CNT/PEDOT core/shell nanostructures as a counter electrode for dye-sensitized solar cells

CNT/PEDOT nanostructures composed of carbon nanotube (CNT) cores and poly(3,4-ethylenedioxythiophene) (PEDOT) shells were synthesized by chemical oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) using FeCl₃ and dodecylbenzene sulfonic acid (DBSA) as the oxidant and surfactant, respectively. The resulting CNT/PEDOT nanostructures had a PEDOT layer thickness of 2–5 nm that exhibited not only higher polymerization yield but also enhanced thermal stability and electrical conductivity relative to pure PEDOT. N-Methyl-2-pyrrolidone (NMP)-based CNT/PEDOT paste containing polyvinylidene fluoride (PVDF) as a binder was painted directly onto fluorine-doped tin oxide (FTO) glass for use as a counter electrode (CE) material in dye-sensitized solar cells (DSSCs). While DSSCs made of pure CNT and PEDOT CE exhibited power conversion efficiencies of ～3.88% and 4.32% under standard AM 1.5 sunlight illumination, respectively, the cell efficiency was enhanced to ～4.62% with the CNT/PEDOT CE. This enhancement was due to the improved fill factor of the CNT/PEDOT-based DSSC realized by the increased electrical conductivity of the CNT/PEDOT composite.     

Facile chemical synthesis of DNA-doped PEDOT

Herein we report the template polymerization of poly(3,4-ethylenedioxythiophene) (PEDOT) using a biomacromolecule, DNA, as the polyelectrolyte. The resultant bio-composite material formed a stable, aqueous-dispersible system. Higher conductivities than that of conventionally prepared PEDOT-PSS without conductivity enhancing additives were observed (ca. 1.0 S/cm versus ca. 0.15 S/cm). The material was redox active in the dispersed state and in the solid state. The DNA double helix was found to undergo changes in conformation upon redox switching, resulting in controllable variance of its ability to polarize light. This non-acidic template is non-corrosive and is therefore an ideal candidate for use in device applications. The system described is potentially bioactive and biocompatible, as well, which would further extend its applicability.     

Effects of iron(III) p-toluenesulfonate hexahydrate oxidant on the growth of conductive poly(3,4-ethylenedioxythiophene) (PEDOT) nanofilms by vapor phase polymerization

Study of the effects of evaporation time and oxidant concentration on the polymerization rate and final thickness of poly(3,4-ethylenedioxythiophene) (PEDOT) films found that the polymerization was chemical reaction limited, and that the final film thickness was linearly dependent on oxidant concentration. The influence of spin-coated iron(III) p-toluenesulfonate hexahydrate (Fe(PTS)₃) oxidant solution on the polymerization and resulting PEDOT structures was also investigated. Spin-coated low-density (1.35 g/cm³) Fe(PTS)₃ oxidant solution, consisting of highly anisotropic paracystalline structures, provided open frameworks for the growth of conducting polymers, leading to a-axis oriented paracrystalline PEDOT films. Polymerization proceeded through the oxidant framework, increasing the density of PEDOT-filled oxidant solution to 2.01 g/cm³ with negligible change of thickness. Furthermore, the formation of a surface organic layer, as a result of spinning, promoted uniform and smooth PEDOT films.     

Highly conducting free-standing poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) films with improved thermoelectric performances

Highly conducting free-standing poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) films with room-temperature electrical conductivity of about 300 S cm^?1 were successfully prepared from PEDOT/PSS solution containing additives (DMSO or EG) on the smooth and flexible polypropylene (PP) film substrate with contact angle of 87°. As formed free-standing PEDOT/PSS films possess good flexibility and can be easily cut into various shapes with a knife. The contact angle of substrate has significant effect on the preparation of free-standing PEDOT/PSS films. Additionally, the process of adding DMSO or EG did not result in the change of carrier concentration but the increase of carrier mobility. The free-standing PEDOT/PSS film showed high electrical conductivity and stable Seebeck coefficient and its figure of merit (ZT) with high environment stability can be up to 10^?2, one order of magnitude higher than that of pressed PEDOT/PSS pellets (10^?3).     

