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Thermoelectric properties of PEDOT: PSS and acid-treated SWCNT composite films
Materials Today Communications 23 (2020) 100867
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Thermoelectric properties of PEDOT: PSS and acid-treated SWCNT composite films
T
Seok-Hwan Chung*, Dong Hwan Kim, Hanna Kim, Hoyoung Kim, Sang Won Jeong Materials Research Institute, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: PEDOT:PSS SWCNT Composite film Thermoelectric property In-plane thermal conductivity
We investigated the thermoelectric properties of composite films that consist of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and single-walled carbon nanotubes (SWCNTs). Thin films with 5−10 μm thickness of PEDOT:PSS/SWCNT and PEDOT:PSS/acid-treated SWCNT (AC-SWCNT) were prepared by varying the CNT content to determine their power factor. Both PEDOT:PSS/SWCNT and PEDOT:PSS/ AC-SWCNT films showed an increase in their power factors with CNT content. The power factor values of PEDOT:PSS/AC-SWCNT films were higher than those of PEDOT:PSS/SWCNT films over the entire experimental range, which was mainly attributed to the significantly increased electrical conductivity. Thus, a series of PEDOT:PSS/AC-SWCNT freestanding films with 100−250 μm thickness were synthesized, and the same sample was used to measure electrical conductivity, Seebeck coefficient, and thermal conductivity to calculate the inplane figure-of-merit ZT values. The composite films exhibited lower ZT values than those of PEDOT:PSS and ACSWCNT films because of the high in-plane thermal conductivity caused by the dimensional anisotropy of ACSWCNTs.
1. Introduction Thermoelectric materials have been studied extensively because wasted heat energy can be recovered as useful electric energy through the Seebeck effect [1]. The thermoelectric energy conversion efficiency of the thermoelectric material is evaluated by a dimensionless figure-ofmerit ZT= S 2σT / κ , where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. The thermal conductivity can be expressed as κ = κ e + κL where κ e and κL are the electronic and lattice contribution to the thermal conductivity, respectively. Conventional thermoelectric materials include inorganic semiconducting materials such as Bi2Te3, PbTe, silicide, and skutterudite. Although these materials exhibit a high figure-of-merit value (ZT∼1) over a specific temperature range, the broad application of the inorganic thermoelectric materials is limited because of the high cost of the raw material, heavy pollution during processing, and poor processability. Recently, organic conducting polymers such as poly(3,4-ethylenedioxythiophene (PEDOT), polyaniline (PANI), and poly(3-hexylthiophene) (P3HT) have gained considerable attention as alternative thermoelectric materials. They could be utilized in low-cost organic thermoelectric generators (TEGs), which are used in self-powered wearable devices and sensor networks [2,3]. Among such polymers, ⁎
PEDOT doped with poly(styrene sulfonate) (PEDOT:PSS) has been investigated intensively because of its high electrical conductivity, transparency, flexibility, water solubility, and air stability [3,4]. PEDOT:PSS films have been recently reported to show a high thermoelectric power factor (S 2σ ) after chemical pre- or post-treatment and appropriate control of its oxidation levels [5–7]. However, at present, organic materials show lower thermoelectric performance than that of inorganic thermoelectric materials. A promising strategy to improve the thermoelectric properties of the conducting polymer PEDOT:PSS is its hybridization with inorganic nanomaterials such as Te nanocrystals [8,9], Bi2Te3 powders [10], and carbon nanotubes (CNTs) [11–15]. Several research groups have reported the enhancement of the power factors of PEDOT:PSS composites by the use of CNTs. Yu et al. reported a high thermoelectric power factor of 160 μW m−1 K-2 from 60 wt% of single-walled CNTs (SWCNTs) within a matrix of PEDOT:PSS and polyvinyl acetate (PVA) [12]. Song et al. reported that a bilayer of SWCNT and PEDOT:PSS, prepared by spin-coating, can achieve a power factor of 21.1 μW m−1 K-2, which is four orders of magnitude higher than that of the PEDOT:PSS control sample [14]. Lee et al. demonstrated the enhanced thermoelectric power factor of double-walled CNTs (DWCNTs) and PEDOT:PSS nanocomposite films by post-treatment with ethylene glycol (EG) [15]. They obtained the highest power factor of 151 μW m−1 K-2 with an EG-
Corresponding author. E-mail address: [email protected] (S.-H. Chung).
https://doi.org/10.1016/j.mtcomm.2019.100867 Received 2 November 2019; Received in revised form 19 December 2019; Accepted 19 December 2019 Available online 20 December 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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Hitachi, SU8230) at 1.6 kV and 10 μA. The crystalline structure was recorded by X-ray diffraction (XRD, MiniFlex II, Rigaku) using Cu Kα radiation (30 kV, 15 mA). The Raman spectra of the samples were observed using a Raman spectroscopy system (Nicolet Almega XR, Thermo Scientific) with a laser wavelength of 532 nm and a spot size of 1 μm. The Seebeck coefficient (S = ΔV/ΔT) and in-plane electrical conductivity of the prepared films were simultaneously measured under a helium atmosphere (LSR-3, Linseis). The temperature and voltage of the films, at the contact points of the two K-type thermocouple probes, were recorded while a constant temperature gradient (30 °C) and a constant current (1 mA) were applied to the films. The Seebeck coefficient of the samples was obtained by subtracting the Seebeck coefficient of the Alumel electrode (18.9 μV K−1 at 25 °C) from the measured relative Seebeck coefficient. The in-plane and through-plane thermal diffusivity of the samples were measured by the laser flash method (LFA457, Netzsch) with an inplane and a through-plane sample holder, respectively. A disk-shaped freestanding PEDOT:PSS/AC-SWCNT film with 24 mm in diameter was used in order to measure the in-plane thermal diffusivity. A laser pulse at the center of the sample created a radial heat flow through the sample, and an infrared signal was measured at a position located on the rear face of the sample at 5 mm radius. In order to determine the through-plane thermal diffusivity, an infrared signal was measured at the center of the rear face of an 8 × 8 mm square-shaped film. The thermal conductivity κ of the composite films was evaluated using the relationship κ = ρ × Cp × α, where ρ is the mass density, Cp is the specific heat capacity, and α is the thermal diffusivity of the sample [19,20]. ρ was calculated from the measured thickness and the mass of the film, and Cp was measured using a differential scanning calorimeter (DSC, DSC 200 F3, Netzsch).
