High Performance Polymer Thermoelectric Composite Achieved by

Apr 1, 2019 - Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)/carbon nanotube (PEDOT:PSS/CNTs) composite films have been demonstrated to ...
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Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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High Performance Polymer Thermoelectric Composite Achieved by Carbon-Coated Carbon Nanotubes Network Delong Li,†,‡ Chengzhi Luo,‡ Yuexing Chen,† Dan Feng,† Youning Gong,‡ Chunxu Pan,‡ and Jiaqing He*,† †

Shenzhen Key Laboratory of Thermoelectric Materials and Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China ‡ School of Physics and Technology, Wuhan University, Wuhan 430072, China

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 91.243.191.143 on 04/04/19. For personal use only.

S Supporting Information *

ABSTRACT: Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)/carbon nanotube (PEDOT:PSS/CNTs) composite films have been demonstrated to offer an excellent thermoelectric performance. In this work, p-type thermoelectric films prepared by a simple dipping coating process based on the carbon-coated carbon nanotubes (C-CNTs) continuous network are investigated. The highest power factor obtained in this work is 504.8 μW/mK2 (at 300 K) due to the relatively high electrical conductivity and Seebeck coefficient. The excellent TE properties of the C-CNTs/PEDOT:PSS composite film are attributed to the interfacial carrier scattering introduced by the unique structure. Composite materials based on a continuous network would open up new opportunities to develop high performance organic thermoelectric materials.

KEYWORDS: conducting polymer, carbon nanotubes, thermoelectric, nanocomposites, poly(3,4-ethylenedioxythiophene)

1. INTRODUCTION Thermoelectric (TE) materials, which can harvest electrical energy directly from thermal energy, have attracted tremendous research interest due to their great potential for power generators, sensors, and waste heat recovery systems.1 Conductive polymers have been widely studied as high performance TE materials due to their unique advantages, including light weight, low thermal conductivity, solution processability, and flexibility.2−5 Due to their potential applications in flexible and wearable TE devices, many studies have been done to improve the TE performance of conductive polymers and conductive polymer-based composites.4−6 The efficiency of TE materials is determined by the dimensionless figure of merit ZT, ZT = S2δT/κ (where S, δ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature).7 As the thermal conductivity of typical conductive polymers materials is about 1−3 orders of magnitude lower than the thermal conductivity of inorganic semiconductor TE materials and it is still difficult to accurately measure the thermal conductivity of thin film samples, usually we use the power factor (S2δ) to gauge the TE performance of conductive polymer TE materials.2,8 For typical conductive polymer TE materials, the power factors are in the range 10−4−102 μW/mK2 at room temperature, which is much lower than the range for the power factors for the traditional inorganic semiconductor TE materials.2,9 Among the conductive polymers, PEDOT:PSS is © XXXX American Chemical Society

widely studied in organic solar cells, transistors, and optoelectronic devices due to its aqueous solution processability and commercial availability. It is also the most studied conductive polymer among the organic TE materials because of its potential for very high electrical conductivity (∼over 3000 S/cm) and power factor (about 470 μW/mK2) when doped with suitable dopants.10,11 For comparison, the electrical conductivities of other conductive polymers (such as polyaniline, polypyrrole, and polyphenylenevinylene) are usually lower than 400 S/cm, and the power factors are usually lower than 100 μW/mK2.3,7,11 Generally, doping and fabricating composite blends are the most efficient methods that can be used to improve the TE performance of PEDOT:PSS. In 2011, Bubnova et al. studied the TE performance of doped PEDOT:PSS-Tos and reported a power factor of 324 μW/mK2 (at room temperature) via accurate control of the oxidation level on PEDOT:PSS-Tos.12 Kim et al. reported the highest power factor of 469 μW/mK2 at room temperature for doped PEDOT:PSS by minimizing the total dopant volume in PEDOT:PSS.13 As polymer blending is one of the most efficient methods for enhancing the power factors of polymers, the TE performance of conductive polymer-based TE materials can be enhanced by adding nanostructure filler.14−22 Polymer blends employing high Received: February 17, 2019 Accepted: April 1, 2019 Published: April 1, 2019 A

