Highly Integrated and Flexible Thermoelectric Module Fabricated by

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Highly Integrated and Flexible Thermoelectric Module Fabricated by Brush-Cast Doping of a Highly Aligned Carbon Nanotube Web Cheng Jin An, Young Hun Kang, Hyeonjun Song, Youngjin Jeong, and Song Yun Cho ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01673 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Applied Energy Materials

Highly Integrated and Flexible Thermoelectric Module Fabricated by Brush-Cast Doping of a Highly Aligned Carbon Nanotube Web Cheng Jin An,† Young Hun Kang,† Hyeonjun Song,‡ Youngjin Jeong,‡ and Song Yun Cho*,† † Division

of Advanced Materials, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea. ‡ Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdoro, Dongjak-gu, Seoul 06978, Republic of Korea.

ABSTRACT: With increasing attention on flexible or wearable power-conversion devices, intensive research efforts have been devoted to flexible organic thermoelectric (TE) modules to replace the brittle inorganic ones. In this study, a highly integrated and flexible TE module with a novel device architecture based on a carbon nanotube (CNT) web is proposed. The pristine CNT web shows great electrical conductivity of 998.3 S cm-1 with the highly aligned structure, owing to the increased carrier mobility in the longitudinal direction. To realize optimal TE property, the pristine CNT web is alternately doped with p- and n-type carriers using FeCl3 and benzyl viologen, respectively, via a brush-casting method. Brush-casting is the simple doping process that enables large-area and continuous fabrication of flexible TE modules by allowing precise doping of the localized area without a shadow mask. Flexible TE modules were then fabricated by repeated brushing and folding of the CNT webs. Owing to the synergic effect of the highly integrated high-performance TE material (highly aligned CNT web) and the facile doping process (brush-casting), flexible TE modules consisting of 120 p-n couples over an area of 8 cm2 show a maximum power output of 5.3 μW for a temperature difference of 11.7 K. KEYWORDS: directly spun carbon nanotube web, doping process, brush-casting, flexible thermoelectric module, thermal sensor

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INTRODUCTION Thermoelectric (TE) devices have received considerable attention because of their use in various energy harvesting applications by converting heat into usable electricity directly.1,2 With the increase in practical demand for flexible power-conversion devices, it is essential to develop high-performance flexible TE modules to replace the conventional ones, which are typically composed of heavy, toxic, rigid, and expensive inorganic materials.3,4 Over the past few decades, organic polymers, nanocarbon-based materials, and their hybrids have mainly been demonstrated as potential candidates for flexible TE materials, owing to their flexible, lightweight, inexpensive, easily processable, and scalable properties.5-10 In particular, the TE performance of conductive organic polymers has been significantly enhanced, such as poly(3,4-ethylenedioxythiophene)

(PEDOT),

poly(3-hexylthiophene),

and

polyaniline

(PANI).11-13 However, these polymeric TE materials are sensitive to moisture in ambient conditions and their TE performance is still insufficient, thus limiting their practical applications. A more significant problem is the difficulty in obtaining polymer films with an adequate thickness of a few millimeters in the vertical direction, according to the configuration of the TE device to realize a large temperature gradient. Carbon nanotube (CNT) is also considered to be a flexible TE material with high TE performance due to its superior charge carrier mobility, narrow energy bandgap, and controllable Seebeck coefficient.14,15 In addition, it is well known that CNT has a low density of approximately 1 g cm-3 with great mechanical properties.16 More importantly, its carrier concentration and carrier type can be easily controlled by simple chemical doping, to simultaneously obtain p- and n-type TE property from an identical CNT film for a p-n junctioned TE module.17-20 Although many previous studies on carbon materials have demonstrated their excellent TE properties, novel methodologies are still required for facile module fabrication and proper performance evaluation. For example, the TE modules based on CNT have mostly been fabricated by alternately stacking p- and n-type films or printing of CNT pastes followed by 2 ACS Paragon Plus Environment