Electro-conductive properties of poly(3,4-ethylenedioxythiophene)/poly(ionic liquid) films with respect to its structure and morphology

Poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(1-vinyl-3-ethylimidazolium⁺ (trifluoromethane sulfonyl)imide^?) (PIL) complexes were prepared at various PEDOT/PIL molar ratios and dispersed in propylene carbonate at a concentration of 1 wt%. After casting, the maximum conductivity was measured to be 1.2 × 10^?1 S/cm, which could be explained by the 3D variable range hopping model. The optimum surface roughness of the PEDOT/PIL film was measured, showing S_a and S_q values of 5.92 and 11.0 nm, respectively. The conductivity of the polymerized PEDOT without a template process had low conductivity due to its poor surface roughness and large particle size. Therefore, the conductivity of PEDOT/PILs is determined by the particle size, crystallinity and surface morphology. These results were supported by surface mapping microscopy, X-ray photon spectroscopy, and X-ray diffraction.     

Fabrication and catalytic property of an Ag@poly(3,4-ethylenedioxythiophene) yolk/shell structure

Poly(3,4-ethylenedioxythiophene) (PEDOT) hollow spheres with the size ranged from 210 to 850 nm and a conductivity of 6 × 10^?2 S cm^?1 have been prepared via electrochemical polymerization of 3,4-ethylenedioxythiophene in the presence of poly(vinyl pyrrolidone) as a soft template. Then, a novel Ag@PEDOT yolk/shell structure has been prepared simply and directly by a penetration and reduction approach. The morphology, formation, and the catalytic activity of the as-prepared Ag@PEDOT yolk/shell structure have been investigated. The experimental result shows that the as-prepared Ag@PEDOT yolk/shell structure exhibits excellent catalytic activity on reduction of p-nitrophenol. The employed approach may shed some light on preparing other noble metal@conductive polymer core/shell structure.     

One-step fabrication of conductive poly(3,4-ethylenedioxythiophene) hollow spheres in the presence of poly(vinylpyrrolidone)

Poly(3,4-ethylenedioxythiophene) (PEDOT) hollow spheres with the size ranged from 130 to 820 nm and a conductivity of 8 × 10^?2 S cm^?1 were prepared simply and directly via a one-step self-assembly approach in the presence of poly(vinylpyrrolidone) (PVP) as a soft template. It was found that the formation probability of PEDOT hollow spheres depended on the concentration of PVP. Bulk quantities of PEDOT hollow spheres can be obtained readily under the optimal conditions such as the concentration of PVP > 0.175 mM and PVP/EDOT molar ratio <1.4. The electron-rich oxygen atoms on the lactam groups in PVP were proposed to act as the chemically active sites to anchor EDOT cation radicals by virtue of electrostatic interaction and cause the self-assembly of PEDOT hollow spheres. Our investigation for the formation mechanism of PEDOT hollow spheres may shed some light on preparing of other hollow materials by proper molecular design and experimental condition optimization.     

Electrosynthesis of 2,7-linked polycarbazole derivatives to realize low-bandgap electroactive polymers

Poly(2,7-carbazole) derivatives have been synthesized because they have more extended conjugation lengths and lower energy bandgaps than poly(3,6-carbazole)s; however, few studies regarding electrochemical synthesis of poly(2,7-carbazole)s have been reported. Here, a series of N-alkylated-2,7-di(2-furyl)carbazoles, N-butyl-2,7-di(2-thienyl)carbazole, and N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole, were synthesized to obtain electroactive films of poly(2,7-carbazole) derivatives by electrochemical polymerization. Cyclic voltammetry revealed that these monomers have excellent polymerization activity, due to their low oxidation potentials (<0.57 V vs. ferrocene (Fc/Fc⁺)), and the corresponding polymers exhibit good redox properties. The energy bandgaps of the polymers obtained from optical absorption spectra range from 2.1 to 2.3 eV. Among the polymers, poly[N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole] shows the lowest bandgap energy of 2.1 eV, which is lower than that of previously reported poly(3,6-carbazole) analogues (2.4 eV). This polymer exhibits a significant color change from red in the oxidized state to blue in the reduced state during an electrochemical redox process. The electrochemical and optical properties of the monomers are dependent on external heteroaromatic rings attached to the 2 and 7 positions of the carbazole unit.     

