Enhanced Thermoelectric Performance of Conjugated Polymer

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Enhanced Thermoelectric Performance of Conjugated Polymer/SWCNT Composites with Strong Stacking Luhai Wang, Chengjun PAN, Zhongming Chen, Wenqiao Zhou, Chunmei Gao, and Lei Wang ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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ACS Applied Energy Materials

Enhanced Thermoelectric Performance of Conjugated Polymer/SWCNT Composites with Strong Stacking

Luhai Wang,† Chengjun Pan,*, †, ‡ Zhongming Chen,§ Wenqiao Zhou,† Chunmei Gao,† and Lei Wang*, †, ‡

†

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials

Science and Engineering, Shenzhen University, Shenzhen 518060, China ‡

Guangdong Research Center for Interfacial Engineering of Functional Materials,

Shenzhen University, Shenzhen 518060, China §

School of environment and civil engineering, Dongguan University of Technology,

Dongguan 523808, China

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract We design and synthesize a novel donor-acceptor-type BDT-based conjugated polymer, PBDTDTffBT, and prepare composite films containing single-walled carbon nanotubes (SWCNTs) and the PBDTDTffBT polymer. Strong π-π interfacial interactions are present between the PBDTDTffBT polymer and the SWCNTs, and we systematically study the effects of different mass ratios of PBDTDTffBT to SWCNTs on the thermoelectric (TE) properties. The maximum electrical conductivity, Seebeck coefficient, power factor and ZT value of the composite films are 529.3 S cm-1, 68.1 µV K-1, 80.9 µW m-1 K-2 and 0.028 at room temperature, respectively. Additionally, the composite film with a PBDTDTffBT:SWCNT mass ratio of 1:10 exhibits the highest power factor of 116.7 µW m-1 K-2 at approximately 95 oC. This study has a large potential to broaden the research scope of composite TE materials.

Keywords:

Thermoelectric, Conjugated polymer, Carbon nanotubes, Composite

film, Strong stacking

2


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1. Introduction Since the industrial revolution, the global economy has sustained rapid growth. At the same time, methods for addressing the energy crisis and environmental pollution have become a focus of social and economic development in every country around the world, which has accelerated the exploitation of green energy and the development of new energy materials. Thermoelectric (TE) materials can directly convert heat and electricity through the mobility of the solid interior; moreover, they do not possess moving parts and produce no noise.1-5 TE materials will play important role in eliminating waste heat from daily life and industry and will gradually replace traditional energy materials. The TE conversion efficiency is generally determined via the dimensionless figure of merit (ZT) according to the equation ZT = S2σT/κ, where σ, S, T, and κ are the electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity, respectively. Furthermore, a power factor (PF = S2σ) is usually used to evaluate the TE performance of organic TE materials. Obviously, a high Seebeck coefficient value, good electrical conductivity, and low thermal conductivity are beneficial for improving the conversion efficiency of TE materials. However, achieving a high TE conversion in traditional bulk materials remains a challenge because all of these TE parameters are strongly correlated; for example, an enhancement in the electrical conductivity usually leads to a decrease in the Seebeck coefficient.6-8 To date, organic and organic/inorganic composite TE materials have attracted considerable attention due to their light weight, flexibility, easy processability, and 3



low cost. Furthermore, compared to inorganic TE materials, the thermal conductivity of organic and organic/inorganic composite TE materials is usually less than 1 W m -1 K-1, which is also an important advantage for improving the ZT value.9-12 The TE properties of pure organic TE materials are still poor because of their low conductivity, and introducing inorganic particles into organic polymer matrices is an effective way to improve their conductivity. The inorganic particles applied for this purpose mainly include carbon nanotubes (CNTs), graphene, silver nanowires, and bismuth telluride due to their good conductivity.13-18 Meanwhile, new composite preparation methods, such as in situ polymerization, layer-by-layer (LBL) assembly deposition and surface wrapping, have also been developed to improve the TE properties of composite TE materials with good results.19-21 Although numerous composite TE systems have been investigated, the organic matrix in the composite TE materials are still limited to several traditional conducting polymers, such as polyaniline (PANI), poly (3,4-ethylenedioxythiophene) (PEDOT), polythiophene (PTh), polypyrrole (PPy), and their derivatives.22-27 High-performance conducting polymers are still lacking in the field of composite TE materials, which may hinder the development of composite TE materials in the future. Therefore, the design and synthesis of novel conducting polymers for application in composite TE materials is urgently needed. Conjugated polymers based on benzo-[1,2-b:4,5-b′]dithiophene (BDT) units have attracted considerable attention in the fields of polymer solar cells (PSCs) and organic field effect transistors (OFETs), mainly due to the planar conjugated structure, 4


