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Novel Hybrid Organic Thermoelectric Materials: Three-Component Hybrid Films Consisting of a Nanoparticle Polymer Complex, Carbon Nanotubes, and Vinyl Polymer Naoki Toshima,* Keisuke Oshima, Hiroaki Anno, Takahiko Nishinaka, Shoko Ichikawa, Arihiro Iwata, and Yukihide Shiraishi About two thirds of the chemical energy for the consumption of fossil fuels is lost as waste heat, and roughly two thirds of these heat losses occur below 150 °C. Organic thermoelectric materials are promising functional materials for providing flexible devices that could be used to recover electric energy from waste heat below 150 °C with a printing processes. Despite the fact that the performance of organic thermoelectric materials has recently improved tremendously, the high performance has mainly been attributed to polymers with high electrical conductivity, such as PEDOT (poly(3,4-ethylenedioxythiophene)), with ionic counter ions,[1,2] but the devices made from PEDOT-based films are sensitive to humidity and therefore of limited practical use in an ambient environment. Here, we have first developed high-performance hybrid organic thermoelectric materials (dimensionless thermoelectric figure-of-merit, ZT ≈ 0.3) by using nanotechnology without using a conducting polymer such as PEDOT. The hybrid organic thermoelectric materials described in this paper are composed of nanoparticles of a polymer complex, carbon nanotubes (CNTs) and poly(vinyl chloride) (PVC). We have successfully prepared nanoparticles of the n-type semiconducting polymer complex, poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT) (Figure 1), which was well dispersed in Prof. N. Toshima, Prof. Y. Shiraishi Department of Applied Chemistry Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756–0884, Japan E-mail: [email protected] Prof. N. Toshima, Prof. H. Anno, S. Ichikawa, Prof. Y. Shiraishi Advanced Materials Institute Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756-0884, Japan K. Oshima, T. Nishinaka Graduate School of Engineering Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756-0884, Japan Prof. H. Anno Department of Electrical Engineering Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756-0884, Japan Dr. A. Iwata Yamaguchi Prefecture Industrial Technology Institute Asutopia, Ube-shi, Yamaguchi 755–0195, Japan
DOI: 10.1002/adma.201405463
Adv. Mater. 2015, DOI: 10.1002/adma.201405463
NMP (N-methyl-2-pyrrolidone) and which could be used for dispersion of CNTs in hybrid films instead of PEDOT-PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)). The nanoparticles made the dispersion of CNTs easier and enhanced the carrier transport between CNTs to improve the electrical conductivity of the hybrid organic films, resulting in the high thermoelectric performance. This novel concept is important for the development of a hybrid type of organic thermoelectric materials, which have high electrical conductivity and low thermal conductivity, and which show a high potential for general applications in organic electronics and photovoltaics. Since organic thermoelectric materials with reasonably high thermoelectric performance (ZT = 0.1) were first demonstrated by us in 2007 by using a conducting polymer, i.e., a stretched copolymer of poly(phenylenevinylene) derivatives,[3] researchers on thermoelectric materials have recognized that organic thermoelectric materials have the potential to be used for practical purpose and may be used instead of inorganic semiconducting thermoelectric materials.[4] If thermoelectric technology will be further developed for practical purposes such as recovering electric energy from waste heat in low grade, organic thermoelectric films will have many advantages, such as flexibility, light weight, easy processability, and low cost.[5] Thus, not only the researchers in the field of thermoelectrics but also those working on organic electronics are now becoming interested in organic thermoelectric materials because the thermal properties of organic conducting polymers are recognized to be quite different from those of inorganic semiconductors. For example, the increase in electrical conductivity in conducting polymers is not accompanied by an increase in thermal conductivity.[6,7] In order to improve the thermoelectric performance, i.e., thermoelectric figure-of-merit, ZT, represented by ZT = (σS2/κ)T, where σ, S, κ and T designate the electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively, the electrical conductivity of the conducting polymer was enhanced by controlling the alignment of polymer chains with stretching.[8] Now, the highly electro-conducting PEDOT films are considered effective organic materials with high thermoelectric performance. Crispin and co-workers reduced the electrical conductivity of PEDOT-Tos by tuning the oxidation level to obtain the optimal thermoelectric performance.[1] Kim et al. increased the electrical conductivity of PEDOT-PSS films by reducing the content of insulating dopant PSS.[2] Grunlan and co-workers added single-walled (SW) and
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xNa
+
Figure 1. The chemical structure of poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT).
