Effect of solvent on PEDOT/PSS nanometer-scaled thin films: XPS and STEM/AFM studies

We have investigated effect of solvent on poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) nanometer-scaled thin films by means of a scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) in terms of thickness and PEDOT:PSS ratio of the films. As a result, the PEDOT:PSS ratio, surface roughness, number of highly conductive grain, and thickness of the PEDOT/PSS thin film coincidently increased with the addition of ethylene glycol (EG). It suggests that primary nanoparticles decrease in size but aggregate by removing excess PSS after the addition of the EG.     

Vapor phase polymerization of poly (3,4-ethylenedioxythiophene) on flexible substrates for enhanced transparent electrodes

Recently, conducting polymer thin films have been investigated as transparent electrodes in photovoltaic devices and organic light emitting diodes. Due to its relatively high conductivity and excellent transmission in the visible region, poly (3, 4-ethyelenedioxythiophene) (PEDOT) has been shown to be a viable option for such applications. Herein described is a method for the vapor phase polymerization (VPP) of transparent PEDOT thin film electrodes on flexible polyethylene naphthalate (PEN) substrates and the comparison of this VPP method with two current approaches to PEDOT deposition: solution-based in situ polymerization and spin coating a dispersion of PEDOT:PSS. Electrical conductivities and UV–vis transmittances were measured for films produced by each of these methods, with VPP PEDOT showing both the highest conductivity (approx. 600 S/cm) and transmittance (>94% at 550 nm). The surface morphologies of the films were compared using AFM and SEM imaging. The stability of these PEDOT films, stored under ambient conditions, was investigated by monitoring the conductivity and transmittance of the thin films over time.     

Electrically driven PEDOT/PSS actuators

The film made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) was prepared by casting and electrical conductivity, tensile properties, electromechanical response, and moisture sorption isotherm of the PEDOT/PSS film were investigated. A linear expansion of the PEDOT/PSS film occurred with increasing ambient humidity, where the strain of the film in a RH range from 20 to 90%RH was 3.3%. Upon application of an electric field, the film underwent significant contraction in ambient air. The degree of contraction increased as the applied voltage became higher and the value attained 2% at 35 V, which was nearly twice as that of polypyrrole films. The principle lay in the desorption of water vapor due to local Joule heating, where the electric field controlled an equilibrium of water vapor sorption. The moisture sorption isotherm of the PEDOT/PSS film indicated that the interaction between water and PEDOT/PSS was superior to that between water molecules, in which isosteric heat of sorption decreased with an increase in the sorption degree and the value reached to the heat of water condensation.     

“One-pot” synthesis,characterization, and NH3 sensing of Pd/PEDOT:PSS nanocomposite

In this letter, we report a novel route to prepare stable Pd/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (Pd/PEDOT:PSS) colloid. With addition of PSS, the Pd/PEDOT:PSS aqueous dispersion was formed by simultaneous oxidation–reduction reaction between Pd(NO₃)₂ and ethylenedioxythiophene (EDOT) at room temperature. The morphologies of the Pd/PEDOT and Pd/PEDOT:PSS nanocomposites were characterized by transmission electron microscope. The Pd nanoparticles in the composites were identified with X-ray powder diffraction and UV/vis/NIR spectrum. The chemical structures of PEDOT were studied using FTIR spectra and UV/vis/NIR spectra. The results confirmed the formation of oxidized PEDOT polymer. The sensing property of the Pd/PEDOT:PSS thin film to the NH₃ vapor was investigated.     

Growth mechanisms,morphology, and electroactivity of PEDOT layers produced by electrochemical routes in aqueous medium

The electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) in sodium poly(styrene-4-sulfonate) (NaPSS) polyelectrolyte aqueous solution was studied in order to establish a direct relationship between the synthesis conditions and the growth mechanisms of the polymeric film. The final morphology, the polymer structure and the electroactivity of the produced PEDOT:PSS layers have also been investigated in order to achieve a comprehensive understanding of the physico-chemical properties of the material.At this aim the current–time transients referred to the growth best conditions have been fitted using a mathematical equation that considers two contributions corresponding to 3D progressive nucleation under diffusion control and to 3D instantaneous nucleation controlled by charge transfers. The atomic force microscopy study performed on the polymeric films at various stages of electrodeposition supported the proposed growth model. The structural feature and the electrochemical performance of the PEDOT:PSS systems have been studied by Raman spectroscopy and cyclic voltammetry. These studies showed that the adopted experimental conditions allow one to grow PEDOT:PSS films in their oxidized state and characterized by high-capacitive properties.     

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).     