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high stability against oxidation, and good electron donating ability of the BDT unit.28-32 In the OFET field, the highest reported value of the hole mobility of 0.25 cm2 V-1 s-1 was achieved by a BDT-based polymer.33 Although BDT-based conjugated polymers have been widely used in both PSCs and OFETs, their TE properties have been scarcely investigated. Based on our previous research, conjugated polymers with donor-acceptor (D-A) structures generally are generally beneficial to their TE properties.34,

35

Therefore, in this study, a novel D-A type

conjugated conducting polymer, PBDTDTffBT, based on BDT units was designed and synthesized. The synthetic route and chemical structure of this polymer, in which BDT and benzothiadiazole (BT) act as the donor (D) and the acceptor (A) units, respectively, are shown in Figure 1a. Structural modification of the BDT and BT units can also be carried out. For example, alkylthienothiophene-substituted BDT units can improve the planar conjugated structure of the molecules, and fluorine-substituted BT units can increase the intermolecular interactions,36 which are beneficial for improving the transport of the carrier in the conjugated polymer backbone. Meanwhile, a thiophene unit with 2-octyldodecyl side chains that acts as a π bridge between the modified BDT and BT units can increase the solubility of the polymers. Herein, we prepared composite films based on PBDTDTffBT and SWCNTs, and applied them as TE materials. We systematically studied the effects of different mass ratios of PBDTDTffBT to SWCNTs on the TE properties, and the presence of strong π-π interfacial interactions between PBDTDTffBT and the SWCNTs gave the 5



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composite films a good TE performance. When the mass ratio of PBDTDTffBT to SWCNTs was 1:10, a high power factor of 80.9 µW m

-1

K-2 was obtained at room

temperature, and this value of power factor reached 116.7 µW m

-1

K-2 at

approximately 95 oC. This study indicated that the design and synthesis of novel conjugated polymers is an effective way to enhance the TE performance of composite TE materials; therefore, this study can provide a good example or serve as a reference for further improving the TE properties of composite TE materials.

2. Experimental Section 2.1 Materials Commercial SWCNTs (diameter: 1~2 nm, length: 5~30 µm, purity: > 95.0 wt%) were purchased from XFNANO Materials Technology Co., Ltd. (Nanjing, China). 4,8-Bis(5-(2-ethylhexyloxy)thiophene-2-yl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl)b is(trimethylstannane) was purchased from Sun Tech Inc. (Suzhou, China). 4,7-Bis(5-bromo-4-(2-ethylhexyl)thiophene-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiaz ole was purchased from Derthon Optoelectronic Materials Science Technology Co., Ltd. (Shenzhen, China), while tris(dibenzylideneacetone) dipalladium(0) (Pd2(dba)3) and tris(2-methylphenyl)phosphine (P(o-tol)3) were obtained from Greenchem Technlogy Co., Ltd. (Beijing, China). Anhydrous chlorobenzene was received from Sun Chemical Technology Co., Ltd (Shanghai, China). Other chemical reagents, including methanol, acetone, hexane, and deionized water, were used as received without further treatment. 6


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2.2 Synthesis of the PBDTDTffBT polymer PBDTDTffBT

was

synthesized

by

Stille

coupling

polymerization.

4,8-Bis(5-(2-ethylhexyloxy)thiophene-2-yl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl)b is (trimethylstannane) (0.3000 g, 0.2951 mmol), 4,7-bis(5-bromo-4-(2-ethylhexyl) thiophene-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (0.3114 g, 0.2951 mmol), Pd2(dba)3 (0.0135 g, 0.0148 mmol) and P(o-tol)3 (0.0225 g, 0.0738 mmol) were added to a two-necked flask, which was protected by an atmosphere of nitrogen after removing the air. Anhydrous chlorobenzene (10 ml) was then added via a syringe, and the reaction solution was stirred and heated to 110 oC for 72 h. Then, the polymer was precipitated by the addition of methanol after the reaction solution has cooled to room temperature and then washed with methanol, acetone, hexane, and deionized water until the solvents become colorless. Finally, the purified polymer was collected and dried under vacuum overnight, and PBDTDTffBT was obtained as a black powder (93 % yield). 1H NMR (600 MHz, CDCl3, ppm): δ 7.62 (s, 4H), 6.98 (s, 2H), 2.83 (s, 8H), 1.31 (s, 4H), 1.27-0.95 (s, 80H), 0.77 (s, 24H).