double-walled (DW) CNTs to PEDOT-PSS to form composites with high electrical conductivity,[9] because CNTs are known for their excellent electrical conductivity in addition to good thermal conductivity depending on their chirality.[10,11] Thus, PEDOT appears to be a key molecule for obtaining organic thermoelectric materials with high performance.[12] In contrast, PEDOT-PSS-based films have several disadvantages; their thermoelectric performance fluctuates depending on the humidity of the direct environment and devices fabricated from PEDOTPSS are difficult to treat in air because PEDOT-PSS involves hydrophilic counter ions, which makes it difficult to use them for practical purposes.[13] Here, novel hybrid organic thermoelectric materials were developed without conducting polymers such as PEDOT-PSS. The new system contained nanoparticles of a polymer complex that served as a charge transporting dopant in addition to CNTs and insulating PVC. Previously CNTs were thought not to be very good thermoelectric materials because of their high thermal conductivity.[10] However, CNTs with high Seebeck coefficients were reported to have a good thermoelectric performance.[14] In addition, CNTs with organic ligands, such as tetraphenylphosphine, were recently demonstrated to have a relatively high negative Seebeck coefficient and low thermal conductivity, resulting in ZT = 0.07.[15] Nevertheless, these CNT buckypapers have disadvantages as practical thermoelectric materials, because they could not be obtained by a printing process. Independently, we have planned to develop the organic–inorganic hybrid thermoelectric materials with high performance, by using an approach in which thermoelectric devices with designed patterns can easily be produced by printing. In our method the mixed CNT dispersions and the usual insulating vinyl polymers such as PVC (which has a low thermal conductivity) were drop-cast to produce hybrid films. The obtained two-component hybrid organic films had a relatively high Seebeck coefficient and low thermal conductivity, but the electrical conductivity was never high enough. The low electrical conductivity of the hybrid films could
be understood by assuming that the insulating polymers located in between CNTs decreased electrical conductivity as well as the thermal conductivity. Since the presence of a small amount of metal nanoparticles between conducting polymer chains could enhance the electrical conductivity of the hybrid polymer films,[16] charge transporting dopants, instead of metal nanoparticles, were considered to assist the increase of electrical conductivity of the hybrid films of CNTs and insulating polymers. After many trials, we discovered that nanoparticles of the thermoelectric polymer complex, n-PETT, can enhance the electrical conductivity of the hybrid films. In other words, a three-component hybrid organic film, composed of n-PETT, CNTs and insulating PVC, showed a high thermoelectric performance. The nanodispersed PETT (n-PETT), composed of nanoparticles having diameters from 10 to 50 nm, was prepared in the presence of surfactant by modification of a previously reported method[17] and used as a charge-transporting dopant in the hybrid materials that consisted of CNTs and insulating polymers. Although a highly negative Seebeck coefficient (−121.6 µV K−1) has been reported for solid PETT at room temperature,[17] the absolute value of the Seebeck coefficient for the pressed block of n-PETT was moderately large, i.e., –43 µV K−1 at 340 K. Casting dispersed solutions of n-PETT and PVC provided smooth hybrid films. However, the thermoelectric performance, especially the electrical conductivity of the hybrid films, was very low. Recently, similar findings were reported for the composites of solid PETT particles, which were prepared by a ball-milling method (average diameter = 456–850 nm), and poly(vinylidene fluoride) in DMSO.[18] In our study CNTs were used to increase the electrical conductivity of n-PETT/PVC hybrid films. Addition of CNTs to the hybrid films to produce three-component hybrids surprisingly increased the electrical conductivity as well as the Seebeck coefficient, as shown in Table 1, although the polarity of the Seebeck coefficient was altered from negative to positive values by addition of only a small amount of CNTs. This is probably because the used CNT was a p-type semiconductor. Recently, Maniwa and co-workers reported that purified semiconducting SWCNTs had a very high Seebeck coefficient of 170 µV K−1 at 350 K, which would make them suitable for use in flexible thermoelectric materials.[14] The thermoelectric properties of the sheet of our CNTs used in this research are shown in Table 1, indicating that the CNTs themselves had a high enough electrical conductivity of ca. 690 S cm−1. When they are dispersed in PVC, however, the electrical conductivity