treated composite film with 20 wt% DWCNTs. The power factor enhancement was explained by the decoupling of the competing thermoelectric parameters—the Seebeck coefficient and electrical conductivity—in the composites. Although previous studies report the enhancement of the power factor of PEDOT:PSS by the hybridization with CNTs and the dependence of the power factor on the CNT content [11–13,15], the thermoelectric figure-of-merit ZT values have not been extensively investigated. This is probably due to the difficulty in the measurement of the in-plane thermal conductivity of thin organic films [16,17]. Even for the studies reporting the ZT values, the samples for thermal conductivity measurements were processed differently as compared to the samples for electrical conductivity and Seebeck coefficient measurements [5,6]. The ZT values were determined indirectly by using through-plane thermal conductivity [7] or theoretical estimation [11,12]. Materials with high dimensional anisotropy may exhibit significant differences in electrical or thermal transport in different directions [18]. Therefore, the in-plane thermal conductivity, electrical conductivity, and Seebeck coefficient should be measured using the same sample for the direct determination of the ZT value. In this study, we prepared PEDOT:PSS/SWCNT and PEDOT:PSS/ acid-treated SWCNT (AC-SWCNT) composite films by drop-casting the mixture solution. Compared to PEDOT:PSS/SWCNT films, the power factor of PEDOT:PSS/AC-SWCNT films improved significantly due to the enhanced electrical conductivity caused by acid treatment. Furthermore, we studied the thermoelectric properties of PEDOT:PSS/ SWCNT and PEDOT:PSS/AC-SWCNT films as a function of CNT content and compared the experimental data with the theoretical model of binary composites. In addition, we directly determined the ZT value of a PEDOT:PSS/AC-SWCNT freestanding film by measuring the in-plane thermal conductivity, electrical conductivity, and Seebeck coefficient of the same sample. The transport properties and the ZT value of PEDOT:PSS/AC-SWCNT composite film were found to be strongly dependent on the dimensional anisotropy of AC-SWCNTs.
3. Results and discussion Fig. 1 shows the high-resolution FE-SEM images of the PEDOT:PSS/ AC-SWCNT freestanding film with different AC-SWCNT content. The films are formed after drying the mixture solution of PEDOT:PSS and AC-SWCNT. The PEDOT:PSS polymer may coat the surface of ACSWCNTs and could infiltrate into the spaces within the AC-SWCNT network. Fig. 1(a) shows that the AC-SWCNT film has a nanowire network with AC-SWCNT bundles with a high aspect ratio. Compared to the AC-SWCNT film, the PEDOT:PSS/AC-SWCNT composite film, with 30 wt% of PEDOT:PSS, shows finer SWCNT bundles (Fig. 1(b)). This indicates that the AC-SWCNTs can be well-dispersed in a PEDOT:PSS aqueous solution [13]. The surface of the films becomes smoother as the PEDOT:PSS content increases (Fig. 1(c) and (d)). We note that the XRD results (data are not shown) indicated no evidence of crystalline ordering in the composite films of this study. The Raman spectra provided chemical and structural information on the composite films. Fig. 2 shows the Raman spectra of the pristine SWCNT, AC-SWCNT, PEDOT:PSS/SWCNT (50:50 wt%), PEDOT:PSS/ AC-SWCNT (50:50 wt%), and PEDOT:PSS films. For the pristine SWCNT, the G (in-plane stretching E2g mode), D (sp3 vibration mode), and 2D bands appear at 1593 cm−1, 1342 cm−1, and 2673 cm−1, respectively, as shown in Fig. 2(a). The low D/G intensity ratio (∼0.023) indicates that the pristine SWCNT film has a large grain size and low density of defects [21]. The Raman spectrum of the AC-SWCNT film in Fig. 2(b) does not exhibit significant changes compared to that of the pristine SWCNT film, which indicates that the SWCNTs are resistant to the structural changes caused by the acid treatment. We observed only a slight increase in the D/G intensity ratio (∼0.035) and a small shift of the G peak from 1593 to 1597 cm−1 that is associated with the intercalation of the acid molecules in the interstitial channels of the SWCNT bundles [22]. The DMSO-doped PEDOT:PSS film in Fig. 2(e) has a peak at 1425 cm−1, which is known to be associated with the symmetric stretching of the thiophene ring on the PEDOT chains [14,23]. The Raman peak of the symmetric stretching blue-shifts to 1433 cm−1 for
2. Material and methods The PEDOT:PSS aqueous solution (CLEVIOS PH1000) was purchased from Heraeus. The solid content of the solution was 1.0–1.3 wt %. The solution was mixed with 5 wt% of the organic polar solvent dimethyl sulfoxide (DMSO, Duksan pure chemicals) using a magnetic stirrer to increase electrical conductivity [5,14]. Unless otherwise stated, the PEDOT:PSS refers to the PEDOT:PSS mixed with DMSO. The SWCNT powder was purchased from Avention (> 98 wt% of purity, 1−2 nm in diameter, and 5−30 μm in length). For acid treatment, the SWCNT powder (100 mg) was magnetically stirred in a 3:1 solution (100 ml) of H2SO4 and HNO3 for 24 h and dried at 80 °C after being washed with de-ionized (DI) water. The PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT mixture solution was prepared by dispersing SWCNTs or AC-SWCNTs in an ultrasonic bath at 300 W for 3 h. Several weight fractions of SWCNT, or AC-SWCNT for composite films, were selected between 0 wt% (PEDOT:PSS film) and 100 wt% (SWCNT or AC-SWCNT film). Thin composite films of PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT were formed by drop-casting the mixture solution on a polyimide (PI) substrate. The films were dried on a hot plate at 120 °C for 10 min. The thickness of the films, measured by a profilometer (Alpha-Step, KLA Tencor), was in a range between 5 and 10 μm. In order to measure all the thermoelectric properties of the same sample, PEDOT:PSS/AC-SWCNT freestanding films, with thickness in a range between 100 and 250 μm, were prepared. The PEDOT:PSS/ACSWCNT mixture solution was drop-casted on a polyvinylidene fluoride (PVDF) sheet. The freestanding composite films were obtained after drying the solution at 80 °C and delaminating the PVDF sheet. The surface morphology of the composite films was measured by a high-resolution field emission scanning electron microscope (FE-SEM, 2
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Fig. 1. FE-SEM images of (a) AC-SWCNT film, and PEDOT:PSS/AC-SWCNT composite films with (b) 70 wt%, (c) 50 wt%, and (d) 30 wt% of AC-SWCNTs. The scale bar is 5 μm.
