- Email: [email protected]
conjugated polymer thermoelectric films and power generators
Accepted Manuscript Title: Hot-pressing for improving performance of CNT/conjugated polymer thermoelectric films and power generators Author: Woohwa Lee Young Hun Kang Jun Young Lee Kwang-Suk Jang Song Yun Cho PII: DOI: Reference:
S2352-4928(16)30109-X http://dx.doi.org/doi:10.1016/j.mtcomm.2016.12.002 MTCOMM 144
To appear in: Received date: Revised date: Accepted date:
7-9-2016 29-11-2016 21-12-2016
Please cite this article as: Woohwa Lee, Young Hun Kang, Jun Young Lee, KwangSuk Jang, Song Yun Cho, Hot-pressing for improving performance of CNT/conjugated polymer thermoelectric films and power generators, Materials Today Communications http://dx.doi.org/10.1016/j.mtcomm.2016.12.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot-pressing for improving performance of CNT/conjugated polymer thermoelectric films and power generators
Woohwa Leea,b, Young Hun Kanga, Jun Young Leeb, Kwang-Suk Jangc*, Song Yun Choa**
a
Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon
34114, Republic of Korea b
School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of
Korea c
Department of Chemical Engineering and Research Center of Chemical Technology,
Hankyong National University, Anseong 17579, Republic of Korea
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.Y. Cho), [email protected] (K.-S. Jang).
Graphical Abstract
Abstract This study demonstrates a post-treatment method for enhancing the thermoelectric power factor of carbon nanotube (CNT)/conjugated polymer nanocomposite films. For the fabrication of the nanocomposite films a mixture of P3HT (50 wt%) and few-walled CNT (50 wt%) in o-dichlorobenzene (oDCB), or a mixture of PEDOT:PSS (50 wt%) and few-walled CNT (50 wt%) in distilled deionized (DDI) water was used. The uniaxial hot-pressing treatment of the as-prepared nanocomposite films resulted in their densification and increased the electrical conductivity and power factor. It was found that the thermoelectric power factor of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased from 150 ± 9 to 217 ± 30 µW m-1K-2, and from 371 ± 44 to 572 ± 92 µW m-1K-2, respectively, after hot-pressing. We fabricated organic thermoelectric generators using the hot-pressed CNT/conjugated polymer nanocomposites and evaluated their capabilities of electrical power generation. The maximum power outputs of the hot-pressed CNT/P3HT and CNT/PEDOT:PSS OTEGs were
46.0 and 36.3 nW, respectively, under a temperature difference of 10 K.
Keywords: Thermoelectric, Organic, Nanocomposites, Hot-pressing, Power generator
1. Introduction Thermoelectric materials have attracted much attention because they can be used in thermoelectric coolers and thermoelectric generators harvesting electrical energy from waste heat. Thermoelectric performance of a material is evaluated using the dimensionless figure of merit, ZT = S2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. In the case of film-type materials on substrates, the precise measurement of their in-plane thermal conductivity is difficult. Therefore, the thermoelectric power factor, S2σ, has been often used for evaluating the thermoelectric performance of the film-type materials. In the past few decades, organic thermoelectric materials have been extensively studied because they are potentially suitable for the fabrication of flexible, non-toxic, light-weight, and low-cost organic thermoelectric generators (OTEGs) operating at near room temperature [1-20]. Among organic thermoelectric materials, carbon nanotube (CNT)/conjugated polymer nanocomposites are considered to be highly promising candidates for the fabrication of high-performance OTEGs [6-20]. By using CNTs as an electrically conductive filler, the electrical conductivity and thermoelectric power factor of organic materials could be improved. Indeed, it has been reported that single-walled CNT/conjugated polymer nanocomposite films exhibit excellent
thermoelectric properties. In particular, single-walled CNT/poly(3-hexylthiophene) (P3HT) nanocomposite films exhibit a power factor of 95 ± 12 µW m-1K-2 [6]. Single-walled CNT/poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate)
(PEDOT:PSS)
nanocomposite films exhibit a power factor of up to 140 µW m-1K-2 [7], and single-walled CNT/polyaniline nanocomposite films exhibit a power factor of up to 176 µW m-1K-2 [8]. Double- and few-walled CNTs have also been used as the fillers. It was found that the double-walled CNT/polyaniline nanocomposite films exhibit a power factor of ~220 µW m1
K-2 [9], whereas the few-walled CNT/P3HT nanocomposite films exhibit a power factor of
325 ± 101 µW m-1K-2 [10]. However, the thermoelectric performance of CNT/conjugated polymer nanocomposites has to be further enhanced for improving the performance of OTEGs. In this work, we have studied how hot-pressing influences thermoelectric properties of CNT/conjugated polymer nanocomposite films. The as-prepared few-walled CNT/P3HT and few-walled CNT/PEDOT:PSS nanocomposite films exhibited power factors of 150 ± 9 µW m-1K-2 and 371 ± 44 µW m-1K-2, respectively. After hot-pressing, the power factors of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased to 217 ± 30 µW m-1K-2 and 572 ± 92 µW m-1K-2, respectively. These values are the highest among all values reported for thermoelectric CNT/conjugated polymers, to the best of our knowledge. Nanocomposite film densification due to the hot-pressing treatment results in the power factor increase up to values of 140%. It is expected that the volume fraction of the CNTs might be increased and the 1-dimensional CNT bundles might be slightly aligned under uniaxial hot-pressing of CNT/polymer nanocomposite films. Using the hot-pressed CNT/conjugated polymer nanocomposites as p-type active materials, OTEGs were fabricated and their capabilities of electric power generation were demonstrated.