Table 1. Thermoelectric properties of three-component hybrid films and related samples at 340 K.a) Sample n-PETT
S [µV K−1]
σ [S cm−1]
Block
−39.6 ± 0.7
3.6 × 10−2 ± 0.7 × 10−2 −3
5.7 × 10−3 ± 1.4 × 10−3 4.7 × 10−4 ± 0.6 × 10−4
−32.9 ± 1.0
CNT
Sheet
38.2 ± 0.4
CNT/PVCc)
Film
31.9 ± 2.8
23.2 ± 5.2
2.3 ± 0.4
n-PETT/CNT/PVCd)
Film
30.2 ± 0.6
429.3 ± 7.1
39.1 ± 1.6
n-PETT/CNT/PVCd) (MeOH treatment)
Film
30.5 ± 0.5
629.9 ± 28.6
58.6 ± 1.5
a)Measured
b)
4.4 ×
10−3
PF [µW m−1 K−2]
Film
n-PETT/PVC
2
Form
± 0.9 × 10
690.6 ± 16.2
100.7 ± 1.9
with ULVAC ZEM-3. b)Mass ratio: 10/3; c) Mass ratio: 8/13; d)Mass ratio: 10/8/3.
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of the hybrids of CNT/PVC (8/13 in weight ratio) was as low as 23 S cm−1. Nevertheless, addition of n-PETT enhanced the electrical conductivity to 430 S cm−1 as shown in Table 1. The Seebeck coefficient of our three-component films was 30.2 µV K−1, which is acceptably high. Thus, the thermoelectric power factor (PF = σS2) was as high as 39.1 µW m−1 K−2. Figure 2a,b show the effect of the CNT concentration on the Seebeck coefficient and electrical conductivity, respectively, of the three-component hybrid organic films composed of n-PETT, CNT, and PVC. The Seebeck coefficient does not depend on the CNT concentration and nearly equals that of CNT/PVC hybrid films. In contrast, the electrical conductivity increases with increasing concentration. It should be emphasized that the Seebeck coefficient did not increase that much with increasing CNT concentration in the case of the twocomponent CNT/PVC hybrid films. Thus, it is striking that n-PETT is very effective to obtain a high electrical conductivity for the hybrid films. Because PVC is insulating and CNT in contrast is conducting, n-PETT should assist the electrical
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Figure 2. CNT concentration dependence of the Seebeck coefficient (a, S) and electrical conductivity (b, σ) of the three-component hybrid films. All films have a thickness of about 6–7 µm, and the mass ratio of the components varied as n-PETT/CNT/PVC = 10/(2–8)/3. The Seebeck coefficient of the film without CNTs is negative but changes to positive by addition of the CNTs, while the electrical conductivity increases linearly with increasing CNT concentration until 38%.
conduction between the CNT chains in the three-component hybrid organic films. Although we have no direct evidence of this, the charge transfer interaction between p-type CNT and n-PETT with negative Seebeck coefficient could provide the strong contact, which might enhance the electrical conductivity between the CNT bundles via n-PETT. In fact, it was observed by eye that the addition of n-PETT to the suspensions of CNT bundles in NMP did improve the dispersions of CNTs in NMP. This dispersion was confirmed by a decrease in the average diameter of the CNT bundles (about 6.1 ± 3.6 nm in Figure 3d) by SEM in the n-PETT-containing system, compared with CNTs without n-PETT (about 10.9 ± 5.4 nm in Figure S1a, Supporting Information). In addition, the CNTs could not be well dispersed if Cu-containing PETT, a p-type semiconductor,[17] was used instead of the n-PETT containing Ni. This observation suggested that the electronic interaction between p-type CNT and n-type n-PETT could provide the good contact, which might result in the improved electrical conductivity. In addition we found that treatment of the three-component hybrid films with methanol could enhance the electrical conductivity, as shown in Figure 2b. A similar enhancement in electrical conductivity by treatment with a solvent such as DMSO and EG has been reported in the case of PEDOT films, where the solvent treatment was considered to enhance the alignment of the conducting polymer chains[19] and/or to remove the insulating materials from the surface of the films.