Synthesis of discus-like Poly(3,4-ethylenedioxythiophene) nanoparticles using a bicellar template

Poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles with discus-like morphology were synthesized by chemical oxidation polymerization using bicelles as a soft template. The bicelles were assembled from the surfactant molecules of sodium perfluorooctanoate (FC7) and cetyltrimethylammonium bromide (CTAB). The PEDOT nanodiscuses have an average diameter around 200 nm and a central thickness of about 50 nm. The effects of synthetic parameters, such as surfactant concentration, molar ratio of oxidant and monomer, reaction temperature on PEDOT nanostructures have been investigated.     

Electrochemical investigation of PEDOT films deposited via CVD for electrochromic applications

A patterned solid-state electrochromic device on an ITO-coated plastic substrate was demonstrated that incorporates poly-3,4-ethylenedioxythiophene (PEDOT) deposited via a solventless oxidative chemical vapor deposition (oCVD) technique. In this paper, we present a thin-film electrochemical and optical analysis of oCVD PEDOT. oCVD PEDOT films about 100 nm thick on ITO/glass had optical switching speeds of 13 and 8.5 s, for light-to-dark and dark-to-light transitions, respectively. The color contrast was 45% at 566 nm and is 85% stable over 150 redox cycles. An Anson plot indicates that oCVD PEDOT color transition speeds are limited by ion diffusion rates, rather than electron or hole conductivity. Dimensionless analysis predicts gains of up to in oCVD PEDOT redox switching speeds by reducing the film thickness an order of magnitude to 10 nm. oCVD is a temperature-controlled process capable of conformal conductive polymer depositions onto a range of substrates from the vapor phase. Compatible substrates include plastic, paper and fabric. Non-conductive dispersion additives are not needed with oCVD, eliminating a potential source of defect-causing corrosion. oCVD offers powerful capabilities that may overlap with key challenges for the designers and fabricators of organic thin-film electronics, including OLED lighting and displays, electrochromics, photovoltaics, and semiconductors.     

Effects of various imidazole-based weak bases and surfactant on the conductivity and transparency of poly(3,4-ethylenedioxythiophene) films

Poly(3,4-ethylenedioxythiophene) (PEDOT)-based conductive thin films with improved transparency and surface resistance can be prepared by using adequate weak bases and non-ionic surfactants. Among the imidazole-based weak bases, 2-ethyl-4-methyl-substituted imidazole showed the best performance in terms of surface resistance and transparency. It is speculated that 2-ethyl-4-methyl-imidazole causes reduced particle size because polymerization kinetics are retarded by the electron donating effect of ethyl and/or methyl groups in the imidazole ring. Addition of Triton X-100 may stabilize nanoscopic PEDOT particles in the coating solution resulting in low surface resistance (172 Ω/□) and enhanced transparency (>90%) of the PEDOT film.     

Multilayer blue polymer light-emitting devices with spin-coated interlayers

Employment of multilayer heterostructures is a common approach to achieve efficiency and stable organic light emitting diodes (OLEDs). In this work, we report multilayer blue polymer light-emitting devices (PLEDs) by using spin-coated fluorene-triarylamine copolymers as interlayers between the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT) and the emitting layer. A blue PLED with stepped hole injection profile yields an external quantum efficiency of 6.0% at a luminance of 9500 cd/m² at 5.5 V and an extrapolated lifetime of more than 18,000 h from 100 cd/m².     

Synthesis and photovoltaic properties of heteroaromatic low-band gap oligomers for bulk heterojunction solar cells

Two new low band gap oligomers, TPyTDzQ-P1 and TPyTDzQ-P2, comprised of electron rich 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole or 1-(p-octylphenyl)-2,5-di(2-thienyl)pyrrole (TPyT) and electron deficient 6,7-di-2-methylpropane-4,9-bis-(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (DzQ) units were synthesized using the Suzuki polycondensation reaction. The oligomers were found to harvest the solar flux from the ultraviolet (300 nm) to near infrared (1200 nm) regions. The optical bad gaps of the two oligomers were calculated to be 1.08 eV and 1.10 eV, respectively, while the electrochemical band gaps were calculated to be 1.17 eV and 1.19 eV, respectively. The two oligomers were used as electron donor materials in bulk heterojunction (BHJ) solar cells. The solution processable bulk heterojunction solar cells with ITO/PEDOT:PSS/TPyTDzQ-P1 or TPyTDzQ-P2:PC₇₀BM/TiOx/Al configuration showed a maximum power conversion efficiency (PCE) of 0.43% with a short-circuit current density (J_sc) of 3.41 mA/cm², open-circuit voltage (V_oc) of 0.39 V and fill factor (FF) of 32%.     