2. Experimental Purified few-walled CNTs (purity >98%, XNM-UP-11050) were purchased from XinNano Materials. Regioregular P3HT (Mw 37,685 g mol-1, regioregularity 98.5%), PEDOT:PSS (dry re-dispersible pellets, OrgaconTM DRY) and o-dichlorobenzene (oDCB) were purchased from Sigma-Aldrich. All chemicals in this study were used as received. Fig. 1 shows a schematic diagram of the fabrication process of the CNT/conjugated polymer nanocomposite film. For the fabrication of the nanocomposite films a mixture of P3HT (50 wt%) and few-walled CNT (50 wt%) in o-dichlorobenzene (oDCB), or a mixture of PEDOT:PSS (50 wt%) and fewwalled CNT (50 wt%) in distilled deionized (DDI) water was used. After dissolving P3HT in oDCB or PEDOT:PSS in distilled deionized (DDI) water, the CNTs were added into the solution. Total solid concentrations of the CNT/P3HT and CNT/PEDOT:PSS inks were 2 mg mL-1 and 1 mg mL-1, respectively. The mixture was sonicated in an ice bath using a probe sonicator (VCX-750 Vibra-Cell, Sonics & Materials) at 10 W for 1 hr. The prepared inks were drop-cast onto polyimide Kapton substrates (20 mm × 20 mm). The thicknesses of the as-prepared CNT/P3HT and CNT/PEDOT:PSS nanocomposite films were 2620 ± 260 and 1220 ± 70 nm, respectively. For enhancing the thermoelectric performance, the prepared nanocomposite films were pressed at 50 MPa and 100 °C for 5 min using a high-temperature constant thickness film maker (Atlas, Specac), a high stability temperature controller (4000 Series, Specac), and a compression molding press (MH-15, Masada Seisakusho). For preparation of single-walled and few-walled CNT films, the mixture of single-walled CNTs (purified HiPco, Unidym) or few-walled CNTs (30 mg) and chloroform (30 mL) was sonicated in an ice bath using a probe sonicator at 10 W for 1 hr. The as-prepared mixture was then vacuum-filtrated with a nylon membrane filter (pore size: 0.2μm, diameter: 47 mm).
The thicknesses of the single-walled and few-walled CNT films were 24.9 and 28.0 µm, respectively.
The Seebeck coefficient of the nanocomposite films has been measured under dark ambient conditions using a custom-built system. Silver paste was printed onto the nanocomposite films through a shadow mask prior to the measurement. Two electrodes of 2 mm in width were placed 15 mm apart. The temperature difference between the two electrodes was varied from 1 K to 10 K. The Seebeck voltage generated by the temperature difference was measured using a Keithley 2182A Nanovoltmeter. The Seebeck coefficient was estimated from the slope of the straight line fit of ΔV/ΔT (Fig. S1). The electrical conductivity was measured by the standard van der Pauw direct current four-probe method [21]. All measurements were performed at room temperature using a Keithley 195A digital multimeter and a Keithley 220 programmable current source. The film thickness was measured using an alpha-step surface profiler (α-step DC50, KLA Tencor). Surface morphologies of the films were observed using a field-emission scanning electron microscope (SEM, MIRA3, TESCAN) operating at 20 kV. For fabrication of OTEGs, 14 lines of the hot-pressed nanocomposites, with a width of 3 mm and a length of 15 mm, were connected in series by silver electrodes (Fig. S2). The silver electrode lines were dispenser-printed with a Musashi Shotmaster 200DS-s. Electrical characteristics of the fabricated OTEGs were evaluated under dark ambient conditions by varying the load resistance using a custom-built system. The temperature difference between two edges of the nanocomposite lines was controlled to be 10 K using two Peltier plates [10]. The output voltage-output current curves and output power-output current curves were obtained using a Keithley 2450 sourcemeter.
3. Results and discussions For the preparation of nanocomposite films, few-walled CNTs with 2-4 carbon walls were used. The vacuum-filtrated few-walled CNT film possesed a Seebeck coefficient of 51 µV K1
and an electrical conductivity of 3920 S cm-1, which are higher than those of the vacuum-
filtrated single-walled CNT film. This is in line with a previous report that few-walled CNT/P3HT nanocomposite films exhibit excellent thermoelectric properties [10]. The fewwalled CNT/P3HT nanocomposite films, drop-cast on glass substrates and spray-printed on polyimide substrates, exhibited power factors of 231 ± 19 µW m-1K-2 and 325 ± 101 µW m1
K-2, respectively [10]. The hot-pressing temperature was higher than the glass transition
temperatures of regioregular P3HT and PEDOT [22,23]. The thicknesses of the as-prepared CNT/P3HT and CNT/PEDOT:PSS nanocomposite films were 2620 ± 260 and 1220 ± 70 nm, respectively (Fig. 2a). After hot-pressing, the thicknesses of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films decreased to 2450 ± 180 and 1040 ± 150 nm, respectively (Fig. 2a); that is, the average thicknesses of the films decreased 7 % and 15 %, respectively. Zhou et al. reported that PEDOT:PSS grains are weakly connected in the film at room temperature owing to the adsorption of water molecules by the surface HSO3 groups of the grains [24]. At temperatures above 100 °C, the water molecules desorb from the grain surface and hydrogen-bonding interactions between HSO3 groups of the PSS chains are stronger. Consequently, the PEDOT:PSS grains become more densely connected [24]. There might be more room for densification due to the fixation of water molecules in the CNT/PEDOT:PSS nanocomposite films.
