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).     

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.     

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.     

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.     

Enhanced power factor of poly (3,4-ethyldioxythiophene):poly (styrene sulfonate) (PEDOT:PSS)/RTCVD graphene hybrid films

The thermoelectric generator has been an attractive alternative power source to operate a wireless sensor node. Usually, inorganic compounds are most often used in thermoelectric devices, and hence, are extensively studied due to their superior thermoelectric performance. We have investigated a novel interfacial technique to fabricate a hybrid film of highly conductive PEDOT:PSS (poly 3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) and graphene. Organic materials PEDOT doped with PSS exhibits outstanding electrical properties due to its high conductivity, low bandgap, and energy migration. Furthermore, we utilized graphene fabricated by rapid thermal chemical vapor deposition (RTCVD) as a thermoelectric material. Our results show that the interfacial technique between substrate and hybrid film could be clearly improved due to the UV plasma treatment. The thermoelectric hybrid film of PEDOT:PSS and RTCVD graphene (P/RTG) exhibited an enhanced power factor of 56.28 μW m⁻¹ K⁻² with a Seebeck coefficient of 54.0 μV K⁻¹.     

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.     

The Thermoelectric Performance of Poly(3,4-ethylenedi oxythiophene)/Poly(4-styrenesulfonate) Thin Films

The thermoelectric performance of synthetic conductive polymers, poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), is explored. The effects of the dielectric solvent, dimethyl sulfoxide (DMSO), and of the ratio of PEDOT to PSS in the polymeric PEDOT/PSS thin films were studied systematically. Within the parameter range studied in this work, the variation of electrical conductivity overwhelmed the variation of the Seebeck coefficient, and the optimal power factor is basically determined by the highest electrical conductivity, because the Seebeck coefficient remains small over a wide range of DMSO concentrations and PEDOT:PSS ratios.     

Preparation and thermoelectric properties of PEDOT:PSS coated Te nanorod/PEDOT:PSS composite films

PEDOT:PSS coated Te (PCTe) nanorod/PEDOT:PSS composite films were prepared by a drop-casting technique. H₂SO₄ treatment was employed to enhance thermoelectric (TE) properties of the composite films. The addition of PCTe nanorods increased both the electrical conductivity and the Seebeck coefficient of the composite films. An optimized power factor of 141.9 μW/mK² was obtained for the film containing 90 wt% PCTe nanorods treated with 12 M H₂SO₄ at room temperature, which was 2.75 times as high as that of the untreated composite film, corresponding to the electrical conductivity and Seebeck coefficient of 204.6 S/cm and 83.27 μV/K, respectively. XPS and GIWAXS analysis revealed the removal of insulating PSS units and the rearrangement of PEDOT chains after the H₂SO₄ treatment. Finally, a 9-leg TE generator prototype was fabricated using the optimized composite film. The maximum output power and area output power density produced from the prototype were 47.7 nW and 57.2 μW/cm², respectively, at the temperature difference of 40 K.     

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.     

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.     

Doped PEDOT:PSS electrodes,patterned through wettability control,and their effects on the electrical properties of polymer thin film transistors

The water−based conductive polymer, poly(3,4−ethylenedioxythiophene), doped with poly(styrene sulfonate) (PEDOT:PSS), has received much attention for its utility as a printable electrode due to its transparency, thermal stability, and processability; however, the electrical properties of devices prepared with printed PEDOT:PSS electrodes are generally inferior to those of devices fabricated with evaporated metal electrodes or their inorganic counterparts. Here, we show that the electrical performances of polymer thin film transistors could be improved by doping the PEDOT:PSS chains used as source and drain electrodes. The addition of HAuCl₄ to the PEDOT:PSS solution increased the electrical conductivity and work function of the electrodes. The PEDOT:PSS film doped with 10 mM HAuCl₄ provided a field effect mobility exceeding 0.01 cm²V⁻¹s⁻¹, a factor of 7 greater than the value obtained from the device prepared with pristine PEDOT electrodes.     

Improved Thermoelectric Performance of Free-Standing PEDOT:PSS/Bi2Te3 Films with Low Thermal Conductivity

Free-standing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS)/Bi₂Te₃ thermoelectric (TE) composite films have been successfully prepared by a simple physical mixing method with different contents of Bi₂Te₃. x-Ray diffraction (XRD) and scanning electron microscopy were used to analyze the phase composition and microstructure of the composite films. Their TE performance from 100 K to 300 K was systematically investigated. The maximum electrical conductivity of the composite polymer film reached up to 421 S/cm when the film contained 10 wt.% Bi₂Te₃, corresponding to the highest power factor of 9.9 μW/m/K², while their Seebeck coefficient fluctuated smoothly in a tiny range (14.2 μV/K to 18.6 μV/K). In addition, a relatively low thermal conductivity of 0.07 ± 0.02 W/m/K has been obtained. The maximum figure of merit of the composite reached up to 0.04 at room temperature, which is a relatively high value in the organic TE field.     

ITO-free organic solar cells using highly conductive phenol-treated PEDOT:PSS anodes

Phenol as one of the most polar solvent was used to enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. The conductivity of PEDOT:PSS films improved to 1193 S/cm after treatment with phenol vapor and 1054 S/cm after treatment with phenol drop. The treated films also showed high transmittance in the visible region which is one of the crucial factors for optoelectronic devices such as organic solar cells and light emitting diodes. The mechanism of conductivity enhancement of treated thin PEDOT:PSS films was investigated by atomic force microscopy (AFM) and UV/Vis spectrophotometer. The AFM images showed that the ratio of PEDOT to PSS at top most of the surface was increased for treated film. Rearrangement of PEDOT segment throughout the film and hence conformational changes are the reasons for enhancement of conductivity. The modified PEDOT:PSS films were used as electrode for ITO-free organic solar cells (OSCs). These ITO-free OSCs showed almost equal operation to those for ITO electrodes.     

Thermoelectric Properties of In<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>N Alloys

We report on the experimental investigation of the potential of InGaN alloys as thermoelectric (TE) materials. We have grown undoped and Si-doped In_0.3Ga_0.7N alloys by metalorganic chemical vapor deposition and measured the Seebeck coefficient and electrical conductivity of the grown films with the aim of maximizing the power factor (P). It was found that P decreases as electron concentration (n) increases. The maximum value for P was found to be 7.3 × 10⁻⁴ W/m K² at 750 K in an undoped sample with corresponding values of Seebeck coefficient and electrical conductivity of 280 μV/K and 93␣(Ω cm)⁻¹, respectively. Further enhancement in P is expected by improving the InGaN material quality and conductivity control by reducing background electron concentration.     

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).     

Conductivity,work function,and environmental stability of PEDOT:PSS thin films treated with sorbitol

The electrical properties of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) thin films deposited from aqueous dispersion using different concentrations of sorbitol have been studied in detail. Although it is well known that sorbitol enhances the conductivity of PEDOT:PSS thin films by three orders of magnitude, the origin and consequences of sorbitol treatment are only partly understood and subject of further study. By thermal annealing of spin coated PEDOT:PSS/sorbitol films and simultaneously monitoring the conductivity, we demonstrate that the strong increase in conductivity coincides with evaporation of sorbitol from the film. Hence, sorbitol is a processing additive rather than a (secondary) dopant. Scanning Kelvin probe microscopy reveals that sorbitol treatment causes a reduction of the work function from 5.1 eV to 4.8–4.9 eV. Sorbitol also influences the environmental stability of the films. While the conductivity of the pristine PEDOT:PSS films increases by about one order of magnitude at ∼50% RH due to an ionic contribution to the overall conductivity, films prepared using sorbitol exhibit an increased environmental stability with an almost constant conductivity up to 45% RH and a slight decrease at 50% RH. The higher stability results from a reduced tendency to take up water from the air, which is attributed to a denser packing of the PEDOT:PSS after sorbitol treatment.     