[2] In the three-component hybrid systems, the films were dried at 80 °C for 30 min after the treatment with methanol, and were found to have higher electrical conductivity than the corresponding pristine films (Figure 2b). The Seebeck coefficient of the threecomponent hybrid films was at a level similar to the pristine ones, resulting in power factors as high as 58.6 µW m−1 K−1. We found that methanol can slightly solve n-PETT and PVC under the present conditions. In fact, EDX (energy dispersive X-ray spectroscopy) measurements in SEM of the three-component hybrid films before and after the methanol treatment indicated that both atomic ratios of sulfur and nickel (both present in n-PETT), and chlorine (in PVC) against carbon (mainly in CNT) decreased from 5.47 ± 0.31 to 2.58 ± 0.31 for S/C, from 1.75 ± 0.11 to 0.88 ± 0.15 for Ni/C, and from 2.77 ± 0.17 to 1.16 ± 0.18 for Cl/C, respectively, with the methanol treatment, suggesting the removal of n-PETT and PVC from the films. The SEM pictures (Figure 3 and Figure S1, Supporting Information), current mode AFM images (Figure S2b, Supporting Information) as well as XRD patterns (Figure S4, Supporting Information) suggest the enhancement of smooth contact between the CNT bundles by methanol treatment and addition of n-PETT, although the measurements could not provide direct evidence. The thermal conductivity was as low as ≈0.06 m−1 K−1, as shown in Table 2 in the Experimental section, although the thermal diffusivity α was based on the measurement through the film. Thus, the maximum thermoelectric figure-of-merit was anticipated to be ca. 0.31 at 340 K for the present threecomponent hybrid films, although the thermal conductivity through plane was used for the calculation in spite of the presence of the anisotropy in the film. Temperature dependence of ZT values is also shown in Figure S3, Supporting Information. In order to understand the low thermal conductivity and the effect of the methanol treatment, scanning electron microscopy
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Figure 3. SEM images of the films of CNT / PVC (a,b) and n-PETT/CNT/PVC (c,d) before (a,c) and after (b,d) the methanol treatment. Mass ratios are CNT/PVC = 8/13 and n-PETT/ CNT/PVC = 10/8/3. The CNT bundles are clearly observed after the methanol treatment, especially in the case of the three-component hybrid organic films.
thermoelectric elements, Ro = NL/(σwt), and Pmax = Vo2/(4Ro). In these calculations, we used S = 38 µV K−1, σ = 425 S cm−1, ΔT = 100 K, N = 5, L = 5 mm, w = 4 mm, and t = 5.68 µm, and neglected the contact resistance and silver electrode resistance. Assuming an optimum packing density of the single legs, defined as the direction of heat transport, we estimated the expected electrical power density from such the devices to be ≈3400 µW cm−2 at ΔT = 100 K, by dividing the measured Pmax values by the crosssectional area of legs. The lateral geometry used here has the advantage that large temperature gradient can be achieved. However, from the point of view of conversion efficiency, the heat flow through the polyimide substrate will lead to a loss in efficiency; this is a drawback of the lateral architecture. In this study, we were not able to measure the heat flow in our homemade measurement system, thus the efficiency was not known. In the lateral geometry, a substrate with very low thermal conductivity should be used for obtaining high efficiency. On the other hand,
(a)
Table 2. Thermal conductivity, κ, and related data of hybrid films at 290 K. α [mm2 s−1]
Film
ρ [g cm−3]
κ [W m−1 K−1]
n-PETT/ CNT/ PVCa)
0.09 ± 0.01
0.85 ± 0.03
0.89 ± 0.19
0.07 ± 0.025
CNT/ PVCb)
0.08 ± 0.01
0.88 ± 0.03
0.83 ± 0.12
0.06 ± 0.003
a)Mass
4
Cp [J g−1 K−1]
ratio: 10/8/3; b)Mass ratio: 8/13
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(b) 20
Voltage (mV)
4 15 3 10
2
5 0
1
0
Power (µW)
(SEM) of the films was carried out before and after the methanol treatment (Figure 3). The SEM images clearly show the removal of the rather insulating materials that cover the CNTs by the methanol treatment, suggesting a smooth contact and formation of gaps or voids between the CNT bundles, which could possibly be one of the reasons for the high electrical conductivity and low thermal conductivity. Since the present three-component hybrid materials had relatively good thermal properties, the thermoelectric devices with a uni-leg structure having five legs (Figure 4a and Figure S5, Supporting Information) were constructed by printing technology on a polyimide substrate. When the temperature difference of approximately 100 K was applied between the two edges of the devices in the in-plane direction (Figure S6, Supporting Information), the output voltage, V, and power, P, changed with electric current as shown in Figure 4b, and the maximum power, Pmax = 3.88 µW was obtained at a matching condition from the current–power curves. The open-circuit voltage Vo = 19.9 mV, internal resistance of the device R0 = 25.6 Ω, and maximum power Pmax = 3.88 µW was in reasonable agreement with the calculated values (Vo = 19.0 mV, R0 = 25.9 Ω, Pmax = 3.49 µW) using the following equations: Vo = NSΔT, where N is the number of