Thermoelectric voltage (ΔV) induced by the temperature difference (ΔT) across the nanocomposite films was measured under dark ambient conditions by using a custom built system. The Seebeck coefficient was obtained as the slope of ΔV/ΔT (Fig. S1). All nanocomposite films used in this study exhibited positive values of the Seebeck coefficient, because CNTs, P3HT, and PEDOT:PSS have p-type characteristics. As can be seen in Fig. 2, the Seebeck coefficient, electrical conductivity, and power factor of the as-prepared CNT/P3HT nanocomposite films are 74.8 ± 1.1 µV K-1, 268 ± 11 S cm-1, and 150 ± 9 µW m1
K-2, respectively, which are comparable to the corresponding values of the drop-cast few-
walled CNT/P3HT nanocomposite films [10]. The Seebeck coefficient, electrical conductivity, and power factor of the as-prepared CNT/PEDOT:PSS nanocomposite films are 42.1 ± 1.9 µV K-1, 2090 ± 100 S cm-1, and 371 ± 44 µW m-1K-2, respectively (Fig. 2). The high value of the power factor in the case of the CNT/PEDOT:PSS nanocomposite films originate from the extremely high electrical conductivity of the films. Recently, we reported that the electrical conductivity of single-walled CNT/P3HT nanocomposite films could be greatly enhanced by doping of the P3HT matrix [11]. By doping, the electrical conductivity of the P3HT matrix can be increased from ~10-5 to 61.9 S cm-1. As a result, the electrical conductivity of the single-walled CNT/P3HT nanocomposite films increased from 141 to 2760 S cm-1. In the nanocomposite films, the non-conductive polymer matrix presents a barrier to the inter-CNT bundle hopping because the electrical conductivity of CNTs is much higher than that of the polymer matrix. An increase in the electrical conductivity of the polymer matrix by doping can reduce the resistance between the CNT bundles in nanocomposite films. Therefore, electrical conductivity of a nanocomposite film could be greatly enhanced by using PEDOT:PSS with an electrical conductivity of ~0.1 S cm-1 instead of using P3HT with an electrical conductivity of ~10-5 S cm-1. Since there is a trade-off between the Seebeck coefficient and electrical conductivity of a thermoelectric material, the Seebeck coefficient of
CNT/PEDOT:PSS nanocomposite films is lower than that of CNT/P3HT nanocomposite films. Correspondingly, the power factor of CNT/PEDOT:PSS nanocomposite films is substantially higher than that of CNT/P3HT nanocomposite films. We found that the thermoelectric performance of both the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films can be improved using hot-pressing. As can be seen in Fig. 2, the Seebeck coefficient, electrical conductivity, and power factor of the hot-pressed CNT/P3HT nanocomposite films increase to 84.1 ± 3.8 µV K-1, 307 ± 33 S cm-1, and 217 ± 30 µW m-1K-2, respectively. Similarly, the Seebeck coefficient, electrical conductivity, and power factor of the CNT/PEDOT:PSS nanocomposite films increase to 49.3 ± 1.5 µV K-1, 2350 ± 330 S cm-1, and 572 ± 92 µW m-1K-2, respectively, after the hot-pressing treatment. This treatment increases the Seebeck coefficient and electrical conductivity of both the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased. A conjugated polymer matrix and voids present barriers to the CNT bundle-to-bundle hopping. The electrical properties of the nanocomposite films with 1-dimensional conductive fillers can be affected by the volume fraction and alignment of the fillers [25,26]. Under uniaxial hot-pressing of CNT/polymer nanocomposite films, the volume fraction of a polymer matrix and voids can be reduced by densification. As a result, the volume fraction of CNTs can be increased and the CNTs can be slightly aligned [25]. Besides hot-pressing, hot-rolling can also be used for densification and enhancement of electrical conductivity of nano-carbon/polymer nanocomposite films [27,28]. To demonstrate electric power generation capabilities of the hot-pressed CNT/conjugated polymer nanocomposites, OTEGs composed of only p-type active lines were fabricated (Fig. S2). The 14 lines of the hot-pressed nanocomposites, with a width of 3 mm and a length of 15 mm, on a glass substrate were connected in series by the dispenser-printed silver lines. Fig. 3 shows output voltage-output current curves and output power-output current curves of the
hot-pressed CNT/P3HT and CNT/PEDOT:PSS OTEGs. The temperature difference between two edges of the nanocomposite lines was controlled to be 10 K [10]. Power factor of the hotpressed CNT/P3HT nanocomposites is lower than that of the hot-pressed CNT/PEDOT:PSS nanocomposites. However, the maximum power output of the hot-pressed CNT/P3HT OTEG, 46.0 nW, is higher than that of the hot-pressed CNT/PEDOT:PSS, 36.3 nW. This might be due to the higher open-circuit voltage of the hot-pressed CNT/P3HT nanocomposites. The open-circuit voltage is directly related to the Seebeck coefficient. The Seebeck coefficient of the hot-pressed CNT/P3HT nanocomposites, 84.1 ± 3.8 µV K-1, is higher than that of the hotpressed CNT/PEDOT:PSS nanocomposites, 49.3 ± 1.5 µV K-1. By developing compatible ntype active materials and optimizing the device structure, the performance of the hot-pressed CNT/conjugated polymer OTEGs might be enhanced.
4. Conclusion In summary, we report the results of a hot-pressing treatment of few-walled CNT/conjugated polymer nanocomposite films aimed at the enhancement of the thermoelectric properties of the nanocomposites. It was found that the thermoelectric power factor of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased from 150 ± 9 to 217 ± 30 µW m-1K-2, and from 371 ± 44 to 572 ± 92 µW m-1K-2, respectively, after hot-pressing. Under uniaxial hotpressing of CNT/polymer nanocomposite films, the volume fraction of the CNTs might be increased and the 1-dimensional CNT bundles might be slightly aligned. Our results suggest that the post-treatment of the CNT/conjugated polymer nanocomposite films can significantly affect their thermoelectric properties, and the processing method should be highly considered in order to improve the thermoelectric properties of nanocomposite materials. Using the hotpressed CNT/P3HT and CNT/PEDOT:PSS nanocomposites, OTEGs composed of 14 p-type
active lines were fabricated. The electric power generation of the OTEGs was demonstrated and the OTEGs with the hot-pressed CNT/P3HT and CNT/PEDOT:PSS nanocomposites were found to exhibit maximum output powers of 46.0 nW and 36.3 nW, respectively, under a temperature difference of 10 K.
Acknowledgements This work was supported by a grant from the R&D Convergence Program of the National Research Council of Science & Technology (NST) and a grant from the KRICT Core Project.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/.
References [1] O. Bubnova, Z.U. Khan, A. Malti, S. Braun, M. Fahlman, M. Berggren, X. Crispin, Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4ethylenedioxythiophene), Nat. Mater. 10 (2011) 429-433. [2] G-H. Kim, L. Shao, K. Zhang, K.P. Pipe, Engineered doping of organic semiconductors for enhanced thermoelectric efficiency, Nat. Mater. 12 (2013) 719-723. [3] M. Chabinyc, Thermoelectric polymers: behind organics' thermopower, Nat. Mater. 13 (2014) 119-121.
