Transverse charge transport in inkjet printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)

Transverse (z) alignment of PEDOT grains was demonstrated in inkjet printed PEDOT:PSS. This explained the superior transverse charge conduction mode in inkjet printed PEDOT:PSS films, best fitted by the Efros-Shklovskii 1D-VRH (variable range hopping) model in this study compared with spin coated PEDOT:PSS films, which have demonstrated layers of generally in-plane aligned PEDOT:PSS grains. The findings of this study, regarding the microstructure of inkjet printed PEDOT:PSS films and their transverse charge transport model, justify measurements of the transverse conductivity of inkjet printed films in this study being 600 times higher than that of spin coated films. In addition, it was found that the addition of 5 wt% DMSO in the printing PEDOT:PSS ink lowers the workfunction by 3% approximately.     

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSS_ADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSS_IMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSS_IMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSS_IMM electrode out-performed the PEDOT:PSS_ADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm⁻² to 7.15 mA cm⁻². The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.     

Modified physico–chemical properties and supercapacitive performance via DMSO inducement to PEDOT:PSS active layer

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was treated with dimethyl sulfoxide (DMSO) in order to modify it physico–chemical properties in an effort to improve its electrochemical performance. In this study, the conductivity of PEDOT:PSS was controlled through post-solvent treatment in DMSO for various dipping times. The maximum conductivity of the DMSO-treated PEDOT:PSS electrode was ∼1890 S cm⁻¹. The electrical conductivity, PEDOT to PSS ratio, chemical compositions, wettability, and surface roughness are correlated and suitably explained. The supercapacitive performance of the DMSO-treated PEDOT:PSS electrodes was studied in 1 M H₂SO₄ within an optimized potential window of 0 to −0.6 V versus Ag/AgCl. The maximum specific capacitance of the PEDOT:PSS electrode treated in DMSO for 60 min was found to be ∼153 F g⁻¹ at a 1 mA cm⁻² discharge current density (∼145 F g⁻¹ at a 10 mV s⁻¹ scan rate), along with an energy density of 93 W h kg⁻¹ and a power density of 4.6 kW kg⁻¹.     

A comparative assessment of surface microstructure and electrical conductivity dependence on co-solvent addition in spin coated and inkjet printed poly(3,4-ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS)

This study focuses on the fabrication of poly(3,4-ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) thin films by inkjet printing and investigates the developed surface morphology and electrical conductivity of the printed films as a function of the concentration of dimethyl sulfoxide (DMSO), added as conduction enhancing co-solvent, and Surfynol, added as a surfactant. The printed films are compared with PEDOT:PSS films fabricated by the traditional spin coating technique. Measurements of the surface tension justify including surfactant as a processing additive, where addition of 1% Surfynol results in substantial decrease of the surface tension of the PEDOT:PSS solution, whilst it also increases film surface roughness by an order of magnitude for both fabrication methods. The addition of 5 wt% DMSO is shown to result in a 10³ decrease in sheet resistance for both spin coated and inkjet printed films with both processing routes demonstrating decrease in surface roughness and coarsening of PEDOT grains as a function of the co-solvent concentration, whilst X-ray photon spectroscopy showed an increase in the surface PEDOT to PSS ratio from 0.4 to 0.5. Inkjet printed films have lower sheet resistance than the corresponding spin coated films, whilst atomic force microscopy reveals a coarser surface morphology for the inkjet printed films. The findings in this work point out at the decrease of sheet resistance due to coarsening of PEDOT grains which is linked to a decrease of surface roughness for small RMS values associated with the PEDOT grains. However, the higher surface roughness generated when Surfynol surfactant was added was not detrimental to the film’s in-plane conductivity due to the fact that these higher roughness values were unrelated to the PEDOT grains.     

Thermoelectric Performance of Poly(3,4-Ethylenedioxy-thiophene)/Poly(Styrenesulfonate) Pellets and Films

The thermoelectric performance of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) pellets and free-standing PEDOT/PSS films, prepared from PEDOT/PSS solution containing the additives dimethyl sulfoxide or ethylene glycol, have been systematically investigated. It has been found that the electrical conductivity of free-standing PEDOT/PSS films is invariably much higher than that of PEDOT/PSS pellets, while there is no distinct change in the Seebeck coefficient. The highest electrical conductivity of a free-standing PEDOT/PSS film can be up to 300 S cm⁻¹, five to six times higher than that of PEDOT/PSS pellets (55 S cm⁻¹). The thermal conductivity was measured over a wide temperature range, indicating that PEDOT/PSS has extremely low thermal conductivity. The figure of merit (ZT) of free-standing PEDOT/PSS films with good environmental stability can be up to 10⁻², an order of magnitude higher than that of pressed PEDOT/PSS pellets (10⁻³).     