DOI: 10.1021/acsaem.9b00334 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 1. (a) Raman spectra of CCP0 and CP0, (b) Raman spectra of CCP0 and CCP20 (inset is the enlarged spectra of RBM peak in the range 100−300 cm−1), (c) XPS spectra of CCPx (x = 0, 10, 20, 30), and (d) S 2p spectra of CCP0 and CCP20.

junction resistance and retained the excellent inherent high electrical conductivity of the CNTs. The composite with two heterojunction interfaces with appropriate interfacial energy barriers led to a remarkable enhancement in Seebeck coefficient, and the optimum C-CNTs/PEDOT:PSS films exhibit a great enhanced power factor of 504.8 μW/mK2 at 300 K.

Seebeck coefficient components (usually inorganic TE materials) and high δ components (usually conductive polymer) could be an efficient method for improving the TE performance of these polymer blends. However, previous studies have shown that it is difficult for the TE performances of the as-synthesized composite to exceed those of individual components.6,18 Recently, the energy filtering effect has been widely studied due to the introduction of carrier energy filtering at the interface of the heterojunction which can overcome this limitation.6,16,17 CNTs have been proven to be efficient fillers for the improvement of the TE performance of conductive polymers because of their excellent electronic properties.19−24 However, in previous studies, most of the CNT films were fabricated from the dispersed CNT solution via a filtration process. The electrical conductivities of this kind of CNT film are very low due to the numerous innertube junctions and residual insulating surfactant.25,26 In addition, the Seebeck coefficient of pristine CNTs is relatively small as compared with inorganic semiconductor TE materials, so modification of CNTs should be done to further enhance the Seebeck coefficient of the conductive polymer/CNT composite.27−29 In this paper, C-CNTs/PEDOT:PSS composite films were prepared via a simple process. The power factor of the asprepared sample exhibits a remarkable enhancement due to the increase in Seebeck coefficient. In this kind of composite, carbon was introduced to build a double interface (carbon/ CNTs and carbon/PEDOT:PSS interface) with an appropriate energy barrier across the interface, which can selectively scatter low energy carriers. In addition, the interconnections of continuous networks were directly formed during the synthesis process without using any surfactant and post-treatment process. This kind of interconnection structure decreased the

2. EXPERIMENTAL SECTION 2.1. Synthetic Procedure of C-CNT Films. The carbon-coated carbon nanotube (C-CNT) films were synthesized in a homemade magnetic field assisted floating catalyst chemical vapor deposition (FCCVD) system. Sulfur/ferrocene and methane were chosen as catalyst and carbon source. The C-CNTs were directly synthesized via the FCCVD method at 1373 K. The tube was filled with Ar/H2 gas. For comparison, the carbon nanotubes (CNTs) continuous network film was prepared by calcining the C-CNTs film at 673 K for 5 h in the air. The C-CNT (or CNT) films were transferred to a glass slide or other flexible substrate for further use. The details of the transfer method can be found in our previous study.30 2.2. Preparation of C-CNTs/PEDOT:PSS Films. The C-CNTs/ PEDOT:PSS network film was prepared by fully immersing the CCNT film in PEDOT:PSS (PH1000) dilute solution for a certain time (range from 0 to 30 min, denoted as CCP0, CCP10, CCP20, and CCP30). The PEDOT:PSS dilute solution was obtained by adding 1 mL of Clevios PH1000 to 4 mL of ethanol with gentle stirring for 1 h. 2.3. Characterization. The binding energy of the materials was measured by X-ray photoelectron spectroscopy (XPS). The morphology of the materials was observed by scanning electron microscopy (SEM). Thermoelectric properties of the samples were investigated in a LINSEIS Seebeck and electric resistivity unit (LSR3) at 300 K. Throughout this paper, all properties data were described as averaged values, and error bars represent the variation in measurement between as-prepared samples. B

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Figure 2. SEM morphology of the samples: (a, b) CCP0, (c, d) CP0, and (e, f) CCP20. (Scale bar indicates 500 nm.)

