Dohyuk Yoo

Direct synthesis of highly conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/graphene composites and their applications in energy harvesting systems

AbstractWe report for the first time highly conductive poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS)/graphene composites fabricated by in situ polymerization and their applications in a thermoelectric device and a platinum (Pt)-free dye-sensitized solar cell (DSSC) as energy harvesting systems. Graphene was dispersed in a solution of poly(4-styrenesulfonate) (PSS) and polymerization was directly carried out by addition of 3,4-ethylenedioxythiophene (EDOT) monomer to the dispersion. The content of the graphene was varied and optimized to give the highest electrical conductivity. The composite solution was ready to use without any reduction process because reduced graphene oxide was used. The fabricated film had a conductivity of 637 S·cm−1, corresponding to an enhancement of 41%, after the introduction of 3 wt.% graphene without any further complicated reduction processes of graphene being required. The highly conductive composite films were employed in an organic thermoelectric device, and the device showed a power factor of 45.7 μW·m−1K−2 which is 93% higher than a device based on pristine PEDOT:PSS. In addition, the highly conductive composite films were used in Pt-free DSSCs, showing an energy conversion efficiency of 5.4%, which is 21% higher than that of a DSSC based on PEDOT:PSS.

Effects of one- and two-dimensional carbon hybridization of PEDOT: PSS on the power factor of polymer thermoelectric energy conversion devices

We investigated the thermoelectric properties of polymer composites based on a conducting polymer and carbon materials with various dimensionalities. PEDOT:PSS as a conducting polymer matrix was successfully hybridized with graphene sheets and multi-walled carbon nanotubes through in situ polymerization of 3,4-ethlyenedioxythiophene monomers in an aqueous solution in the presence of the carbon materials dispersed by using a polymeric dispersant. The hybrid structures of PEDOT:PSS, graphene, and carbon nanotubes in the composite showed an electrical conductivity, Seebeck coefficient, and power factor of 689 S cm−1, 23.2 μV K−1, and 37.08 μW mK−2, respectively, values that are much higher than those of pristine PEDOT:PSS, PEDOT:PSS/graphene, or PEDOT:PSS/carbon-nanotube composites. The thermoelectric figure of merit increased from 0.017 in the pristine PEDOT:PSS to 0.031 in the composite, corresponding to an 80% enhancement. We believe that the enhanced thermoelectric performance comes from the synergic effects of multi-component systems with excellent electrical bridging and electronic coupling between PEDOT and carbon materials.

Gradual thickness-dependent enhancement of the thermoelectric properties of PEDOT:PSS nanofilms

We have investigated the thickness-dependent change in the thermoelectric properties of nanofilms of the conducting polymer, PEDOT:PSS. Films with varying thickness were prepared by spin coating the polymer solution at different speeds. Because of its relatively facile processing, good electrical conductivity, and environmental stability, PEDOT:PSS is considered to be one of the most promising candidates for application in thermal to electric energy conversion devices. Electrical conductivity is attributed to the enhanced carrier mobility in the ordered chain structures of the polymer. The Seebeck coefficient is influenced by the energy derivative of electronic energy density. This approach can be used to study the dependence of conductivity and the Seebeck coefficient at room temperature with varying film thickness. Both the conductivity and Seebeck coefficient improved with increasing thickness of the polymer nanofilms. This can be attributed to the change in the conformation of PEDOT, which exposes the PEDOT on the surface of the PEDOT:PSS phase. The PEDOT:PSS thin films were characterized by UV-Vis spectroscopy, tapping-mode atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. This study suggests that variation of film thickness is an effective way of improving the thermoelectric properties of PEDOT:PSS.

Synthesis of conductive and transparent PEDOT:P(SS-co-PEGMA) with excellent water-, weather-, and chemical-stabilities for organic solar cells

The water-, weather- and chemical-resistant conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)-co-poly(ethylene glycol methacrylate) (PEDOT:P(SS-co-PEGMA)) copolymer was successfully synthesized with thermally curable P(SS-co-PEGMA) copolymers. The PSS and P(SS-co-PEGMA) copolymers were synthesized by solution polymerization and PEDOT:PSS and PEDOT:P(SS-co-PEGMA) were synthesized by Fe+-catalyzed oxidative polymerization. PSS and P(SS-co-PEGMA) were characterized by Fourier-transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR). The electrical properties of the conductive PEDOT:P(SS-co-PEGMA) thin films were characterized in two parts; first, the mechanism and characterization of the conductivity change, and second, the characterization of the water-, chemical-, and weather-stability of the films. The conductivity and transmittance, respectively, of PEDOT:P(SS-co-PEGMA) at 550 nm under optimized conditions were maintained at the levels found in PEDOT:PSS, 160.3 S cm−1 and 86.7%. The introduction of PEGMA to the PSS copolymer improved the mechanical properties and weather stability. The PEDOT:P(SS-co-PEGMA) was highly stable to chemical solvents and independent of the type of solvents used for stability analysis. The conductivity in the weather stability test of PEDOT:PSS decreased by 44.9%, on the other hand, the conductivity of PEDOT:P(SS-co-PEGMA) was decreased by only 22.2%. The PEDOT:PSS and PEDOT:P(SS-co-PEGMA) copolymers were used as buffer layers in organic solar cells (OSC) and showed as high efficiency as conventional PEDOT:PSS materials. The decrease of OSC efficiency with PEDOT:P(SS-co-PEGMA) was 30% less than the OSCs with the commercial and reference PEDOT:PSS buffer layers.

N-type organic thermoelectric materials based on polyaniline doped with the aprotic ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate

The thermopower performance of polyaniline doped with the considerably reliable ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate was investigated to determine its potential as an alternative to fossil fuels. The ionic liquid, a stable ion complex, was used as the chemical dopant to confer n-type electrical properties to the polyaniline conducting polymer. The ionic liquid-doped polyaniline had an electrical conductivity, n-type Seebeck coefficient, and thermopower execution of 0.23 S m−1, −138.8 μV K−1, and 4.43 × 10−3 μW m−1 K−2, respectively as a result of the chemical doping method and thermal treatment. In addition, the ionic liquid-doped polyaniline realized a stable n-type thermoelectric performance under ambient conditions for 15 days.

Large-scalable RTCVD Graphene/PEDOT:PSS hybrid conductive film for application in transparent and flexible thermoelectric nanogenerators

Poly(3,4-ethyldioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), as an thermoelectric(TE) material, exhibits a high electrical conductivity and ZT value (10−1–100). Nevertheless, a low thermovoltage of the organic thermoelectric materials must be overcome, in comparison to that of semi metals. Recently, to address these challenges, several researchers have investigated PEDOT:PSS/carbon material composites. Herein, a transparent and flexible hybrid film made up of rapid thermal chemical vapor deposition (RTCVD) graphene and PEDOT:PSS results in enhanced TE performance. The PEDOT:PSS was synthesized by oxidative polymerization, and the hybrid process of the graphene film and PEDOT:PSS film was conducted using the layer-by-layer method. The results of AFM and Raman spectroscopy revealed that the synergistic effect through composite films improved the electrical properties. The maximum electrical conductivity and power factor of the RTCVD graphene/PEDOT:PSS (RCG/P) hybrid film were 1096 S cm−1 and 57.9 μW m−1 K−2, respectively. In addition, the RCG/P hybrid film exhibited excellent mechanical flexibility and stability.