Desktop inkjet printer as a tool to print conducting polymers

In order to exploit mechanical flexibility of organic-based electronic devices, conducting polymer anodes, such as polyaniline or poly(3,4-ethylenedioxy)-thiophene-poly(styrene sulfonate) (PEDOT-PSS), have been extensively studied. Along with the use of solution based processing techniques, conducting polymers can simplify the device fabrication procedure and yield themselves easily to printing techniques. In this paper, we present the results of utilizing desktop inkjet printer as a tool for direct printing and patterning of conducting polymer. Design of printable patterns and adjustment of printing parameters can be performed using any software such as Power Point. PEDOT-PSS suspension can be loaded into an inkjet cartridge and deposited on a given substrate in any designed pattern. The gray-scale color scheme can be employed to control the layer thickness and sheet resistivity of the inkjet printed layers. These layers are then used as anodes in organic light-emitting devices (OLEDs).     

Solid-state polymer light-emitting electrochemical cells

Solid-state polymer light-emitting electrochemical cells have been developed using thin films of conjugated polymers blended with solid electrolytes. The cells contain three parts: the polymer blend films as the active medium, and two contact electrodes: indium-tin oxide and aluminum. When externally biased, the conjugated polymers are p-doped and n-doped on opposite sides of the polymer layer, and a dynamic light-emitting p-n junction is formed between the doped regions. The admixed solid electrolyte provides the dopant counterions and the ionic conductivity necessary for the doping. The p-n junction is dynamic and reversible, with an internal built-in potential close to the bandgap of the redox-active conjugated polymers. Orange, green and blue lights emitted from the p-n junction have been obtained with turn-on voltage less than 3 V and external quantum efficiency higher than 2% photons/electron. In addition, a two-color light-emitting electrochemical cell has also been fabricated based on a bilayer structure consisting of two different luminescent polymers. The light-emitting p-n junction can be switched from one layer to another under different bias conditions.     

Light-emitting electrochemical cells and light-emitting diodes based on ionic conductive poly(phenylene vinylene): a new chemical sensor system

Red-orange light-emitting electrochemical cells (LECs) based on poly[1,4-(2,5-bis(1,4,7,10-tetraoxaundecyl))phenylene vinylene] also named poly[2,5-bis (triethoxy-methoxy)-1,4-phenylene vinylene], (BTEM-PPV) are fabricated and characterized. BTEM-PPV combines good electronic conductivity with ionic conductivity in the oxidized and reduced state due to its conjugated backbone and oligo(ethylene oxide) attached as side chains. When applying this polymer in LECs one obtains devices of moderate brightness and with fast response times. This high performance is achieved without blending an additional ionic conductive polymer into the film. The response time of such devices driven with square waveform voltage pulses was determined to be 480 μs. The turn-on voltage for electroluminescence occurs at 2 V and at 3 V a brightness of about 35 cd/m² was obtained. Due to the covalent linkage of glyme like side chains to the PPV backbone, BTEM-PPV, complexed with metal ions, shows an ionochromic effect in the absorption spectra and also in the electroluminescence spectra, which can be a new approach to chemical sensors.     

Study of electrolyte effects on electrochemical synthesis and p-doping of poly(thienyl biphenyl) films

The electrochemical synthesis and charging–discharging process of a copolymer consisting of 3-octylthiophene (3-OT) and biphenyl units have been studied in different electrolytic media. The polymer material has been characterized by electrochemical and spectroscopic methods: cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), chronopotentiometry, chronoamperometry and FTIR spectroscopy. The diffusion coefficients of different ions in poly(thienyl biphenyl) (PTB) films have been determined by chronoamperometry and compared with corresponding values in poly(3-octylthiophene) (POT) and poly(paraphenylene) (PPP). The best electrolytic conditions for synthesis of poly(thienyl biphenyl) concerning the copolymer structure was found to be in 0.1 M lithium hexafluoro arsenate (LiAsF₆) in acetonitrile. In this electrolyte solution, the content of phenylene segments compared to thienylene segments is highest resulting in a higher degree of cross-linking compared with films made in the presence of the other electrolyte salts studied.     

Organic electroluminescence devices fabricated with chemical vapour deposited polyazomethine films

We demonstrate the use of chemical vapour deposition polymerization as an effective method to fabricate conjugated polymer films as constituents in organic electroluminescence (EL) devices. This technique provides excellent compatibility with vacuum sublimation deposition of low molecular weight materials for construction of hybrid devices. The specific polyazomethine polymer studied here, namely, poly (1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne), is shown to be an addition to the range of polymer structures that provide electron transport functionality and, in combination with suitable emissive materials, provides the basis for new device structures. From measurements on a series of devices we propose an energy level structure for this material. Despite remaining isoelectronic with poly (1,4-phenylenevinylene) the energy levels of the polyazomethine are strongly shifted through the replacement of a vinylene CH with a N atom.     