3. RESULTS AND DISCUSSION The C-CNTs (CCP0) continuous network film was synthesized via an FCCVD method. As shown in the TEM images (Figure S1) of the carbon-coated carbon nanotube network, we can confirm that a thin layer of carbon coated the surface of the CNTs. The overall schematic representation of the preparation process for the p-type composite samples including CCPx and CNTs/PEDOT:PSS (denoted as CPy (y means the dipping time, and y = 0 and 20)) is shown in Figure S2. First, the pristine CCP0 film was transferred onto a glass slide. Then, the CCP0 film was immersed into the dilute PEDOT:PSS solution for a certain time. Environmentally friendly ethanol was used as the solvent of PEDOT:PSS to infiltrate well into the hydrophobic CCP0 network. The CPy composite film was prepared with the same method mentioned above, and the pure CNTs film was prepared via a calcine process. Raman spectra of the CP0, CCP0, and CCP20 film are shown in Figure 1a,b. The peak located at 1584.6 cm−1 corresponds to the G band of the CNTs. Compared with the CNTs, a relatively strong peak located at 1329.4 cm−1 (D band) for the Raman spectrum of CCP0 was observed which was ascribed to amorphous carbon coated on the surface of CNTs, as shown in Figure 1a. The intensity ratio of the D band and G band (ID/IG) for the CCP0 film is 0.06, which indicated a low content of defects.30,31 The normalized Raman spectra of CCP0 and CCP20 are shown in Figure 1b for comparison, and the intensity of the D band and the G band was almost unchanged for samples coated by PEDOT:PSS. These results demonstrate that there are no structural defects

introduced during the dipping coating process. The radial breathing mode (RBM) band (in the range 100−300 cm−1) represents the bond-stretching out of plane phonon mode for which all the carbon atoms move coherently in the radial direction.32,33 As shown in Figure 1b and the corresponding inset, the intensity of the RBM band for CCP20 decreased compared with that of CCP0. The intensity reduction of the RBM band for the CCPx composite indicates the presence of PEDOT:PSS, because the oscillation of the carbon atom in the radial direction was hindered as the PEDOT:PSS attached on the C-CNT surfaces.32,33 As shown in Figure S3, the same results can be concluded from the Raman spectra of CPy. The intensity of the G band remains virtually unchanged. The RBM band also exhibits an intensity reduction in the CP20 composite compared with CP0. To further verify the existence of PEDOT:PSS in the composite, the XPS of the as-prepared samples was measured. Figure 1c shows that the pristine CCP0 films have predominant C 1s and O 1s peaks. Also, the CCPx (x = 10, 20, 30) composite films have C 1s, O 1s, and S 2p peaks. The atomic ratios of carbon, oxygen, and sulfur calculated on the basis of XPS spectra are shown in Table S1. As the chemical environments of sulfur atoms in PEDOT and PSS are totally different, the binding energy peak should be clearly distinguishable. As shown in Figure 1d, the S 2p spectrum of CCP20 is fitted considering four peaks by using Gaussian fitting which originated from the PEDOT chain and the PSS chain. The peaks located in the range 163−166 eV originated from the spin-split components of the sulfur atoms in the PEDOT molecular chain.34 The other peaks located in the range 166− 172 eV originated from the sulfur in the PSS molecular chain. C

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Figure 3. (a) Electrical conductivity and Seebeck coefficient, and (b) power factor of CCPx as a function of dipping time. (c) Electrical conductivity and Seebeck coefficient, and (d) power factor of the PEDOT:PSS, CP0, CCP0, CP20, and CCP20.

