XPS and IR study of electrochemically or chemically redoped poly(3-methylthienylene) films

XPS (X-ray photoemission spectroscopy) and IR (infrared) analyses of poly(3-methylthienylene) films, electrochemically or chemically redoped0 with various chemical species at varying doping levels, have been investigated. On the basis of these results, the major chemical species of dopants have been identified. As a results, the dopant content was determined and the poly(3-methylthienylene) films were classified as light or heavy by doping level. The conductivity ranged from about 10^?12 to 10₂ S cm^?1 for all the films investigated. The conductivity and the activation energy of conduction for the heavily-doped films vary as a function of the dopant content, independent of the different chemical species of dopants. In particular, a sudden change is observed in the dependence of activation energy on dopant content. This sudden change may be associated with the semi-conductor-metal transition. Furthermore, it is shown that the specific absorption bands in the infrared are induced by the doping, intensified with increasing dopant uptake and accompanied by an increase in conductivity.     

13C NMR spectrum analysis of electrochemically prepared poly(3-methylthienylene) films

The ¹³C NMR spectrum analysis of electrochemically prepared poly(3-methylthienylene) films has been investigated. Both a film doped with ClO₄ ions (P3MT-ClO₄) and an undoped film (P3MT₀) give only two distinct absorption bands through the electrochemical reduction of P3MT-ClO₄. These distinct bands are definitely attributable to the methyl group (upper field) and the thiophene ring (lower field), respectively, and are characteristic of well-defined poly(3-methylthienylene). The correlation between conductivity and ¹³C NMR spectra will be discussed.     

Conductive Poly(3-methylthiophene) Thin Films

The work described herein represents an efficient method in the deposition of poly(3-methylthiophene),P3MeTh,thin films utilizing a microwave plasma system in combination with a simultaneous doping with iodine.It was envisaged that,an alternative poly(3-methylthiophene),P(3MeTh),with an electron donating methyl substituent,would reduce the degree of ring opening which reportedly occur to a certain extent during the plasma polymerization process of its parent compound polythiophene.An alkyl substituent would also increase the solubility of the materials.P(3MeTh)thin film deposition has been performed utilizing microwave-induced plasma polymerization in order to directly grow films on glass substrates.Moreover,simultaneous doping of the so-formed polymer with iodine has been carried out as opposed to the post-synthesis doping method.This is aimed to prolong electrical conducting lifetime of the materials.The synthesized films were characterized by attenuated total reflection fourier transform infrared(ATR FT-IR)spectroscopy and energy dispersive X-ray spectroscopy(EDS)to confirm the incorporation of iodine dopant into the films.Scanning electron microscopy showed uniformly deposited films.It has been observed that the electrical conductivity of the doped film is 2 orders of magnitude higher than the undoped counterpart.The doped fabricated films exhibited UV-vis spectra indicative of increased π-conjugation(536 nm).Furthermore,electrical conductivity of the in situ doped P(3MeTh)is more highly sustained over a longer period of time.     

Investigation of solution processed poly(4,4-dioctylcyclopentadithiophene) thin films as transparent conductors

Thin films of poly(4,4-dioctylcyclopentadithiophene) were obtained by processing from solution. These films were doped by treatment with iodine or DDQ and their conductivity determined by electrical measurements. The iodine doped polymer films show conductivities of up to 0.35 S/cm but the conductivities decreased on standing due to reversible dedoping of the films. Polymers doped with DDQ are more stable and conductivities up to 1.1 S/cm are reported. The doped polymers show little absorption in the visible region of the spectrum, suggesting possible applications in plastic electronics.     

Electrochemical preparation and properties of poly(3-methoxy-2,5-thiophenediyl) and poly(3-methylthio-2,5-thiophenediyl)