Spectroscopic and conductivity studies of doping in chemically synthesized poly(3,4-ethylenedioxythiophene)

Spectroscopic methods (Raman (514.5 nm excitation), infrared (IR), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR)) and electrical conductivity measurements were used to characterize the electrically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by persulfate oxidation under aqueous conditions. Elemental analysis was carried out on the resulting sulfate-doped PEDOT to determine the sulfate doping level. Due to the difficulty in dedoping highly stable p-doped PEDOT, the sulfate-pre-doped PEDOT was directly treated with iodine solution in order to investigate the secondary doping processes between PEDOT and iodine. Two processes were observed: (1) the further oxidation of the polymer by iodine to produce an increase in the doping level, with triiodide as the dopant and (2) the replacement of sulfate by triiodide in an ion-exchange process involving triiodide in equilibrium with iodine in the iodine solution. Conductivity studies, in conjunction with XPS and EPR experiments, were used to analyse the relationship between the doping level and conductivity that was explained by differences in the nature of SO₄²⁻ and I₃⁻ as dopant ions and a shift in the polaron/bipolaron equilibrium. The Raman spectra (785 nm excitation) of these PEDOT samples were also investigated and a significant change has been observed in the wavenumber of the symmetric C_α = C_β stretching bands with varying levels of dopants. A correlation was observed between the ratio of the integrated intensities of the symmetric C_α = C_β stretching bands and the doping level in PEDOT that can be useful for estimating the doping level of a PEDOT sample from its Raman spectrum.     

Electrodeposited PEDOT films on ITO with a flower-like hierarchical structure

Poly(3,4-ethylenedioxythiophene) (PEDOT) films with a flower-like nanostructure have been electrochemically deposited on indium tin oxide (ITO) electrode simply by a one-step cyclic voltammetry (CV) method in an aqueous media in the absence of any surfactant. The PEDOT films deposited on the substrates were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The results indicate that the flower-like hierarchical structure of the PEDOT film is composed of nanosheets with a thickness of even less than 2 nm at the edge. The resulted porous structure of the PEDOT coating leads to a two-order increase of magnitude in the electrochemical capacitance-current. The growth process of the electrodeposited PEDOT as well as the effect of polymerization potential on the properties of products was studied.     

Ablation of PEDOT/PSS with excimer lasers for micro structuring of organic electronic devices

In this paper we present a potentially fast method for high resolution micro structuring of organic electronics via laser patterning. An investigation of the absorption spectrum in the UV/VIS regime of poly (3,4-ethylene dioxythiophene) poly (styrene-sulfonate) (PEDOT/PSS) has shown that UV-laser radiation should be used for optimal laser ablation of the material. Hence, the ablation characteristics of PEDOT/PSS with two different excimer lasers are compared with each other. The optimal fluence for the ablation of the material has been determined. The lasers used in this study are ArF (λ = 193 nm) and KrF (λ = 248 nm) excimer lasers.     

Synthesis of a benzothiadiazole/thiophene-based oligomer for bulk heterojunction photovoltaic cells

An electron-donor–acceptor-type oligomer, oligo(B–EDOT₂–HT₃), composed of 2,1,3-benzothiadiazole (B), 3,4-ethylenedioxythiophene (EDOT) and 3-hexylthiophene (HT) units was synthesized via a multi-step procedure. The UV–vis absorption and photoluminescence emission peaks of the material in chloroform were observed at 510 and 660 nm, respectively. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of the material were estimated as ?3.47 and ?5.49 eV, respectively, corresponding to a band-gap of 1.97 eV. Bulk heterojunction photovoltaic cells were fabricated, using oligo(B–EDOT₂–HT₃) and PCBM, in the device configuration of ITO/PEDOT:PSS:6% glycerol/oligo(B–EDOT₂–HT₃):PCBM/LiF/Ag. The short-circuit current density, open-circuit voltage and fill factor of the device were estimated to be 1.44 mA/cm², 0.33 V and 0.36, respectively, with a 1:4 ratio of oligo(B–EDOT₂–HT₃) to PCBM, corresponding to an energy conversion efficiency of 0.17% under AM 1.5 illumination. One main reason for the low efficiency of the device was attributed to the low absorptivity of oligo(B–EDOT₂–HT₃) in the UV–vis spectral range.     

