Design and electrosynthesis of push-pull substituted polythiophenes

A series of bithiophenes substituted with donor and acceptor groups has been synthesized. The relationship between the electrochemical behaviour of these bithiophenes and their ability to electropolymerize is analysed and discussed. The antagonistic electronic effect of both substituents can be controlled in order to allow stereoregular polymerization to take place. The obtained polymer shows unusual electrochemical and electrooptical properties, associated with the presence of strong accepting and donating groups alternating along the conjugated backbone.     

Synthesis and properties of novel fluorinated ester substituted polythiophenes

The electronic properties of the conjugated backbone of polythiophene can be tailored by the incorporation of perfluoroalkyl ester substituents. A polythiophene substituted with two different kinds of semifluoro and perfluoroalkyl esters were prepared by FeCl₃ oxidative polymerization. Both polymers were found to be highly soluble in common organic solvents, such as CHCl₃, THF and acetone. The influence of bulky fluoroalkyl substitutions on electrical conductivity, electronic absorption, fluorescence and surface properties of polymer films were investigated, and the properties were compared with poly(3-octylthiophene) synthesized under similar experimental conditions.     

Organic single- and double-layer electroluminescent devices based on substituted phthalocyanines

Organic electroluminescent diodes containing tetra-tert-butylphthalocyanine, tetrakis(3-phenylneopentyloxy)-phthalocyanine and tetrakis(3-phenylneopentyloxy)-phthalocyaninatocopper(II) were prepared by the vacuum deposition method. The current–voltage (I–U) characteristics and the electroluminescence (EL) spectra were measured.     

Photovoltaic devices based on a novel poly(phenylene ethynylene) derivative

A novel poly(phenylene ethynylene) derivative containing benzothiadiazole (BT) co-monomer (PPE-BT) was synthesized, characterized and used for photovoltaic devices. Photovoltaic devices of ITO/PEDOT-PSS/pristine PPE-BT/Ba/Al and ITO/PEDOT-PSS/PPE-BT+PCBM (1/4 w/w)/Ba/Al were fabricated and their photovoltaic performances were measured and discussed. The energy conversion efficiency and incident photo-current conversion efficiency of photovoltaic device based on PPE-BT+PCBM reached 0.022% (78.2 mW/cm², AM1.5) and 1% (391 nm), respectively. Its photosensitivity reached 3.1 mA/W, which is almost one order of magnitude higher compared to that of the pure PPE-BT. The electroluminescence of pristine PPE-BT was almost completely quenched when it was blended with PCBM.     

Photovoltaic devices fabricated with a p-type conjugated multi-branched molecules

New multi-branched crystalline molecules were synthesized through Horner(Emmons reactions using hexyl-substituted thiophene-based carbaldehydes as dendrons and [1,2,4,5-tetra-(diethoxy-phosphorylmethyl)-benzyl]-phosphonic acid diethyl ester as the core unit. 1,2,4,5-Tetrakis((E)-2-(5′-hexyl-2,2′-bithiophen-5-yl)vinyl)benzene (HPBT) and 1,2,4,5-tetrakis((E)-2-(5′-dodecyl-2,2′-bithiophen-5-yl)vinyl)benzene (DPBT) have different alkylene peripheral moities such as the hexyl and dodecyl group. The molecules were employed to fabricate organic photovoltaic (OPV) cells with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As a result, HPBT-based bulk heterojunction with PCBM exhibited higher short-circuit current density and open circuit voltage, resulting in a higher power conversion efficiency of around 0.944%.     

Electrosynthesis and characterization of alkylester-substituted polythiophenes

Thiophenes containing alkylester groups in the β position of the ring have been electropolymerized on Pt. The fraction of the polymer films that became soluble in chloroform has been analysed by spectroscopic techniques and by size exclusion chromatography, with the aim of comparing them with the chemically prepared corresponding macromolecules. The electrochemical behaviour, i.e., the charge process of the polythiophene coatings, has been studied and the relevant parameters have been discussed.     

Photovoltaic cells based on ionically self-assembled nanostructures

We use the technique of ionically self-assembled monolayers (ISAMs) to produce photovoltaic devices of well-controlled thickness and composition. The ISAM nanostructure fabrication method simply involves the alternate dipping of a charged substrate into aqueous cationic and anionic solutions at room temperature. We have employed several approaches to combine the tetrahydrothiophenium precursor of poly(para-phenylene-vinylene) (PPV) with fullerenes and other organic materials. We apply modulation spectroscopy for the electro-optical characterization of the ISAM-devices. The modulation frequency dependence of the photocurrent can be assigned to the influence of trapped charges taking part in the photovoltaic process.     