hydroxyl or carboxyl groups on defects [25,26]. However, the enhancement of electrical conductivity is mainly caused by the significant reduction of the junction resistance due to acid treatment [27,28]. Both samples exhibit a positive Seebeck coefficient, or p-type behavior, indicating that the majority carrier is hole. The Seebeck coefficient of the pristine SWCNT film is 43.5 μV K−1 at 25 °C and slightly increases with temperature. The Seebeck coefficient of the AC-SWCNT film is 17.3 μV K−1 at 25 °C and increases with temperature reaching 34.9 μV K-1 at 200 °C showing the typical metallic behavior [29]. The Seebeck coefficient of the AC-SWCNT film is lower than that of the pristine SWCNT film because the Seebeck coefficient is inversely proportional to the electrical conductivity [30]. Although the acid pre-treatment reduces the Seebeck coefficient, the power factor is enhanced as shown in Fig. 3(b) due to the significant increase in electrical conductivity. The power factor of the AC-SWCNT film (73.1 μW m−1 K−2) is enhanced by 63% compared to that of the pristine SWCNT film (44.9 μW m−1 K−2) at 25 °C. We studied the thermoelectric properties of PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT composite films as a function of their SWCNT content. The Seebeck coefficient of the PEDOT:PSS/SWCNT composite films increases with the SWCNT content while the electrical conductivity decreases as shown in Fig. 4(a). The electrical conductivity of the DMSO-doped PEDOT:PSS film (515 S cm−1) is higher than that of the pristine SWCNT film (237 S cm−1) at 25 °C. The power factor of the PEDOT:PSS/SWCNT films increases with the SWCNT content, reaching 45.0 μW m-1 K-2 for the SWCNT film as shown in Fig. 4(b). Fig. 4(c) and (d) show the thermoelectric properties of the PEDOT:PSS/AC-SWCNT composite films measured at 25 °C as a function of AC-SWCNT content. Unlike the PEDOT:PSS/SWCNT films, both the electrical conductivity and Seebeck coefficient increase with increasing the AC-SWCNT content. The simultaneous increase in the electrical conductivity and the Seebeck coefficient is mainly attributed to a substantial increase in carrier mobility [31] that is induced by the acid treatment. The power factor also increases with the AC-SWCNT content, reaching 67.0 μW m1 -2 K for the AC-SWCNT film. Comparing Fig. 4(b) and Fig. 4(d), the power factor of the PEDOT:PSS/AC-SWCNT film is higher than those of PEDOT:PSS/SWCNT films over the entire experimental range. Besides,
Fig. 2. Raman spectra of (a) pristine SWCNT film, (b) AC-SWCNT film, (c) PEDOT:PSS/SWCNT (50:50 wt%) film, (d) PEDOT:PSS/AC-SWCNT (50:50 wt %) film, and (e) PEDOT:PSS film.
the PEDOT:PSS/SWCNT film (Fig. 2(c)) and to 1440 cm−1 for the PEDOT:PSS/AC-SWCNT film (Fig. 2(d)). This is associated with the conformational changes and the enhanced doping of PEDOT:PSS chains due to the hybridization with SWCNTs and AC-SWCNTs [14,24]. In order to understand the effect of the acid treatment on the thermoelectric properties of the composite film, we first characterized the temperature dependence of the Seebeck coefficient and electrical conductivity of the pristine SWCNT and AC-SWCNT films. Fig. 3(a) shows the Seebeck coefficient and electrical conductivity of the SWCNT and AC-SWCNT films in the temperature range of 25−200 °C. The electrical conductivity of the AC-SWCNT film is enhanced more than 10 times (2444 S cm−1) compared to that of the pristine SWCNT film (237 S cm−1) at 25 °C. The treatment of SWCNTs by oxidizing acids is known to be an effective method to induce charge-transfer doping by physisorption of O2 molecules on SWCNT walls and chemisorption of 3
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Fig. 3. (a) Temperature dependence of the Seebeck coefficient and electrical conductivity for the pristine SWCNT (circle) and AC-SWCNT (square) films. (b) Temperature dependence of the power factor for the pristine SWCNT (circle) and AC-SWCNT (square) films.
both PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT films demonstrate a monotonous increase in their power factors with CNT content. We note that some previous works reported the maximum power factor of PEDOT:PSS/CNT composite at 60 wt% [12] and 20−25 wt% [15] of CNT content. However, the results were explained by either the less effective nanotube dispersion or the film morphologies. The effective thermoelectric properties of a binary composite are expressed as the following theoretical model [32–34]. The effective electrical conductivity and Seebeck coefficient for series coupling of materials 1 and 2 are:
σs =
S1 f / κ1 + S2 (1 − f )/ κ2 σ1 σ2 , Ss = f / κ1 + (1 − f )/ κ2 fσ2 + (1 − f ) σ1
of the interface between SWCNT and PEDOT:PSS, the experimental values of power factor exist within the upper and lower boundaries. The fitting curves for the power factor, represented as the solid lines in Fig. 4(b) and (d), are based on the weighted average of the series and parallel coupling models PF = (1 − α ) Ss2 σs + αSp2 σp where α is a fitting parameter. The α values in Fig. 4(b) and (d) are 0.57 and 0.65, respectively. The fitting parameter α, which is larger than 0.5, indicates that the PEDOT:PSS polymer chains have some tendency of parallel coupling with respect to SWCNTs. This suggests a possible phase separation between the PEDOT:PSS-rich and SWCNT-rich regions during the drop-casting process. In order to directly determine the thermoelectric figure-of-merit ZT value of the composite film, we attempted to measure all the thermoelectric properties, including in-plane thermal conductivity (κin), using the same sample. Only the thick freestanding films (100−250 μm) of PEDOT:PSS/AC-SWCNT were used for the ZT measurements because the power factor is higher than that of the PEDOT:PSS/SWCNT films. Moreover, the thick films exhibit low thermal loss and high structural stability during the radial heat propagation created by the laser pulses. Fig. 5 shows the thermoelectric properties of the PEDOT:PSS/ACSWCNT freestanding films as a function of the AC-SWCNT content. Although the power factor of the thick freestanding films is slightly lower than that of the thin films on PI substrate, due to the reduced electrical conductivity, the dependence on the AC-SWCNT content is
(1)
where σi, Si, and κi are the electrical conductivity, Seebeck coefficient, and thermal conductivity of material i (i = 1, 2). f is the volume fraction of material 1. For parallel coupling:
σp = fσ1 + (1 − f ) σ2 , Sp =
S1 σ1 f + S2 σ2 (1 − f ) σ1 f + σ2 (1 − f )
(2) (Sp2 σp )
The parallel and series coupling models represent the upper and lower (Ss2 σs ) boundaries of the power factor, respectively, as shown in Fig. 4(b) and (d). The curves are plotted after converting the volume fraction in Eqs. (1) and (2) to the mass fraction. Although the parallel and series coupling models are very simple and do not include the effect
Fig. 4. (a) Seebeck coefficient and electrical conductivity, (b) power factor of thin PEDOT:PSS/SWCNT films on PI substrate measured at 25 °C as a function of SWCNT content. (c) Seebeck coefficient and electrical conductivity, (d) power factor of thin PEDOT:PSS/AC-SWCNT films on PI substrate measured at 25 °C as a function of AC-SWCNT content. The red and blue dashed curves in (b) and (d) are based on the parallel and the series coupling models, respectively, and the solid curves are the best fit to the weighted average of the two models. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
4
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Fig. 5. (a) Seebeck coefficient and electrical conductivity, (b) power factor, (c) in-plane and through-plane thermal conductivity, and (d) in-plane ZT value of PEDOT:PSS/AC-SWCNT freestanding films as a function of their AC-SWCNT content.