Evaluation of electrochromic fast-switching behavior of self-doped conducting polymer

The electrochemical and optical fast-switching behavior of a self-doped conducting polymer was investigated by cyclic voltammetry and opto-electrochemical spectroscopy. In comparison with poly(3-hexylthiophene), the self-doped polymer thin film exhibited a reasonable electrochemical response in an acid electrolyte solution, even at fast scan rates. The optical switching phenomenon on the self-doped polymer was observed to take place within about 50 ms, in contrast to polyisothianaphthene which exhibits optical switching within about 500 ms.     

One-step electrochemical process for the formation of poly(N-methylpyrrole) coatings on steel in different media

Poly (N-methylpyrrole) coatings have been successfully formed on steel by a one-step electrochemical process in different media. Electropolymerization was performed in aqueous medium with low monomer concentration and in a mixed aqueous/organic media with higher monomer concentrations. Ethanol (EtOH) and N,N-dimethylformamide (DMF) were chosen as the organic components of the mixed solvents and oxalic acid was used as the electrolyte. The effect of process parameters on the formation of poly(N-methylpyrrole) was systematically investigated. The composition and morphology of the coatings were also studied by elemental analysis, FT-IR and scanning electron microscopy (SEM). Our results reveal that the formation and properties of poly (N-methylpyrrole) coatings were all dependent on the solvent composition and other process parameters. The amount of poly(N-methylpyrrole) formed on steel increased with time and applied current for all the solvent-electrolyte systems. For the electropolymerization performed in distilled water-oxalic acid solution, the amount of polymer coatings formed decreased with increased electrolyte concentration. In contrast, the amount of polymer coatings formed on steel in distilled water:ethanol (1:1)-oxalic acid and distilled water:DMF (1:1)-oxalic acid systems, increased with increasing electrolyte concentration. By controlling the electrochemical parameters, smooth, uniform and strongly adherent coatings could be formed onto steel substrates.     

Ionic conductivities of polymeric solid electrolyte films containing rare earth ions

Polymeric solid electrolyte films containing rare earth metal ions (Ce³⁺, La³⁺, Yb³⁺) have been prepared, with a view to applying them to solid-state electrochemical devices. The films, composed of poly(ethylene oxide)-grafted poly(methylmethacrylate) (PEO–PMMA), were prepared by dissolving rare earth salts with appropriate amounts of poly(ethylene glycol) dimethyl ether (PEG) that have ethylene oxide units in their structure. The ionic conductance behaviour of the polymeric composite electrolyte systems was investigated by AC impedance and DC polarization methods in an ambient temperature range. The oligo(ethylene oxide) units in the polymer matrix and the PEG components ensured the dissociation of the salts and the high mobility of the resulting ionic species in the solid films. About 10⁻⁵ S cm⁻¹ or above of ionic conductivity was obtained for a PEO–PMMA/PEG/Ce(ClO₄)₃ system at room temperature. The addition of a liquid plasticizer in the composite improved the conductivity by about two orders of magnitude.     

Temperature-dependent kinetics of printed polymer light-emitting electrochemical cells

The thermal activation energy for ion transport and related junction formation in a polymer light-emitting electrochemical cell (LEC) has been determined by measuring the transient behavior of light emission as a function of temperature from 257 K to 297 K. We find the initial time derivative of the light intensity to have an Arrhenius temperature dependence, with an activation energy of 1.6 eV. Our devices utilize a flourene-based polymer emitter which fluoresces at red/near-infrared wavelengths. The polymer matrix includes phenyl alkoxy-substituted PPV and an electrolyte/dopant system based on both solid polymer and ionic liquid dopants. Transient measurements of current and light emission have been used to capture the kinetics of both junction formation and junction decay and to measure variations in optical external quantum efficiency during device turn-on.     

Electrophoretic deposition of conjugated polymer: Deposition from dilute solution and PEDOT coating effect

It has been shown that the films of a soluble conjugated polymer poly[(9,9-dioctyl-2,7-divinylene fluorenylene)-alt-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}] for electronic devices can be prepared by the electrophoretic deposition with polymer suspensions derived from dilute polymer solutions which are so dilute that the conventional spin-coating technique is not applicable. For example, a 100 nm-thick film can be prepared on an indium-tin-oxide (ITO) electrode from a suspension from solution containing 0.1 g/l of the polymer. The thickness of the polymer film deposited is found to be almost proportional to the concentration of the polymer, and the linearity down to 5.0 × 10^?3 g/l is confirmed. On the other hand, it has been found that coating the ITO electrode with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) salt results in low and nonlinear deposition rate.     