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the deposition of a metal electrode to connect the p- and n-type units.21,22 Moreover, the traditional p- or n-doping method for CNT generally involves dipping the CNT film in the dopant solution for a long time, whereby selective doping of the specific CNT area is nearly impossible. Hence, some critical issues of the carbon-based TE module need to be resolved to satisfy the basic requirements of practical application. First, reliable materials with enhanced TE performance and improved mechanical flexibility should be developed. Excellent TE property of a material is essential for a high-performing TE module, because usable power obtained from the TE module directly relates to the power factor of the TE material. Second, an easy fabrication method for large scale production, such as a simple selective doping process to lower the fabrication cost, should be established. Third, a novel module should be designed by taking into account the flexibility, portability, and high integration for high power density. In this study, we demonstrate a repeated brush-casting and folding technique for doping highly aligned CNT webs for the mass-production of efficient and flexible TE modules. On the basis of the highly conducting CNT web, high performance TE module was fabricated by alternate brush-casting of p- and n-type dopants, FeCl3 and benzyl viologen (BV), respectively. A flexible TE module composed of 120 p-n pairs of the doped CNT web shows a maximum power output of 5.3 μW at a temperature difference of 11.7 K.

RESULTS AND DISCUSSION The large-area CNT web (over 8 cm × 30 cm) was prepared by direct spinning via chemical vapor deposition, as reported previously (See Figure S1a).23,24 The conductive and continuous CNT bundles are highly aligned by rolling with some stress during the collecting process at a high temperature (See Figure S1b,c), which results in distinctive bundle structures and superior electrical properties. The high degree of CNT bundle alignment was also confirmed by the high polarized Raman intensity factor of 6.46 (IG∥/IG⊥), where the intensity factor is 3 ACS Paragon Plus Environment


the ratio between the G band intensity in the parallel and vertical directions to the incident Raman laser (See Figure S1d). Figure 1 schematically illustrates the fabrication process of the flexible TE module by alternate brush-casting of p- and n-type dopants on the pristine CNT web at regular intervals. The brush-casting method is an attractive way for mass-production based on the roll-to-roll process, because it allows localized doping without an expensive shadow mask. A partial 5-mm-wide area of the web, which corresponds to the height of the TE leg, was painted by brush-casting with one type of dopant solution from the edge of the CNT web and then blow-dried (Figure 1a). Thin double-sided PET adhesive tape with a width of 5 mm was then attached to the locally doped CNT web. An equivalent area next to the attached PET tape was brushed with another type of dopant solution and blow-dried similarly. Along the boundary line between the p- and n-doped regions, which corresponds to the edge line of the PET tape, the doped CNT web was folded up. Finally, a bar-shaped TE module with 20 pairs of continuous p-n couple was obtained by brushing p- and n-dopants alternately at 5 mm intervals and folding the locally functionalized CNT web repeatedly. This continuous folding process is allowed by the outstanding mechanical stability of the CNT web, and the web shows negligible change up to 100 folding cycles in the internal resistance (less than 1.58%) (See Figure S2). The formation of the bar-shaped TE module requires no gold or silver top electrodes for metal interconnection between the p- and n-type areas, which frees the TE module from the influence of the contact resistance unlike in the case of a conventional p-n junctioned TE module. Furthermore, the long bar-shaped TE module was cut into several small TE cells with 4 mm widths (hereafter referred to as the TE cell) (Figure 1b). Therefore, the harsh and costly steps in the fabrication of a conventional TE module, such as sintering, cutting, and soldering of TE materials, aligning TE legs, and connecting the p- and n-type segments with metal electrodes, are avoided in our fabrication process. The side-view image of the TE cell clearly exhibits the distinct layer-by-layer structure formed from the folded CNT web and double-sided adhesive tape (Figure 1c). The adhesive tape acts 4 ACS Paragon Plus Environment