of the continuous network film much more than that of the filtration-based CNT film. As the PEDOT:PSS coated the surface of C-CNTs, the film cannot be dispersed again even with a long ultrasonication time in polar solvent. As shown in Figure 2e,f, the SEM morphology of CCP20 after a long ultrasonication time is almost the same with the morphology shown in Figure S5. This result confirms that the PEDOT:PSS absorbed tightly on the surface of C-CNT bundles due to the strong π−π interaction that existed between the C-CNTs and PEDOT:PSS molecules.35,36 The TE performances of the samples were investigated, and the results were shown in Figure 3. Due to the interconnected continuous network structure, the pristine CCP0 films exhibit relatively high electrical conductivity (∼912.6 S/cm). The Seebeck coefficient of the CCP0 film is 65.6 μV/K, and the obtained power factor of the CCP0 film is as high as 394.1 μW/mK2. As the carbon was burned out during the calcine process, the electrical conductivity of the CP0 films increased to about 2690.8 S/cm, but the Seebeck coefficient decreased to 35.8 μV/K. The positive Seebeck coefficient of the CCP0 and CP0 films resulted from oxygen doping in air, which is suggestive of holelike carriers.32,37 The TE properties of the CCPx film were investigated, and the results were presented as a function of dipping time. The properties critically changed with the dipping time, as illustrated in Figure 3a,b. As the dipping time increased from 0 to 30 min, the electrical conductivity of the CCPx film decreased monotonically from 912.6 to 547.2 S/cm, but the Seebeck coefficient first increased and then decreased. As shown in Figure 3, the highest Seebeck coefficient (∼82.9 μV/ K) was obtained for the CCP20 film, and the obtained highest power factor is 504.8 μW/mK2. As the dipping time increased to 30 min, the Seebeck coefficient, electrical conductivity, and power factor of the CCPx film decreased simultaneously due to more and more

The two fitting peaks near 168 eV correspond to sulfur atoms in neutral and ionic PSS molecular chains.34 The strong S 2p signals from the sulfonate group in the PSS molecular chain (166−172 eV) and the thiophene unit in the PEDOT molecular chain (162−166 eV) verify the existence of PEDOT:PSS in the CCPx composite. Nearly identical results for the CP0 films and the CP20 films were also observed (shown in Figure S4a). The comparison of the S 2p spectra of CCPx (x = 0, 10, 20, 30) is shown in Figure S4b. The SEM morphologies of CCP0, CCP20, CP0, and CP20 films on the glass slide substrate are shown in Figure S5. The pristine CCP0 network (Figure S5a) and CP0 network (Figure S5c) exhibit excellent continuity and relatively flat surface morphology. For the PEDOT:PSS coated C-CNT and CNT samples, Figure S5b,d clearly shows that PEDOT:PSS coated the surface of C-CNTs and CNTs. In addition, the bundles of PEDOT:PSS coated C-CNTs and CNTs exhibit a relatively thicker diameter than those of the pristine C-CNT and CNT films. As shown in Figure S6, more PEDOT:PSS molecularly absorbed on the surface of C-CNTs bundles with an increase in dipping time (from 0 to 30 min). To further investigate the morphology characteristics of the as-prepared samples, the films were peeled off from the glass slide and then dispersed in N-methyl pyrrolidone (NMP) with ultrasonic dispersion. As shown in Figure 2a,b, the C-CNTs show a rough surface with some particles coated evenly on the surface of CNTs. As the C-CNTs calcined in the air, the pure CNT networks (Figure 2c,d) show a smooth surface. Above all, the interconnected C-CNT continuous network was clearly observed. The red arrows in Figure 2 point out the X- or Ytype interbundle junction of the C-CNT and CNT continuous network. Compared with the CNT film prepared via a solution-based filtration process, which usually introduced defects and surfactant, this kind of interconnected network reduced the junction resistance and the electrical conductivity D