similar to the thin film samples in Fig. 5(b). Thermal conductivity in Fig. 5(c) was obtained from the same sample used for the simultaneous measurement of Seebeck coefficient and electrical conductivity without further processing. The κin of the PEDOT:PSS/AC-SWCNT film increases with the AC-SWCNT content, while the through-plane thermal conductivity (κth) is within a range between 0.14 and 0.28 W/mK. The measured values of κin and κth for the AC-SWCNT film are 24.3 and 0.24 W/mK, respectively. The in-plane to through-plane thermal conductivity ratio (κin/κth) for the AC-SWCNT film is 101.3. CNT with a smaller diameter is known to have a very high thermal conductivity anisotropy [35,36]. The SWCNTs used in this work had a very high aspect ratio (1−2 nm in diameter, 5−30 μm in length). Therefore, the high anisotropy in thermal conductivity mainly originated from the dimensional anisotropy of AC-SWCNTs. Due to the high aspect ratio of AC-SWCNTs and the conductive junction between AC-SWCNTs, the PEDOT:PSS/AC-SWCNT composite films also have a high anisotropy in their thermal conductivity (κin/κth > 60). Even the polymeric PEDOT:PSS film shows anisotropy in its thermal conductivity. The measured values of κin and κth of a DMSO-doped PEDOT:PSS film are 2.4 and 0.26 W/mK, respectively. The thermal conductivity anisotropy κin/κth of the PEDOT:PSS film is 9.2. Anisotropy of thermal conductivity in PEDOT:PSS films have been reported in previous works using different measurement techniques [37,38]. Liu et al. reported that the κin/κth of DMSO-doped PEDOT:PSS film (σ ≈ 500 S cm−1) is 3.3 [37], and Wei et al. reported that the κin/κth of EG-doped PEDOT:PSS film (σ ≈ 810 S cm−1) is 4.7 [38]. We believe that the differences in the κin/κth values are caused by the different sample processing conditions because the transport properties of the PEDOT:PSS samples are known to be very sensitive to chemical treatment and film morphology [17,18,39]. The anisotropy in the transport properties of a doped-PEDOT:PSS film was explained to originate from the molecular ordering with respect to a substrate and the significant electronic contribution to thermal conductivity [17,37–39]. Fig. 5(d) shows the calculated ZT values of the PEDOT:PSS/ACSWCNT films as a function of their AC-SWCNT content. The ZT value of the PEDOT:PSS/AC-SWCNT films initially becomes lower than that of the PEDOT:PSS film, with 10 wt% addition of AC-SWCNTs, and
increases with the AC-SWCNT content. The minimum ZT value near 10 wt% of AC-SWCNT content may be associated with thermal percolation [40]. The parallel and series coupling model predict the monotonous changes of the thermoelectric properties as the composition of a binary composite varies [16,32]. However, the PEDOT:PSS/AC-SWCNT composite films in this study show non-monotonous changes of the ZT value. Furthermore, they exhibit lower ZT values than those of the PEDOT:PSS and AC-SWCNT films, which is mainly due to the high κin caused by the dimensional anisotropy of AC-SWCNTs. Since the thermoelectric figure-of-merit ZT= S 2σT / κ is inversely proportional to thermal conductivity, low thermal conductivity is desirable for thermoelectric applications. However, we obtained low ZT values, on the order of 10−4, for the PEDOT:PSS/AC-SWCNT composite films due to high κin. The ZT value is 7.2 × 10−4 for the AC-SWCNT film. We note that the measured κin and the ZT value of the pristine SWCNT film is 19.1 W/mK and 4.7 × 10−4, respectively. In the existing literature, 2–3 orders higher ZT values have been reported for spin-coated PEDOT:PSS thin films and PEDOT:PSS/SWCNT composite films [5,11]. However, the ZT values were obtained without the direct measurement of the inplane thermal conductivity, electrical conductivity and Seebeck coefficient using the same sample in the same direction. Our study suggests the importance of the direct determination of the in-plane ZT value, which allowed the observation of the significant effect of the dimensional anisotropy of SWCNTs on the thermoelectric properties of the composite film. 4. Conclusions We systematically studied the thermoelectric properties of PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT films as a function of their SWCNT content. The power factor values of the PEDOT:PSS/ACSWCNT films were higher than those of the PEDOT:PSS/SWCNT films over the entire experimental range due to the significant increase in the electrical conductivity by the acid pre-treatment. Furthermore, the power factor of both PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT films increased with the SWCNT content, and the values were within the boundaries calculated by the parallel and series coupling model of 5
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binary composites. In addition, we directly determined the ZT value of a PEDOT:PSS/AC-SWCNT freestanding film by measuring all the thermoelectric properties of the same sample in the same direction. The PEDOT:PSS/AC-SWCNT composite films showed non-monotonous changes of the ZT values, and exhibited lower ZT values than those of the PEDOT:PSS and AC-SWCNT films due to the high κin caused by the dimensional anisotropy of AC-SWCNTs.
[13]
[14]
[15]
Data availability
[16]
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
[17]
CRediT authorship contribution statement
[18]
Seok-Hwan Chung: Methodology, Formal analysis, Writing - original draft. Dong Hwan Kim: Resources, Writing - review & editing. Hanna Kim: Investigation. Hoyoung Kim: Supervision. Sang Won Jeong: Writing - review & editing, Conceptualization.
[19] [20] [21]
Declaration of Competing Interest
[22]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[23]
[24]
Acknowledgements [25]
This work was supported by the DGIST R&D Program of the Ministry of Science and ICT of Korea (19-ET-02). We would also like to thank the DGIST Center for Core Research Facilities for technical support.