Thermistor behavior of PEDOT:PSS thin film

The temperature-dependent resistance changing characteristics (thermistor behaviors) of a poly(3,4-ethylenedioxythophene):poly(4-styrenesulfonate) (PEDOT:PSS) thin film are investigated in the 30–100 °C range using Greek-cross and bar patterns. The PEDOT:PSS film was spin-coated onto a Si wafer passivated with a SiO₂ layer, and a conventional dry etching technique was used to pattern the PEDOT:PSS film in conjunction with a nitride etch mask layer. Cr/Au was used for the electrode material. It was found that the characteristic temperature (T₀) and resistivity of the PEDOT:PSS film have an inversely proportional relationship with the number of coatings and the number of interfaces between multiply coated PEDOT:PSS layers. It was also found that as the number of coatings and the number of the interfaces increase, lower temperature-dependent resistance changes are observed. The temperature coefficient of resistance (TCR) value of 60 nm thick PEDOT:PSS film was slightly larger than or comparable to that of a conventional metal (Au or Pt) thermistor. The possibility of utilizing the PEDOT:PSS thin film in thermistor applications is discussed.     

PEDOT/PSS bending actuators for autofocus micro lens applications

This paper reports on the development of micro autofocus lens actuators using conducting polymer actuators. We propose a hydrophilic treatment for polyvinylidene difluoride (PVDF) membranes and a polyethylenedioxythiophene/poly(styrene sulfonic acid) (PEDOT/PSS) casting method for easily producing bending conducting polymer actuators with high mechanical strength.We first designed micro autofocus lens actuators using bending conducting polymer actuators, and then determined experimentally that the electrical conductivity and breaking strain of the PEDOT/PSS films could be improved by the addition of polyethylene oxide (PEO). Furthermore, we made hydrophobic PVDF membrane surfaces hydrophilic using polyethylene glycol methacrylate (PEGMA) and coated a solution of a mixture of PEDOT/PSS and PEO on the membrane surfaces to form a laminated film. We produced bending conducting polymer actuators by processing this film. Tests simulating the actuator's use in micro autofocus lenses showed the actuators to operate stably for more than a million cycles.     

A flexible textile structure based on polymeric photovoltaics using transparent cathode

This paper presents results for photovoltaic performance obtained from the application of different bulk heterojunction blends onto flexible polypropylene (PP) substrates for textile applications. Organic photovoltaic devices were fabricated onto non-transparent PP tapes and ITO coated glasses. The layer of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) and PEDOT:PSS with 100 nm of silver (Ag) metal layer constituted anode structure and substituted indium tin oxide (ITO) layer in this study. The blends of poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl(6,6)C₆₁ (P3HT:PCBM) or poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene] and 1-(3-methoxycarbonyl)-propyl-1-phenyl(6,6)C₆₁ (MDMO-PPV:PCBM) were utilized as the absorbing materials. The thin metal layers of lithium fluoride (LiF) and aluminum (Al) were deposited on top of the devices by evaporation. All photovoltaic devices were characterized by measuring current–voltage (I–V) characteristics under simulated AM 1.5 conditions. The morphology of these devices using MDMO-PPV:PCBM and P3HT:PCBM blends was also investigated using atomic force microscopy (AFM).     

Enhanced optical and electrical properties of PEDOT: PSS films by the addition of MWCNT-sorbitol

A thin layer of carbon nanotubes (CNTs) presents a strong candidate for application as a transparent conducting electrode and a high frequency Schottky diode. Multiwalled carbon nanotubes (MWCNTs) were modified using nitric acid to form –OH and –COOH groups on the MWCNT surface. Functionalized MWCNTs (FMWCNTs) were further modified using sorbitol molecules. N, N′-carbonyldiimidazole (CDI) was utilized as an activating agent for carboxylic acids in a homogeneous one-pot reaction of FMWCNTs in N,N-dimethylacetamide (DMAc). The activated FMWCNTs were mixed with sorbitol and heated up to 60 °C with stirring. Due to the mild conditions and efficiency of the reaction, a large amount of sorbitol molecules were covalently attached with increasing reaction time. The FMWCNTs with sorbitol (FMWCNTSORs) were mixed with poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate) (PEDOT:PSS). The FMWCNTSORs were homogeneously dispersed into PEDOT:PSS solution without any precipitation. The FMWCNTSORs/PEDOT:PSS film showed stronger FTIR absorption peaks in the case of samples reacted for longer time. The UV–vis transmittance and the conductivity of the FMWCNTSORs/PEDOT:PSS film was increased as the reaction time increased. Although the field emission scanning electron microscope (FESEM) surface image of the 2 h reacted FMWCNTSORs/PEDOT:PSS film showed large number of small aggregated particles, only a small number of aggregated particles was found for the sample reacted for 6 h. These results indicate that the appropriate amount of sorbitol molecules on the MWCNT can increase the conductivity and transmittance of the PEDOT:PSS film.     