[4] T. Park, C. Park, B. Kim, H. Shin, E. Kim, Flexible PEDOT electrodes with large thermoelectric power factors to generate electricity by the touch of fingertips, Energy Environ. Sci. 6 (2013) 788-792. [5] C. Cho, B. Stevens, J.-H. Hsu, R. Bureau, D.A. Hagen, O. Regev, C. Yu, J.C. Grunlan, Completely organic multilayer thin film with thermoelectric power factor rivaling inorganic tellurides, Adv. Mater. 27 (2015) 2996-3001. [6] W. Lee, C.T. Hong, O.H. Kwon, Y. Yoo, Y.H. Kang, J.Y. Lee, S.Y. Cho, K.-S. Jang, Enhanced thermoelectric performance of bar-coated SWCNT/P3HT thin films, ACS Appl. Mater. Interfaces 7 (2015) 6550-6556. [7] G.P. Moriarty, S. De, P.J. King, U. Khan, M. Via, J.A. King, J.N. Coleman, J.C. Grunlan, Thermoelectric behavior of organic thin film nanocomposites, J. Polymer. Sci. Part B: Polym. Phys. 51 (2013) 119-123. [8] Q. Yao, Q. Wang, L. Wang, L. Chen, Abnormally enhanced thermoelectric transport properties of SWNT/PANI hybrid films by the strengthened PANI molecular ordering, Energy Environ. Sci. 7 (2014) 3801-3807. [9] H. Wang, S.-I. Yi, X. Pu, C. Yu, Simultaneously improving electrical conductivity and thermopower of polyaniline composites by utilizing carbon nanotubes as high mobility conduits, ACS Appl. Mater. Interfaces 7 (2015) 9589-9597. [10] C.T. Hong, Y.H. Kang, J. Ryu, S.Y. Cho, K.-S. Jang, Spray-printed CNT/P3HT organic thermoelectric films and power generators, J. Mater. Chem. A 3 (2015) 21428-21433. [11] C.T. Hong, W. Lee, Y.H. Kang, Y. Yoo, J. Ryu, S.Y. Cho, K.-S. Jang, Effective doping by spin-coating and enhanced thermoelectric power factors in SWCNT/P3HT hybrid films, J. Mater. Chem. A 3 (2015) 12314-12319.
[12] C. Bounioux, P. Díaz-Chao, M. Campoy-Quiles, M.S. Martín-González, A.R. Goñi, R. Yerushalmi-Rozen, C. Müller, Thermoelectric composites of poly(3-hexylthiophene) and carbon nanotubes with a large power factor, Energy Environ. Sci. 6 (2013) 918-925. [13] C. Yu, K. Choi, L. Yin, J.C. Grunlan, Light-weight flexible carbon nanotube based organic composites with large thermoelectric power factors, ACS Nano 5 (2011) 78857892. [14] B. Endrődi, G.F. Samu, D. Fejes, Z. Németh, E. Horváth, A. Pinsoni, P.K. Matus, K. Hernádi, C. Visy, L. Forró, C. Janáky, Challenges and rewards of the electrosynthesis of macroscopic aligned carbon nanotube array/conducting polymer hybrid assemblies, J. Polymer. Sci. Part B: Polym. Phys. 53 (2015) 1507-1518. [15] Y. Du, S. Z. Shen, W. D. Yang, K. F. Cai, P. S. Casey, Preparation and characterization of multiwalled carbon nanotube/poly(3-hexylthiophene) thermoelectric composite materials, Synth. Met. 162 (2012) 375-380. [16] Y. Du, K.F. CAi, S.Z. Shen, P.S. Casey, Preparation and characterization of graphene nanosheets/poly(3-hexylthiophene) thermoelectric composite materials, Synth. Met., 162 (2012) 2102-2106. [17] C. Meng, C. Liu, S. Fan, A promising approach to enhanced thermoelectric properties using carbon nanotube networks, Adv. Mater. 22 (2010) 535-539. [18] Q. Yao, L. Chen, W. Zhang, S. Liufu, X. Chen, Enhanced thermoelectric performance of single-walled carbon nanotubes/polyaniline hybrid nanocomposites, ACS Nano 4 (2010) 2445-2451. [19] Q. Wang, Q. Yao, J. Chang, L. Chen, Enhanced thermoelectric properties of CNT/PANI composite nanofibers by highly orienting the arrangement of polymer chains, J. Mater. Chem. 22 (2012) 17612-17618.
[20] J. Chen, X. Gui, Z. Wang, Z. Li, R. Xiang, K. Wang, D. Wu, X. Xia, Y. Zhou, Q. Wang, Z. Tang, L. Chen, Superlow thermal conductivity 3D carbon nanotube network for thermoelectric applications, ACS Appl. Mater. Interfaces 4 (2012) 81-86. [21] L.T. Van der Pauw, Method of measuring specific resistivity and hall effect of discs of arbitrary shape. Philips Res. Rep. 13 (1958) 1-9. [22] K. Yazawa, Y. Inoue, T. Yamamoto, N. Asakawa, Twist glass transition in regioregulated poly(3-alkylthiophene), Phys. Rev. B 74 (2006) 094204. [23] C.A. Ng, D.H. Camacho, Polymer electrolyte system based on carrageenan-poly(3,4ethylenedioxythiophene) (PEDOT) composite for dye sensitized solar cell, IOP Conf. Series: Materials Science and Engineering 79 (2015) 012020. [24] J. Zhou, D.H. Anjum, L. Chen, X. Xu, I.A. Ventura, L. Jiang, G. Bubineau, The temperature-dependent microstructure of PEDOT/PSS films: insights from morphological, mechanical and electrical analyses, J. Mater. Chem. C 2 (2014) 9903-9910. [25] F. Du, J.E. Fischer, K.I. Karen, I. Winey, Effect of nanotube alignment on percolation conductivity in carbon nanotube/polymer composites, Phys. Rev. B 72 (2005) 121404. [26] M. Pooreman, M. Traianidis, G. Bister, F. Cambier, Colloidal processing, hot pressing and
characterisation
of
electroconductive
MWCNT-alumina
composites
with
compositions near the percolation threshold, J. Eur. Ceram. Soc. 29 (2009) 669-675. [27] H. Wu, L. T. Drzal, Graphene nanoplatelet paper as a light-weight composite with excellent electrical and thermal conductivity and good gas barrier, Carbon 50 (2012) 11351145. [28] K.-Y. Chun, Y. Oh, J. Rho, J.-H. Ahn, Y.-J. Kim, H.R. Choi, S. Baik, Highly conductive, printable and stretchable composite films of carbon nanotubes and silver, Nat. Mater. 5 (2010) 853-857.
Fig. 1. A schematic diagram of the fabrication process of CNT/conjugated polymer nanocomposite films.
Fig. 2. (a) Thickness, (b) Seebeck coefficient, (c) electrical conductivity, and (d) power factor of the as-prepared and hot-pressed CNT/P3HT and CNT/PEDOT:PSS nanocomposite films.