[26]
[27]
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Status Solidi B 246 (11-12) (2009) 2717–2720. M. Łapkowski, A. Proń, Electrochemical oxidation of poly(3,4-ethylenedioxythiophene)-“in situ” conductivity and spectroscopic investigations, Synth. Met. 110 (2000) 79–83. S. Garreau, G. Louarn, J.P. Buisson, G. Froyer, S. Lefrant, In situ spectroelectrochemical Raman studies of poly(3,4-ethylenedioxythiophene) (PEDT), Macromolecules 32 (1999) 6807–6812. M. Grujicic, G. Cao, R. Singh, The effect of topological defects and oxygen adsorption on the electronic transport properties of single-walled carbon-nanotubes, Appl. Surf. Sci. 211 (2003) 166–183. R. Graupner, J. Abraham, A. Vencelová, T. Seyller, F. Hennrich, M.M. Kappes, et al., Doping of single-walled carbon nanotube bundles by Brønsted acids, Phys. Chem. Chem. Phys. 5 (2003) 5472–5476. H.-Z. Geng, K.K. Kim, K.P. So, Y.S. Lee, Y. Chang, Y.H. Lee, Effect of acid treatment on carbon nanotube-based flexible transparent conducting films, J. Am. Chem. Soc. 129 (2007) 7758–7759. P.N. Nirmalraj, P.E. Lyons, S. De, J.N. Coleman, J.J. Boland, Electrical connectivity in single-walled carbon nanotube networks, Nano Lett. 9 (11) (2009) 3890–3895. C. Dun, C.A. Hewitt, H. Huang, J. Xu, D.S. Montgomery, W. Nie, et al., Layered Bi2Se3 nanoplate/polyvinylidene fluoride composite based n-type thermoelectric fabrics, ACS Appl. Mater. Interfaces 7 (2015) 7054–7059. M. Cutlee, N.F. Mott, Observation of Anderson localization in an electron gas, Phys. Rev. 181 (1969) 1336–1340. Y. Du, S.Z. Shen, W. Yang, R. Donelson, K. Cai, P.S. Casey, Simultaneous increase in conductivity and Seebeck coefficient in a polyaniline/graphene nanosheets thermoelectric nanocomposite, Synth. Met. 161 (2012) 2688–2692. D.J. Bergman, O. Levy, Thermoelectric properties of a composite medium, J. Appl. Phys. 70 (11) (1991) 6821–6833. Y. Gelbstein, Thermoelectric power and structural properties in two-phase Sn/SnTe alloys, J. Appl. Phys. 105 (2) (2009) 023713. Y. Yang, S.H. Xie, F.Y. Ma, J.Y. Li, On the effective thermoelectric properties of layered heterogeneous medium, J. Appl. Phys. 111 (1) (2012) 013510. Z. Ma, Z. Guo, H. Zhang, T. Chang, Extremely high thermal conductivity anisotropy of double-walled carbon nanotubes, AIP Adv. 7 (6) (2017) 065104. T. Kodama, M. Ohnishi, W. Park, T. Shiga, J. Park, T. Shimada, et al., Modulation of thermal and thermoelectric transport in individual carbon nanotubes by fullerene encapsulation, Nature Mater. 16 (2017) 892–898. J. Liu, X. Wang, D. Li, N.E. Coates, R.A. Segalman, D.G. Cahill, Thermal conductivity and elastic constants of PEDOT:PSS with high electrical conductivity, Macromolecules 48 (2015) 585–591. Q. Wei, C. Uehara, M. Mukaida, K. Kirihara, T. Ishida, Measurement of in-plane thermal conductivity in polymer films, AIP Adv. 6 (4) (2016) 045315. A.M. Nardes, M. Kemerink, R.A.J. Janssen, J.A.M. Bastiaansen, N.M.M. Kiggen, B.M.W. 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Thermoelectric properties of PEDOT: PSS and acid-treated SWCNT composite films
T
Seok-Hwan Chung*, Dong Hwan Kim, Hanna Kim, Hoyoung Kim, Sang Won Jeong Materials Research Institute, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: PEDOT:PSS SWCNT Composite film Thermoelectric property In-plane thermal conductivity
We investigated the thermoelectric properties of composite films that consist of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and single-walled carbon nanotubes (SWCNTs). Thin films with 5−10 μm thickness of PEDOT:PSS/SWCNT and PEDOT:PSS/acid-treated SWCNT (AC-SWCNT) were prepared by varying the CNT content to determine their power factor. Both PEDOT:PSS/SWCNT and PEDOT:PSS/ AC-SWCNT films showed an increase in their power factors with CNT content. The power factor values of PEDOT:PSS/AC-SWCNT films were higher than those of PEDOT:PSS/SWCNT films over the entire experimental range, which was mainly attributed to the significantly increased electrical conductivity. Thus, a series of PEDOT:PSS/AC-SWCNT freestanding films with 100−250 μm thickness were synthesized, and the same sample was used to measure electrical conductivity, Seebeck coefficient, and thermal conductivity to calculate the inplane figure-of-merit ZT values. The composite films exhibited lower ZT values than those of PEDOT:PSS and ACSWCNT films because of the high in-plane thermal conductivity caused by the dimensional anisotropy of ACSWCNTs.
1. Introduction Thermoelectric materials have been studied extensively because wasted heat energy can be recovered as useful electric energy through the Seebeck effect [1]. The thermoelectric energy conversion efficiency of the thermoelectric material is evaluated by a dimensionless figure-ofmerit ZT= S 2σT / κ , where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. The thermal conductivity can be expressed as κ = κ e + κL where κ e and κL are the electronic and lattice contribution to the thermal conductivity, respectively. Conventional thermoelectric materials include inorganic semiconducting materials such as Bi2Te3, PbTe, silicide, and skutterudite. Although these materials exhibit a high figure-of-merit value (ZT∼1) over a specific temperature range, the broad application of the inorganic thermoelectric materials is limited because of the high cost of the raw material, heavy pollution during processing, and poor processability. Recently, organic conducting polymers such as poly(3,4-ethylenedioxythiophene (PEDOT), polyaniline (PANI), and poly(3-hexylthiophene) (P3HT) have gained considerable attention as alternative thermoelectric materials. They could be utilized in low-cost organic thermoelectric generators (TEGs), which are used in self-powered wearable devices and sensor networks [2,3]. Among such polymers, ⁎
PEDOT doped with poly(styrene sulfonate) (PEDOT:PSS) has been investigated intensively because of its high electrical conductivity, transparency, flexibility, water solubility, and air stability [3,4]. PEDOT:PSS films have been recently reported to show a high thermoelectric power factor (S 2σ ) after chemical pre- or post-treatment and appropriate control of its oxidation levels [5–7]. However, at present, organic materials show lower thermoelectric performance than that of inorganic thermoelectric materials. A promising strategy to improve the thermoelectric properties of the conducting polymer PEDOT:PSS is its hybridization with inorganic nanomaterials such as Te nanocrystals [8,9], Bi2Te3 powders [10], and carbon nanotubes (CNTs) [11–15]. Several research groups have reported the enhancement of the power factors of PEDOT:PSS composites by the use of CNTs. Yu et al. reported a high thermoelectric power factor of 160 μW m−1 K-2 from 60 wt% of single-walled CNTs (SWCNTs) within a matrix of PEDOT:PSS and polyvinyl acetate (PVA) [12]. Song et al. reported that a bilayer of SWCNT and PEDOT:PSS, prepared by spin-coating, can achieve a power factor of 21.1 μW m−1 K-2, which is four orders of magnitude higher than that of the PEDOT:PSS control sample [14]. Lee et al. demonstrated the enhanced thermoelectric power factor of double-walled CNTs (DWCNTs) and PEDOT:PSS nanocomposite films by post-treatment with ethylene glycol (EG) [15]. They obtained the highest power factor of 151 μW m−1 K-2 with an EG-
Corresponding author. E-mail address: [email protected] (S.-H. Chung).