2.3 Preparation of PBDTDTffB/SWCNT composite films SWCNTs were well-dispersed in anhydrous chlorobenzene through vigorous stirring. PBDTDTffBT was then added to the dispersion, which was continuously stirred

to

completely

dissolve

the

polymer

in

the

solution.

The

PBDTDTffBT/SWCNT composite films were obtained by drop-casting the mixture 7



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on glass substrates (15 mm x 15 mm) under ambient conditions until the chlorobenzene evaporated completely. PBDTDTffBT/SWCNT composite films with different mass ratios of polymer to SWCNTs were prepared by changing the amounts of

the

polymer

and

SWCNTs.

In

this

study,

we

prepared

seven

PBDTDTffBT/SWCNT composite films with polymer:SWCNT mass ratios of 10:1, 10:3, 10:7, 10:10, 7:10, 3:10, and 1:10.

2.4 Measurements 1

H nuclear magnetic resonance (NMR) spectra were collected on a Bruker

ADVANCE III 600 MHz spectrometer with deuterated chloroform (CDCl3) as the solvent and tetramethylsilane as the reference. The molecular weight and distribution of the polymer were determined by gel permeation chromatography (GPC) (Waters e2695 Separations Module), where polystyrene was used as a standard and THF was used as the eluent. Thermal gravimetric analysis (TGA) measurements were performed on a TGA-Q50 instrument (TA, USA) at a heating rate of 10 oC/min from room temperature to 600 oC under a nitrogen flow of 40 mL/min. Fourier transform infrared (FT-IR) spectra were collected using an FT-IR spectrometer (Thermo Scientific Nicolet 6700). Raman spectra were measured with a Raman spectrometer (Renishaw

inVia-Reflex)

with

an

excitation

wavelength

of

514.5

nm.

Ultraviolet-visible (UV-Vis) absorption spectra were acquired using a UV-Vis spectrophotometer

(Thermo

Evolution

220).

The

ultraviolet

photoelectron

spectroscopy (UPS) was carried out in a photoelectron spectrometer (Thermo Fisher 8


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ESCALAB 250X). X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) data were obtained with a field emission auger spectrometer (Thermo Fisher MICROLAB 350). The sample morphology was examined by scanning electron microscopy (SEM, Hitachi S-4700) and transmission electron microscopy (TEM, FEI-Tecnai G2 TF20). The thickness of the composite films was determined using a stylus profilometer and thickness information of samples were shown in Table S1. The Seebeck coefficients and electrical conductivities of the composite films were tested using an MRS-3 thin film TE test system (Wuhan Joule Yacht Science & Technology Co., Ltd., China). Thermal conductivities were obtained by thermal conductivity tester (KY-DRX-RW, Shanghai).

3. Results and Discussion The synthetic route to PBDTDTffBT, the fabrication procedure of the PBDTDTffBT/SWCNT composite films and the mode of binding between the SWCNT and polymer are shown in Figure 1. PBDTDTffBT was synthesized by Stille coupling polymerization under anhydrous and anaerobic conditions, and the chemical structure of the polymer was confirmed by 1H NMR spectroscopy (Figure S1). The number average molecular weight (Mn) of PBDTDTffBT was 65.44 kDa with a polydispersity index (PDI) of 2.12; this high molecular weight gave the polymer a good film-forming ability. The TGA curve of PBDTDTffBT (Figure S3) indicates that the polymer has good thermal stability, as the thermal decomposition temperature 9



exceeds 400 oC. As shown in Figure 1b, PBDTDTffBT/SWCNT composite films were prepared by a drop-casting method, in which composite films with a uniform surface were tightly attached to the glass substrates after complete evaporation of the chlorobenzene solvent, and these films could be used to test the TE performance and for other characterization techniques. In addition, the mode of binding between the SWCNT and polymer are shown in Figure 1c.