DOI: 10.1021/acsaem.9b00334 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials PEDOT:PSS being absorbed on the surface of the C-CNT bundles. For the CCP60 film, the TE performance exhibited an obvious decrease, as shown in Table S2. It can be concluded that the variety of electrical conductivity and Seebeck coefficient values are strongly related to the mass content of PEDOT:PSS absorbed on the CCPx film. It is difficult to measure the mass content of PEDOT:PSS in the CCPx composite accurately as the weight of the components cannot be accurately weighed directly with electronic balance, so we estimated them on the basis of the XPS data. As shown in Table S1, it can be seen that only a small amount of PEDOT:PSS absorbed on the surface of C-CNT networks in the initial stage of the self-assembly process. The surrounding PEDOT:PSS coated layer introduced a strong energy filtering effect by scattering low energy carriers. However, with the increase in dipping time, more and more PEDOT:PSS absorbed on the surface of C-CNTs. As shown in Figure S7, when the dipping time increased to 60 min (denoted as CCP60), a thin layer of PEDOT:PSS film coated on the surface. As the PEDOT:PSS film formed on the surface of the C-CNT film with an increase in dipping time, the structure of the CCPx composite film changed from a coated structure to a layer-by-layer structure. The properties of the composite are controlled by the traditional series- and parallel-connected models rather than by the energy filtering effect. The increase in thickness of the PEDOT:PSS coating layer attenuated the size dependent energy filtering effect, which resulted in a decrease of the Seebeck coefficient.16,35,38,39 Figure 3c,d shows the comparison of the TE performance of the PEDOT:PSS, CP0, CCP0, CP20, and CCP20 samples. The power factor (as high as 504.8 μW/mK2) obtained in the CCP20 film is much higher than that of the binary composite film (CCP0 and CP20) and those of the single component films (PEDOT:PSS and CP0). The values of electrical conductivity, Seebeck coefficient, and the power factor of the as-prepared samples can be found in Table S2. In order to further understand the nature of the enhanced TE properties of the CCPx film, the energy filtering mode was proposed to illustrate the mechanism. The energy diagrams of the carbon, CNTs, and PEDOT:PSS are shown in Figure 4a. Also, Figure 4b shows the energy filtering at the two junctions of carbon/CNTs and carbon/PEDOT:PSS. The value of the work function of carbon, CNTs, and PEDOT:PSS was obtained from previous reports in the literature.40,41 It is worth noting that the analysis is predicated on the basis of the assumption of the work function values being the same as the values from the literature. However, it is difficult for us to directly test the actual work function of this kind of material, so the values from the literature are used for the analysis. As shown in Figure 4b, the equilibrium band diagram for the CCPx (x = 10, 20, and 30) films indicates that the energy barriers of the carbon/CNTs and carbon/PEDOT:PSS junctions are 0.25 and 0.09 eV, respectively. The carrier mobility and concentration in CP0, CCP0, CP20, and CCP20 were measured via Hall measurements to clarify the energy filtering effect, as shown in Table S3. The CNT networks had high electrical conductivity based on a high carrier concentration. Because of the lower carrier concentration, the binary composite film (CP20 and CCP) and ternary composite film (CCP20) showed a higher Seebeck coefficient and a lower electrical conductivity. It has been proven that the low energy charge carriers are more strongly scattered than the high energy charge carriers at the interface, leading to an

Figure 4. (a) Energy diagram of the CNTs/carbon/PEDOT:PSS, (b) energy filtering at CNTs/carbon junction, and (c) energy filtering at the carbon/PEDOT:PSS junction (red ball, high energy carriers; blue ball, low energy carriers).