Soluble and conductive poly(3-methylthio-2,5-thiophenediyl) and poly(3-methoxy-2,5-thiophenediyl) films were prepared by the electrochemical polymerization of 3-(methylthio)thiophene (MTT) and 3-methoxy-thiophene (MOT), respectively. The polymers were identified from their i.r. spectra and found to be soluble in chloroform, propylene carbonate (PC), dimethyl sulfoxide (DMSO) and 1-methyl-2-pyrrolidinone (NMP). When doped, they were soluble in PC, DMSO and NMP. Cast films could be obtained from their solutions. The degrees of polymerization of poly-MTT and poly-MOT were 7.4 and 79, respectively. The vis-near i.r. specta of either polymer solution had characteristic peaks due to bipolarons. The doped poly-MTT solution became undoped after four days in air. The doped poly-MOT solution was stable for a few months, and the conductivity of the pressed pellet of doped poly-MOT did not change for as long as 11 months. With regard to differences in the absorption spectra between the films and solutions of the undoped state, the films had bands at longer wave-length than the solutions. The shifts may possibly have been due to internal rotation about single bonds, probably occurring more freely in solution than in films. Solid-state samples retained the coplanar configuration. In cyclic voltammograms, the oxidation potential of poly-MOT was lower than 0 V versus Ag/Ag⁺. It is thus evident that the doped state of poly-MOT is stable in air.     

Electrical properties of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl] benzene

The increase of the electrical conductivity of the arylenevinylene (AV) system 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl] benzene was measured as a function of the doping level of iodine. Compressed powders and films blended with polystyrene (PS) containing up to 30 wt.% of the oligomer were used. At the highest doping level the conductivity of the compressed powder reaches a value of 3.3×10⁻⁴ Ω⁻¹ cm⁻¹. The conductivity, measured between 90 and 300 K on the powder with the highest conductivity, is thermally activated with an activation energy of ∼0.1 eV. The thermopower is positive in agreement with a charge transfer from the organic chain to an iodine atom. Its value is small (25 μV K⁻¹) and temperature independent. The results are interpreted in terms of small-polaron hopping.     

Electrochemical synthesis and electronic properties of poly(3,4-dibutyl-α-terthiophene)

Electrochemically prepared poly(3,4-dibutyl-α-terthiophene) has been investigated and the relationship between the electronic properties, the chemical structure of the monomer and the electrolyte used during the electropolymerization was studied. Doping studies were carried out with various electrolytes. We have electrochemically oxidized (p-doping) and reduced (n-doping) thin films of this polymer on platinum electrode under the same electrochemical conditions. We find that the films show a reversible oxidation wave yielding an electrically conducting polymer and a reversible reduction wave which is sensitive to the nature of the electrolyte. A value for the band gap derived electrochemically compares well with that obtained by optical absorption measurements and X-ray photoelectron spectroscopy. The electrical conductivity varies by 10¹² S cm⁻¹ between the doped and undoped states of the polymer. The temperature-independent magnetic susceptibility above 100 K is consistent with the conducting properties of this polymer. The electrochemically prepared material is compared with the chemically prepared one using the same monomer.     

Enhancement of the electrical conductivity of polypyrrole films on applying higher voltage

The effect of voltage on the conductivity of polypyrrole films has been investigated. It is found that the conductivity of the films increases with some structural change after the applied voltage reaches a higher value. The degree and the stability of the conductivity enhancement are related to the nature of the counter-anions doped in the polypyrrole films. A 65% increase in conductivity of the film doped with TsO⁻ is observed on applying higher voltage, and it remains stable after the treatment. The conductivity of the film doped with NO₃⁻ increases 47% on applying higher voltage, but it drops back to its original value quickly after the higher voltage is turned off. The structural change of PPy(TsO⁻) is studied through Vis and NIR absorption spectra.     

Electrically conducting poly[3-(ω-hydroxyalkyl)thiophenes]

A series of poly[3-(ω-hydroxyalkyl)thiophenes] was synthesized by chemical oxidative polymerization using FeCl₃ and characterized by FT-IR, thermal gravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). Upon doping with iodine the polymers attained electrical conductivities of between 0.5 and 38 S cm⁻¹. Electrical conductivities of doped samples were found to increase with alkyl chain lengths, being optimum for the octyl pendant moiety. Thereafter, the conductivity decreased, as the doping and dedoping for the polymers became more difficult. Increasing the hydrocarbon chain length of the pendant substituent also resulted in decreased thermal stability of the polymers. XPS results showed that some C–OH groups in the polymers were oxidized to CO in the polymerization process.     

Polarization properties of the Raman spectra in neutral and iodine doped cis-trans (CH)x

Polarized resonant Raman spectra of oriented cis-trans -(CH)_x are presented for undoped and iodine doped samples. For undoped films the depolarization ratios of cis lines are consistent with a fibrils misorientation of about 20°. Similar values of depolarization ratios have been measured in iodine doped films as well as for the cis and trans lines than for I₃⁻ mode. These results mean that doping does not destroy the orientational ordering of polymer chains and that under doping the iodine takes the orientation of these chains.     