Micropatterning of a stretchable conductive polymer using inkjet printing and agarose stamping

A highly conducting stretchable polymer material has been patterned using additive inkjet printing and by subtractive agarose stamping of a deactivation agent (hypochlorite). The material consisted of elastomeric polyurethane combined in an interpenetrating network with a conductive polymer, poly(3,4-ethylenedioxythiophene) (PEDOT). The agarose stamping produced 50 μm wide conducting lines with high spatial fidelity. The deactivation agent was found to cause some degradation of the remaining conducting lines, as revealed by a stronger increase in resistance upon straining compared to the pristine polymer material. Inkjet printing of the material was only possible if a short-chain polyurethane was used as elastomer to overcome strain hardening at the neck of the droplets produced for printing. Reproducible line widths down to 200 μm could be achieved by inkjet printing. Both methods were used to fabricate test patterns that allowed the electrical resistance parallel and perpendicular to the elongation direction to be measured. Electrical resistance increased both parallel and perpendicular to the direction of strain, with a faster increase observed parallel to the straining.     

Electrochemical studies of self-doped conducting polymers: Verification of the ‘cation-popping’ doping mechanism

Cyclic voltammetry in conjunction with X-ray microprobe analysis has been used to investigate the doping mechanism in covalently-bonded-anion (self-doped) conducting polymers. The polymers investigated are the acid and sodium salt forms of poly 3-(4-butanesulfonate) thiophene (P3-BTSH and P3-BTSNa, respectively) and poly 3-(2-ethane sulphonate) thiophene (P3-ETSNa). These water-soluble polymers, whose synthesis is described in detail, were used (in the form of films cast from solution onto ITO glass or platinum sheets) as working electrodes in cyclic voltammetry experiments in which the proton or Na⁺ concentration of the electrolyte was monitored. The results directly verify the proposed doping mechanism: charge injection into the π-electron system of the conjugated polymer accompanied by proton or sodium ion ejection, without incorporation of electrolyte anion, leaving behind the oppositely charge counterion.     

Effect of electrolyte and monomer concentration on anticorrosive properties of poly(N-methylaniline) and poly(N-ethylaniline) coated mild steel

The electrosynthesis of poly(N-methylaniline) (PNMA) and poly(N-ethylaniline) (PNEA) coatings on mild steel in aqueous oxalic acid solutions was carried out by potentiodynamic synthesis technique. The effects of monomer and electrolyte concentrations on electrochemical growth of PNMA and PNEA coatings on mild steel substrates were investigated. Repassivation peak did not appear during electrosynthesis of PNMA and PNEA coatings from solutions containing 0.1 M monomer and 0.1 M electrolyte. The tests for corrosion protection of the polymer coated and uncoated mild steel substrates were done in 3% NaCl solutions by dc polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion tests revealed that PNMA and PNEA coatings exhibited effective anti-corrosive properties. The acidity of the polymerization solution was found to influence the anticorrosive behavior of the polymer coating.     

Electrochemical impedance studies on the electrochemical properties of poly(3-methylthiophene) in aqueous solutions

Electrochemical impedance spectroscopy (EIS) measurements in a wide frequency range were used to study the electrochemical properties of poly(3-methylthiophene) (PMT) films. PMT was synthesized in acetonitril (AC) using BF₄⁻, ClO₄⁻ and CF₃SO₃⁻ as doping anion and Li⁺ as cation. The electrochemical characterization of these films was performed in aqueous solution using the same electrolyte for synthesis. Different parameters as electrolyte resistance, double layer capacitance, pore resistance, fractal dimensions, internal capacitance and charge resistance. SEM observation of PMT films provide strong evidence of the close relationship between type of electrolyte and their morphology, also is in good agreement with ac impedance results interpretation. The results are interpreted on the basis of models developed for intercalation electrodes in relation to the geometry and morphology of the polymer electrode. The results are discussed on the basis of the structural and chemical parameters of the anions and their interaction with the polymer. The discussion shows that the nature of the anion plays a major role on the electrochemical behavior of the polymer.     

Oxadiazole-containing phenylene vinylene ether linkage copolymer as blue-green luminescent and electron transport material in polymer light-emitting diodes

We report studies on a new ether-type poly(phenylene vinylene) (PPV) copolymer containing oxadiazole groups in the conjugated main chain. It can be used as a blue-green electroluminescent material and as an electron transport/hole blocking material in polymer light-emitting diodes using PPV as the emitting material. The bilayer devices with aluminum cathode show a maximum brightness of about 300 cd/m² at about 21 V and a maximum external quantum efficiency of 0.1%. The quantum efficiency of the bilayer device is enhanced by a factor of 195 in comparison with that of the single layer device of PPV.     