Fig. 3. Output voltage-output current and output power-output current curves of the hotpressed (a) CNT/P3HT and (b) CNT/PEDOT:PSS OTEGs.
S2352-4928(16)30109-X http://dx.doi.org/doi:10.1016/j.mtcomm.2016.12.002 MTCOMM 144
To appear in: Received date: Revised date: Accepted date:
7-9-2016 29-11-2016 21-12-2016
Please cite this article as: Woohwa Lee, Young Hun Kang, Jun Young Lee, KwangSuk Jang, Song Yun Cho, Hot-pressing for improving performance of CNT/conjugated polymer thermoelectric films and power generators, Materials Today Communications http://dx.doi.org/10.1016/j.mtcomm.2016.12.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot-pressing for improving performance of CNT/conjugated polymer thermoelectric films and power generators
Woohwa Leea,b, Young Hun Kanga, Jun Young Leeb, Kwang-Suk Jangc*, Song Yun Choa**
a
Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon
34114, Republic of Korea b
School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of
Korea c
Department of Chemical Engineering and Research Center of Chemical Technology,
Hankyong National University, Anseong 17579, Republic of Korea
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.Y. Cho), [email protected] (K.-S. Jang).
Graphical Abstract
Abstract This study demonstrates a post-treatment method for enhancing the thermoelectric power factor of carbon nanotube (CNT)/conjugated polymer nanocomposite films. For the fabrication of the nanocomposite films a mixture of P3HT (50 wt%) and few-walled CNT (50 wt%) in o-dichlorobenzene (oDCB), or a mixture of PEDOT:PSS (50 wt%) and few-walled CNT (50 wt%) in distilled deionized (DDI) water was used. The uniaxial hot-pressing treatment of the as-prepared nanocomposite films resulted in their densification and increased the electrical conductivity and power factor. It was found that the thermoelectric power factor of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased from 150 ± 9 to 217 ± 30 µW m-1K-2, and from 371 ± 44 to 572 ± 92 µW m-1K-2, respectively, after hot-pressing. We fabricated organic thermoelectric generators using the hot-pressed CNT/conjugated polymer nanocomposites and evaluated their capabilities of electrical power generation. The maximum power outputs of the hot-pressed CNT/P3HT and CNT/PEDOT:PSS OTEGs were
46.0 and 36.3 nW, respectively, under a temperature difference of 10 K.
Keywords: Thermoelectric, Organic, Nanocomposites, Hot-pressing, Power generator
1. Introduction Thermoelectric materials have attracted much attention because they can be used in thermoelectric coolers and thermoelectric generators harvesting electrical energy from waste heat. Thermoelectric performance of a material is evaluated using the dimensionless figure of merit, ZT = S2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. In the case of film-type materials on substrates, the precise measurement of their in-plane thermal conductivity is difficult. Therefore, the thermoelectric power factor, S2σ, has been often used for evaluating the thermoelectric performance of the film-type materials. In the past few decades, organic thermoelectric materials have been extensively studied because they are potentially suitable for the fabrication of flexible, non-toxic, light-weight, and low-cost organic thermoelectric generators (OTEGs) operating at near room temperature [1-20]. Among organic thermoelectric materials, carbon nanotube (CNT)/conjugated polymer nanocomposites are considered to be highly promising candidates for the fabrication of high-performance OTEGs [6-20]. By using CNTs as an electrically conductive filler, the electrical conductivity and thermoelectric power factor of organic materials could be improved. Indeed, it has been reported that single-walled CNT/conjugated polymer nanocomposite films exhibit excellent
thermoelectric properties. In particular, single-walled CNT/poly(3-hexylthiophene) (P3HT) nanocomposite films exhibit a power factor of 95 ± 12 µW m-1K-2 [6]. Single-walled CNT/poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate)
(PEDOT:PSS)
nanocomposite films exhibit a power factor of up to 140 µW m-1K-2 [7], and single-walled CNT/polyaniline nanocomposite films exhibit a power factor of up to 176 µW m-1K-2 [8]. Double- and few-walled CNTs have also been used as the fillers. It was found that the double-walled CNT/polyaniline nanocomposite films exhibit a power factor of ~220 µW m1
K-2 [9], whereas the few-walled CNT/P3HT nanocomposite films exhibit a power factor of
325 ± 101 µW m-1K-2 [10]. However, the thermoelectric performance of CNT/conjugated polymer nanocomposites has to be further enhanced for improving the performance of OTEGs. In this work, we have studied how hot-pressing influences thermoelectric properties of CNT/conjugated polymer nanocomposite films. The as-prepared few-walled CNT/P3HT and few-walled CNT/PEDOT:PSS nanocomposite films exhibited power factors of 150 ± 9 µW m-1K-2 and 371 ± 44 µW m-1K-2, respectively. After hot-pressing, the power factors of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased to 217 ± 30 µW m-1K-2 and 572 ± 92 µW m-1K-2, respectively. These values are the highest among all values reported for thermoelectric CNT/conjugated polymers, to the best of our knowledge. Nanocomposite film densification due to the hot-pressing treatment results in the power factor increase up to values of 140%. It is expected that the volume fraction of the CNTs might be increased and the 1-dimensional CNT bundles might be slightly aligned under uniaxial hot-pressing of CNT/polymer nanocomposite films. Using the hot-pressed CNT/conjugated polymer nanocomposites as p-type active materials, OTEGs were fabricated and their capabilities of electric power generation were demonstrated.