Electrochemical synthesis of polythiophenes and polyselenophenes

Thiophene and selenophene, when substituted in the 3-position by an electron-donor group such as methyl or methoxy, lead to the formation of a polymeric film when electrochemically oxidized. Thus, poly(3-methylthiophene) (PMeT), poly)3-methoxythiophene) (PMeOT), poly(3-methylselenophene) (PMeS) and poly(3-methoxyselenophene) (PMeOS) have been synthesized either by electrolysis at fixed potential or by repetitive cyclic voltammetry. These polymers are electrical conductors and can be cycled between the oxidized state (doped) and the neutral state (undoped). This charge-discharge process is accompagnied by reversible changes in colour, which can be seen by u.v.-visible spectroscopy.     

Polarized Raman spectra of polyacetylenes and polythiophenes

The results of an extensive study of the polarized resonant Raman scattering from cis polyacetylene and from polythiophene are presented.Depolarization ratios measured on isotropic samples appear to be inconsistent with the view that the visible absorption band is dominated by a single transition polarized along the chain axis.     

Pentacene based photovoltaic devices

Photovoltaic diodes based on iodine- and bromine-doped pentacene single crystals as well as thin films (TFs) have been prepared. Two different device structures, a Schottky diode with a Mg/Al back contact and a heterojunction with a ZnO:Al window layer, are investigated. An immense increase of the photovoltaic conversion efficiency of these devices is observed upon doping with iodine as well as bromine, which is ascribed to a reduction of the series resistance, an increase in quantum efficiency, and a red shift of the absorption spectrum. AM1.5 efficiencies of 2.7 and 4.5% are obtained for single crystal Schottky diodes and heterojunctions, respectively. In addition, efficiencies exceeding 2% are realized in TF devices on flexible plastic substrates.     

The influence of electron-deficient comonomer on chain alignment and OTFT characteristics of polythiophenes

The structure–property relations of polythiophene derivatives have been the subject of research for both materials and electronic device applications. We have designed and prepared two kinds of polythiophene derivatives with similar chemical structures but different electronic properties by Stille polymerization. One is composed of only electron-rich thiophenes (PQT2T), while the other is composed of both electron-rich thiophenes and electron-deficient thiazoles (PTZT2T). The number-average molecular weights of PQT2T and PTZT2T were 23,000 (PDI = 1.5) and 13,000 (PDI = 1.7), respectively. PTZT2T showed a large bathochromic shift of absorption maximum from solution to film by 57 nm. Further investigation by differential scanning calorimetry and X-ray diffraction revealed the formation of well-ordered interdigitated structure of PTZT2T in the solid state (d₁ = 20.7 Å). From absorption onset, optical band-gap of PQT2T and PTZT2T was calculated to be 2.22 eV and 1.92 eV, respectively. A mobility of 1.1 × 10^?3 cm²/Vs, an on/off ratio of 4.7 × 10⁴ have been obtained from organic thin-film transistors (OTFTs) using PTZT2T as a channel. By comparison with PQT2T, the mobility was 18 times and the on/off ratio was 52 times higher for PTZT2T, which shows the usefulness of electron-deficient thiazoles as comonomer units of polythiophene derivatives for OTFT applications.     

Synthesis and characterization of polythiophenes prepared in the presence of surfactants

In this study homopolymers of thiophene were synthesized by chemical oxidative polymerization in the presence of three different surfactants (anionic, cationic and non-ionic) and an oxidant in an anhydrous medium. An anionic surfactant, sodium dodecylbenzenesulfonate (DBSNa), a cationic surfactant, tetradecyltrimethylammonium bromide (TTAB) and a non-ionic surfactant, poly(ethylene oxide) (20) sorbitan monolaurate (Tween 20), were used as additives. The properties of the polythiophenes (PTs) prepared were studied and compared with those of surfactant-free polythiophene (PT). PTs were characterized by different techniques: FTIR spectroscopy, elemental analysis, scanning electron microscopy, thermogravimetry and conductivity measurement. The scanning electron micrographs of PT prepared in the presence of surfactants showed differences in the surface morphologies for different surfactants. PT prepared in the presence of cationic and non-ionic surfactants exhibited smooth surfaces, while a typical globular structure was found for surfactant-free PT. PT synthesized with a cationic surfactant showed the highest thermal stability of all the samples.     

Synthesis of supramolecular complexes based on tetracrown–substituted zinc porphyrinates

New metal-porphyrin receptors functionalized by aza-crown-ether groups were synthesized. Main paths of supramolecular self-assembly of porphyrin aza-crown-ether conjugates to bis-porphyrin sandwich complexes with metal and ammonium cations were studied using spectral methods of analysis (EAS, ¹H, ¹³CNMR, MALDI-ToF mass spectrometry). Principal regularities of axial coordination of the obtained bis-porphyrin complexes with exobidentate ligands (1,4-diazabicyclo[2.2.2]octane and pyrazine) were studied.     