Gold Nanoparticle and Gold Nanorod Embedded PEDOT:PSS Thin Films as Organic Thermoelectric Materials

We report the thermoelectric properties of organic–inorganic hybrid thin films composed of conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and inorganic gold nanomaterials. Two kinds of material with different shapes, namely rod-shaped gold nanorods (AuNRs) and spherical gold nanoparticles (AuNPs), were used in this study. The PEDOT:PSS/AuNR hybrid films showed an enhancement in electrical conductivity (σ ≈ 2000 S cm^?1) and concurrently a decrease in the Seebeck coefficient (S ≈ 12 μV K^?1) with increase in the AuNR concentration. This behavior indicates the presence of the hybrid effect of AuNR on the thermoelectric properties. From scanning electron microscopy (SEM) observation of the highly concentrated PEDOT:PSS/AuNR hybrid films, the formation of a percolated structure of AuNRs was confirmed, which probably contributed to the large enhancement in σ. For the highly concentrated PEDOT:PSS/AuNP films, a dense distribution of AuNPs in the film was also observed, but this did not lead to a major change in the σ value, probably due to the less conductive connections between NPs. This suggests that one-dimensional particles with larger aspect ratio (rods and wires) are favorable nanocomponents for development of highly conductive hybrid materials.     

Optical properties and conductivity of PEDOT:PSS films treated by polyethylenimine solution for organic solar cells

We report on conductivity and optical property of three different types of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films [pristine PH1000 film (PH1000-p), with 5 wt.% ethylene glycol additive (PH1000-EG) and with sulfuric acid post-treatment (PH1000-SA)] before and after polyethylenimine (PEI) treatment. The PEI is found to decrease the conductivity of all the PEDOT:PSS films. The processing solvent of 2-methoxyethanol is found to significantly enhance the conductivity of PH1000-p from 1.1 up to 744 S/cm while the processing solvent of isopropanol or water does not change the conductivity of PH1000-p much. As for the optical properties, the PEI treatment slightly changes the transmittance and reflectance of PH1000-p and PH1000-EG films, while the PEI leads to an substantial increase of the absorptance in the spectral region of 400–1100 nm of the PH1000-SA films. Though the optical property and conductivity of the three different types of PEDOT:PSS films vary with the PEI treatment, the treated PEDOT:PSS films exhibit similar low work function. We demonstrate solar cells with a simple device structure of glass/low-WF PEDOT:PSS/P3HT:ICBA/high-WF PEDOT:PSS cells that exhibit good performance with open-circuit voltage of 0.82 V and fill factor up to 0.62 under 100 mW/cm² white light illumination.     

Optimization of electrohydrodynamic-printed organic electrodes for bottom-contact organic thin film transistors

In this study, we investigate the optimization of printed (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as source/drain electrodes for organic thin film transistors (OTFTs) through electrohydrodynamic (EHD) printing process. The EHD-printed PEDOT:PSS electrodes should fulfill the prerequisites of not only high conductivity but also optimum surface tension for successful jetting. The conductivity of PEDOT:PSS was dramatically enhanced from 0.07 to 352 S/cm by the addition of dimethylsulfoxide (DMSO). To use the DMSO-treated PEDOT:PSS solution in the EHD printing process, its surface tension was optimized by the addition of surfactant (Triton X-100), which was found to enable various jetting modes. In the stable cone-jet mode, the patterning of the modified PEDOT:PSS solution was realized on the surface-functionalized SiO₂ substrates; the printed line widths were in the range 384 to 81 μm with a line resistance of 8.3 × 10³ Ω/mm. In addition, pentacene-based OTFTs employing the EHD-printed PEDOT:PSS as source and drain electrodes were found to exhibit electrical performances superior to an equivalent vacuum-deposited Au-based device.     

Highly conductive PEDOT:PSS film by doping p-toluenesulfonic acid and post-treatment with dimethyl sulfoxide for ITO-free polymer dispersed liquid crystal device

In this paper, the highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film realized by applying the doping technique and the post-treatment process is demonstrated. The conductivity of the spin coated PEDOT:PSS film enhanced greatly from 0.7 S/cm to 736 S/cm after 1.25% of p-toluenesulfonic acid solution (50 wt%) was doped into the PEDOT:PSS aqueous dispersion. The post-treatment using dimethyl sulfoxide further improved the conductivity to 1549 S/cm. The highly conductive PEDOT:PSS film was used as transparent electrode to fabricate ITO-free polymer dispersed liquid crystal (PDLC) cell. The experimental results showed that the electro-optical properties of the PDLC cell fabricated by the highly conductive PEDOT:PSS film were comparable to those of the PDLC cell constructed by ITO. This study reveals that the highly conductive PEDOT:PSS film is a prospective material for manufacturing ITO-free liquid crystal devices.