2. Experimental Purified few-walled CNTs (purity >98%, XNM-UP-11050) were purchased from XinNano Materials. Regioregular P3HT (Mw 37,685 g mol-1, regioregularity 98.5%), PEDOT:PSS (dry re-dispersible pellets, OrgaconTM DRY) and o-dichlorobenzene (oDCB) were purchased from Sigma-Aldrich. All chemicals in this study were used as received. Fig. 1 shows a schematic diagram of the fabrication process of the CNT/conjugated polymer nanocomposite film. For the fabrication of the nanocomposite films a mixture of P3HT (50 wt%) and few-walled CNT (50 wt%) in o-dichlorobenzene (oDCB), or a mixture of PEDOT:PSS (50 wt%) and fewwalled CNT (50 wt%) in distilled deionized (DDI) water was used. After dissolving P3HT in oDCB or PEDOT:PSS in distilled deionized (DDI) water, the CNTs were added into the solution. Total solid concentrations of the CNT/P3HT and CNT/PEDOT:PSS inks were 2 mg mL-1 and 1 mg mL-1, respectively. The mixture was sonicated in an ice bath using a probe sonicator (VCX-750 Vibra-Cell, Sonics & Materials) at 10 W for 1 hr. The prepared inks were drop-cast onto polyimide Kapton substrates (20 mm × 20 mm). The thicknesses of the as-prepared CNT/P3HT and CNT/PEDOT:PSS nanocomposite films were 2620 ± 260 and 1220 ± 70 nm, respectively. For enhancing the thermoelectric performance, the prepared nanocomposite films were pressed at 50 MPa and 100 °C for 5 min using a high-temperature constant thickness film maker (Atlas, Specac), a high stability temperature controller (4000 Series, Specac), and a compression molding press (MH-15, Masada Seisakusho). For preparation of single-walled and few-walled CNT films, the mixture of single-walled CNTs (purified HiPco, Unidym) or few-walled CNTs (30 mg) and chloroform (30 mL) was sonicated in an ice bath using a probe sonicator at 10 W for 1 hr. The as-prepared mixture was then vacuum-filtrated with a nylon membrane filter (pore size: 0.2μm, diameter: 47 mm).
The thicknesses of the single-walled and few-walled CNT films were 24.9 and 28.0 µm, respectively.
The Seebeck coefficient of the nanocomposite films has been measured under dark ambient conditions using a custom-built system. Silver paste was printed onto the nanocomposite films through a shadow mask prior to the measurement. Two electrodes of 2 mm in width were placed 15 mm apart. The temperature difference between the two electrodes was varied from 1 K to 10 K. The Seebeck voltage generated by the temperature difference was measured using a Keithley 2182A Nanovoltmeter. The Seebeck coefficient was estimated from the slope of the straight line fit of ΔV/ΔT (Fig. S1). The electrical conductivity was measured by the standard van der Pauw direct current four-probe method [21]. All measurements were performed at room temperature using a Keithley 195A digital multimeter and a Keithley 220 programmable current source. The film thickness was measured using an alpha-step surface profiler (α-step DC50, KLA Tencor). Surface morphologies of the films were observed using a field-emission scanning electron microscope (SEM, MIRA3, TESCAN) operating at 20 kV. For fabrication of OTEGs, 14 lines of the hot-pressed nanocomposites, with a width of 3 mm and a length of 15 mm, were connected in series by silver electrodes (Fig. S2). The silver electrode lines were dispenser-printed with a Musashi Shotmaster 200DS-s. Electrical characteristics of the fabricated OTEGs were evaluated under dark ambient conditions by varying the load resistance using a custom-built system. The temperature difference between two edges of the nanocomposite lines was controlled to be 10 K using two Peltier plates [10]. The output voltage-output current curves and output power-output current curves were obtained using a Keithley 2450 sourcemeter.
3. Results and discussions For the preparation of nanocomposite films, few-walled CNTs with 2-4 carbon walls were used. The vacuum-filtrated few-walled CNT film possesed a Seebeck coefficient of 51 µV K1
and an electrical conductivity of 3920 S cm-1, which are higher than those of the vacuum-
filtrated single-walled CNT film. This is in line with a previous report that few-walled CNT/P3HT nanocomposite films exhibit excellent thermoelectric properties [10]. The fewwalled CNT/P3HT nanocomposite films, drop-cast on glass substrates and spray-printed on polyimide substrates, exhibited power factors of 231 ± 19 µW m-1K-2 and 325 ± 101 µW m1
K-2, respectively [10]. The hot-pressing temperature was higher than the glass transition
temperatures of regioregular P3HT and PEDOT [22,23]. The thicknesses of the as-prepared CNT/P3HT and CNT/PEDOT:PSS nanocomposite films were 2620 ± 260 and 1220 ± 70 nm, respectively (Fig. 2a). After hot-pressing, the thicknesses of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films decreased to 2450 ± 180 and 1040 ± 150 nm, respectively (Fig. 2a); that is, the average thicknesses of the films decreased 7 % and 15 %, respectively. Zhou et al. reported that PEDOT:PSS grains are weakly connected in the film at room temperature owing to the adsorption of water molecules by the surface HSO3 groups of the grains [24]. At temperatures above 100 °C, the water molecules desorb from the grain surface and hydrogen-bonding interactions between HSO3 groups of the PSS chains are stronger. Consequently, the PEDOT:PSS grains become more densely connected [24]. There might be more room for densification due to the fixation of water molecules in the CNT/PEDOT:PSS nanocomposite films.