enhanced power factor.42 The introduction of energy barriers in the ternary and binary composite film can selectively scatter low energy charge carriers while allowing high energy charge carriers to cross over the interface, which results in asymmetric carrier distribution. In addition, due to the different work functions of the carbon, CNTs, and PEDOT:PSS, the band bending at the interface may partially eliminate the interfacial barrier to facilitate the charge carrier transfer.42,43 The binary composite film (CP20) without the carbon layer had a higher energy barrier of 0.34 eV but a lower Seebeck coefficient compared with those of CCPx (x = 0, 10, 20, 30), as shown in Figure 3. Therefore, the energy barriers of 0.09 and 0.25 eV were more efficient for carrier filtering. Owing to the energy filtering effect, the Seebeck coefficient of the CCPx composite film exhibited an obvious enhancement, thus leading to a great improvement in the TE performance. We plotted the Seebeck coefficient and power factor as a function of electrical conductivity in this work and literature values for a composite made of CNTs and commercially available PEDOT,21,35,44−57 as shown in Figure 5. The relevant data are listed in Table S2. As shown in Figure 5, the electrical conductivity and Seebeck coefficient of the composite seem unable to improve simultaneously. In this work, the CCPx can produce a decent Seebeck coefficient and a high electrical conductivity, and an extraordinary power factor of 504.8 μW/mK2 at 300 K was achieved. The power factor obtained in this work is among the highest values for the PEDOT/CNT composite based on a commercially available PEDOT:PSS solution.

4. CONCLUSIONS In this work, continuous network films composed of carbon, CNTs, and PEDOT:PSS were successfully prepared via combining the FECVD with a “dip coating” process. The PEDOT:PSS homogeneously coated the surface of C-CNTs. A maximum power factor of 504.8 μW/mK2 at 300 K for the optimum C-CNTs/PEDOT:PSS films was obtained due to the obvious improvement in Seebeck coefficient. The introduction of a double interface (carbon/CNTs and carbon/PEDOT:PSS E

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Figure 5. Comparison of (a) Seebeck coefficient vs electrical conductivity and (b) power factor vs electrical conductivity for the p-type CNT/ PEDOT composite in the literature and in this work. (2) He, M.; Qiu, F.; Lin, Z. Q. Towards High Performance Polymerbased Thermoelectric Materials. Energy Environ. Sci. 2013, 6, 1352− 1361. (3) Yao, C. J.; Zhang, H. L.; Zhang, Q. C. Recent Progress in Thermoelectric Materials Based on Conjugated Polymers. Polymers 2019, 11, 107. (4) Wu, J.; Sun, Y.; Pei, W.-B.; Huang, L.; Xu, W.; Zhang, Q. Polypyrrole Nanotube Film for Flexible Thermoelectric Application. Synth. Met. 2014, 196, 173−177. (5) Wu, J.; Sun, Y.; Xu, W.; Zhang, Q. Investigating Thermoelectric Properties of doped polyaniline nanowires. Synth. Met. 2014, 189, 177−182. (6) Choi, J.; Lee, J. Y.; Lee, S. S.; Park, C. R.; Kim, H. J. A. E. M. High-Performance Thermoelectric Paper Based on Double CarrierFiltering Processes at Nanowire Heterojunctions. Adv. Energy Mater. 2016, 6, 1502181. (7) Khan, Z. U.; Edberg, J.; Hamedi, M. M.; Gabrielsson, R.; Granberg, H.; Wagberg, L.; Engquist, I.; Berggren, M.; Crispin, X. Thermoelectric Polymers and Their Elastic Aerogels. Adv. Mater. 2016, 28, 4556−4562. (8) Chen, Y. N.; Zhao, Y.; Liang, Z. Q. Solution Processed Organic Thermoelectrics: Towards Flexible Thermoelectric Modules. Energy Environ. Sci. 2015, 8, 401−422. (9) Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. A. Organic Thermoelectric Materials for Energy Harvesting and Temperature Control. Nat. Rev. Mater. 2016, 1, 16050. (10) Mengistie, D. A.; Chen, C.-H.; Boopathi, K. M.; Pranoto, F. W.; Li, L.-J.; Chu, C.-W. Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants. ACS Appl. Mater. Interfaces 2015, 7, 94−100. (11) McGrail, B. T.; Sehirlioglu, A.; Pentzer, E. Polymer Composites for Thermoelectric Applications. Angew. Chem., Int. Ed. 2015, 54, 1710−1723. (12) Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-ethylenedioxythiophene). Nat. Mater. 2011, 10, 429−433. (13) Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Doping of Organic Semiconductors for Enhanced Thermoelectric Efficiency. Nat. Mater. 2013, 12, 719−23. (14) Zhang, P.; Gao, C.; Xu, B.; Qi, L.; Jiang, C.; Gao, M.; Xue, D. Structural Phase Transition Effect on Resistive Switching Behavior of MoS2-Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices. Small 2016, 12, 2077−2084. (15) Culebras, M.; Igual-Munoz, A. M.; Rodriguez-Fernandez, C.; Gomez-Gomez, M. I.; Gomez, C.; Cantarero, A. Manufacturing Te/ PEDOT Films for Thermoelectric Applications. ACS Appl. Mater. Interfaces 2017, 9, 20826−20832.