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as an insulator to electrically separate the p- and n-doped layers of the CNT web, which allows the electrical connection only on the folded edge between two differently doped layers of the CNT web, as shown in the top-view image of the TE cell (Figure 1c). Finally, TE cells were embedded in a pre-patterned silicon pad and then electrically connected in series with a silver paste (Figure 1d). The highly integrated and flexible TE module with a thickness of 5 mm shows excellent flexibility without substantial change in the internal resistance (less than 3 %) on the bending radius of 25 mm (See Figure S3), enables it to adhere thoroughly to the bent surface. Its high flexibility is attributed to inherently flexible organic TE materials and its module configuration, in which electrode-free TE cells are tightly embedded in a silicon pad and electrically connected with CNT webs. The Seebeck coefficient and electrical conductivity of the pristine CNT web (before doping) were first investigated, as indicated in the ellipse in Figure 2a, and then the corresponding power factor was calculated (Figure 2b). The pristine CNT web exhibits a Seebeck coefficient of 39.2 μV K-1 with high electrical conductivity of 998.3 S cm-1, which is attributed to the highly oriented configuration of the extremely long and innately interconnected CNT bundles. To obtain n-type TE property from the pristine CNT web, BV was used as n-type dopant, due to its lowest reduction potential amongst the electron-donating organic molecules.25 To optimize the doping level, BV solutions with various concentrations ranging from 0.01 to 100 mM were brush-cast on the pristine CNT webs. At a relatively low BV concentration of 0.01 mM, slight decrease in the electrical conductivity and a positive Seebeck effect, which are induced from the reduced hole concentration in the CNT web, were simultaneously observed owing to the limited electron transfer from the BV dopant to CNT web. When the BV concentration is increased to 5 mM, the Seebeck coefficient of the CNT web becomes negative (−78.9 μV K-1) with n-type TE property. With a further increase in the BV concentration from 5 to 100 mM, the electrical conductivity begins to increase gradually owing to a more effective electron transfer from BV. In contrast, the negative Seebeck 5 ACS Paragon Plus Environment


coefficient changes drastically, reaching a maximum value (−99.7 μV K-1) at the concentration of 50 mM. Based on the results of the power factor, the optimized concentration of BV for n-type doping is determined to be 50 mM. The stability of the n-doped CNT web without any encapsulation was also examined because the air stability of n-type TE materials is critical for their practical application.26,27 The BV-doped CNT web exhibits long-term stability with negligible variation in the Seebeck coefficient and electrical conductivity, less than 5% over one week (Figure 2e). Such outstanding stability may be attributed to the efficient wrapping of the CNT bundles by the BV molecules, which efficiently block oxygen from the atmosphere.28,29 Although the pristine CNT web can be used as a p-type counterpart because it inherently possesses excellent TE property with a positive Seebeck Coefficient, it was further modified by brush-casting with a FeCl3 solution to effectively transfer charges from the CNT to FeCl3.30 The Seebeck coefficient, electrical conductivity, and power factor were investigated depending on the FeCl3 concentration (Figure 2c,d). The electrical conductivity of the doped CNT web increases from 998 to 3689 S cm-1 with an increase in the FeCl3 concentration. In contrast, the Seebeck coefficient of the FeCl3-doped CNT web increases up to 93.8 μV K-1 at 1 mM FeCl3, and then decreases slightly with a further increase in the FeCl3 concentration. The optimum power factor was determined to be 2163 μV m-1 K-2 at 5 mM FeCl3, which is one of the highest performance for a flexible TE material.31-35 According to the measured thermal conductivity (Table S1), the thermoelectric figure-of-merit (ZT) was calculated to be 0.1 and 0.07 for p- and n-doped CNT web, respectively. The electrical properties depending on temperature were also characterized for the BV- and FeCl3-doped CNT web, respectively (See Figure S4). The electrical conductivity of the FeCl3-doped CNT web increases slightly at the beginning and decreases rapidly with increase of the temperature, possibly because of the dissociation of the dopant. However, the BV-doped CNT web shows an additional inflection