Highly conductive PEDOT:PSS films prepared through a treatment with geminal diols or amphiphilic fluoro compounds

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with high conductivity can have important application as the transparent electrode of optoelectronic devices. In this paper, we report the significant conductivity enhancement of PEDOT:PSS through a treatment with germinal diols which have two hydroxyl groups connected to one carbon atom or amphiphilic fluoro compounds which have hydrophobic fluorocarbon groups and hydrophilic hydroxyl or carboxylic groups. Several compounds, including hexafluoroacetone, cyclohexanehexone, formaldehyde, acetaldehyde, and perfluorobenzophenone, which could convert into geminal diols, were used to treat PEDOT:PSS films. The conductivity enhancements are generally consistent with the equilibrium constants for the conversion of these compounds into geminal diols. PEDOT:PSS films were also treated with several amphiphilic fluoro compounds. The conductivity was significantly enhanced when PEDOT:PSS films were treated with hexafluoroisopropanol, trifluoroacetic acid and heptafluorobutyric acid, while it hardly changed when they were treated with 2,2,2-trifluoroethanol. Conductivities of more than 1000 S cm⁻¹ were observed on the treated PEDOT:PSS films. The mechanism for the conductivity enhancement of PEDOT:PSS through the treatment with geminal diols or amphiphilic fluoro compounds is attributed to the phase segregation of PSSH from PEDOT:PSS and conformational change of the PEDOT chains as the results of the compounds-induced reduction in the Coulombic attraction between the positively charged PEDOT and negatively charged PSS chains.     

Neutral-pH PEDOT:PSS as over-coating layer for stable silver nanowire flexible transparent conductive films

The silver nanowire (AgNW) mesh film with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the over-coating layer is a promising flexible transparent conductive film technology. In this work, experimental studies show that the hygroscopic and acid properties of the common PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS. By using the modified PEDOT:PSS of neutral pH as the over-coating layer, the long term shelf-life time, thermal and current stressing stabilities are all significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm², no obvious change of the conductivity is observed. The results clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability for flexible optoelectronic devices.     

A Morphological Model for the Solvent‐Enhanced Conductivity of PEDOT:PSS Thin Films

The well‐known enhanced conductivity of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films that is obtained by addition of high‐boiling solvents like sorbitol to the aqueous dispersion used for film deposition is shown to be associated with a rearrangement of PEDOT‐rich clusters into elongated domains, as evidenced from STM and AFM. Consistently, temperature dependent conductivity measurements for sorbitol‐treated films reveal that charge transport occurs via quasi 1D variable range hopping (VRH), in contrast to 3D VRH for untreated PEDOT:PSS films. The typical hopping distance of 60–90 nm, extracted from the conductivity measurements is consistent with hopping between the 30–40 nm sized grains observed with scanning probe microscopy.     

Organic Semiconductors for Thermoelectric Applications

The thermoelectric performance of thin films fabricated from two commercially available, highly conductive polymer formulations based on poly (3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated. In order to enhance the electrical conductivity, the high-boiling solvent dimethyl sulfoxide (DMSO) was added. By changing the content of DMSO the electrical conductivity was increased by a factor of two without changing the Seebeck coefficient or the thermal conductivity. We achieved ZT = 9.2 × 10⁻³ at room temperature upon the addition of 5 vol.% DMSO to the PEDOT:PSS formulation.     

Explaining the effects of processing on the electrical properties of PEDOT:PSS

By simultaneously measuring the Seebeck coefficient and the conductivity in differently processed PEDOT:PSS films, fundamental understanding is gained on how commonly used processing methods improve the conductivity of PEDOT:PSS. Use of a high boiling solvent (HBS) enhances the conductivity by 3 orders of magnitude, as is well-known. Simultaneously, the Seebeck coefficient S remains largely unaffected, which is shown to imply that the conductivity is improved by enhanced connectivity between PEDOT-rich filaments within the film, rather than by improved conductivity of the separate PEDOT filaments. Post-treatment of PEDOT:PSS films by washing with H₂SO₄ leads to a similarly enhanced conductivity and a significant reduction in the layer thickness. This reduction strikingly corresponds to the initial PSS ratio in the PEDOT:PSS films, which suggests removal and replacement of PSS in PEDOT:PSS by HSO₄⁻ or SO₄²⁻ after washing. Like for the HBS treatment, this improves the connectivity between PEDOT filaments. Depending on whether the H₂SO₄ treatment is or is not preceded by an HBS treatment also the intra-filament transport is affected. We show that by characterization of S and σ it is possible to obtain more fundamental understanding of the effects of processing on the (thermo)electrical characteristics of PEDOT:PSS.     

