strength of polymer–solvent interaction forces, observed experimentally through thermogravimetric analysis experiments. This solvent effect was not observed in other properties as neither the UV–Vis absorbance nor the hole mobility was observed to depend significantly on the wetting solvent. It is believed that the rigid rod structure and large side chain limited the degree of increase in the effective conjugation length and preventing even the aligned chains from being more tightly packed as would be necessary for an increase in inter-chain π-bond interactions sufficient to impact these performance characteristics of the material.     

Nanostructured layers of a new cross-linkable poly(3-hexylthiophene) in organic photovoltaic cells

We report on the synthesis of a new cross-linkable poly(3-hexylthiophene) copolymer, P3HT-Ox10, consisting on regioregular blocks of 3-hexylthiophene and 10% molar of an oxetane-functionalized thiophene comonomer. This copolymer was used in the fabrication of organic photovoltaics (OPVs) upon combination with 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C₆₁ (PCBM), either as blends or in bilayer structures. Bilayer devices with a nanostructured donor/acceptor interface were fabricated using a patterning process based on the phase separation in P3HT-Ox10:polystyrene blends. Columnar-grain films of cross-linked P3HT-Ox10 with diameters of ca. 140 nm were thereby obtained. These are the lowest dimensions achieved so far by this approach. Despite the modest power conversion efficiencies achieved, we believe that this easily implemented approach is very effective to prepare OPVs with well-defined morphologies.     

Picosecond time-resolved infrared absorption study on photoexcited dynamics of regioregular poly(3-hexylthiophene)

Picosecond time-resolved infrared absorption spectra of the photoexcited states of a spin-coated film of regioregular poly(3-hexylthiophene) (RR-P3HT) have been recorded at 77 K. The time-resolved photoinduced infrared absorptions due to electronic transitions at 2500 cm^?1 and vibrational transitions at 1575 cm^?1 have fast and slow decay components. By comparing the picosecond time-resolved infrared absorption spectrum of long-lived (slow decay component) transient species with the doping-induced infrared difference spectrum of RR-P3HT, the long-lived transient species are assigned to polarons. The short-lived transient species are assigned to singlet excitons and/or polaron pairs.     

The application of soluble and regioregular poly(3-hexylthiophene) for organic thin-film transistors

We have investigated the effect of solvent medium, thermal annealing and dedoping reducing agent such as hydrazine on the performance of poly(3-hexylthiophene) (P3HT) device. The performance of P3HT device varied significantly from different solvents, among them, p-xylene proved to be the best solvent medium, and also hydrazine as dedoping agent made progress in the mobilities by one or two orders of magnitude. Especially, in our experiments, solution preparing, deposition and device measurements were all performed in the air for the application of P3HT without further purification to the ink-jet printing, spin coating or solution casting process for large area and flexible organic thin-film transistors (OTFTs).     

Preparation and characterization of graphene nanosheets/poly(3-hexylthiophene) thermoelectric composite materials

Graphene nanosheets/poly(3-hexylthiophene) (GNs/P3HT) composites were prepared by oxidative polymerization of 3-hexylthiophene in a GNs dispersed chloroform solution. The phase composition of the composite materials was analyzed by X-ray diffraction and Fourier transform infrared spectra. The thermoelectric properties of the cold pressed composite pellets with different GNs loadings were measured at room temperature. As the GNs loading increased from 0 to 30 wt.%, the electrical conductivity of the composites dramatically increased from ∼10⁻⁶ to ∼1.2 S/cm while the Seebeck coefficient slightly increased from 33.15 to 35.46 μV/K. The highest power factor (∼0.16 μW m⁻¹ K⁻²) was obtained in the 30 wt.% GNs/P3HT composite material.     

Unique structural features and electrical properties of electrospun conjugated polymer poly(3-hexylthiophene) (P3HT) fibers

This study examines and compares the internal structure conjugated polymeric fibers fabricated by electrospinning with cast films. Despite rigidity of polymer chain and the inability of its molecular chains to entangle to form viscoelastic jets, regioregular poly(hexyl-3-thiophene) (P3HT) exhibited the remarkable capability to be electrospun when the solution was subjected to gelation. In order to investigate the influence of additional mechanical stretching on the fibers, P3HT was electrospun with the aid of a rotating disc collector. Structure of electrospun fibers was probed via characterization techniques such as differential scanning calorimetry (DSC), Fourier transform infrared and photoluminescence (PL) spectroscopies. The findings indicated internal structural modifications developed within P3HT fibers, as a consequence of additional mechanical stretching induced by the rotating collector. Polarized FTIR and PL spectroscopies suggested that the molecular chains were aligned along the fiber axis. Electrical conductivity of iodine doped P3HT electrospun fibers was between 3 × 10³ S/m and 6 × 10³ S/m.     

High energy ion irradiation effect on light-emitting poly(3-methylthiophene) (P3MT) nanowires

Light-emitting poly(3-methylthiophene) (P3MT) nanowires with a diameter of ∼200 nm were fabricated using an electrochemical polymerization method based on an anodic alumina oxide (Al₂O₃) nanoporous template. The relatively high energy (3 MeV) Cl²⁺ ions were irradiated onto the P3MT nanowires with dosages from 1 × 10¹³ to 1 × 10¹⁵ ions/cm². To study the effect of high energy ion irradiation on P3MT nanowires, we measured UV/vis absorbance, Raman spectra, and laser confocal microscope photoluminescence (PL) on the nanometer scale. The relative atomic concentrations of carbon elements in the pristine and the 3 MeV Cl²⁺ ion-irradiated P3MT nanowires were investigated through XPS experiments. As the dosage of the 3 MeV Cl²⁺ ion irradiation increased, we observed that the P3MT nanowires changed to a partially carbonized form, and that the PL efficiency of the systems decreased.