Figure 1. (a) Synthetic route to PBDTDTffBT. (b) Schematic illustrations of the fabrication procedure of the PBDTDTffBT/SWCNT composite film. (c) The mode of binding between the SWCNT and polymer.

10


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Figure 2. (a) FT-IR spectra of PBDTDTffBT and the PBDTDTffBT/SWCNT composites.

(b)

Raman

spectra

of

PBDTDTffBT,

SWCNTs,

and

the

PBDTDTffBT/SWCNT composite films. (c) UV-Vis absorption spectra of PBDTDTffBT, SWCNTs, and the PBDTDTffBT/SWCNT composite films. (d) XPS spectra of PBDTDTffBT and the PBDTDTffBT/SWCNT composite films. The FT-IR spectra of PBDTDTffBT and the PBDTDTffBT/SWCNT composites are displayed in Figure 2a. The main absorption peaks of the polymer are as follows: the absorption band in the range of 2800~3000 cm-1 arises from the stretching vibration of unsaturated C–H bonds. The C=N stretching vibration absorption peak 11



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appears at 1629 cm-1. The absorption peaks at 1399 cm-1 and 718 cm-1 are attributed to the bending and rocking vibrations of methylene, respectively. In addition, the absorption peaks at 1156 cm-1 and 627 cm-1 are caused by the C–F bonds and C-S bonds of thiophene, respectively. Similar absorption peaks are observed in all of the PBDTDTffBT/SWCNT composites without the appearance of new peaks, which indicates that the addition of SWCNTs does not damage the chemical structure of the polymer; therefore, the polymer is stable in the composite. Figure 2b shows the Raman spectra of pure PBDTDTffBT, SWCNTs, and the PBDTDTffBT/SWCNT composite films with different mass ratios of PBDTDTffBT to SWCNTs. The spectrum of the pure SWCNTs showed characteristic peaks at 1563 and 1591 cm-1, which are assigned to the G band of SWCNTs that originates from the splitting of the E2g stretching mode of graphite.37 Some important characteristic peaks were clearly observed for the pure PBDTDTffBT film, such as the benzene ring stretching vibrations of the BDT and BT units at 1537 cm-1 and 1480 cm-1, respectively.38 The peak at 1432 cm-1 corresponds to the C=C symmetrical stretching mode of the thiophene units, and the peaks at 1335 and 856 cm-1 are attributed to C-C and C-F stretching vibrations, respectively. In the Raman spectra of the PBDTDTffBT/SWCNT composite films, the intensity of the characteristic peaks of PBDTDTffBT gradually decreased and eventually disappeared with increasing content of SWCNTs; meanwhile, the peaks at 1537 cm-1, 1335 cm-1, and 850 cm-1 of PBDTDTffBT

showed

redshifts

of

approximately

2~6

cm-1

in

the

PBDTDTffBT/SWCNT composite films, resulting from the strong π-π interfacial 12


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interactions between PBDTDTffBT and the SWCNTs. To examine the π-π interfacial interactions between PBDTDTffBT and the SWCNTs in more detail, UV-Vis absorption spectra and XRD patterns of the PBDTDTffBT/SWCNT composites were obtained. Pure PBDTDTffBT and SWCNT films were also measured for comparison (Figure 2c and Figure S4). The absorption maximum (λmax) of the PBDTDTffBT film occurred at 585 nm with a strong shoulder at 638 nm, indicating the presence of strongly conjugated accumulation in the PBDTDTffBT film. The work function value of PBDTDTffBT was obtained as 3.83 eV from UPS spectra (Figure S5). The band gap was estimated from the absorption onset (λoneset) of the thin films which was determined as 1.75 eV by the formula Eg =