Thermoelectric Performance Enhancement of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Composite Films by Addition of Dimethyl Sulfoxide and Urea

Significant enhancement of thermoelectric (TE) performance was observed for free-standing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) composite films obtained from a PEDOT:PSS aqueous solution by simultaneous addition of dimethyl sulfoxide (DMSO) and different concentrations of urea. The electrical conductivity was enhanced from 8.16?S?cm^?1 to over 400?S?cm^?1, and the maximum Seebeck coefficient reached a value of 18.81???V?K^?1 at room temperature. The power factor of the PEDOT:PSS composite films reached 8.81???W?m^?1?K^?2. The highest thermoelectric figure of merit (ZT) in this study was 0.024 at room temperature, which is at least one order of magnitude higher than most polymers and bulk Si. These results indicate that the obtained composite films are a promising thermoelectric material for applications in thermoelectric refrigeration and thermoelectric microgeneration.     

Effects of tetramethylene sulfone solvent additives on conductivity of PEDOT:PSS film and performance of polymer photovoltaic cells

A solvent additive in PEDOT:PSS solution is one of many methods to improve the conductivity of the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. We explore a new type of the solvent additive, namely tetramethylene sulfone (TMS), for the fabrication of the PEDOT:PSS conductive layer in the ITO/PEDOT:PSS/P3HT:PCBM/TiO_x/Al polymer photovoltaic cells, in comparison to a more common dimethyl sulfoxide (DMSO) solvent additive. At optimal conditions, the TMS additive at 10 wt.% has been found to enhance the conductivity of pristine PEDOT:PSS films from 0.04 S/cm up to approximately 189 S/cm, compared with the highest conductivity for the case of the DMSO additive at 15 wt.% of 117 S/cm. Possible mechanisms of this conductivity enhancement, relating to the polymer conformation and the film morphology, have been investigated by Raman spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The performance of the polymer photovoltaic cells fabricated with the solvent additives PEDOT:PSS films follows a similar trend to the conductivity of the films as a function of the additive concentration. The additives mainly lead to greater short circuit current density (J_sc) of the photovoltaic cells. The highest power conversion efficiency (PCE) of 2.24% of the device has been obtained with the 10 wt.% TMS additive of, compared to the PCE of 1.48% for the standard device without solvent additive.     

Effect of water-soluble vitamins on the structure and properties of poly(3,4-ethylenedioxythiopehene):poly(styrenesulfonate)

Intrinsically conducting polymers can have important application in biology because they can be conductive and have good biological compatibility. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) has been the most popular conductive polymer in biological application due to its solution processability in water. PEDOT:PSS can be used as electrode materials or active materials of biological devices or circuits. It is important to study the effect of biomaterials on the structure and properties of PEDOT:PSS films. In this work, water-soluble vitamins that are biomaterials needed for organisms are used to treat PEDOT:PSS. They can significantly enhance the conductivity of PEDOT:PSS from 0.3 S cm⁻¹ up to higher than 1000 S cm⁻¹. The conductivity enhancement depends on the structure of vitamins. The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3. The vitamin-induced changes in the structure and properties of PEDOT:PSS were studied by UV–Vis absorption spectroscopy, temperature-dependence of resistance measurements, atomic force microscopy and cyclic voltammetry. The characterizations indicate that vitamins can induce phase segregation between PEDOT and PSS and the conformational change of the PEDOT chains. These discoveries are important to understand the application of PEDOT:PSS in biology and the development of new biological application of PEDOT:PSS.     

Thermal degradation mechanisms of PEDOT:PSS

The thermal stability of thin (50 nm) PEDOT:PSS films, was investigated by dc conductivity measurements, X-ray and UV photoelectron spectroscopies as a function of heating temperature and heating time. The mechanism of electrical conductivity as a function of temperature is consistent with a hopping type carrier transport. The electrical conductivity decreased, as a function of time, in agreement with a granular metal type structure, in which aging is due to the shrinking of the PEDOT conductive grains. XPS and UPS spectra indicate that conformational changes of the PEDOT:PSS film are responsible for this behaviour and a model for these modifications is proposed.     

High resolution electrical characterisation of organic photovoltaic blends

In this work, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) are applied to perform high-resolution electrical characterisation of organic photovoltaic films. These films are composed of the C₆₀-derivative PCBM blended with hole conductive conjugated polymers PPV derivatives or P3HT. It is demonstrated that both EFM and C-AFM are able to electrically evidence phase separation in the blends, suggesting in addition higher density of carriers along interfaces. Correlation between the EFM contrast and the photovoltaic properties of the blends was observed. Local spectroscopy (I-V curves) completes the C-AFM investigations, analysing charge transport mechanisms in the P3HT:PCBM blend. Significant modifications of the local electrical properties of P3HT are shown to occur upon blending. Space charge limited current is evidenced in the blend and a hole mobility of 1.7 × 10⁻² cm² V⁻¹ s⁻¹ is determined for P3HT.     

Wide range thickness effect of hole-collecting buffer layers for polymer:fullerene solar cells

Here we report the wide range thickness effect of hole-collecting buffer layers (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)) on the performance of polymer:fullerene solar cells with blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The thickness of the PEDOT:PSS layers was controlled from 3 nm to 625 nm, followed by characterizations such as optical transmittance, electrical resistances in the in-plane and out-of-plane directions, work functions, contact angles, and device performances. Results showed that the optical transmittance was gradually decreased with the PEDOT:PSS thickness but a maximum value was measured for other properties in the thickness range of 10–30 nm. The device performance was noticeably improved with only 3 nm-thick PEDOT:PSS layer, while it was almost similar in the thickness range of 30–225 nm in the presence of gradual decrease in the surface roughness. The similar device performance between 30 nm and 225 nm has been assigned to the compensation effect between the reduced electrical resistance (increased conductivity) and the decreased optical transmittance as the thickness of the PEDOT:PSS layer increased.     

Metal salt modified PEDOT:PSS as anode buffer layer and its effect on power conversion efficiency of organic solar cells

The effects of metal chlorides such as LiCl, NaCl, CdCl₂ and CuCl₂ on optical transmittance, electrical conductivity as well as morphology of PEDOT:PSS films have been investigated. Transmittance spectra of spun PEDOT:PSS layers were improved by more than 6% to a maximum of 94% in LiCl doped PEDOT:PSS film. The surface of the PEDOT:PSS films has exhibited higher roughness associated with an increase in the electrical conductivity after doping with metal salts. The improvement in the physical properties of PEDOT:PSS as the hole transport layer proved to be key factors towards enhancing the P3HT:PCBM bulk heterojunction (BHJ) solar cells. These improvements include significantly improved power conversion efficiency with values as high as 6.82% associated with high fill factor (61%) and larger short circuit current density (∼18 mA cm⁻²).     

Highly improved thermoelectric performances of PEDOT:PSS/SWCNT composites by solvent treatment

Composites of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and single-wall carbon nanotube (SWCNT) were prepared by mixing aqueous dispersions of PEDOT:PSS and SWCNT at different weight ratios. By being soaked with DMSO for 2 min at room temperature, the PEDOT:PSS/SWCNT composite with an optimized SWCNT weight ratio of 74 wt% exhibited a high electric conductivity of 3800 S cm⁻¹ and a reasonable Seebeck coefficient of 28 μV K⁻¹, leading to a promising power factor of 300 μW m⁻¹ K⁻² and a hopeful ZT value of 0.13. Possible reasons for the highly improved properties are carefully discussed.     

Improvement of electrical conductivity of PEDOT:PSS films by 2-Methylimidazole post treatment

The electrical conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films was significantly improved without losing the optical transparency by treating the films with solution of 2-Methylimidazole in ethanol. The maximum electrical conductivity of such a thin film reached 930 S cm⁻¹, more than 1150 order of magnitude higher than that of pure PEDOT:PSS film. The mechanism of conductivity enhancement of treated thin PEDOT:PSS films was explored by atomic force microscopy (AFM) and UV/VIS spectrophotometer. The AFM scans show that the surface of the 2-Methylimidazole treated PEDOT:PSS layer is smoother than that of the pristine PEDOT:PSS thin film. Improvement in the morphology, electrical and optical properties of PEDOT:PSS films makes them highly suitable for numerous applications in optoelectronic devices.     

Simultaneous Increases in Electrical Conductivity and Seebeck Coefficient of PEDOT:PSS Films by Adding Ionic Liquids into a Polymer Solution

The electrical conductivity and Seebeck coefficient of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films were simultaneously improved by adding an ionic liquid (IL) into a polymer solution of the polymers. The maximum electrical conductivity of such a PEDOT:PSS/IL film reached 174 S cm⁻¹, more than an order of magnitude higher than that of pure PEDOT:PSS film, and the maximum Seebeck coefficient was up to 30 μV K⁻¹, more than twice the value for pure PEDOT:PSS film. This behavior is different from conventional thermoelectric (TE) materials, whose TE properties are strongly correlated, such as increasing electrical conductivity with increasing carrier concentration, usually resulting in a logarithmic decrease in Seebeck coefficient. Atomic force microscopy images of the PEDOT:PSS/IL films indicated that the ILs induced formation of a particular three-dimensional structure of highly conducting PEDOT grains, resulting in improvement of the TE performance of PEDOT:PSS films.