Doped Polythiophene Films Research Articles

Molecularly doped polythiophene film as an efficient photocathode for oxygen reduction to hydrogen peroxide

AbstractThe molecularly doped poly(3-hexylthiophene) (P3HT) was used for the first time as a photocathode for reducing oxygen to H2O2. For this purpose, a P3HT film was doped with hexaazatriphenylenehexacarbonitrile, which increased the oxygen reduction current at an applied negative potential in the dark. Visible light illumination of the doped P3HT film significantly facilitated the oxygen reduction with a high current density and shifted the onset potential beyond the reaction equilibrium potential. The oxygen reduction performance of the doped P3HT film is discussed in relation to the energy level diagram. Graphical abstract

Efficiency enhancement of solid-state dye-sensitized solar cells by doping polythiophene films photoelectrochemically grown onto TiO2 nanoparticles covered with cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)

We recently reported on the effect of dopant sizes on the performance of polythiophene (PT)-based solar cells wherein the PT film was polymerized with 3-{5-[N,N-bis(4-diphenylamino)phenyl]thieno[3,2-b]thiophen-2-yl}-2-cyano-acrylic acid (C207) pre-adsorbed onto the electrode. Herein we utilized n- and p-doped polythiophene (PT) films as part of the solid-state dye-sensitized solar cell (ssDSSCs) based on cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N3), the most commonly used dye sensitizer. Although the structure of the N3 is different from the chain-like C207, a smooth and homogeneous PT film can still be grown photoelectrochemically into the pores and onto the surface of the N3-covered TiO2 nanoparticles. The PT films were electrochemically doped with anions (ClO4− or PF6−) and cations (tetrabutylammonium, TBA+, or tetramethylammonium, TMA+). TMA+, being smaller, leads to a higher doping level in the PT films and a power conversion efficiency of 7.57 ± 0.33%, which is a 33% increase over that constructed with undoped PT films. However, the efficiency of the anion-doped PT films is reduced when compared to that of the undoped PT films. The N3-sensitized solar cell comprising a TMA+-doped PT film is 15% more efficient and costs at least 57% less than the PT-based solar cell comprising C207. This study demonstrates that the n-doped PT film can be a useful interfacial modifier, and pre-adsorbed N3 not only assists the growth of smooth PT films, but also enhances light absorption and power conversion efficiency of the resultant DSSC.

Anisotropic terahertz complex conductivities in oriented polythiophene films

We investigate polarization-resolved terahertz (THz) transmission through a doped polythiophene film consisting of partially oriented polymer chains. The THz complex conductivities are found to be significantly larger for polarization parallel to the principal direction of orientation than for polarization perpendicular to it, but involve no change in spectral shape with polarization. This indicates that charge transport occurs mainly along polythiophene chains with their in-plane angle distribution, ruling out a possible interchain contribution, whose spectral shape should be sensitive to polarization.

Study of dye molecule orientation and configuration in dye molecule doped polythiophene films

Dye molecules doped polythiophene (PT) films prepared by electrochemical doping were investigated by Fourier transform infrared spectroscopy (FTIR) reflection absorption spectroscopy (RAS) and transmission in order to investigate whether the presence of an electric field could control the ordering of the dye molecule assembly in the polymer. The present work focuses on organic dye molecules, which contain three benzene rings in plane with a high symmetry such as crystal violet (CV) and brilliant green (BG) as a dopant. In the case of CV doped PT by applying triangle-shaped negative voltage, the orderings of CV molecules were found by FTIR measurements. Since the ring stretching mode due to the componential benzene ring around 1585 cm−1 was observed only in the FTIR transmission spectra while not observed in the spectrum of FTIR RAS spectra, this result shows that CV molecules are doped in the polymer so that the ring plane of CV is inclined to be parallel to the polymer film plane. In the case of CV doped PT samples by field-free casting method, the ring stretching modes were observed in both spectra of FTIR RAS and transmission, indicating that the dopant molecules were randomly oriented.

Soluble polymers with constant π-conjugation length: polymers containing thiophene tetramers and hexamers

Soluble polymers (PQE and PSE) with each constant π-conjugation length, containing thiophene tetramers and hexamers in the main chains, were prepared by reductive condensation, respectively. The obtained polymers were soluble in common organic solvents. Cast films could be obtained from their solutions. The PQE and PSE films doped with FeCl 3 showed considerable conductivities, 9-7×10 −3 and 0.4-0.2 S(cm, respectively. The polymer solutions and films were oxidized (doped) by stepwise addition of FeCl 3 and characterized by electronic absorption spectra. The spectral change of the PQE solutions and films showed one-step oxidation to produce cation radicals, whereas that of the PSE solutions and films did two-step oxidation to yield cation radicals in the first step and plural-electron-oxidized species such as dications in the second step. In atmosphere, the plural-electron-oxidized species of the PSE films are extremely labile. The spectra of the oxidized PSE films in atmosphere were almost similar to that of doped polythiophene films, indicating that oxidized species existing in doped polythiophenes are mainly cation radicals.