hc / λoneset. These results indicated that PBDTDTffBT has relatively narrow band gap which is usually beneficial to the thermoelectric performance. 34, 35 After the addition of SWCNTs, the absorption peaks of the PBDTDTffBT film at 266, 585, and 638 nm redshifted by approximately 4~15 nm in the PBDTDTffBT/SWCNT composite films, which confirmed the formation of π-π interactions between the interface of PBDTDTffBT and the SWCNTs. In Figure S4, PBDTDTffBT shows a wide diffraction peak at approximately 2θ = 26.7°, which corresponds to the typical π-stacking spacing of the conjugated polymer backbone.39 The pure SWCNT film exhibited a typical diffraction peak at 2θ = 26.5°, and the other two diffraction peaks observed at 2θ = 14.0° and 2θ = 33.6° are attributed to intercalated graphite and the catalyst from the SWCNT preparation process, respectively.40, 41 In the XRD patterns of the PBDTDTffBT/SWCNT composite films, 13



the typical diffraction peak of pure SWCNTs at 2θ = 26.5° moved to a slightly lower angle, and the intensity of the diffraction peak also decreased, which further proved the presence of strong π-π interfacial interactions between PBDTDTffBT and the

SWCNTs. To investigate the electronic environment of the polymer and composite films, we performed XPS. Figure 2d shows the XPS results for the pristine polymer and the PBDTDTffBT/SWCNT composite films. The binding energies of the C 1s and S 2p levels shifted from 286.8 eV and 166.1 eV, respectively, for the pure PBDTDTffBT film to 284.5 eV and 163.9 eV for the PBDTDTffBT/SWCNT composite film with a PBDTDTffBT:SWCNT mass ratio of 1:10. Furthermore, as the content of SWCNTs increased, an obvious shift of the peaks was observed. This shift in the binding energies shows that electron transfer occurs between PBDTDTffBT and the SWCNTs, which is also attributed to the π-π interfacial interactions between PBDTDTffBT and the SWCNTs.

14


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Figure 3. (a) SEM images of the pure PBDTDTffBT film and (b-h) the PBDTDTffBT/SWCNT composite films with various PBDTDTffBT:SWCNT mass ratios : (b) 10:1, (c) 10:3, (d) 10:7, (e) 10:10, (f) 7:10, (g) 3:10, and (h) 1:10; (i-l) TEM images of the PBDTDTffBT/SWCNT composite films with various PBDTDTffBT:SWCNT mass ratios: (i) 10:3, (j) 10:10, (k) 3:10, and (l) 1:10.

The surface morphologies of the pure PBDTDTffBT and PBDTDTffBT/SWCNT films were observed by SEM, as shown in Figure 3. The surface of the pure PBDTDTffBT film was very smooth and dense (Figure 3 a), and no particles or fibers were found, which is attributed to the good solubility of PBDTDTffBT in chlorobenzene. When a low content of SWCNTs was present in the composite film, such as with the polymer:SWCNT mass ratio of 10:1 (Figure 3b), all the SWCNTs were embedded in the PBDTDTffBT matrix, and a small amount of SWCNTs could 15



be observed through the thin polymer layer. For the PBDTDTffBT:SWCNT mass ratios of 10:3, 10:7, and 10:10 (Figure 3c, Figure 3d, and Figure 3e), the SWCNTs were partially embedded in the PBDTDTffBT matrix. As the SWCNT content in the composite film increased further, the embedding of the SWCNTs in the PBDTDTffBT matrix disappeared gradually, and an increasing number of SWCNTs become randomly distributed, especially for the PBDTDTffBT: SWCNT mass ratio of 1:10, in which aggregation of the SWCNTs was clearly observed. Thus, a conductive network formed in the composite films with a relatively low content of SWCNTs, where the main matrix of the structure was the polymer and the SWCNTs were evenly dispersed in the polymer matrix network. At higher SWCNT contents, the polymer tightly coated the surface of the SWCNTs through the strong π-π interfacial interactions between PBDTDTffBT and the SWCNTs, which can also be clearly observed in the TEM images.

16


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Figure 4. (a) Electrical conductivities, (b) Seebeck coefficients, (c) power factors, and (d) ZT values of the PBDTDTffBT/SWCNT composite and SWCNT films.

Figure 4 presents the electrical conductivities, Seebeck coefficients, power factors, and ZT values of the SWCNT and PBDTDTffBT/SWCNT films with different polymer: SWCNT mass ratios at room temperature. As shown in Figure 4b, the Seebeck coefficients of all the composite films were positive, indicating that these composite films are P-type TE materials, and thus, holes are the main source of carrier transmission. In the PBDTDTffBT/SWCNT films, the large Seebeck coefficient was dominated by the PBDTDTffBT polymer, and the addition of SWCNTs was 17



beneficial

for

increasing

the

electrical

conductivity.