Thermoelectric voltage (ΔV) induced by the temperature difference (ΔT) across the nanocomposite films was measured under dark ambient conditions by using a custom built system. The Seebeck coefficient was obtained as the slope of ΔV/ΔT (Fig. S1). All nanocomposite films used in this study exhibited positive values of the Seebeck coefficient, because CNTs, P3HT, and PEDOT:PSS have p-type characteristics. As can be seen in Fig. 2, the Seebeck coefficient, electrical conductivity, and power factor of the as-prepared CNT/P3HT nanocomposite films are 74.8 ± 1.1 µV K-1, 268 ± 11 S cm-1, and 150 ± 9 µW m1
K-2, respectively, which are comparable to the corresponding values of the drop-cast few-
walled CNT/P3HT nanocomposite films [10]. The Seebeck coefficient, electrical conductivity, and power factor of the as-prepared CNT/PEDOT:PSS nanocomposite films are 42.1 ± 1.9 µV K-1, 2090 ± 100 S cm-1, and 371 ± 44 µW m-1K-2, respectively (Fig. 2). The high value of the power factor in the case of the CNT/PEDOT:PSS nanocomposite films originate from the extremely high electrical conductivity of the films. Recently, we reported that the electrical conductivity of single-walled CNT/P3HT nanocomposite films could be greatly enhanced by doping of the P3HT matrix [11]. By doping, the electrical conductivity of the P3HT matrix can be increased from ~10-5 to 61.9 S cm-1. As a result, the electrical conductivity of the single-walled CNT/P3HT nanocomposite films increased from 141 to 2760 S cm-1. In the nanocomposite films, the non-conductive polymer matrix presents a barrier to the inter-CNT bundle hopping because the electrical conductivity of CNTs is much higher than that of the polymer matrix. An increase in the electrical conductivity of the polymer matrix by doping can reduce the resistance between the CNT bundles in nanocomposite films. Therefore, electrical conductivity of a nanocomposite film could be greatly enhanced by using PEDOT:PSS with an electrical conductivity of ~0.1 S cm-1 instead of using P3HT with an electrical conductivity of ~10-5 S cm-1. Since there is a trade-off between the Seebeck coefficient and electrical conductivity of a thermoelectric material, the Seebeck coefficient of
CNT/PEDOT:PSS nanocomposite films is lower than that of CNT/P3HT nanocomposite films. Correspondingly, the power factor of CNT/PEDOT:PSS nanocomposite films is substantially higher than that of CNT/P3HT nanocomposite films. We found that the thermoelectric performance of both the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films can be improved using hot-pressing. As can be seen in Fig. 2, the Seebeck coefficient, electrical conductivity, and power factor of the hot-pressed CNT/P3HT nanocomposite films increase to 84.1 ± 3.8 µV K-1, 307 ± 33 S cm-1, and 217 ± 30 µW m-1K-2, respectively. Similarly, the Seebeck coefficient, electrical conductivity, and power factor of the CNT/PEDOT:PSS nanocomposite films increase to 49.3 ± 1.5 µV K-1, 2350 ± 330 S cm-1, and 572 ± 92 µW m-1K-2, respectively, after the hot-pressing treatment. This treatment increases the Seebeck coefficient and electrical conductivity of both the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased. A conjugated polymer matrix and voids present barriers to the CNT bundle-to-bundle hopping. The electrical properties of the nanocomposite films with 1-dimensional conductive fillers can be affected by the volume fraction and alignment of the fillers [25,26]. Under uniaxial hot-pressing of CNT/polymer nanocomposite films, the volume fraction of a polymer matrix and voids can be reduced by densification. As a result, the volume fraction of CNTs can be increased and the CNTs can be slightly aligned [25]. Besides hot-pressing, hot-rolling can also be used for densification and enhancement of electrical conductivity of nano-carbon/polymer nanocomposite films [27,28]. To demonstrate electric power generation capabilities of the hot-pressed CNT/conjugated polymer nanocomposites, OTEGs composed of only p-type active lines were fabricated (Fig. S2). The 14 lines of the hot-pressed nanocomposites, with a width of 3 mm and a length of 15 mm, on a glass substrate were connected in series by the dispenser-printed silver lines. Fig. 3 shows output voltage-output current curves and output power-output current curves of the
hot-pressed CNT/P3HT and CNT/PEDOT:PSS OTEGs. The temperature difference between two edges of the nanocomposite lines was controlled to be 10 K [10]. Power factor of the hotpressed CNT/P3HT nanocomposites is lower than that of the hot-pressed CNT/PEDOT:PSS nanocomposites. However, the maximum power output of the hot-pressed CNT/P3HT OTEG, 46.0 nW, is higher than that of the hot-pressed CNT/PEDOT:PSS, 36.3 nW. This might be due to the higher open-circuit voltage of the hot-pressed CNT/P3HT nanocomposites. The open-circuit voltage is directly related to the Seebeck coefficient. The Seebeck coefficient of the hot-pressed CNT/P3HT nanocomposites, 84.1 ± 3.8 µV K-1, is higher than that of the hotpressed CNT/PEDOT:PSS nanocomposites, 49.3 ± 1.5 µV K-1. By developing compatible ntype active materials and optimizing the device structure, the performance of the hot-pressed CNT/conjugated polymer OTEGs might be enhanced.
4. Conclusion In summary, we report the results of a hot-pressing treatment of few-walled CNT/conjugated polymer nanocomposite films aimed at the enhancement of the thermoelectric properties of the nanocomposites. It was found that the thermoelectric power factor of the CNT/P3HT and CNT/PEDOT:PSS nanocomposite films increased from 150 ± 9 to 217 ± 30 µW m-1K-2, and from 371 ± 44 to 572 ± 92 µW m-1K-2, respectively, after hot-pressing. Under uniaxial hotpressing of CNT/polymer nanocomposite films, the volume fraction of the CNTs might be increased and the 1-dimensional CNT bundles might be slightly aligned. Our results suggest that the post-treatment of the CNT/conjugated polymer nanocomposite films can significantly affect their thermoelectric properties, and the processing method should be highly considered in order to improve the thermoelectric properties of nanocomposite materials. Using the hotpressed CNT/P3HT and CNT/PEDOT:PSS nanocomposites, OTEGs composed of 14 p-type
active lines were fabricated. The electric power generation of the OTEGs was demonstrated and the OTEGs with the hot-pressed CNT/P3HT and CNT/PEDOT:PSS nanocomposites were found to exhibit maximum output powers of 46.0 nW and 36.3 nW, respectively, under a temperature difference of 10 K.
Acknowledgements This work was supported by a grant from the R&D Convergence Program of the National Research Council of Science & Technology (NST) and a grant from the KRICT Core Project.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/.
References [1] O. Bubnova, Z.U. Khan, A. Malti, S. Braun, M. Fahlman, M. Berggren, X. Crispin, Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4ethylenedioxythiophene), Nat. Mater. 10 (2011) 429-433. [2] G-H. Kim, L. Shao, K. Zhang, K.P. Pipe, Engineered doping of organic semiconductors for enhanced thermoelectric efficiency, Nat. Mater. 12 (2013) 719-723. [3] M. Chabinyc, Thermoelectric polymers: behind organics' thermopower, Nat. Mater. 13 (2014) 119-121.