interface) can scatter low energy charge carriers because of the energy barrier that existed at the interface, thus leading to enhanced TE performance due to the energy filtering effect. Also, this work offers an efficient and practical strategy to prepare high performance PEDOT:PSS-based TE materials. The C-CNTs/PEDOT:PSS polymer composites exhibit enormous application potential for wearable thermoelectric generators and thermoelectric sensors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00334. Additional characterization results from XPS, TEM, SEM, and Hall measurements of the as-prepared samples, schematic of the synthesis procedure, and summary of TE performance in this work and literature studies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Youning Gong: 0000-0002-8114-8095 Chunxu Pan: 0000-0001-9007-8562 Jiaqing He: 0000-0003-3954-6003 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Guangdong Province (2015A030308001); the Leading Talents of Guangdong Province Program (00201517); the Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20150831142508365, KQTD2016022619565991 and ZDSYS 20141118160434515); the National Natural Science Foundation of China (Grant No. 51632005); and the China Postdoctoral Science Foundation (2017M612492).

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REFERENCES

(1) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105−114. F

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Letter

ACS Applied Energy Materials

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DOI: 10.1021/acsaem.9b00334 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials (52) Song, H.; Qiu, Y.; Wang, Y.; Cai, K.; Li, D.; Deng, Y.; He, J. Polymer/carbon Nanotube Composite Materials for Flexible Thermoelectric Power Generator. Compos. Sci. Technol. 2017, 153, 71−83. (53) Hu, X.; Chen, G.; Wang, X. An Unusual Coral-like Morphology for Composites of Poly(3,4-ethylenedioxythiophene)/carbon Nanotube and the Enhanced thermoelectric performance. Compos. Sci. Technol. 2017, 144, 43−50. (54) Fan, W.; Guo, C.-Y.; Chen, G. Flexible Films of Poly(3,4ethylenedioxythiophene)/carbon Nanotube Thermoelectric Composites Prepared by dDnamic 3-phase Tnterfacial Electropolymerization and Subsequent Physical Mixing. J. Mater. Chem. A 2018, 6, 12275− 12280. (55) Zhang, L.; Harima, Y.; Imae, I. Highly Improved Thermoelectric Performances of PEDOT:PSS/SWCNT Composites by Solvent Treatment. Org. Electron. 2017, 51, 304−307. (56) Choi, K.; Kim, S. L.; Yi, S.-i.; Hsu, J.-H.; Yu, C. Promoting Dual Electronic and Ionic Transport in PEDOT by Embedding Carbon Nanotubes for Large Thermoelectric Responses. ACS Appl. Mater. Interfaces 2018, 10, 23891−23899. (57) Wang, J.; Cai, K. F.; Yin, J. L.; Shen, S. Thermoelectric Properties of the PEDOT/SWCNT Composite Films Prepared by a Vapor Phase Polymerization. Synth. Met. 2017, 224, 27−32.

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DOI: 10.1021/acsaem.9b00334 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX