Improved interaction between semiconducting polymer and carbon nanotubes in thermoelectric composites through covalent grafting

Accepted Manuscript Improved interaction between semiconducting polymer and carbon nanotubes in thermoelectric composites through covalent grafting Cheng Jin An, Young Cheul Lee, Young Hun Kang, Song Yun Cho PII:

S0008-6223(17)30899-0

DOI:

10.1016/j.carbon.2017.09.022

Reference:

CARBON 12352

To appear in:

Carbon

Received Date: 28 March 2017 Revised Date:

29 June 2017

Accepted Date: 8 September 2017

Please cite this article as: C.J. An, Y.C. Lee, Y.H. Kang, S.Y. Cho, Improved interaction between semiconducting polymer and carbon nanotubes in thermoelectric composites through covalent grafting, Carbon (2017), doi: 10.1016/j.carbon.2017.09.022. 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.

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GRAPHICAL ABSTRACT

ACCEPTED MANUSCRIPT Improved interaction between semiconducting polymer and carbon nanotubes in thermoelectric composites through covalent grafting

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Cheng Jin An, Young Cheul Lee, Young Hun Kang, Song Yun Cho*

Division of Advanced Materials, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea.

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*Corresponding author. E-mail address: [email protected] (S. Y. Cho)

ABSTRACT

We developed an unconventional route to produce uniform and intimately interfaced nanocomposite films for their potential application in thermoelectric (TE) devices. Here, amino-terminated poly(3-hexylthiophene-2,5-diyl) (P3HT) was chemically adhered onto the surface of a functionalized double-walled carbon nanotube (DWCNT), via an

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amidation reaction, to form P3HT-grafted DWCNT (P3HT-g-DWCNT). As the P3HT chains are intimately and permanently tethered on the DWCNT surface, a well-defined P3HT/DWCNT interface prevents the DWCNT from aggregating by promoting the

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solubility of P3HT-g-DWCNT nanocomposites in organic solvent. Such characteristic changes in DWCNT by polymer grafting can improve the thermoelectric properties of DWCNT films. The covalently grafted P3HT-g-DWCNT film exhibits a significantly

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enhanced Seebeck coefficient of 116. 6 µV K-1, as compared to a physically mixed P3HT/DWCNT composite (69.2 µV K-1), in conjunction with a high power output in TE modules. This facile approach would open a new way to synthesize intimately connected nanocomposites made up of conductive polymers and carbon materials for high performance thermoelectrics.

Keywords: P3HT-grafted DWCNT; thermoelectric property; solubility in organic solvent; printing process; flexible thermoelectric module 1

ACCEPTED MANUSCRIPT 1. Introduction Remarkable progress of high-performance thermoelectric (TE) materials has been achieved in the past several decades. TE materials directly convert temperature difference into an electric voltage for power generation, and can also be used for refrigeration [1-2]. The

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fundamental challenge in developing TE materials showing superior performance lies in delicately adjusting the interrelated TE parameters, such as the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ [3]. In recent years, intensive research efforts have been devoted to improve the efficiency of TE materials through different

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approaches, including reducing the lattice thermal conductivity by suppressing phonon scattering [4-5] and enhancing the TE power factor (S2σ) by quantum confinement [6-9], energy filtering [10-11], and tuning the electronic band structure (i.e. the density of states) of

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the material [12]. A high TE power factor has also been achieved through the formation of nanocomposites with conducting polymers [13-16]. In particular, nanocomposites based on conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI), or poly(3-hexylthiophene) (P3HT) and low-dimensional carbon materials (e.g. carbon nanotube (CNT), and graphene) have received considerable attention as hybrid

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polymer-carbon nanocomposites due to the various advantages from the combined high S and low κ values of the conducting polymer, with the high σ of the carbon material. In 2009, Meng et al. reported that multi-walled carbon nanotube (MWCNT) sheet/PANI nanocomposites prepared by a two-step method showed simultaneously enhanced electrical

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conductivities and Seebeck coefficients [17]. These enhancements were attributed to the sizedependent energy-filtering effect at the nano-interfaces between the PANI coating layer and

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the CNT. In 2011, Yu et al. [18] reported a notable increase in the electrical conductivity of CNT/polymer composites, without a decrease in the Seebeck coefficient, by the addition of CNT. In 2014, Yao et al [19] demonstrated SWCNT/PANI hybrid films with power factor of 176 µW m-1 K-2, by forming highly ordered PANI interface layer on the single-walled carbon nanotube surface. In 2015, Cho et al. [20] prepared PANI/graphene/PANI/DWNT nanocomposites by using layer by layer deposition and this ordered molecular structure enabled the maximum power factor to reach 1825 µW m−1 K−2. Although such nanocomposites have been demonstrated to be an effective approach to improve the performance of a material by synergistically combining the advantages of each component, some critical issues regarding the nanocomposite formation for TE materials still 2

ACCEPTED MANUSCRIPT need to be resolved. In conventional methods, where the carbon materials are physically mixed

with

conjugated

polymers

to

produce

hybrid

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polymer

nanocomposites, the agglomeration of CNT is likely to take place in CNT/conducting polymer composite films. Even materials with a low loading of CNT lead to unexpected interfaces between the two components, which prevent the development of advantageous

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features from the combination of CNT with conjugated polymers. Moreover, it remains challenging to form a good interfacial connection between carbon materials and conducting polymers, because the intimate interfacial contact between these two constituents could facilitate carrier transport without the scattering of carriers (electrons or holes) at the

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interface.

In this study, P3HT was covalently grafted on to the surface of double-walled carbon

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nanotube (DWCNT), by the amidation reaction between the acyl moieties on the surface of the DWCNT, and the amine groups in the amino-terminated P3HT. Such surface modification increases the dispersibility of DWCNT in organic solvents, thus leading to much smaller bundle sizes by avoiding the formation of big agglomerates, as compared to directly mixed P3HT/DWCNT composites. Furthermore, the well-dispersed combination at the nanometer level can maximize the integration effect, and the TE performance, by fully

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taking advantage of the benefits from each component, such as the high Seebeck coefficient and low thermal conductivity. The increased figure of merit (ZT) with high power output of the TE modules is not only affected by the enhanced dispersity of the material, but also by the

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2. Experimental

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intimate interactions between the DWCNT and P3HT.

2.1 Materials

The

pristine

DWCNT

was

purchased

from

XinNano

Materials

(www.xinnanomaterials.com). DWCNT was adopted due to its relatively low price and high electrical conductivity. The pristine DWCNTs are in the form of powder with 3 nm of outer diameter and over 10 µm of length. 2,5-Dibromo-3-hexylthiophene, t-BuMgCl, ni(dppp)cl2 and other solvents were purchased from Aldrich and used without any further purification.

2.2 Synthesis of amino-terminated P3HT 3

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Amino-terminated P3HT (P3HT-NH2) was polymerized through the Grignard metathesis reaction. Following the typical reaction, 2,5-dibromo-3-hexylthiophene (1.68 g, 5.2 mmol) and anhydrous THF (10 mL) were mixed in a dry Schlenk flask under the protection of N2. The reaction mixture was cooled to 0 °C by using an ice/water bath, and then t-BuMgCl (1M

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solution in THF, 5.9 mL, 5.2 mmol) was added into the flask. After the reaction was allowed to take place at room temperature for 3.5 h, Ni(dppp)Cl2 (70 mg, dissolved in 40 mL of anhydrous THF) was added into the flask. After the polymerization reaction was allowed to take place for 6 h, 3-(bis[trimethylsilyl]amino)phenylmagnesium chloride (1.0 M solution in

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THF, 6 mL) was added into the reaction mixture, and stirred for 10 min. The reaction was terminated by adding a HCl solution (12 mL of a 5 N aqueous solution). Finally, the polymer

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was precipitated in a methanol/DI water mixture (7/3, 1000 mL). In this process, the alcoholysis reaction of the bis(trimethylsilyl)amino groups led to the formation of amine groups. The precipitate was obtained by centrifugation and drying in a vacuum oven. The polymer was further purified by a Soxhlet extraction with chloroform. The solution was concentrated and precipitated in methanol again.

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2.3 Synthesis of P3HT-g-DWCNT

To graft P3HT onto the surface of the DWCNT, they were first oxidized using nitric acid to produce carboxyl groups. In a typical reaction, pristine DWCNT (150 mg) were first

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dispersed in a diluted HNO3 solution (150 mL) by sonication for 2 h, followed by refluxing at 120 °C for 48 h. The reaction mixture was then additionally sonicated for 1 h, and refluxed

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for 48 h. To obtain the oxidized DWCNT the solution was filtered under vacuum, and purified by redispersing in a 1 M HCl solution (30 mL), washing with deionized water, and drying in a vacuum oven at 60 °C overnight. For an acylation reaction of the oxidized DWCNT, the oxidized DWCNT (30 mg) were dispersed in DMF (10 mL) under sonication for 3 h. Then, SOCl2 (30 mL) was added into the reaction mixture, followed by refluxing for 24 h. The acyl chloride activated DWCNT (DWCNT-COCl) were obtained by centrifuging, and washed by cycles of dispersing the DWCNT-COCl in anhydrous THF and centrifuging. For the amidation reaction on the surface of the DWCNT, the DWCNT-COCl suspension in anhydrous THF (30 mL) was added into a P3HT-NH2 in anhydrous THF (P3HT-NH2 200 mg, THF 50 mL). To the reaction mixture, triethylamine (3 mL) was slowly added at 0 °C and the 4

ACCEPTED MANUSCRIPT reaction was allowed to carry out at room temperature for 9 h, and at 50 °C for 48 h. Finally, the P3HT-g-DWCNT was obtained by centrifuging, and purified by removing any ungrafted P3HT by repeated centrifugation.

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2.4 Preparation of the P3HT-g-DWCNT and P3HT/DWCNT film

The suspension was prepared by dispersing P3HT-g-DWCNT (30 mg) in chloroform (30 mL, 10 mg mL−1), with the aid of a sonication bath, for 3 h. Afterwards, the nanocomposite film was retained with a PMMA membrane by using the vacuum filtration method. The

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P3HT-g-DWCNT composite film on the PMMA membrane was dried in an oven at 60 °C overnight, and detached spontaneously from the supporting substrate. The thickness of

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resulting P3HT-g-DWCNT film was measured to be 30 µm with a variation of 5 µm and weight fraction of DWCNT in P3HT-g-DWWNT film was 89 wt%. For the physically mixed suspension, the P3HT (3.3 mg) and DWCNT (26.7 mg) was dispersed simultaneously in chloroform (30 mL), followed with bath sonication for 3 h. The film with thickness of

2.5 Characterization

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approximately 40 µm was fabricated using identical process of vacuum filtration method.

The surface morphology was characterized by scanning electron microscopy (SEM) (SigmaHD, Carl Zeiss). FT-IR spectra were recorded on an Excalibur 3100 spectrometer

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(Varian, Inc.) with a resolution of 2 cm-1 using KBr pellets. Raman images were obtained by Raman spectroscopy (excitation at 514 nm using a high-resolution dispersive Raman

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microscope, LabRAM HR UV/vis/NIR, Horiba). Thermalgravimetric analyses (TGA) were performed with a Q50 TGA (TA instruments) at a scanning rate of 5 °C min-1 under a N2 atmosphere. Transmission electron microscopy (TEM) observations were performed on a JEM-2100 (JEOL) operated at 200 kV. The TEM samples were prepared by dropping the suspension of pristine DWCNT, or P3HT-g-MWNT, in chloroform on 400 mesh Cu grids with a supporting carbon film.

2.6 Experimental setup for Seebeck coefficient measurements

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temperature difference across the sample was generated by applying an electrical current through the two Peltier plates (Fig. S1(a)). A Keithley 2460 sourcemeter was used to control the temperature of the Peltier plate (Fig. S1(b)). A pair of thermocouples was pointed onto the surface of the sample to detect the local temperature difference. The temperature gradient

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between the electrodes was varied gradually from 2 to 10 °C. Meanwhile, a pair of probes was pointed towards the previously deposited Au electrodes to measure the thermoelectric

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voltage, thus ensuring that the measured voltage corresponded exactly to the temperature difference. By varying the temperature difference through the control of the current in the Peltier devices, the data of the thermoelectric voltage (V) versus the temperature difference (∆T) was automatically recorded by the Keithley 2182A nanovoltmeter (Fig. S1). The Seebeck coefficient was then obtained by analyzing the linear regression slope of the ∆V-∆T

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curve.

2.7 Measurements of the electrical and thermal conductivity

The electrical conductivity was measured by the standard van der Pauw direct current four-

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probe method [21]. All measurements were conducted at room temperature using a Keithley 195A digital multimeter, and a Keithley 220 programmable current source. The thickness of

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the films was determined by an alpha-step surface profiler (α-step DC 50, KLA Tencor). The thermal conductivity, κ, was calculated using the equation κ = αρCp, where α is the thermal diffusivity, ρ is the density calculated by measuring the weight and volume of the films, and Cp is the specific heat capacity at constant pressure. The in-plane thermal diffusivity, α, of the free-standing films was measured using a laser flash analysis at 25 °C. The Cp was measured using a modulated differential scanning calorimeter (MDSC Q200, TA Instruments). The experimental results of the thermal conductivity measurements are summarized in Table S1.

2.8 Fabrication of the thermoelectric generator

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terephthalate (PET) substrate.

2.9 Power-generation characteristics

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The power generation characteristics of the TE module were measured using a custom system, which has been reported previously [22-24]. Simultaneously, while one side of the

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TE module was heated by applying an electrical current through the hot Peltier plate, the other side was cooled with a cooling fan at a constant temperature of 25 °C. We set the temperature difference ∆T to ≈20 °C in an in-plane direction between the two sides of the module. The temperature difference ∆T of the surface of the device was measured by sensitive thermocouples. This system measured the voltage–current, and power output–

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3. Results and discussion

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current curves, by varying the load resistance from 0 to 600 KΩ.

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Fig. 1. Reaction route for the synthesis of (a) P3HT-NH2 and (b) P3HT-g-DWCNT.

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A P3HT-NH2 was synthesized by the Grignard metathesis polymerization, which is a general method for the preparation of polythiophenes with high regularity and a narrow molecular weight distribution (Fig. 1(a)). Then the P3HT-NH2 was grafted onto the surface of functional DWCNT through the amidation reaction (Fig. 1(b)).

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The P3HT-g-DWCNT and intermediate grafting products were characterized using FT-IR spectroscopy (Fig. 2(a)). Compared to the FT-IR spectrum of the pristine DWCNT, the

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oxidized DWCNT showed enhanced peaks at 1730 and 1440 cm-1, corresponding to the vibrations of C=O and C–O–H in the carboxylic acid groups, respectively. Upon amidation, The C=O absorption of amides occurs at lower frequencies than acidic absorption due to the resonance effect. The C=O in acids absorbs near 1730 cm-1, while the C=O absorption from amide in the range of 1610-1660 cm-1. Moreover, strong aliphatic C-H stretching vibration and aromatic C-H bending vibration peaks were found at 2954, 2924, 2854, and 841 cm-1, which correspond to the C–H bonds of the hexyl groups of the grafted P3HT, thus verifying the incorporation of P3HT chains on the surface of DWCNT. To further characterize the interface interactions between P3HT and DWCNT in P3HT-gDWCNT, Raman spectroscopy was investigated at the 514 nm excitation wavelength (Fig. 8

ACCEPTED MANUSCRIPT 2(b)). The high quality of the pristine DWCNT was confirmed by the high intensity ratio of the G and D bands. Upon oxidation, the intensity of the D band increased, and the G band was shifted to a higher frequency (1570 cm-1) due to the introduction of oxygen-containing groups on the surface of the DWCNT. The amino-terminated P3HT exhibited two Raman peaks at 1449 and 1379 cm-1, corresponding to C–C skeletal stretching vibrations and C=C

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skeletal stretching vibrations, respectively [25,26]. As expected, the Raman spectrum of P3HT-g-DWCNT included peaks from both the DWCNTs and P3HT. In addition, the G band of the DWCNT in the P3HT-g-DWCNT was up-shifted to 1573 cm-1, in comparison to the pristine and oxidized DWCNT (1567 and 1570 cm-1). This Raman shift reflects the charge

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transfer from the DWCNT to the P3HT in P3HT-g-DWCNT [26].

The thermal stability and compositional analysis of the P3HT-g-DWCNT were determined

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by TGA (Fig. S2). The pristine DWCNT appeared to be thermally stable up to 500 °C, and a rapid mass loss was detected above this temperature. The oxidized DWCNT showed a slight mass loss before 500 °C, which might be due to the thermal degradation of the functional groups, and small carbonaceous fragments that formed during the oxidation [27]. P3HT-NH2 shows a remarkable mass loss between 370 and 470 °C. The mass loss of P3HT-g-DWCNT

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encompasses features of the oxidized DWCNT and P3HT. For example, the gradual mass loss

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Fig. 2. (a) FT-IR spectra, (b) Raman spectra for the pristine DWCNT, oxidized DWCNT, P3HT-NH2, and P3HT-g-DWCNT. (c) Fluorescence spectra of the P3HT and P3HT-g-

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DWCNT solutions in chloroform, with the enlarged spectra shown in the inset. (d) Energyband diagrams of P3HT and DWCNT, showing the band difference between the LUMO of

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P3HT and the Fermi level of the DWCNT.

before 370 °C was mainly attributed to oxidized DWCNT, and the major mass loss between 370 and 470 °C was attributed to P3HT, because the mass loss of oxidized DWCNT is relatively slow within this range. Thus, the weight content of P3HT in P3HT-g-DWWNT was estimated to be 11%, which is a value similar to that reported by a previous work [28]. The reduced interfacial contact of P3HT with DWCNT in the nanocomposite was verified by fluorescence spectroscopy (Fig. 2(c)).

A solution of P3HT (0.01 mg mL–1 in chloroform)

exhibited a fluorescence emission at 571 nm, which corresponds to the relaxation of excited electrons from the lowest unoccupied molecular orbital (LUMO) to the highest occupied 10

ACCEPTED MANUSCRIPT molecular orbital (HOMO). By contrast, the fluorescence emission of the P3HT-g-DWCNT solution (0.09 mg mL–1 in chloroform with 0.01 mg mL–1 of P3HT concentration) revealed that the fluorescence of P3HT was effectively quenched, due to the photoinduced electron transfer from P3HT to the DWCNT, as illustrated in the Fig. 2(d). As a control experiment, a mixture of P3HT and DWCNT was prepared with the same ratio between the two

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components as that of P3HT-g-DWCNT. The fluorescence emission from the physically mixed P3HT/DWCNT solution containing 11 wt% of P3HT component was relatively less quenched than that of P3HT-g-DWCNT. The results support the idea that an intimate interface between P3HT and DWCNT was formed to facilitate the electron transfer in P3HT-

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g-DWCNT.

The surface morphology of the P3HT-g-DWCNT films fabricated by vacuum filtration

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was characterized by SEM (Fig. 3). Chunky bundles with large diameters of ca. 50 nm were observed in the pristine DWCNT and directly mixed P3HT/DWCNT composite (Fig. 3(a)

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and (b)), indicating that DWCNT readily exists in an aggregated form. In contrast, P3HT-g-

Fig. 3. SEM images of the (a) pristine DWCNT, (b) directly mixed P3HT/DWCNT, and (c) P3HT-g-DWCNT films with the insets showing the optical images of their corresponding suspension in chloroform. (d) EDXS mapping of the P3HT-g-DWCNT film (sulphur, red; carbon, green; EDXS spectrum in inset). 11

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DWCNT had a relatively smooth morphology with a reduced bundle diameter (Fig. 3(c)). This result means that the DWCNT was concealed by the grafted P3HT, and therefore the aggregation of the DWCNTs by Van der Waals interactions was efficiently prevented, resulting in their stable solubility in physiological solutions. For example, some aggregated

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bundles could be easily observed in the chloroform solution of pristine DWCNT and directly mixed P3HT/DWCNT. On the other hand, P3HT-g-DWCNT was dispersed stably without any aggregated bundles in the solution.

To investigate the dispersity of P3HT in the film,

elemental sulfur and carbon were detected in the energy dispersive X-ray spectroscopy

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(EDXS) mapping (Fig. 3(d)). From the well-distributed red dots (sulfur element) in the mapping, it was concluded that the grafted P3HT was homogeneously distributed within the

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P3HT-g-DWCNT film as compared with the physically mixed composite (Fig. S3). In addition, the sulfur peak on the EDXS spectrum further clarified the existence of P3HT on the P3HT-g-DWCNT films (inset of Fig. 3(d)).

The TE properties of P3HT-g-DWCNT were investigated and compared to those of pristine DWCNT, and directly mixed DWCNT/P3HT composite films (Fig. 4). In order to investigate the chain length effect of P3HT on the TE performance, an amino-terminated P3HT with a

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longer chain length was synthesized. The P3HT with a higher molecular weight of 16K was then covalently attached on the DWCNT surface using the same grafting method previously described, and this sample is hereafter referred to as long P3HT-g-DWCNT. Another P3HT-gDWCNT with lower molecular weight P3HT (8K) is denoted as short P3HT-g-DWCNT. The

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evaluation of the Seebeck coefficient was carried out with a computer controlled multimeter and sourcemeter, connected to a heating and cooling Peltier to generate a temperature

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difference across the electrodes (Fig. 4(a)). Once the temperature difference was in a thermally steady-state mode, the TE voltage (V) versus temperature difference (∆T) was automatically recorded by a Keithley 2182A nanovoltmeter. The Seebeck coefficient was then obtained by dividing the generated voltage with a certain ∆T. While the directly mixed DWCNT/P3HT composite film possessed a low Seebeck coefficient of 69.2 µV K-1, which is similar to that of pristine CNTs at low temperature gradients from 2 to 10 °C, the short and long P3HT-g-DWCNT exhibited extremely higher Seebeck coefficients of 114 and 120 µV K1

, respectively (Fig. S5 and Fig. 4(b)). The results indicate that intimate contact and excellent dispersity with a large interface by

covalent bonding played a crucial role in improving the Seebeck coefficient of the P3HT-g12

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DWCNT composite, as it facilitated the carrier transport by reducing the scattering of carriers

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Fig. 4. (a) Schematic set-up for the Seebeck coefficient measurements; (b) average Seebeck coefficient (red column), electrical conductivity (blue column), and corresponding power

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factor (green column); (c) thermal conductivity, and (d) figure of merit, ZT, of pristine DWCNT, P3HT-g-DWCNT, and P3HT/DWCNT nanocomposites.

at the interface. In addition, the electrical conductivity was measured by the standard van der Pauw direct current four-probe method, and the corresponding power factor was calculated from the Seebeck coefficient and electrical conductivity (Fig. 4(b)). As compared to pristine DWCNT, the conductivity of the P3HT/DWCNT composite was slightly degraded from 99.03 to 81.58 S cm-1, while that of the P3HT-g-DWCNT was significantly reduced by the uniformly dispersed P3HT, which had a rather lower conductive property in the matrix. Meanwhile, the lower electrical conductivity of the long P3HT-g-DWCNT can be understood 13

ACCEPTED MANUSCRIPT by assuming that the less electrically conductive P3HT, with its long chain, more significantly interfered during the electron transfer between the DWCNT bundles. As a result of the significant reduction of electrical conductivity, the power factor of P3HT-g-DWCNT was not much higher than that of the pristine DWCNT and directly mixed DWCNT/P3HT composite. To investigate the figure of merit (ZT) of P3HT-g-DWCNT, the thermal

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conductivity was calculated from the factors of in-plane thermal diffusivity, specific heat capacity, and density of the films (Table S1). The thermal conductivities of both short and long P3HT-g-DWCNTs were reduced to approximately 2.0 W m-1 K-1 compared to that of P3HT/DWCNT composite (2.9 W m-1 K-1) (Fig. 4(c)). Such decreases in thermal

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conductivity result from the reduced diameter of CNT bundle. According the previous study, CNT bundles with smaller diameters would provide larger boundary for the phonon

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scattering in the same volume [29]. As a result of the significant decrease in the thermal conductivity, and the increase in the Seebeck coefficient, the in-plane ZT value increased from 0.0038, for the pristine DWCNT, to 0.0069 for the short P3HT-g-DWCNT at 25 °C (Fig. 4(d)), whose property is approximately twofold superior to the directly mixed P3HT/DWCNT nanocomposite of 0.0039. Therefore, the covalent graft of polymers onto the

thermoelectric materials.

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surface of the highly conductive CNT is a promising approach to provide high performance

A flexible TEG composed of only p-type P3HT-g-DWCNT legs was fabricated on a PET substrate. The 15 p-Type active legs, which were 2 mm in width and 15 mm in length, were connected in series by dispenser printed silver electrodes (Fig. 5(a)). To investigate the power

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generation characteristics of the flexible TEG, a custom system was utilized as previously reported [22-25] (Fig. S1), where one side of the TEG was heated by applying an electrical

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current through a hot Peltier plate; and the other side was simultaneously cooled to maintain a constant temperature of 20 °C (Fig. 5(b)). Fig. 5(c) shows the output power–output current, and output voltage–output current curves of the flexible TEG at the temperature difference of 20 °C. The maximum power output of 14.5 nW was generated with a high open circuit voltage (Vo) of 33.5 mV, which is slightly lower than the ideal value of 34.2 mV (V = N S ∆T, where N is the number of thermoelectric elements). Moreover, the maximum output power at the output power-load resistance curve exhibited a matched load resistance of 18.2, which is in close agreement with the internal resistance of 18.5 kΩ (Fig. 5(d)). Even though the device performance of flexible TEGs still needs to be improved to meet the requirements for practical applications, the performance of flexible TEGs could be enhanced by properly 14

ACCEPTED MANUSCRIPT selecting highly conductive polymers, and high quality CNT with long lengths, optimizing

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the structure of the device, and introducing compatible n-type counterparts.

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Fig. 5. (a) Photograph of the flexible TEG composed of 15 legs of small P3HT-g-DWCNT films, and a silver electrode on a PET substrate. (b) Schematic drawing for setting the

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temperature difference between the two sides of the legs to 20 K using two Peltier plates. (c) Power output-current output, and voltage output– current output curves. (d) Power outputload resistance curve of the flexible TEG.

4. Conclusions

A P3HT-g-DWCNT composite was prepared through covalently grafting P3HT on the surface of DWCNTs. TEM observations of the composite verified the grafting of P3HT on the surface of the P3HT-g-DWCNT composite. The intimate interaction between the P3HT and DWCNT components was characterized through FT-IR, Raman, and fluorescence spectroscopy.

The surface modification of the P3HT-g-DWCNT nanocomposite enhanced 15

ACCEPTED MANUSCRIPT its dispersity in chloroform, allowing the formation of relatively small bundles in the resulting films by suppressing the aggregation of the DWCNT. Such property changes can provide P3HT-g-DWCNT with a higher Seebeck coefficient and lower thermal conductivity than pristine DWCNT or P3HT/DWCNT composites. Consequently, the covalently grafted P3HT-g-DWCNT film exhibited an enhanced figure of merit (ZT) along with a high power

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output in the TE module.

Acknowledgment

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This work was supported by a grant from the KRICT Core Project and the R&D Convergence Program of National Research Council of Science and Technology of the

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Republic of Korea.

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[4] M. Zebarjadi, K. Esfarjani, M. S. Dresselhaus, Z. F. Ren, G. Chen, Perspectives on thermoelectrics: From fundamentals to device applications, Energy Environ.

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