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point at around 250 °C, possibly owing to the change to the original p-type from n-type property. To further verify the charge transfer and doping effect induced by BV or FeCl3 doping, G band shift and the ratio of the G and D band (G/D) intensities were investigated in the Raman spectra (Figure 3a). A weak D band and strong G band of the pristine CNT web are observed at 1350 cm-1 and 1580 cm-1, respectively, which indicates that the pristine CNT web possesses superior crystal structure with negligible defects.36 After BV or FeCl3 doping, the G/D ratio is nearly unchanged, suggesting that no structural defect is induced by the brush-cast doping method.37 However, the G band is slightly upshifted with regard to that of the pristine CNT web at the optimized FeCl3 concentration of 5 mM. This G band shift indicates that the charge was transferred from the CNT web to FeCl3. In contrast, the downshift of the G band means charge transfer from BV to the CNT web in the BV-doped CNT web, leading to phonon softening. These G band shifts due to doping are consistent with the results of previous reports.38-40 The radial breathing mode (RBM) was also investigated at a wavenumber of approximately 170 cm-1, where all the carbon atoms in the CNT web vibrate in the radial direction coherently (Figure 3b).41 Accordingly, a reduction in the intensity of the RBM reveals the presence of BV or FeCl3 because the dopants disturb the oscillation of the carbon atoms by attaching onto the CNT surfaces. To further explain the effect of doping on the differently doped CNT web, the work function (Φ) was measured by analyzing the UV photoelectron spectra (UPS), as shown in Figure 3c, because Φ of organic semiconductors is commonly associated with the charge carrier injection and collection.42-44 In general, Φ is directly related to the onset of secondary photoelectrons, which is consistent with the vacuum level with regard to sample’s Fermi energy. The onset energy of the pristine CNT web is measured to be 9.8 eV, which corresponds to Φ = 4.7 eV, agreeing with the value reported in previous literature.45 However, Φ of the FeCl3-doped CNT web significantly increases to 4.9 eV due to electron donation from the pristine CNT web to FeCl3. In contrast, Φ of the BV7 ACS Paragon Plus Environment


doped CNT web decreases to 4.5 eV, which is induced by electron transfer from the negatively charged BV ions to the CNT web because BV0 has a lower reduction potential (−1.12 V vs. a standard hydrogen electrode) than that of the CNT web (+0.14).38 The morphology of the pristine and doped CNT webs was observed by SEM (Figure 4). The pristine CNT web shows continuous bundles and a homogenous surface with excellent uniformity (Figure 4a). The interconnected networks with highly aligned bundles are also clearly observed in the magnified SEM image (Figure 4b), and this contrasts with the randomly oriented CNT buckypaper with high tortuosity shown in Figure 4c for comparison. The n-type CNT web doped with 50 mM BV solution apparently shows that the surfaces of the CNT bundles are coated by the dopant molecules with some aggregation (Figure 4d). In addition, bundles of the BV-doped CNT web are slightly thicker than that of the pristine CNT web, owing to the attachment of the conjugated dopant molecules onto the surfaces of the CNT bundles via π-bonding.46 The 3D network porous structure of the BV-doped CNT web with a thickness of approximately 13 μm is also confirmed by cross-sectional SEM images (Figure 4e). The unique porous structure would extremely benefit the charge carrier transport through the network, whereas efficiently suppressing the thermal conduction.47 The surface morphology of the FeCl3-doped CNT web is not much different from that of the BV-doped CNT web (Figure 4f), and it completely retains the bundle alignment required for efficient carrier transport. Using the efficiently doped CNT webs, we fabricated highly integrated and flexible TE modules by a brush-casting and folding method (Figure 5a). A TE module with 6 TE cells consisting of 120 p-n couples (20 couples in each TE cell) on a silicon pad exhibits a total internal resistance of 96.8 Ω with a nearly constant resistance of 16 Ω for each TE cell (See Figure S5). Such low internal resistance would allow a numerous connection of TE cells with enhanced power generation for target applications. The voltage output of the flexible TE module increases with increasing temperature gradient (Figure 5b), showing a linear 8 ACS Paragon Plus Environment