Page 18 of 31

Therefore,

the

PBDTDTffBT/SWCNT film with the mass ratio of PBDTDTffBT to SWCNTs of 10:1 had the largest Seebeck coefficient of 68.1 µV K-1, and with the continuous addition of SWCNTs to the PBDTDTffBT matrix, the electrical conductivity of the PBDTDTffBT/SWCNT films increased drastically. The highest electrical conductivity of 529.3 S cm-1 was achieved for a mass ratio of PBDTDTffBT to SWCNTs of 1:10. The power factor was determined from the Seebeck coefficient and the electrical conductivity. Because the degree of change in the conductivity was much larger than that in the Seebeck coefficient, a trend similar to that of the electrical conductivity was observed for the power factor, in which the value of the power factor also increased with the addition of SWCNTs. The largest power factor was 80.9 µW m

-1

K-2 at room temperature, which is about two times higher than that of the pristine SWCNT film. In addition, we compared the TE properties of PBDTDTffBT/SWCNT composites and other conjugated polymer/inorganic composites as shown in Table S2, we found that PBDTDTffBT/SWCNT composites show good TE performance compared to other composite systems. To evaluate the TE performance of PBDTDTffBT/SWCNT composites more accurately, we measured the thermal conductivity of PBDTDTffBT/SWCNT composites with different mass ratios, as shown in Figure S6. Thermal conductivities of PBDTDTffBT/SWCNT composites slightly enhanced from 0.42 to 0.87 W m-1 K-1 as the mass ratio of PBDTDTffBT to SWCNTs increased from 10:3 to 1:10, this could be attributed to the strong interfacial interactions between PBDTDTffBT and the SWCNTs through π−π stacking, which 18


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imped the thermal energy transport. The thermal conductivities of the composites are much lower than those of the conventional inorganic materials,42,43 which is beneficial to the enhancement of TE performance. ZT values of PBDTDTffBT/SWCNT composites were obtained based on Seebeck coefficient, electric conductivity, and thermal

conductivity,

as

shown

in

Figure

4d.

The

ZT

values

of

PBDTDTffBT/SWCNT composites show an increasing trend with the increase of SWCNTs content, when the mass ratio of PBDTDTffBT to SWCNT reach to1:10, the composite films exhibited an optimized ZT value of 0.028.

Figure 5. Temperature-dependent (a) electrical conductivities, (b) Seebeck 19



coefficients, and (c) power factors of the PBDTDTffBT/SWCNT composite and SWCNT films. (d) The fitting curve of Inσ vs T-1/2 according to the VRH model for the PBDTDTffBT/SWCNT composite film.

Figure 5 shows the temperature-dependent electrical conductivities, Seebeck coefficients, and power factors of the SWCNT and PBDTDTffBT/SWCNT composite films with different mass ratios of polymer to SWCNTs in the range of 25~140 oC. The electrical conductivities of the PBDTDTffBT/SWCNT films decreased with increasing temperature except for the composite film with a PBDTDTffBT:SWCNT mass ratio of 10:1. This difference might be due to the high content of polymer in this composite film; the polymer plays an important role in the carrier transport of the composites films, and with increasing temperature, the polymer chain arrangement gradually becomes regular, which can facilitate electron transfer and increase the carrier mobility. As a result, the electrical conductivity of the PBDTDTffBT/SWCNT film increased at the beginning of the temperature rise. However, when the temperature reached 80 oC, the arrangement of the polymers chains changed, which weakened the interactions between the polymer and the SWCNTs. In this case, when the temperature continued to rise, the electrical conductivity showed a downward trend. The PBDTDTffBT/SWCNT films with other mass ratios showed a decreasing trend in electrical conductivity with increasing temperature, and this trend is particularly obvious for the PBDTDTffBT/SWCNT film with a high SWCNT content. Furthermore, this trend is similar to that observed for the pure SWCNTs, suggesting 20