Determination of mobilities of charge carriers in electrochemically anion-doped polythiophene film

Mobilities of positive charge carriers in a polythiophene film at different oxidation stages are evaluated on the bases of elaborate electrochemical measurements. The mobility in the low doping regime, most likely that of polarons, is 10 −6 cm 2 V −1 s −1. The mobility starts to increase at around 0.2 mol% doping level and reaches 0.5×10 −2 cm 2 V −1 s −1 in the heavily doped region. The drastic mobility increase by doping manifests changes in nature and/or kind of charge-carrying species, or the switching of conduction mechanisms in the ClO 4 --doped polythiophene film.

Raman Studies of Doped Polythiophene and the Radical Cation and Dication of Quinquethiophene

Abstract The near-infrared Raman and electronic absorption spectra of an aspolymerized BF4− -doped polythiophene film have been measured and analyzed by referring to the data of the radical cation and dication of α-quinquethiophene, which are models of a positive polaron and a positive bipolaron, respectively. Similarities between the Raman spectrum of doped polythiophene and that of the radical cation suggest that the observed Raman bands of the doped polymer arise from positive polarons. It is shown that the two characteristic bands in the electronic absorption spectrum of the doped polymer are assignable to positive polarons.

Dynamics of polythiophene with defects

New infrared and Raman experiments on undoped and doped polythiophene films, obtained in various synthesis conditions, are reported. The samples present a broad distribution of conjugation length. To explain all the observed spectra the authors develop a discrete force field model. Then, using this model and a new powerful computing process, the spectral moments method, they have been able to interpret practically all principal features observed: not only are the calculated frequencies very close to the experimental data but the intensities and the behaviour with conjugation length of the infra-red and Raman lines are in fair agreement for undoped and doped polythiophene. In particular, in infrared, the emergence upon doping of four intense lines at 1032, 1117, 1203 and 1334 cm-1, and other features less intense, and in isotropic Raman scattering the reduction of the intensity of all lines, are nicely reproduced. Taking into account previous results obtained for the perfect chain it is possible to furnish a reasonable interpretation of the infrared and Raman spectra of the polythiophene. In particular, the infrared activity is very well represented with a simple tensor charge model without explicitly taking into account the fluctuation of the charge density.

AC Conductivity of ClO4− Doped Polythiophene Films

AC Conductivity of ClO4− Doped Polythiophene Films

Studies on iodine doped thiophene oligomers

Individual thiophene oligomers, n=2–5, have been isolated and doped by iodine in the gas phase. Their electrical and spectroscopic properties were compared with iodine doped polythiophene film. It is shown that long conjugation length is not requisite to achieve metallic conductivity and interchain carrier transport plays an important role to the transport mechanism. In the doped oligomers the mid-gap absorption shifted from visible region for n=2 to near infra red for n=4 and polythiophene film, while another absorption appeared below the band gap in doped polythiophene at 1.63 eV. We tentatively attribute the lower energy absorption observed for both doped oligomers and the doped polymer to charge transfer band. Four new IR modes in the region of 1000–1300 cm −1 observed in doped polythiophene film were present also in the doped oligomers. We prefer to attribute these IR absorptions to the vibration of charge transferred conjugated thiophene rings accompanied by a great enhancement of absorption intensity.

Electrical and optical properties of polaronic states in conducting polymer, polythiophene

The ESR spin density and linewidth in a slightly BF 4 − doped polythiophene film decrease upon illumination of an infrared light. The photocurrent response is observed at the infrared region too. The results are discussed in terms of the photoexcitation of polarons to free state and the formation of spinless bipolarons. The evolution of polaronic states during the electrochemical cycle was investigated in detail by the in situ measurement of absorption spectra combined with cyclic voltammogram.

IR and Raman studies of polythiophene prepared by electrochemical polymerization

IR and Raman studies have been carried out for polythiophene prepared by electrochemical polymerization and subsequently treated with aq. NH 4OH solution to neutral film. Four additional strong absorption bands at 1330, 1200, 1100 and 1030 cm −1 appeared upon doping irrespective of the nature of dopants and their intensities increased with increasing dopant concentration. Polydeuterothiophene obtained by the same method gave similar additional strong bands shifting slightly to lower frequencies. These four intense IR bands did not appear in the Raman spectra of doped polythiophene films. Origin of these IR bands can be attributed to ring stretching of the charged polythiophene chain.

ESR and Transport Studies in Electrochemically Doped Polythiophene Film

Electron spin resonance (ESR), electrical conductivity and thermoelectric power are studied in electrochemically prepared and doped polythiophene films. The ESR spin density increases by two orders of magnitude up to dopant concentration of ca. 3 mol%, then decreases, while the conductivity increases drastically by more than ten orders of magnitude until this doping level, followed by a rather mild increase. The ESR linewidth decreases sharply from 9 G to 0.4 G upon slight doping with shape change from Gaussian to Lorenzian. The contribution of Pauli-paramagnetism to the spin succeptibility and the positive thermoelectric power of ca. 20 µV/K which depends linearly on the temperature are observed at heavily doped region. These results are discussed in terms of shallow and deep polarons, and formation of bipolaron up to intermediate doping levels and the hole conduction at the metallic regime.