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relationship. Additionally, as the number of TE cells is increased from 1 to 6 through electrical connection, the voltage output increases rapidly, approaching 40.8 mV at ΔT = 11.7 K. Further, for the TE module consisting of 120 p-n couples, the power output and current output are also enhanced with increasing temperature gradient. The maximum power output reaches 3.1 μW at ΔT = 11.7 K (Figure 5c). To deliver the thermal energy homogenously, aluminum foil-covered thermal tapes were attached to both sides of the flexible TE module (Figure 5d). Interestingly, both voltage and current output were enhanced simultaneously, and a maximum power output increased to 5.3 μW at ΔT = 11.7 K, which is 70% higher than that without the thermal tape (Figure 5e). We believe that the thermally conductive layer may homogenize the heat diffusion across the thickness considering no further barriers. To more accurately compare the TE performance, the power density of 0.66 μW cm-2 was obtained by dividing the power output by the actual area of the TE module (silicon pad area, 8 cm2) (Figure 5f), which is much higher than that of the previously reported organic TE module, 0.02 μW cm-2 at ΔT = 10 K.4 The power density of the flexible TE module can be enhanced further up to 11.0 μW cm-2 by increasing the packing density, however, with some loss of flexibility. The potential for harvesting biothermal power is demonstrated in Figure 6. When one side of the TE module is touched with a finger at room temperature, a large Seebeck voltage of 22 mV is promptly generated (Figure 6a). After withdrawing the finger, the voltage vanishes gradually. In particular, the current response of the flexible TE module was observed for possible application in thermal sensors because the current response has rarely been demonstrated owing to the low current output by organic TE materials. The current output is a crucial factor along with the voltage output to realize the full potential of TE devices. With thermal energy input by a finger touch, a large current difference of 179 μA is generated rapidly (Figure 6b), which indicates that our flexible TE module can convert the human body heat into electrical energy in the ambient environment. After the finger was removed from the 9 ACS Paragon Plus Environment


top of the TE module, the current output was found to drastically reduce to 0.72 μA in a very short time. Moreover, the stability of the TE module was also confirmed by repeated finger touching, which shows very stable current responses up to 5 times (Figure 6c). As a next step, the large-scale and highly integrated TE module composed of 48 TE cells (960 p-n couples) over a module area of 9 cm × 9 cm with the expected power output of 139 μW is being developed for industrial production (See Figure S6). We believe that further process development for synthesis of the continuous CNT web with high yield, automatic folding procedure following the roll-to-roll collection of the CNT web, and a precise technique for electrically connecting the TE cells can facilitate the fabrication of wearable TE modules in the near future.

CONCLUSION On the basis of the outstanding TE material, the highly aligned CNT web, flexible p- and ntype films were rapidly and conveniently formed by brush-casting solutions of FeCl3 and BV. Twenty p-n couples could be directly connected without any electrodes in one CNT web, and their optimized power factors were 2163 and 1124 μW m-1 K-2 for p- and n- type, respectively. A highly integrated and flexible TE module was also fabricated with a novel device configuration using alternately p- and n-doped CNT webs with a repeated folding process. Our TE module exhibits excellent TE performance with a power output of 5.3 μW and power density of 0.66 μW cm-2, which is considerably superior to those of the flexible TE modules reported previously. In addition, the TE module with highly integrated p-n TE cells also has great potential in terms of both current and voltage responses for thermal sensing applications as well as portable and flexible TE conversion.