[4] T. Park, C. Park, B. Kim, H. Shin, E. Kim, Flexible PEDOT electrodes with large thermoelectric power factors to generate electricity by the touch of fingertips, Energy Environ. Sci. 6 (2013) 788-792. [5] C. Cho, B. Stevens, J.-H. Hsu, R. Bureau, D.A. Hagen, O. Regev, C. Yu, J.C. Grunlan, Completely organic multilayer thin film with thermoelectric power factor rivaling inorganic tellurides, Adv. Mater. 27 (2015) 2996-3001. [6] W. Lee, C.T. Hong, O.H. Kwon, Y. Yoo, Y.H. Kang, J.Y. Lee, S.Y. Cho, K.-S. Jang, Enhanced thermoelectric performance of bar-coated SWCNT/P3HT thin films, ACS Appl. Mater. Interfaces 7 (2015) 6550-6556. [7] G.P. Moriarty, S. De, P.J. King, U. Khan, M. Via, J.A. King, J.N. Coleman, J.C. Grunlan, Thermoelectric behavior of organic thin film nanocomposites, J. Polymer. Sci. Part B: Polym. Phys. 51 (2013) 119-123. [8] Q. Yao, Q. Wang, L. Wang, L. Chen, Abnormally enhanced thermoelectric transport properties of SWNT/PANI hybrid films by the strengthened PANI molecular ordering, Energy Environ. Sci. 7 (2014) 3801-3807. [9] H. Wang, S.-I. Yi, X. Pu, C. Yu, Simultaneously improving electrical conductivity and thermopower of polyaniline composites by utilizing carbon nanotubes as high mobility conduits, ACS Appl. Mater. Interfaces 7 (2015) 9589-9597. [10] C.T. Hong, Y.H. Kang, J. Ryu, S.Y. Cho, K.-S. Jang, Spray-printed CNT/P3HT organic thermoelectric films and power generators, J. Mater. Chem. A 3 (2015) 21428-21433. [11] C.T. Hong, W. Lee, Y.H. Kang, Y. Yoo, J. Ryu, S.Y. Cho, K.-S. Jang, Effective doping by spin-coating and enhanced thermoelectric power factors in SWCNT/P3HT hybrid films, J. Mater. Chem. A 3 (2015) 12314-12319.
[12] C. Bounioux, P. Díaz-Chao, M. Campoy-Quiles, M.S. Martín-González, A.R. Goñi, R. Yerushalmi-Rozen, C. Müller, Thermoelectric composites of poly(3-hexylthiophene) and carbon nanotubes with a large power factor, Energy Environ. Sci. 6 (2013) 918-925. [13] C. Yu, K. Choi, L. Yin, J.C. Grunlan, Light-weight flexible carbon nanotube based organic composites with large thermoelectric power factors, ACS Nano 5 (2011) 78857892. [14] B. Endrődi, G.F. Samu, D. Fejes, Z. Németh, E. Horváth, A. Pinsoni, P.K. Matus, K. Hernádi, C. Visy, L. Forró, C. Janáky, Challenges and rewards of the electrosynthesis of macroscopic aligned carbon nanotube array/conducting polymer hybrid assemblies, J. Polymer. Sci. Part B: Polym. Phys. 53 (2015) 1507-1518. [15] Y. Du, S. Z. Shen, W. D. Yang, K. F. Cai, P. S. Casey, Preparation and characterization of multiwalled carbon nanotube/poly(3-hexylthiophene) thermoelectric composite materials, Synth. Met. 162 (2012) 375-380. [16] Y. Du, K.F. CAi, S.Z. Shen, P.S. Casey, Preparation and characterization of graphene nanosheets/poly(3-hexylthiophene) thermoelectric composite materials, Synth. Met., 162 (2012) 2102-2106. [17] C. Meng, C. Liu, S. Fan, A promising approach to enhanced thermoelectric properties using carbon nanotube networks, Adv. Mater. 22 (2010) 535-539. [18] Q. Yao, L. Chen, W. Zhang, S. Liufu, X. Chen, Enhanced thermoelectric performance of single-walled carbon nanotubes/polyaniline hybrid nanocomposites, ACS Nano 4 (2010) 2445-2451. [19] Q. Wang, Q. Yao, J. Chang, L. Chen, Enhanced thermoelectric properties of CNT/PANI composite nanofibers by highly orienting the arrangement of polymer chains, J. Mater. Chem. 22 (2012) 17612-17618.
[20] J. Chen, X. Gui, Z. Wang, Z. Li, R. Xiang, K. Wang, D. Wu, X. Xia, Y. Zhou, Q. Wang, Z. Tang, L. Chen, Superlow thermal conductivity 3D carbon nanotube network for thermoelectric applications, ACS Appl. Mater. Interfaces 4 (2012) 81-86. [21] L.T. Van der Pauw, Method of measuring specific resistivity and hall effect of discs of arbitrary shape. Philips Res. Rep. 13 (1958) 1-9. [22] K. Yazawa, Y. Inoue, T. Yamamoto, N. Asakawa, Twist glass transition in regioregulated poly(3-alkylthiophene), Phys. Rev. B 74 (2006) 094204. [23] C.A. Ng, D.H. Camacho, Polymer electrolyte system based on carrageenan-poly(3,4ethylenedioxythiophene) (PEDOT) composite for dye sensitized solar cell, IOP Conf. Series: Materials Science and Engineering 79 (2015) 012020. [24] J. Zhou, D.H. Anjum, L. Chen, X. Xu, I.A. Ventura, L. Jiang, G. Bubineau, The temperature-dependent microstructure of PEDOT/PSS films: insights from morphological, mechanical and electrical analyses, J. Mater. Chem. C 2 (2014) 9903-9910. [25] F. Du, J.E. Fischer, K.I. Karen, I. Winey, Effect of nanotube alignment on percolation conductivity in carbon nanotube/polymer composites, Phys. Rev. B 72 (2005) 121404. [26] M. Pooreman, M. Traianidis, G. Bister, F. Cambier, Colloidal processing, hot pressing and
characterisation
of
electroconductive
MWCNT-alumina
composites
with
compositions near the percolation threshold, J. Eur. Ceram. Soc. 29 (2009) 669-675. [27] H. Wu, L. T. Drzal, Graphene nanoplatelet paper as a light-weight composite with excellent electrical and thermal conductivity and good gas barrier, Carbon 50 (2012) 11351145. [28] K.-Y. Chun, Y. Oh, J. Rho, J.-H. Ahn, Y.-J. Kim, H.R. Choi, S. Baik, Highly conductive, printable and stretchable composite films of carbon nanotubes and silver, Nat. Mater. 5 (2010) 853-857.
Fig. 1. A schematic diagram of the fabrication process of CNT/conjugated polymer nanocomposite films.
Fig. 2. (a) Thickness, (b) Seebeck coefficient, (c) electrical conductivity, and (d) power factor of the as-prepared and hot-pressed CNT/P3HT and CNT/PEDOT:PSS nanocomposite films.
Fig. 3. Output voltage-output current and output power-output current curves of the hotpressed (a) CNT/P3HT and (b) CNT/PEDOT:PSS OTEGs.