Polyaniline modified electrodes for detection of dyes

Polyaniline (PANI) is one of the most extensively used conjugated polymers in the design of electrochemical sensors. In this study, we report electrochemical dye detection based on PANI for the adsorption of both anionic and cationic dyes from solution. The inherent property of PANI to adsorb dyes has been explored for the development of electrochemical detection of dye in solution. The PANI film was grown on electrode via electrochemical polymerization. The as grown PANI film could easily adsorb the dye in the electrolyte solution and form an insulating layer on the PANI coated electrode. As a result, the current intensity of the PANI film was significantly altered. Furthermore, PANI coated stainless steel (SS) electrodes show a change in the current intensity of Fe²⁺/Fe³⁺ redox peaks due to the addition of dye in electrolyte solution. PANI films coated on both Pt electrodes and non-expensive SS electrodes showed the concentration of dye adsorbed is directly proportional to the current intensity or potential shift and thus can be used for the quantitative detection of textile dyes at very low concentrations.     

Charge trapping in bilayers of conducting polymers under open circuit conditions: a protection of conducting polymer films from spontaneous charging/discharging

Spontaneous charging/discharging processes of polymers: polypyrrole and poly(N-methylpyrrole), doped with perchlorate (anion exchanging PPy and PMPy, respectively) and poly(4-styrenesulphonate) ions (cation exchanging PPy(PSS) and PMPy(PSS)) occurring in aqueous electrolyte solutions were studied using different electrochemical techniques. These reactions (oxidation of the polymer by dissolved oxygen and discharge of the polymer redox capacitance) are usually undesired processes leading to alteration of the charge accumulated in the polymer films. This paper points out that these processes can be significantly limited in polymer bilayers due to effect of charge trapping.     

Electrochemical properties of luminescent polymers and polymer light-emitting electrochemical cells

The electrochemical p-doping potentials (φ_p) and n-doping potentials (φ_n) of 10 soluble poly(1,4-phenylene vinylene) (PPV) derivatives and of the conjugated polymer blend in a light-emitting electrochemical cell (LEC) were measured by cyclic voltammetry. The energy levels corresponding to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymers were determined from the onset potentials for n-doping (φ_n′) and p-doping (φ_p′), respectively. The electrochemical energy gap, E_g′=Δφ=φ_p′−φ_n′, agrees well with the optical energy gap (E_g). For copolymers of poly(2-methoxy, 5-(2′-ethyl-hexyloxy) paraphenylene vinylene) (MEH-PPV) and poly(2-butyl, 5-(2′-ethyl-hexyl) paraphenylene vinylene) (BuEH-PPV), the n-doping potentials are nearly independent of the ratio of MEH-PPV to BuEH-PPV. However, p-doping potentials depend strongly on the ratio, the higher the MEH-PPV fraction, the lower the p-doping potential of the copolymer. For cyano-PPV (CN-PPV), both p-doping and n-doping potentials were shifted by ca. 0.6 V (more electronegative) as a result of the electron withdrawing effect of cyano side group. The electrochemical p-doping and n-doping processes of the MEH-PPV polymer blend in the LEC device were confirmed from linear sweep voltammetry of the LEC; the doping onset potentials were the same as for an MEH-PPV film measured in a liquid electrolyte.     

Influence of water and electrolyte included in the polymer on the rectifying properties of the junction n-doped silicon/poly(4,4′-dipentoxy-2,2′-bithiophene)

Small amounts of water and electrolyte included in the polymer during the electrochemical polymerization, and detected by thermogravimetric analysis and X-ray diffraction strongly affect the current-voltage characteristics of the system n-doped silicon/poly(4,4′-dipentoxy-2,2′-bithiophene). Current-voltage curves and impedance measurements demonstrated that a double layer is established at the interface relative to the electric contact on the polymer where, probably, the oxidation of water takes place when the rectifying junction is forward biased. These processes affect the transport properties of the junction in different ways, depending on the nature of the contact. It was also found that the properties of the silicon/polymer interface do not depend meaningfully on eventual amounts of water and electrolyte included in the polymer.     

The chemical,electrochemical synthesis and properties of poly[(4-decylthiophene-2,5-diyl)(2,2′-biphenyl)(4-decylthiophene-2,5-diyl)]

A novel blue-emitting conjugated polymer, poly[(4-decylthiophene-2,5-diyl)(2,2′-biphenyl)(4-decylthiophene-2,5-diyl)] (PDTBPDT), has been synthesized via the reductive coupling polymerization using nickel(0) as the catalyst in place of FeCl₃-oxidative polymerization. The chemical structure of the polymer is verified by ¹H and ¹³C NMR, FT-IR and elemental analysis. The polymer exhibits good thermal stability and solubility in common organic solvents. The investigation of its optical properties exhibited that its conjugation efficiency was controlled by the twisted biphenyl group and head-to-head connection between adjacent thiophene rings, and resulted in blue photoluminescence. Its electrochemical property showed that the biphenyl group introduced into the main chain could adjust the HOMO and LUMO energy levels of the polymer. The electrochemical polymerization of the polymer is also investigated.