EXPERIMENTAL SECTION


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Preparation of a Highly Aligned CNT Web. Highly aligned CNT web was formed in a furnace by injecting a liquid feedstock, in which acetone was used as the carbon source, thiophene (0.8 wt%) as the promoter, and ferrocene (0.2 wt%) as the catalytic precursor. The reactor temperature and injection rate of the carrier H2 gas were kept at 1473 K and 1000 sccm, respectively. The liquid feedstock was injected at the rate of 10 mL/h under flowing H2, as reported elsewhere.21,22 A CNT sock in the form of continuous tube was formed and then wound onto a winder. Finally, the densified CNT web was obtained by immersing in dimethyl sulfoxide and drying at 373 K.48 Preparation of the Dopant Solutions and the Doping Process. Solutions of FeCl3 (97%, Sigma-Aldrich) in ethanol at various concentrations were brush-cast on the CNT web and the brushed CNT web was blow-dried to complete the p-type doping process. A 2.5-mm-wide soft paintbrush was utilized to brush-cast dopants. Dopants were brush-cast three times with weak pressure and the amount of dopants for one brush was fixed to 100 µL for one side of the CNT web (5 mm by 80 mm). The BV solution was prepared via a method described elsewhere28. Briefly, a known quantity of BV (97%, Sigma–Aldrich) corresponding to desired concentration was dissolved in deionized water (10 mL) followed by the addition of toluene (10 mL). Then, the water/toluene bilayer solution followed by the addition of sodium borohydride (7.5 g, Sigma–Aldrich) was stored for one day at room temperature. The organic layer on top of the bilayer solution was used for doping. BV doping of the CNT web was conducted following the same procedure as the FeCl3 doping. Evaluation of the Thermoelectric Properties. The standard van der Pauw direct-current four-probe method was used to measure the electrical conductivity using a Keithley 220 programmable current source and Keithley 195A digital multimeter.49 The Seebeck coefficients of the freestanding CNT webs were determined using a commercial equipment (LSR-3, Linseis, Germany). The thicknesses were measured from both cross-sectional SEM images and thickness profiles obtained using an alpha-step surface profiler (α-step DC50, 11 ACS Paragon Plus Environment


KLA Tencor, USA). The air-stability of the BV-doped CNT web without any encapsulation was examined by monitoring the Seebeck coefficient and electrical conductivity over a one week period. The thermal conductivity was obtained using the equation κ = αρCp, where α is the thermal diffusivity, ρ is the density, and Cp is the specific heat capacity at constant pressure. The in-plane thermal diffusivity in the identical direction to the electrical conductivity measurements was measured using a Laser PIT-M2 (Advance Riko, Japan) at 300 K. The density was obtained from the measured volume and weight of all samples. The specific heat capacity was measured using a differential scanning calorimeter (MDSC Q200, TA Instruments, USA). Fabrication of the Flexible TE Module. To fabricate the flexible TE module, the pristine CNT web was alternately brush-cast with 5 mM FeCl3 and 50 mM BV solution at 5 mm distances for p- and n-doping, respectively. One solution was brush-cast with 5 mm width from the edge of the CNT web and then a thin PET tape was attached to the locally doped area on the CNT web as an insulating layer. A 5 mm area next to the PET tape was brushed with another type of dopant solution and the CNT web was folded up along the edge of the tape. Finally, 20 p-n couples in a bar-shaped TE module were obtained by repeatedly brushing and folding the locally doped CNT web. This bar-shaped large TE module was cut into a few small TE cells with widths of 4 mm and these small TE cells were then embedded in a silicon pad and then connected electrically in series. In this study, a flexible TE module composed of 6 TE cells with 120 p-n couples (20 p-n couples in each TE cell) was fabricated and characterized. Power-Generation Characteristics. A customized system was utilized to evaluate the power generation of the TE module, as described in previous literature.50 The temperature difference was generated by heating one side of the TE module through a hot Peltier plate, while maintaining the other side at room temperature (See Figure S7). The temperature


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gradient was measured with thermocouples. The voltage output-current output and power output-current output were measured by changing the load resistance. Characterization of the Structures. The surface morphology was investigated by SEM (SigmaHD, Carl Zeiss, Germany). An ex-situ lift-out technique of focused ion beam machining (FB-2100, Hitachi, Japan) was utilized to prepare the cross-section of the sample. High-resolution dispersive Raman spectroscopy was used to analyze Raman spectra at excitation of 514 nm (XploRATMPLUS, Horiba Jobin Yvon, Japan). UPS were recorded using a base pressure of 5 × 10-10 Torr and He I line (hν = 21.2 eV) (Sigma Probe, Thermo VG Scientific, USA).