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Current (mA) Figure 4. Photograph (a) and voltage–current line and current–power curve (b) of a flexible five-leg thermoelectric device composed of threecomponent n-PETT/CNT/PVC hybrid films and Ag electrodes on a polyimide substrate. The open circuit voltage Vo = 19.9 mV and the internal resistance R0 = 25.6 Ω at the temperature gradient ΔT ≈ 100 K. The maximum power Pmax = 3.88 µW was obtained at a matching condition from the current–power curves.
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Experimental Section Preparation of Nanodispersable PETT (n-PETT): Dispersed nanoparticles of n-type thermoelectric polymer complex, PETT, were prepared from 1,3,4,6-tetrathiapentalene-2,5-dione and nickel(II) chloride in the presence of the surfactant dodecyltrimethylammonium bromide, by modifying the method reported by Sun et al. to produce a PETT polymer complex without surfactants.[18] Nevertheless, the nanoparticles were well dispersed in an NMP solution containing poly(vinyl chloride) (PVC). Fabrication of the Three-component Hybrid Films: The three-component n-PETT/CNT/PVC hybrid films were usually prepared by drop-casting the mixed dispersion of n-PETT, CNTs (PureTubes, NanoIntegris Inc., single-walled carbon nanotubes prepared by a HiPCO (high pressure carbon monoxide) method and mixtures of 30% metallic and 70% semiconducting ones) and PVC (Average degree of polymerization: ca. 1100, Wako Pure Chemical Ind., Ltd., Osaka, Japan) in NMP at the designed ratios on a quartz or polyimide substrate. The films were slowly dried in air on a hot plate at ca. 60 °C for 12 h. The film thickness, measured with a linear gage (model LGK-010, Resolution: 0.1 µm, Mitsutoyo corp., Kawasaki, Japan) by subtracting the substrate thickness, was typically 5–20 µm. Fabrication of the Thermoelectric Devices: The devices consisted of five thermocouples, a leg of the three-component hybrid organic film that was 5 mm long, 4 mm wide and ca. 10 µm thick, and an Ag electrode 5 mm long, 3 mm wide and ca. 15µm thick on a polyimide substrate (UBE Industries, Ltd., Yamaguchi, Japan). Measurements of the Thermal Conductivity: Thermal conductivity, κ, was calculated by the equation κ = αρCp, where α is the thermal diffusivity, ρ is the density calculated by measuring the weight and volume of the films, and Cp is the specific heat capacity at constant pressure measured using a Netzsch DSC 204 F1 Phoenix. The thermal diffusivity α was measured with a Netzsch LFA 447 Nanoflash in a through-plane direction of self-standing films at 290 K.
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a vertical geometry has the advantage that the internal resistance can be lowered, resulting in higher output power than the in-plane device. However, in this case, a very small temperature gradient can be realistically obtained. In a vertical geometry, a very high thermally conductive substrate should be used for high efficiency to reduce the thermal resistance of the heat flow through the device. The direction of the design concept of the thin-film thermoelectric device should be different and should be optimized for its application. In summary, we have demonstrated the fabrication of novel three-component hybrid organic materials of n-PETT/CNT/PVC that show reasonably good thermoelectric properties, without the need for conducting polymers. The nanoparticles of the polymer complexes, n-PETT, can be well dispersed in PVC hybrids, although the powders of solid PETT could not be well dispersed in the polymer composites,[18] and played an important role for the smooth contact and good charge transport between CNT bundles in the films. This discovery could lead to the development of a new field of organic thermoelectric materials, which could have advantages in terms of flexibility and long lifetime. We envision that such materials could be applied for recovering electric energy from waste heat as well as providing electric resources for environmental sensors. The material used in the new system is stable and is likely to be better suited than PEDOT:PSS for practical applications in ambient atmosphere. In addition, this novel concept on the role of nanoparticles may provide a new field in organic electronics and photovoltaics.