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that the film has a metallic or heavily doped semiconductor character.44 The change in temperature also had a little effect on the Seebeck coefficients of the composite films with high polymer contents, such as in the polymer: SWCNT mass ratios of 10:1, 10:3, 10:7, 10:10; the Seebeck coefficients of these composite films were stable from 62~70 µV K-1, and the maximum value of 69.9 µV K-1 was observed for the PBDTDTffBT/SWCNT film with the mass ratio of 10:7 at approximately 80 oC. On the other hand, the transport of carriers may be more sensitive to changes in temperature for the composite films with high contents of SWCNTs, and as a result, the Seebeck coefficients of these composite films exhibited significant changes with changes in the temperature. The power factor of the composite film was the largest for the mass ratio of polymer to SWCNTs of 1:10 even though the electrical conductivity decreased with increasing temperature, and the highest value was 116.7 µW m-1 K-2. To explore the temperature dependence of the electrical conductivity and hence the underlying transport mechanism in the PBDTDTffBT/SWCNT composite films, the variable-range hopping (VRH) model was used. This model usually describes conduction in a disordered system and indicates that electrons possibly hop from one localized site near the Fermi level to another site inside the network.45-47 The description of the VRH model is (1) where σ0 is a constant determined by the density of states of the material, the length of the average transition, and the frequency of the molecular vibration; T0 is the characteristic temperature; T is the measured temperature; and r is the dimensionality 21



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of hopping in the VRH model. In our study, taking the composite film with a PBDTDTffBT:SWCNT mass ratio of 7:10 as an example, the PBDTDTffBT/SWCNT film showed a good linear relationship between In (σ) and T-1/2 (Figure 5d), indicating that charge transport in the PBDTDTffBT/SWCNT films follows a one-dimensional VRH (1D-VRH) model, where r = 1 in Eq. (1). In more detail, in such a disordered system consisting of PBDTDTffBT and SWCNTs, the SWCNTs are tightly coated with the polymer through strong π-π interfacial interactions, and the carriers can hop from one SWCNT to another through the one-dimensional channel of the SWCNTs.

4. Conclusions In summary, a novel conjugated polymer, PBDTDTffBT, was designed and synthesized, and a series of PBDTDTffBT/SWCNT composite films with different mass ratios of polymer to SWCNTs were prepared by drop-casting. The addition of SWCNTs did not damage the chemical structure of the polymer, and strong π-π interfacial interactions formed in the composites, as revealed by Raman, XRD, UV-Vis, SEM, and TEM analyses. With increasing amount of SWCNTs in the composite films, the Seebeck coefficient decreased, while the conductivity of the composites increased gradually. The maximum Seebeck coefficient and electrical conductivity of the composite films were 68.1 µV K-1 and 529.3 S cm-1, respectively, and the maximum power factor and ZT value were 80.9 µW m

-1

K-2 and 0.028 at

room temperature for the composite film with a 1:10 mass ratio of PBDTDTffBT to 22


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SWCNTs. The temperature-dependent TE performance of these composite films was characterized in the range of 25~140 oC, and the composite film with a 1:10 mass ratio of PBDTDTffBT to SWCNTs showed the largest power factor of 116.7 µW m-1 K-2 at approximately 95 oC. The 1D-VRH model effectively explained the mechanism of charge transport in the PBDTDTffBT/SWCNT composite films. The exploration of novel conjugated polymers for TE applications is expected to play an important role in the development of organic or organic/inorganic composite TE materials.

Associated content Supporting information 1

H NMR spectrum, GPC curve, TGA curve and UPS spectra of polymer

PBDTDTffBT.

XRD

patterns

of

PBDTDTffBT/SWCNT

composite

PBDTDTffBT/SWCNT

composite.

PBDTDTffBT, films.

SWCNTs

Thermal

Thickness

and

the

conductivity

of

information

of

the

PBDTDTffBT/SWCNT composite and SWCNT films, and a table about the thermoelectric performance for some conjugated polymer/inorganic thermoelectric composites at room temperature.

Author information Corresponding author: *E-mail: [email protected] *E-mail: [email protected] 23



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Notes The authors declare no conflict of interest.

Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project Nos. 51773118 and 21706030), and Shenzhen Sci & Tech Bureau (Project Nos. JCYJ20170818093417096 and JCYJ20170302150144008), and Shenzhen University (No. 2016003).

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Mechanisms. Appl. Phys. Lett. 2010, 96, 233115.

Table of content

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Enhanced Thermoelectric Performance of Conjugated Polymer

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