ASSOCIATED CONTENT Supporting Information SEM images and polarized Raman spectra of the CNT webs; Folding stability of the CNT web; Flexibility of the TE module; Electrical conductivities of the doped CNT web depending on temperature; Internal resistance of the TE module; Demonstration of a large-area flexible TE module; Photographs of the power measurement system; Thermal conductivity and related data of CNT webs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +82-42-860-6560. Fax: +82-42-860-7200. Author contributions S.Y.C. conceived and supervised the experiments. C.J.A. performed the experiments and analyzed the data. Y.H.K. measured and analyzed the thermoelectric properties. H.S. and Y.J. synthesized the CNT web. S.Y.C. and C.J.A. wrote the manuscript. All authors discussed the results. 13 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest.

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

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Figure 1. Fabrication of the highly integrated and flexible TE module using a highly aligned CNT web by the brush-casting and folding method. (a) Doping of the CNT web with corresponding dopants by the brush-casting method and mechanical folding of the alternately doped CNT web. (b) Cutting the large bar-shaped TE module into several TE cells. (c) Schematic view and scanning electron microscopy (SEM) images of the cut TE cell. (d) Embedding 6 TE cells into a silicon pad and electrically connecting them in series.


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Electrical Conductivity (S cm )

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Figure 2. Thermoelectric properties and stability of the CNT web after doping. (a) The measured Seebeck coefficients and electrical conductivities and (b) the calculated power factors of the BV-doped CNT web as a function of the dopant concentration. (c) The measured Seebeck coefficients and electrical conductivities and (d) the calculated power factors of the FeCl3-doped CNT web as a function of the dopant concentration. The average values and error bars were obtained from measurements on five different samples. (e) Change in the Seebeck coefficient (S) and electrical conductivity (σ) as a function of time in air without encapsulation for the n-type CNT web doped with 50 mM BV solution, where S0 and σ0 are the corresponding values in their original state.



Intensity (a. u.)

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Pristine CNT web BV doped CNT web FeCl3 doped CNT web

Φ = 4.5 eV Φ = 4.9 eV

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Figure 3. Spectroscopic characterization of the CNT webs electrically doped with p and n-type dopants. (a) G and D band region and (b) RBM region in the Raman spectra and (c) the secondary electron threshold region in the UPS spectra for the pristine and doped CNT webs. The inset of a shows the G band shift.


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Figure 4. Structural characterization of the CNT webs. SEM images showing a (a) homogenous and uniform surface and (b) highly aligned bundles of the pristine CNT web. (c) A random network of the CNT buckypaper for comparison; this was prepared using a CNT suspension. (d) SEM image of the surface and (e) cross-sectional SEM images of the BV-doped CNT web. (f) SEM image of the surface of the FeCl3-doped CNT web. The inset of e shows low-magnification SEM image of the cross-section of the BV-doped CNT web with a thickness of approximately 13 μm.



a

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c 20 p-n couples (1 cell) 40 p-n couples (2 cells) 60 p-n couples (3 cells) 80 p-n couples (4 cells) 100 p-n couples (5 cells) 120 p-n couples (6 cells)

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Figure 5. Power generation by the flexible TE module fabricated using the doped CNT web. (a) Photograph, (b) voltage output as a function of temperature gradient, and (c) power output-current output characteristics of the flexible TE module consisting of 6 TE cells. (d) Photograph, (e) power output-current output and voltage output-current output curves, and (f) power density of the flexible TE module with a thermal tape. The power density (red solid line) and the maximum power density (blue dash line) are defined by dividing the power output by the area of the entire TE module and area occupied by the TE cells, respectively.


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Figure 6. Electrical response of the flexible TE module to human body heat. (a) Voltage response and (b) current response of the flexible TE module to finger touch. (c) Current modulation of the flexible TE module by the repeated finger touch in ambient atmosphere.



ToC figure


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Highly Integrated and Flexible Thermoelectric Module Fabricated by

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