https://doi.org/10.1016/j.mtcomm.2019.100867 Received 2 November 2019; Received in revised form 19 December 2019; Accepted 19 December 2019 Available online 20 December 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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Hitachi, SU8230) at 1.6 kV and 10 μA. The crystalline structure was recorded by X-ray diffraction (XRD, MiniFlex II, Rigaku) using Cu Kα radiation (30 kV, 15 mA). The Raman spectra of the samples were observed using a Raman spectroscopy system (Nicolet Almega XR, Thermo Scientific) with a laser wavelength of 532 nm and a spot size of 1 μm. The Seebeck coefficient (S = ΔV/ΔT) and in-plane electrical conductivity of the prepared films were simultaneously measured under a helium atmosphere (LSR-3, Linseis). The temperature and voltage of the films, at the contact points of the two K-type thermocouple probes, were recorded while a constant temperature gradient (30 °C) and a constant current (1 mA) were applied to the films. The Seebeck coefficient of the samples was obtained by subtracting the Seebeck coefficient of the Alumel electrode (18.9 μV K−1 at 25 °C) from the measured relative Seebeck coefficient. The in-plane and through-plane thermal diffusivity of the samples were measured by the laser flash method (LFA457, Netzsch) with an inplane and a through-plane sample holder, respectively. A disk-shaped freestanding PEDOT:PSS/AC-SWCNT film with 24 mm in diameter was used in order to measure the in-plane thermal diffusivity. A laser pulse at the center of the sample created a radial heat flow through the sample, and an infrared signal was measured at a position located on the rear face of the sample at 5 mm radius. In order to determine the through-plane thermal diffusivity, an infrared signal was measured at the center of the rear face of an 8 × 8 mm square-shaped film. The thermal conductivity κ of the composite films was evaluated using the relationship κ = ρ × Cp × α, where ρ is the mass density, Cp is the specific heat capacity, and α is the thermal diffusivity of the sample [19,20]. ρ was calculated from the measured thickness and the mass of the film, and Cp was measured using a differential scanning calorimeter (DSC, DSC 200 F3, Netzsch).
treated composite film with 20 wt% DWCNTs. The power factor enhancement was explained by the decoupling of the competing thermoelectric parameters—the Seebeck coefficient and electrical conductivity—in the composites. Although previous studies report the enhancement of the power factor of PEDOT:PSS by the hybridization with CNTs and the dependence of the power factor on the CNT content [11–13,15], the thermoelectric figure-of-merit ZT values have not been extensively investigated. This is probably due to the difficulty in the measurement of the in-plane thermal conductivity of thin organic films [16,17]. Even for the studies reporting the ZT values, the samples for thermal conductivity measurements were processed differently as compared to the samples for electrical conductivity and Seebeck coefficient measurements [5,6]. The ZT values were determined indirectly by using through-plane thermal conductivity [7] or theoretical estimation [11,12]. Materials with high dimensional anisotropy may exhibit significant differences in electrical or thermal transport in different directions [18]. Therefore, the in-plane thermal conductivity, electrical conductivity, and Seebeck coefficient should be measured using the same sample for the direct determination of the ZT value. In this study, we prepared PEDOT:PSS/SWCNT and PEDOT:PSS/ acid-treated SWCNT (AC-SWCNT) composite films by drop-casting the mixture solution. Compared to PEDOT:PSS/SWCNT films, the power factor of PEDOT:PSS/AC-SWCNT films improved significantly due to the enhanced electrical conductivity caused by acid treatment. Furthermore, we studied the thermoelectric properties of PEDOT:PSS/ SWCNT and PEDOT:PSS/AC-SWCNT films as a function of CNT content and compared the experimental data with the theoretical model of binary composites. In addition, we directly determined the ZT value of a PEDOT:PSS/AC-SWCNT freestanding film by measuring the in-plane thermal conductivity, electrical conductivity, and Seebeck coefficient of the same sample. The transport properties and the ZT value of PEDOT:PSS/AC-SWCNT composite film were found to be strongly dependent on the dimensional anisotropy of AC-SWCNTs.
3. Results and discussion Fig. 1 shows the high-resolution FE-SEM images of the PEDOT:PSS/ AC-SWCNT freestanding film with different AC-SWCNT content. The films are formed after drying the mixture solution of PEDOT:PSS and AC-SWCNT. The PEDOT:PSS polymer may coat the surface of ACSWCNTs and could infiltrate into the spaces within the AC-SWCNT network. Fig. 1(a) shows that the AC-SWCNT film has a nanowire network with AC-SWCNT bundles with a high aspect ratio. Compared to the AC-SWCNT film, the PEDOT:PSS/AC-SWCNT composite film, with 30 wt% of PEDOT:PSS, shows finer SWCNT bundles (Fig. 1(b)). This indicates that the AC-SWCNTs can be well-dispersed in a PEDOT:PSS aqueous solution [13]. The surface of the films becomes smoother as the PEDOT:PSS content increases (Fig. 1(c) and (d)). We note that the XRD results (data are not shown) indicated no evidence of crystalline ordering in the composite films of this study. The Raman spectra provided chemical and structural information on the composite films. Fig. 2 shows the Raman spectra of the pristine SWCNT, AC-SWCNT, PEDOT:PSS/SWCNT (50:50 wt%), PEDOT:PSS/ AC-SWCNT (50:50 wt%), and PEDOT:PSS films. For the pristine SWCNT, the G (in-plane stretching E2g mode), D (sp3 vibration mode), and 2D bands appear at 1593 cm−1, 1342 cm−1, and 2673 cm−1, respectively, as shown in Fig. 2(a). The low D/G intensity ratio (∼0.023) indicates that the pristine SWCNT film has a large grain size and low density of defects [21]. The Raman spectrum of the AC-SWCNT film in Fig. 2(b) does not exhibit significant changes compared to that of the pristine SWCNT film, which indicates that the SWCNTs are resistant to the structural changes caused by the acid treatment. We observed only a slight increase in the D/G intensity ratio (∼0.035) and a small shift of the G peak from 1593 to 1597 cm−1 that is associated with the intercalation of the acid molecules in the interstitial channels of the SWCNT bundles [22]. The DMSO-doped PEDOT:PSS film in Fig. 2(e) has a peak at 1425 cm−1, which is known to be associated with the symmetric stretching of the thiophene ring on the PEDOT chains [14,23]. The Raman peak of the symmetric stretching blue-shifts to 1433 cm−1 for
2. Material and methods The PEDOT:PSS aqueous solution (CLEVIOS PH1000) was purchased from Heraeus. The solid content of the solution was 1.0–1.3 wt %. The solution was mixed with 5 wt% of the organic polar solvent dimethyl sulfoxide (DMSO, Duksan pure chemicals) using a magnetic stirrer to increase electrical conductivity [5,14]. Unless otherwise stated, the PEDOT:PSS refers to the PEDOT:PSS mixed with DMSO. The SWCNT powder was purchased from Avention (> 98 wt% of purity, 1−2 nm in diameter, and 5−30 μm in length). For acid treatment, the SWCNT powder (100 mg) was magnetically stirred in a 3:1 solution (100 ml) of H2SO4 and HNO3 for 24 h and dried at 80 °C after being washed with de-ionized (DI) water. The PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT mixture solution was prepared by dispersing SWCNTs or AC-SWCNTs in an ultrasonic bath at 300 W for 3 h. Several weight fractions of SWCNT, or AC-SWCNT for composite films, were selected between 0 wt% (PEDOT:PSS film) and 100 wt% (SWCNT or AC-SWCNT film). Thin composite films of PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT were formed by drop-casting the mixture solution on a polyimide (PI) substrate. The films were dried on a hot plate at 120 °C for 10 min. The thickness of the films, measured by a profilometer (Alpha-Step, KLA Tencor), was in a range between 5 and 10 μm. In order to measure all the thermoelectric properties of the same sample, PEDOT:PSS/AC-SWCNT freestanding films, with thickness in a range between 100 and 250 μm, were prepared. The PEDOT:PSS/ACSWCNT mixture solution was drop-casted on a polyvinylidene fluoride (PVDF) sheet. The freestanding composite films were obtained after drying the solution at 80 °C and delaminating the PVDF sheet. The surface morphology of the composite films was measured by a high-resolution field emission scanning electron microscope (FE-SEM, 2
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Fig. 1. FE-SEM images of (a) AC-SWCNT film, and PEDOT:PSS/AC-SWCNT composite films with (b) 70 wt%, (c) 50 wt%, and (d) 30 wt% of AC-SWCNTs. The scale bar is 5 μm.