Study of the effect of DMSO concentration on the thickness of the PSS insulating barrier in PEDOT:PSS thin films

One of the most used secondary dopants in thin film processing of PEDOT:PSS is dimethyl sulfoxide (DMSO). In this work, we present results that explain, from the point of view of impedance spectroscopy, the mechanism of the increase in the conductivity observed on films based on PEDOT:PSS. The results obtained with this technique, combined with others such as AFM, and Raman and UV–vis–NIR spectroscopies, clearly show that there is a thinning of the insulating barrier of PSS surrounding conductive grains of PEDOT. It is shown that the thickness of the insulating barrier is related strongly and inversely with the onset frequency of AC conductivity. However, this is not the only existing effect, because for values beyond the optimal concentration of DMSO, we observe a decrease in the conductivity related with an increase of the separation of the PEDOT grains. The AC measurements and the AFM images also show the clear interplay between the increase of the PEDOT average grain size and the separation between them.     

Effects of the FeCl<Subscript>3</Subscript> concentration on the polymerization of conductive poly(3,4-ethylenedioxythiophene) thin films on (3-aminopropyl) trimethoxysilane monolayer-coated SiO<Subscript>2</Subscript> surfaces

This study examined the effects of the FeCl₃ (oxidant) concentration on the vapor phase polymerization (VPP) of conducting poly (3,4-ethylenedioxythiophene) (PEDOT) thin films on (3-aminopropyl)trimethoxysilane (APS)-coated SiO₂ surfaces, in which the interaction between Fe(III) and the -NH₂ groups of APS enabled a uniform distribution of FeCl₃ on the surface. The FeCl₃ concentration has a strong impact on the thickness, surface morphology, and conductivity of the PEDOT films deposited by VPP on an APS monolayer. The thickness of the PEDOT thin films increased linearly as the FeCl₃ concentration increased, as predicted by a model of spun films from a FeCl₃ solution. However, the rate of the increase in PEDOT thin film thickness per unit of FeCl₃ in wt.% was lower than the predicted value. This suggests that the consumption of FeCl₃ not participating in polymerization to produce Fe₂O₃ or FeCl₃ aggregates increased as the FeCl₃ concentration increased. In addition, the surface morphology improved as the FeCl₃ concentration increased from 1 wt.% to 3 wt.% and the conductivity increased to approximately 400 S/cm. However, further increases in the FeCl₃ concentration to 5 wt.% and 7 wt.% significantly degraded the morphology by creating holes in the PEDOT film, which reduced the conductivity.     

PEDOT:PSS films—Effect of organic solvent additives and annealing on the film conductivity

We report results on conductivity of PEDOT:PSS films, which contain different amounts of organic solvents, i.e., dimethyl sulphoxide (DMSO) or ethylene glycol (EG), and annealed at different temperatures. The maximum of conductivity of the resulting films was reached at about 5wt.% of DMSO or EG in the solution. At the same time, the presence of the solvent residue in the film also resulted in the poor control of the film morphology and conductivity. It was found that conductivity of a film prepared with the same DMSO content is sensitive to the surface quality of the substrate used. It was also found that annealing substantially reduces conductivity of the films prepared with and without the additives. The higher the temperature of annealing, the smaller film conductivity was observed. Correlation in changes of the electronic absorption of the PEDOT:PSS film in the near-IR and the film conductivity induced by the solvent additive was also found. Exposure of PEDOT:PSS films to the solvent vapors has been employed as an alternative controlled method to increase film conductivity. This method, in combination with quartz microbalance measurements, showed a great capability of the film to absorb DMSO vapors and that a saturation limit for conductivity is reachable after more than 20 h of exposure. Swelling and interconnection of the polymer chains is suggested to be the main factor responsible for the conductivity increase.     

Hole-injection properties of annealed polythiophene films to replace PEDOT–PSS in multilayered OLED systems

Hole-injection properties of annealed poly(alkoxy- and alkylthiophene) films in OLEDs were studied. Among them, annealed poly(3,3′-dihexyloxy-2,2′-bithiophene) (aPHOBT) film exhibited good hole-injection properties and a triple-layered OLED with the structure ITO/aPHOBT/PVK/Alq3/Mg–Ag (device I) showed much higher performance than a double-layered device without the aPHOBT layer (device II, ITO/PVK/Alq3/Mg–Ag). Device I was slightly inferior to a device having a PEDOT–PSS layer as the hole injector (device III, ITO/PEDOT– PSS/PVK/Alq3/Mg–Ag) in the low-intermediate region of the applied voltage (6–11 V), but gave comparable luminance to III when the applied voltage exceeded 11 V.     

Atomic scale modeling of interfacial structure of PEDOT/PSS

Theoretical calculations are performed in the framework of the interaction between the charged poly(ethylenedioxythiophene) (PEDOT²⁺) and two p-toluensulfonic acid (TSA^?1). The influence of the counterion on the charge distribution in the PEDOT is investigated indicating that a strong influence of the interionic correlation on the stability of PEDOT by TSA. Further several configurations are studied for the interaction between PEDOT and PSS. The calculations indicate that the side assembly is the most stable configuration, however in the presence of the solvent both parallel and side assemblies have similar stability. These results give a new insight about the charge transport conduction in PEDOT/PSS interactions.     