Novel `synthetic metals' based on symmetrically tetrasubstituted nickel phthalocyanines

Nickel(II) 2,9,16,23-tetrasubstituted phthalocyanines, NiPTX (where X=–OH, –NO₂, –NH₂ and –SO₃H) were synthesized in pure state and doped with iodine. Elemental analysis, UV-Vis, infrared, X-ray diffraction, magnetic susceptibility and TGA studies were used to investigate the effects of iodine doping as well as substituents on the properties of the complexes. The electrical conductivity of nickel phthalocyanine derivatives found to depend on the nature of the substituents and showed ∼10³–10⁵ times increased electrical conductivity in comparison to parent nickel phthalocyanine. Further, the substitution and iodine doping show remarkably very high improved electrical conductivity nearly 10¹¹ times the electrical conductivity of nickel phthalocyanine.     

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.     

Impedance spectroscopy properties of polypyrrole doped with boric acid

The electrical conductivity and dielectric properties of polypyrrole doped with boric acid have been investigated. The direct current conductivity (dc) of the polymer increases with increasing temperature. The alternating current (ac) conductivity of the polymer obeys the power law, i.e., σ_ac(ω) = Aω^s. The alternating conductivity of polypyrrole doped with boric acid is controlled by the correlated barrier hopping model. The activation energy for alternating current mechanism decreases with increasing frequency which confirms the hopping conduction to the dominant mechanism as compared with the dc activation energy. The density of localized states N(E_F) for polypyrrole doped with boric acid was in the range of 2.5–9.2 × 10²² cm^?3 for various temperatures. The dielectric relaxation mechanism was explained on the basis of complex dielectric modulus. The imaginary modulus plot at different temperatures shows a dielectric mechanism with non-Debye relaxation. Boric acid can be a good candidate for controlling the electrical conductivity of the conducting polymer.     

Correlation between vibrational spectra and electrical conductivity of polythiophene

Polythiophene films synthesized by electrochemical polymerization under various conditions were undoped and redoped maximally with iodine. The electrical conductivity of the redoped films was in the range 2 × 10² − 5.9 S cm⁻¹. Vibrational spectra of the undoped films were analysed on the basis of those of model compounds including 2,2′-bithiophene, 2,2′:5′,2″-terthiophene and 2,2′:5′,2″:5″,2′-quaterthiophene. First, the intensity ratio of the infrared CH out-of-plane bending bands at 697 (2-substituted thiophene) and 787 cm⁻¹ (2,5-substituted thiophene) is a clue to the degree of polymerization. Secondly, the intensity ratio of the infrared double-bond symmetric and antisymmetric stretching bands reflects the distribution in length of the conjugated segment. Finally, the intensities of three Raman bands at 1155, 682 and 652 cm⁻¹ give information about the amount of distorted structure around the inter-ring C-C bonds. From the comparison of the spectroscopic and conductivity data, it is concluded that a high degree of polymerization alone is not adequate for a doped film to have high electrical conductivity but abundance of long conjugated coplanar segments is the requisite condition.     

Polymer–nanotube composites for transparent,conducting thin films

Highly conductive, highly transparent thin films have been fabricated from polymer–single walled carbon nanotube blends. Using poly-3,4-ethylenedioxythiophene doped with poly(styrenesulfonate) as a host material, excellent dispersion of single wall nanotubes could be achieved enhancing the conductivity with relatively low loadings <3 wt%. Raman spectroscopy indicates that there is little bundling of the single wall nanotubes in the matrix and that the nanotubes are sensitive to residual stress within the film. As the host bulk conductivity is increased, enhancements of the overall composite conductivity are observed to be proportional. These results suggest that the energy barrier to nanotube–nanotube carrier hopping within the matrix can be modified in accordance with a heterogeneous conduction model.     