hydroxyl or carboxyl groups on defects [25,26]. However, the enhancement of electrical conductivity is mainly caused by the significant reduction of the junction resistance due to acid treatment [27,28]. Both samples exhibit a positive Seebeck coefficient, or p-type behavior, indicating that the majority carrier is hole. The Seebeck coefficient of the pristine SWCNT film is 43.5 μV K−1 at 25 °C and slightly increases with temperature. The Seebeck coefficient of the AC-SWCNT film is 17.3 μV K−1 at 25 °C and increases with temperature reaching 34.9 μV K-1 at 200 °C showing the typical metallic behavior [29]. The Seebeck coefficient of the AC-SWCNT film is lower than that of the pristine SWCNT film because the Seebeck coefficient is inversely proportional to the electrical conductivity [30]. Although the acid pre-treatment reduces the Seebeck coefficient, the power factor is enhanced as shown in Fig. 3(b) due to the significant increase in electrical conductivity. The power factor of the AC-SWCNT film (73.1 μW m−1 K−2) is enhanced by 63% compared to that of the pristine SWCNT film (44.9 μW m−1 K−2) at 25 °C. We studied the thermoelectric properties of PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT composite films as a function of their SWCNT content. The Seebeck coefficient of the PEDOT:PSS/SWCNT composite films increases with the SWCNT content while the electrical conductivity decreases as shown in Fig. 4(a). The electrical conductivity of the DMSO-doped PEDOT:PSS film (515 S cm−1) is higher than that of the pristine SWCNT film (237 S cm−1) at 25 °C. The power factor of the PEDOT:PSS/SWCNT films increases with the SWCNT content, reaching 45.0 μW m-1 K-2 for the SWCNT film as shown in Fig. 4(b). Fig. 4(c) and (d) show the thermoelectric properties of the PEDOT:PSS/AC-SWCNT composite films measured at 25 °C as a function of AC-SWCNT content. Unlike the PEDOT:PSS/SWCNT films, both the electrical conductivity and Seebeck coefficient increase with increasing the AC-SWCNT content. The simultaneous increase in the electrical conductivity and the Seebeck coefficient is mainly attributed to a substantial increase in carrier mobility [31] that is induced by the acid treatment. The power factor also increases with the AC-SWCNT content, reaching 67.0 μW m1 -2 K for the AC-SWCNT film. Comparing Fig. 4(b) and Fig. 4(d), the power factor of the PEDOT:PSS/AC-SWCNT film is higher than those of PEDOT:PSS/SWCNT films over the entire experimental range. Besides,
Fig. 2. Raman spectra of (a) pristine SWCNT film, (b) AC-SWCNT film, (c) PEDOT:PSS/SWCNT (50:50 wt%) film, (d) PEDOT:PSS/AC-SWCNT (50:50 wt %) film, and (e) PEDOT:PSS film.
the PEDOT:PSS/SWCNT film (Fig. 2(c)) and to 1440 cm−1 for the PEDOT:PSS/AC-SWCNT film (Fig. 2(d)). This is associated with the conformational changes and the enhanced doping of PEDOT:PSS chains due to the hybridization with SWCNTs and AC-SWCNTs [14,24]. In order to understand the effect of the acid treatment on the thermoelectric properties of the composite film, we first characterized the temperature dependence of the Seebeck coefficient and electrical conductivity of the pristine SWCNT and AC-SWCNT films. Fig. 3(a) shows the Seebeck coefficient and electrical conductivity of the SWCNT and AC-SWCNT films in the temperature range of 25−200 °C. The electrical conductivity of the AC-SWCNT film is enhanced more than 10 times (2444 S cm−1) compared to that of the pristine SWCNT film (237 S cm−1) at 25 °C. The treatment of SWCNTs by oxidizing acids is known to be an effective method to induce charge-transfer doping by physisorption of O2 molecules on SWCNT walls and chemisorption of 3
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Fig. 3. (a) Temperature dependence of the Seebeck coefficient and electrical conductivity for the pristine SWCNT (circle) and AC-SWCNT (square) films. (b) Temperature dependence of the power factor for the pristine SWCNT (circle) and AC-SWCNT (square) films.
both PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT films demonstrate a monotonous increase in their power factors with CNT content. We note that some previous works reported the maximum power factor of PEDOT:PSS/CNT composite at 60 wt% [12] and 20−25 wt% [15] of CNT content. However, the results were explained by either the less effective nanotube dispersion or the film morphologies. The effective thermoelectric properties of a binary composite are expressed as the following theoretical model [32–34]. The effective electrical conductivity and Seebeck coefficient for series coupling of materials 1 and 2 are:
σs =
S1 f / κ1 + S2 (1 − f )/ κ2 σ1 σ2 , Ss = f / κ1 + (1 − f )/ κ2 fσ2 + (1 − f ) σ1
of the interface between SWCNT and PEDOT:PSS, the experimental values of power factor exist within the upper and lower boundaries. The fitting curves for the power factor, represented as the solid lines in Fig. 4(b) and (d), are based on the weighted average of the series and parallel coupling models PF = (1 − α ) Ss2 σs + αSp2 σp where α is a fitting parameter. The α values in Fig. 4(b) and (d) are 0.57 and 0.65, respectively. The fitting parameter α, which is larger than 0.5, indicates that the PEDOT:PSS polymer chains have some tendency of parallel coupling with respect to SWCNTs. This suggests a possible phase separation between the PEDOT:PSS-rich and SWCNT-rich regions during the drop-casting process. In order to directly determine the thermoelectric figure-of-merit ZT value of the composite film, we attempted to measure all the thermoelectric properties, including in-plane thermal conductivity (κin), using the same sample. Only the thick freestanding films (100−250 μm) of PEDOT:PSS/AC-SWCNT were used for the ZT measurements because the power factor is higher than that of the PEDOT:PSS/SWCNT films. Moreover, the thick films exhibit low thermal loss and high structural stability during the radial heat propagation created by the laser pulses. Fig. 5 shows the thermoelectric properties of the PEDOT:PSS/ACSWCNT freestanding films as a function of the AC-SWCNT content. Although the power factor of the thick freestanding films is slightly lower than that of the thin films on PI substrate, due to the reduced electrical conductivity, the dependence on the AC-SWCNT content is
(1)
where σi, Si, and κi are the electrical conductivity, Seebeck coefficient, and thermal conductivity of material i (i = 1, 2). f is the volume fraction of material 1. For parallel coupling:
σp = fσ1 + (1 − f ) σ2 , Sp =
S1 σ1 f + S2 σ2 (1 − f ) σ1 f + σ2 (1 − f )
(2) (Sp2 σp )
The parallel and series coupling models represent the upper and lower (Ss2 σs ) boundaries of the power factor, respectively, as shown in Fig. 4(b) and (d). The curves are plotted after converting the volume fraction in Eqs. (1) and (2) to the mass fraction. Although the parallel and series coupling models are very simple and do not include the effect
Fig. 4. (a) Seebeck coefficient and electrical conductivity, (b) power factor of thin PEDOT:PSS/SWCNT films on PI substrate measured at 25 °C as a function of SWCNT content. (c) Seebeck coefficient and electrical conductivity, (d) power factor of thin PEDOT:PSS/AC-SWCNT films on PI substrate measured at 25 °C as a function of AC-SWCNT content. The red and blue dashed curves in (b) and (d) are based on the parallel and the series coupling models, respectively, and the solid curves are the best fit to the weighted average of the two models. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
4
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Fig. 5. (a) Seebeck coefficient and electrical conductivity, (b) power factor, (c) in-plane and through-plane thermal conductivity, and (d) in-plane ZT value of PEDOT:PSS/AC-SWCNT freestanding films as a function of their AC-SWCNT content.