Single- and dual-type electrochromic devices based on polycarbazole derivative bearing pendent viologen

In relation to electrochromic (EC) device performance, it is important to avoid unwanted power consumption and to minimize electrochemical degradation by using an EC material, which exhibits the desired EC state under no bias voltage in most of the time. Regarding this point, we prepared single- and dual-type EC devices based on poly(N-methyl-N′-(6-carbazole-1-ylhexyl)-4,4′-bipyridinium dihexafluorophosphate) [P(Cz-V)]. Since polycarbazole is oxidative-coloring and viologen is reductive-coloring, the supplementary EC action provides variety in color, which cannot be achieved by individual EC materials. In the single-type device, the polymer became green P(Cz^α+-V²⁺) at 3 V (versus ITO), bleached P(Cz⁰-V²⁺) at 0 V, and violet P(Cz⁰-V⁺) at −3 V. In the dual-type device, the complementary action of (Cz^α+-V²⁺) and P(Cz⁰-V⁺) upon a bias voltage of 3 V showed higher EC contrast compared to the single-type device.     

Switching and memory devices based on a polythiophene derivative for data-storage applications

In this article, we report electrical characteristics of devices based on oriented and unoriented films of a polymer, namely poly[3-(6-methoxyhexyl)thiophene]. The current–voltage characteristics of sandwiched devices, based on unoriented polymer, showed hysteresis behavior, while oriented versions exhibited switching characteristics, i.e. presence of two conducting states depending on sweep direction of voltage scans. The ratio between the device current of two conducting states has been as high as 10⁵. This is comparable, if not better, than the results reported so far with complicated device architecture or doped polymeric materials. We have also demonstrated that the switching devices have an associated memory effect for data-storage applications.     

Dual-type electrochromic devices based on conducting copolymers of thiophene-functionalized monomers

Dual polymer electrochromic devices (ECDs) composed of electrochemically deposited conducting copolymers of thiophene-functionalized monomers, 2-[(3-thienylcarbonyl)oxy] ethyl 3-thiophene carboxylate (TOET), 2,3-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOPT), and 3-[(3-thienylcarbonyl)oxy]-2,2-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOTPT), and polyethylene dioxythiophene (PEDOT) as the counter part were constructed with tetrabutylammonium tetrafluoroborate (TBAFB) doped gelled poly(methyl methacrylate) (PMMA) and evaluated. An electrochromic device with the following configuration was assembled — indium tin oxide (ITO)-coated glass /conducting copolymer || gel electrolyte || PEDOT/ITO. Spectroelectrochemistry, electrochromic switching, stability and open-circuit memory of the devices were examined.     

High contrast solid-state electrochromic devices from substituted 3,4-propylenedioxythiophenes using the dual conjugated polymer approach

Solid-state electrochromic windows from 3,4-propylenedioxythiophene (ProDOT) derivatives using dual polymer electrochromic architecture were fabricated and their electro-optical characteristics were recorded. DibenzylProDOT (DiBz-ProDOT) and biphenylmethyloxymethyl ProDOT (BPMOM-ProDOT) were used as cathodically coloring polymers, whereas bis(2-(3,4-ethylenedioxy)thienyl)-N-methyl carbazole (BEDOT-NMCz) was used as their complementary anodically coloring polymer. Straightforward assembly of the devices was designed by means of using a UV photo-curable gel. These devices exhibited response speeds of approximately 1 s with photopic contrasts as high as 52% for the complete device, and were found to exhibit good stability under open circuit conditions. Convenience of reporting photopic values to characterize electrochromic devices compared to reporting contrasts at a single wavelength is remarked.     

Highly conjugated acrylate-functionalized polythiophenes from anodic coupling of thiophene- and bithiophene-hexyl acrylates

Anodic coupling of thiophene- (1) and cyclopentadithiophene- (2) hexyl acrylates in acetonitrile to polythiophenes is reported. The resulting polymers have been characterized by cyclic voltammetry, UV–vis and FTIR spectroscopy and in-situ conductivity. The produced polythiophene films, for which FTIR analysis show that the acrylate moiety is kept unchanged, display high conjugation lengths (λ_max=520 (1) and 600 (2) nm) and conductivities (20 (1) and 100 (2) S cm⁻¹).     

Blue light electroluminescent devices based on a copolymer derived from fluorene and carbazole

Thin layers of poly(dihexylfluorene-co-N-hexylcarbazole) (poly(DHF-co-NHK) sandwiched between indium tin oxide (ITO) and aluminium electrodes show stable photoluminescence (PL) and electroluminescence (EL) at room temperature and ambient atmosphere. I(V) and EL vs. voltage curves present a much lower threshold than device made up of poly(DHF) homopolymer. The behaviour of both materials has been quantitatively investigated through a Fowler–Nordheim analysis allowing the determination of the energy offsets between the Fermi level of the ITO electrode and the HOMO of polymers. When using the copolymer, the barrier height for hole injection is reduced to a value close to 0.17 eV compared to 0.83 eV for the homopolymer. This difference in offsets has been confirmed by electrochemical investigations.