Humidity-dependent characteristics of thin film poly(3,4-ethylenedioxythiophene) field-effect transistor

π-Conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) based field-effect transistors (FETs) were fabricated in this study. A thin PEDOT layer (thickness ≈500 nm) with the desired pattern was formed as an active and a gate electrode by a vapor polymerization of 3,4-ethylenedioxythiophene (EDOT) on the photolithographically patterned ferric p-toluenesulfonate (FTS) oxidant layer. Crosslinked poly(vinyl cinnamate) (PVCN) insulating layer was formed by spin-coating and UV crosslinking. The currents (I_ds) of the PEDOT active channel decreased with increasing gate bias (V_g), implying p-type FET. The dc conductivities (σ_dc) and I_ds of the PEDOT active channel were measured as a function of V_g under various relative humidities (RHs) ranging from 0% to 55%. The σ_dc and the I_ds of the PEDOT channel rapidly decreased with increasing positive V_g under humid conditions, while those of the PEDOT channel showed no change with V_g in vacuum, i.e. 0% RH. The threshold gate bias, which is defined as the onset of the curve of I_ds versus V_g, rapidly decreased with increase of RH. The moisture absorbed in the PEDOT active channel may assist the dedoping of the system by the screening and/or the relative separation of constituent ions from PEDOT chain upon applying a positive V_g. When the positive V_g was removed (V_g = 0), the σ_dc of the channel recovered slowly and a relatively long relaxation time was observed, which implies that the PEDOT active layer was slowly redoped. For the PEDOT based FET, we observed the moisture assisted dedoping and the redoping processes of the PEDOT channel upon applying and removal of positive V_g under the humid conditions.     

Polycarbonate-based conductive film prepared by coating DBSA-doped PEDOT/sorbitol

A series of conductive polycarbonate films (CPCFs) were prepared by spin coating with an ethanol-based coating solution comprised of dodecylbenzenesulfonic acid-doped poly(3,4-ethylenedioxythiophene) (PEDOT)/sorbitol. The variation of CPCF surface resistivity was then measured as a function of temperature. CPCF surface resistivity decreased as sorbitol content increased, with the minimum values in the temperature range 175–185 °C. Furthermore, the CPCF-1.6 exhibited significantly improved thermal stability with respect to surface resistivity even at 175 °C. FT-IR analysis indicated that, after thermal treatment, hydrogen bonds arising from the hydroxy groups of sorbitol or hydroxypropyl cellulose and carbonyl groups of polycarbonate were established. The combination of surface resistivity data and FT-IR analysis suggests that the introduction of sorbitol accompanied with thermal treatment allows the PEDOT chains to undergo favorable inter-chain interactions leading to charge transfer and physical cross-linkage via hydrogen bonding.     

Synthesis and characterization of red-emitting diketopyrrolopyrrole-alt-phenylenevinylene polymers

Two novel diketopyrrolopyrrole (DPP) and p-phenylenevinylene alternating copolymers, poly(1,4-(2,5-dicyano)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P1) and poly(1,4-(2,5-diethoxy)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P2), were synthesized through Wittig reaction in good yields, and were characterized by FTIR, NMR. Two polymers possessed moderate molecular weights and polydispersities, well-defined structures, and were readily soluble in common organic solvents. In particular, P1 and P2 exhibited excellent thermal stability with T_d = 393 and 398 °C at 5% weight loss, respectively. Both P1 and P2 in solution and in thin films exhibited strong red photoluminescence. Both electroluminescence devices using ITO/PEDOT/polymer/Ba/Al configuration with P1 and P2 as the emitting layer showed nearly pure red emission with the emission peaks at 646 nm for P1 and 648 nm for P2, and exhibited low turn-on voltages of 4.5 and 3.4 V, respectively. The results show that P1 and P2 are promising candidates for red emissive materials in polymer light-emitting diodes.