similar to the thin film samples in Fig. 5(b). Thermal conductivity in Fig. 5(c) was obtained from the same sample used for the simultaneous measurement of Seebeck coefficient and electrical conductivity without further processing. The κin of the PEDOT:PSS/AC-SWCNT film increases with the AC-SWCNT content, while the through-plane thermal conductivity (κth) is within a range between 0.14 and 0.28 W/mK. The measured values of κin and κth for the AC-SWCNT film are 24.3 and 0.24 W/mK, respectively. The in-plane to through-plane thermal conductivity ratio (κin/κth) for the AC-SWCNT film is 101.3. CNT with a smaller diameter is known to have a very high thermal conductivity anisotropy [35,36]. The SWCNTs used in this work had a very high aspect ratio (1−2 nm in diameter, 5−30 μm in length). Therefore, the high anisotropy in thermal conductivity mainly originated from the dimensional anisotropy of AC-SWCNTs. Due to the high aspect ratio of AC-SWCNTs and the conductive junction between AC-SWCNTs, the PEDOT:PSS/AC-SWCNT composite films also have a high anisotropy in their thermal conductivity (κin/κth > 60). Even the polymeric PEDOT:PSS film shows anisotropy in its thermal conductivity. The measured values of κin and κth of a DMSO-doped PEDOT:PSS film are 2.4 and 0.26 W/mK, respectively. The thermal conductivity anisotropy κin/κth of the PEDOT:PSS film is 9.2. Anisotropy of thermal conductivity in PEDOT:PSS films have been reported in previous works using different measurement techniques [37,38]. Liu et al. reported that the κin/κth of DMSO-doped PEDOT:PSS film (σ ≈ 500 S cm−1) is 3.3 [37], and Wei et al. reported that the κin/κth of EG-doped PEDOT:PSS film (σ ≈ 810 S cm−1) is 4.7 [38]. We believe that the differences in the κin/κth values are caused by the different sample processing conditions because the transport properties of the PEDOT:PSS samples are known to be very sensitive to chemical treatment and film morphology [17,18,39]. The anisotropy in the transport properties of a doped-PEDOT:PSS film was explained to originate from the molecular ordering with respect to a substrate and the significant electronic contribution to thermal conductivity [17,37–39]. Fig. 5(d) shows the calculated ZT values of the PEDOT:PSS/ACSWCNT films as a function of their AC-SWCNT content. The ZT value of the PEDOT:PSS/AC-SWCNT films initially becomes lower than that of the PEDOT:PSS film, with 10 wt% addition of AC-SWCNTs, and
increases with the AC-SWCNT content. The minimum ZT value near 10 wt% of AC-SWCNT content may be associated with thermal percolation [40]. The parallel and series coupling model predict the monotonous changes of the thermoelectric properties as the composition of a binary composite varies [16,32]. However, the PEDOT:PSS/AC-SWCNT composite films in this study show non-monotonous changes of the ZT value. Furthermore, they exhibit lower ZT values than those of the PEDOT:PSS and AC-SWCNT films, which is mainly due to the high κin caused by the dimensional anisotropy of AC-SWCNTs. Since the thermoelectric figure-of-merit ZT= S 2σT / κ is inversely proportional to thermal conductivity, low thermal conductivity is desirable for thermoelectric applications. However, we obtained low ZT values, on the order of 10−4, for the PEDOT:PSS/AC-SWCNT composite films due to high κin. The ZT value is 7.2 × 10−4 for the AC-SWCNT film. We note that the measured κin and the ZT value of the pristine SWCNT film is 19.1 W/mK and 4.7 × 10−4, respectively. In the existing literature, 2–3 orders higher ZT values have been reported for spin-coated PEDOT:PSS thin films and PEDOT:PSS/SWCNT composite films [5,11]. However, the ZT values were obtained without the direct measurement of the inplane thermal conductivity, electrical conductivity and Seebeck coefficient using the same sample in the same direction. Our study suggests the importance of the direct determination of the in-plane ZT value, which allowed the observation of the significant effect of the dimensional anisotropy of SWCNTs on the thermoelectric properties of the composite film. 4. Conclusions We systematically studied the thermoelectric properties of PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT films as a function of their SWCNT content. The power factor values of the PEDOT:PSS/ACSWCNT films were higher than those of the PEDOT:PSS/SWCNT films over the entire experimental range due to the significant increase in the electrical conductivity by the acid pre-treatment. Furthermore, the power factor of both PEDOT:PSS/SWCNT and PEDOT:PSS/AC-SWCNT films increased with the SWCNT content, and the values were within the boundaries calculated by the parallel and series coupling model of 5
Materials Today Communications 23 (2020) 100867
S.-H. Chung, et al.
binary composites. In addition, we directly determined the ZT value of a PEDOT:PSS/AC-SWCNT freestanding film by measuring all the thermoelectric properties of the same sample in the same direction. The PEDOT:PSS/AC-SWCNT composite films showed non-monotonous changes of the ZT values, and exhibited lower ZT values than those of the PEDOT:PSS and AC-SWCNT films due to the high κin caused by the dimensional anisotropy of AC-SWCNTs.
[13]
[14]
[15]
Data availability
[16]
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
[17]
CRediT authorship contribution statement
[18]
Seok-Hwan Chung: Methodology, Formal analysis, Writing - original draft. Dong Hwan Kim: Resources, Writing - review & editing. Hanna Kim: Investigation. Hoyoung Kim: Supervision. Sang Won Jeong: Writing - review & editing, Conceptualization.
[19] [20] [21]
Declaration of Competing Interest
[22]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[23]
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Acknowledgements [25]
This work was supported by the DGIST R&D Program of the Ministry of Science and ICT of Korea (19-ET-02). We would also like to thank the DGIST Center for Core Research Facilities for technical support.
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