High thermoelectric performance of poly(2,5-dimethoxyphenylenevinylene) and its derivatives

Poly(2,5-dimethoxyphenylenevinylene) (PMeOPV) and a series of copolymers consisting of both 2,5-dimethoxy-substituted phenylenevinylene units and unsubstituted units (P(MeOPV-co-PV)) were evaluated from the viewpoint of their thermoelectric properties. Their conjugated polymer films were prepared by pyrolysis of stretched or unstretched films of sulfonium salt precursor polymers, and subsequently doped with iodine vapor to provide electrical conductivity. The power factors P (=S²σ), indicating thermoelectric performance, were calculated with the measured electrical conductivity σ and Seebeck coefficient S of the doped films. PMeOPV showed a higher power factor of 7.1 μW m⁻¹ K⁻² at 313 K than that of a camphorsulfonic acid-doped polyaniline as reference. P(MeOPV-co-PV) precursor polymers with less than 20 mol% of MeOPV unit content in the monomer feed were stretchable, therefore providing stretched P(MeOPV-co-PV) films with low MeOPV unit content. The stretching treatment for P(MeOPV-co-PV) enhanced its electrical conductivity, but kept the Seebeck coefficient at nearly the same level as that of unstretched polymers. Consequently a 4.4-fold stretched copolymer exhibited an electrical conductivity of 183.5 S/cm and a Seebeck coefficient of 43.5 μV/K at 313 K, and thus, its power factor at 313 K was over 30 μW m⁻¹ K⁻². To the best of our knowledge, this is the highest thermoelectric performance ever reported among conducting polymers.     

Poly(propargyl alcohol) doped with sulfuric acid,a new proton conductor usable for humidity sensor construction

The electrical properties of poly(propargyl alcohol), chemically doped with sulfuric acid, were investigated as a function of the relative humidity. In the absence of water, the conductivity of the doped polymer is very low (about 3 × 10⁻⁹ S cm⁻¹ at 25 °C) with characteristics of electronic-type charge transfer. In the presence of humidity, the charge transfer inside the polymer is mainly due to proton motion and the conductivity, ionic in character, increases with exponential trend as a function of the relative humidity. An explanation of this behaviour is attempted by taking into account the effect of water on the ion interaction. A preliminary investigation on the possibility of the use of the acid-doped poly(propargyl alcohol) as humidity sensor was also carried out.     

Thermal undoping in poly(3-alkylthiophenes)

Thin solid films of doped poly(3-hexylthiophene) and poly(3-decylthiophene) undergo rapid conductivity degradation at elevated temperatures (110 °C). We have followed the changes in the electronic structure during thermal treatment by optical spectroscopy. We have also studied thin films of the materials, before and after heat treatment, by means of infrared and X-ray photoelectron spectroscopy. The results show that the materials are undoped by thermal treatment. Comparative studies on poly(3-methylthiophene) show that this polymer also undergoes thermal undoping, but at a much slower rate.     

Electrochemical polymerization of 9-cyanophenanthrene and characterization of its polymer

A novel conducting polymer poly(9-cyanophenanthrene) (P9CP) was synthesized electrochemically by direct anodic oxidation of 9-cyanophenanthrene (9CP) in boron trifluoride diethyl etherate (BFEE). The oxidation onset potential of 9CP in the medium was measured to be 1.6 V vs. a saturated calomel electrode (SCE). P9CP films obtained from BFEE showed good electrochemical behavior and good thermal stability with electrical conductivity of 0.13 S cm^?1 in the doped state. The polymer of P9CP may be an attractive candidate material for organic light-emitting material with blue-green-light-emitting upon irradiation with 365 nm UV light. Moreover, the P9CP films on the ITO electrode showed good electrochromic property from dark green to dark yellow, between the doped and dedoped state. FTIR and the quantum chemistry calculation indicated that the 9CP was grown via the coupling of the monomer mainly at the C₃, and C₁₂ positions.     

Gas sensitiveness of polypyrrole films doped chemically in the gas phase

Electropolymerized polypyrrole films were electrochemically reduced and then chemically doped with one of PCl₃, SO₂, NO₂ or iodine vapour in the gas phase. These chemically-doped polypyrrole films showed a resistance decrease when they were exposed to electron-acceptor gases having electron affinities larger than those used in the chemical doping, except for one case of exposure of iodine-doped polypyrrole films to NO₂, where (although iodine has reportedly a larger electron affinity than NO₂) a resistance increase was seen, which was caused by a partial substitution of the doped iodine with exposed NO₂. The phenomenon is discussed in terms of the stabilization of gaseous species on adsorption. The present study suggests that a selective gas sensitivity may be attached to polypyrrole films if they are doped with an appropriate gas in advance.