The experimental results of the thermal conductivity measurements of the films are summarized in Table 2. Measurements of Thermoelectric Properties: The thermoelectric properties were measured by the following methods: a) Seebeck coefficient (S): Measured with a thin-film measurement system ULVAC ZEM-3M8 at 330–380 K under vacuum with He at least three times, as reported elsewhere.[20,21] The Seebeck coefficient was obtained from measuring thermo-electromotive force as a function of temperature difference within a range of 5 K at each temperature. The absolute Seebeck coefficient was obtained by subtracting the Seebeck coefficient of the Pt probe wire from the Seebeck coefficient, calculated from the measured thermo-electromotive force and temperature difference over the entire temperature range. b) Electrical conductivity (σ): Measured with a thin-film measurement system ULVAC ZEM3M8 at 330–380 K under vacuum with He at least three times. The electrical conductivity was measured with a standard four-point probes method with a direct-current power supply of 0.1–0.2 mA, and the thermo-electromoive force was subtracted by alternating the current direction. c) Power factor (PF): Calculated by the equation PF = S2σ. d) Thermal conductivity (κ): Measured as shown above. e) Thermoelectric figure-of-merit (ZT): Calculated by the equation ZT = (σS2/κ)T, where T is the absolute temperature of the measurement. It should be emphasized that although the electrical conductivity σ and Seebeck coefficient S were measured in an in-plane direction, thermal conductivity was measured in a through-plane direction. Since the organic films usually have anisotropic properties, the ZT value calculated by the factors with different directions is not exactly correct. Nevertheless, tentatively we used the above value because of difficulty in the evaluation of thermal diffusivity in an in-plane direction. Measurements of Power-Generation Characteristics: The powergeneration characteristics of an n-PETT/CNT/PVC film flexible device was measured by a homemade system (Figure S7), which has been reported previously.[20,21] We heated one side of the device using a cartridge heater (the temperature of cartridge heater block was up to 473 K); simultaneously, we cooled the other side by circulating water at constant temperature of 293 K. A polyimide sheet was inserted between the n-PETT/CNT/PVC device and cartridge heater block for insulation. Thermally conducting paste was used to reduce the thermal resistance between the interfaces. We set the temperature difference ΔT to ≈100 K in an in-plane direction between the two sides of the device. The temperature difference ΔT of the surface of the device was measured by a thermal imaging camera (Model T640, FILR Systems Japan K. K., Tokyo, Japan). The output power characteristics of the device were evaluated in air using a homemade system; this system measured the voltage–current and output power–current curves by varying a potentiometer (load resistance RL) from 0 to 100 Ω.