The electronic structure of poly(3,4-ethylene-dioxythiophene): studied by XPS and UPS

The electronic structure of poly(3,4-ethylene-dioxythiophene) (PEDOT) has been investigated by X-ray and ultraviolet photoelectron spectroscopies as well as quantum chemical calculations. Significant differences have been observed in the photoelectron spectra between as-prepared chemically neutralized and anion-doped PEDOT thin films. The electronic structures of as-prepared neutral and doped PEDOT obtained from the photoelectron spectra are in good agreement with the results of new quantum chemical electronic structure calculations. No significant thermal-induced effects have been detected for either as-prepared neutral or doped PEDOT films. The concentration of anions on the polymer surface depends upon the size of the anion, with large anions, like polystyrene sulfonate (PSS⁻) base, being much more likely to cover the surface of a PEDOT film than small anion, such as tosylate(p-methyl benzyl sulfonate). This surface concentration effect probably makes the large-anion-doped polymer a more suitable candidate as an electrode in polymer light-emitting diodes (LEDs) than the small-anion-doped polymer.     

Doped polymeric cathodes for PPV/Al based LEDs

The effect of Li-doping in poly(para-phenylenevinylene) (PPV) based light emitting devices has been studied. In a standard structure with an indium tin oxide (ITO) anode, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT–PSS)-layer and an active PPV-layer, the effects of a thin (around 1 Å) Li-layer and a thin layer, (50 Å), of a large bandgap polymer, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP) between the PPV and the aluminum cathode have been studied in terms of IV-characteristics and efficiency. The Li-atoms dope the interfacial layer of the PPV as seen by photoelectron spectroscopy. A thin layer of Li improves the charge balance by decreasing the energy barrier for injection of electrons for the Al/Li/PPV/PEDOT–PSS/ITO device. The efficient electron injection originates from a Fermi level alignment between the doped polymer and the aluminum cathode, which reduces the energy barrier. A thin layer of the large bandgap polymer P14NHP, between the PPV and Al contact, increases the light output and efficiency by blocking the holes. In addition, it may also reduce the light quenching by moving the region of recombination away from the Al-contact. The addition of a Li-layer on top of P14NHP leads to an increase of the quantum efficiency, because of better electron injection.     

Charge carrier mobility in substituted polythiophene-based diodes

We have investigated the transport properties of the semiconducting polymer poly(3-(2′-methoxy-5′-octylphenyl)thiophene) (POMeOPT). We have measured the current–voltage (C–V) characteristics of single polymer layer devices in two regimes contact limited current and bulk-limited current. The passage from one regime to the other was done upon insertion of a conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT–PSS) between the metallic electrode and the semiconducting polymer. With PEDOT–PSS as electrode, the polymer gave space–charge limited current (SCLC) with the mobility dependent on electric field. Fitting the data, we were able to obtain important parameters, such as the zero-field mobility and the characteristic field. We have compared our results with the well-studied polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)–1,4-phenylene vinylene) (MEH–PPV) in similar experiments earlier reported.     

Synthesis and redox behavior of PEDOT/PSS and PPy/DBS structures

Electrochemical synthesis of dodecylbenzenesulfonate (DBS⁻) doped polypyrrole (PPy/DBS) onto polystyrenesulphonate (PSS⁻) doped poly(3,4-ethylenedioxythiophene) (PEDOT/PSS) modified gold EQCM electrode was studied. Monitoring of mass and potential response during PPy growth onto PEDOT underlayer revealed at least three different stages in this process. AFM study confirmed that PEDOT film morphology constantly changed during the synthesis of PPy film onto its surface. Studying of redox properties of PEDOT/PSS + PPy/DBS structures showed that PEDOT influenced the redox processes of the structure after its complete coverage with PPy suggesting the formation of three-dimensional electrode.     

Transparent polymer cathode for organic photovoltaic devices

We demonstrate a prototype solar cell with a transparent polymer cathode, and indium-tin-oxide (ITO)/poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT:PSS) anode. As an active layer, thin film of a bulk heterojunction of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2thienyl-2′,1′3′-benzothiadiazole)] (APFO-3) and an electron acceptor molecule [6], [6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) (1:4 wt.) was sandwiched between the two transparent polymer electrodes. The cathode is another form of PEDOT formed by vapor phase polymerised PEDOT (VPP PEDOT) of conductivity 10²–10³ S/cm. The cathode is supported on an elastomeric substrate, and forms a conformal contact to the APFO-3/PCBM blend. Transparent solar cells are useful for building multilayer and tandem solar cells.