Supporting Information Supporting information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by “Yamaguchi Green Materials Cluster” project registered as Regional Innovation Strategy Support Program (Global Type) by the Ministry of Education, Culture, Sports, Science & Technology (MEXT), Japan. Polyimide films were provided by UBE Industries, Ltd.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: November 29, 2014 Revised: January 7, 2015 Published online:
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Novel Hybrid Organic Thermoelectric Materials: Three-Component Hybrid Films Consisting of a Nanoparticle Polymer Complex, Carbon Nanotubes, and Vinyl Polymer Naoki Toshima,* Keisuke Oshima, Hiroaki Anno, Takahiko Nishinaka, Shoko Ichikawa, Arihiro Iwata, and Yukihide Shiraishi About two thirds of the chemical energy for the consumption of fossil fuels is lost as waste heat, and roughly two thirds of these heat losses occur below 150 °C. Organic thermoelectric materials are promising functional materials for providing flexible devices that could be used to recover electric energy from waste heat below 150 °C with a printing processes. Despite the fact that the performance of organic thermoelectric materials has recently improved tremendously, the high performance has mainly been attributed to polymers with high electrical conductivity, such as PEDOT (poly(3,4-ethylenedioxythiophene)), with ionic counter ions,[1,2] but the devices made from PEDOT-based films are sensitive to humidity and therefore of limited practical use in an ambient environment. Here, we have first developed high-performance hybrid organic thermoelectric materials (dimensionless thermoelectric figure-of-merit, ZT ≈ 0.3) by using nanotechnology without using a conducting polymer such as PEDOT. The hybrid organic thermoelectric materials described in this paper are composed of nanoparticles of a polymer complex, carbon nanotubes (CNTs) and poly(vinyl chloride) (PVC). We have successfully prepared nanoparticles of the n-type semiconducting polymer complex, poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT) (Figure 1), which was well dispersed in Prof. N. Toshima, Prof. Y. Shiraishi Department of Applied Chemistry Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756–0884, Japan E-mail: [email protected] Prof. N. Toshima, Prof. H. Anno, S. Ichikawa, Prof. Y. Shiraishi Advanced Materials Institute Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756-0884, Japan K. Oshima, T. Nishinaka Graduate School of Engineering Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756-0884, Japan Prof. H. Anno Department of Electrical Engineering Tokyo University of Science Yamaguchi SanyoOnoda-shi, Yamaguchi 756-0884, Japan Dr. A. Iwata Yamaguchi Prefecture Industrial Technology Institute Asutopia, Ube-shi, Yamaguchi 755–0195, Japan
DOI: 10.1002/adma.201405463
Adv. Mater. 2015, DOI: 10.1002/adma.201405463
NMP (N-methyl-2-pyrrolidone) and which could be used for dispersion of CNTs in hybrid films instead of PEDOT-PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)). The nanoparticles made the dispersion of CNTs easier and enhanced the carrier transport between CNTs to improve the electrical conductivity of the hybrid organic films, resulting in the high thermoelectric performance. This novel concept is important for the development of a hybrid type of organic thermoelectric materials, which have high electrical conductivity and low thermal conductivity, and which show a high potential for general applications in organic electronics and photovoltaics. Since organic thermoelectric materials with reasonably high thermoelectric performance (ZT = 0.1) were first demonstrated by us in 2007 by using a conducting polymer, i.e., a stretched copolymer of poly(phenylenevinylene) derivatives,[3] researchers on thermoelectric materials have recognized that organic thermoelectric materials have the potential to be used for practical purpose and may be used instead of inorganic semiconducting thermoelectric materials.[4] If thermoelectric technology will be further developed for practical purposes such as recovering electric energy from waste heat in low grade, organic thermoelectric films will have many advantages, such as flexibility, light weight, easy processability, and low cost.[5] Thus, not only the researchers in the field of thermoelectrics but also those working on organic electronics are now becoming interested in organic thermoelectric materials because the thermal properties of organic conducting polymers are recognized to be quite different from those of inorganic semiconductors. For example, the increase in electrical conductivity in conducting polymers is not accompanied by an increase in thermal conductivity.[6,7] In order to improve the thermoelectric performance, i.e., thermoelectric figure-of-merit, ZT, represented by ZT = (σS2/κ)T, where σ, S, κ and T designate the electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively, the electrical conductivity of the conducting polymer was enhanced by controlling the alignment of polymer chains with stretching.[8] Now, the highly electro-conducting PEDOT films are considered effective organic materials with high thermoelectric performance. Crispin and co-workers reduced the electrical conductivity of PEDOT-Tos by tuning the oxidation level to obtain the optimal thermoelectric performance.[1] Kim et al. increased the electrical conductivity of PEDOT-PSS films by reducing the content of insulating dopant PSS.[2] Grunlan and co-workers added single-walled (SW) and
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xNa
+
Figure 1. The chemical structure of poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT).
double-walled (DW) CNTs to PEDOT-PSS to form composites with high electrical conductivity,[9] because CNTs are known for their excellent electrical conductivity in addition to good thermal conductivity depending on their chirality.[10,11] Thus, PEDOT appears to be a key molecule for obtaining organic thermoelectric materials with high performance.[12] In contrast, PEDOT-PSS-based films have several disadvantages; their thermoelectric performance fluctuates depending on the humidity of the direct environment and devices fabricated from PEDOTPSS are difficult to treat in air because PEDOT-PSS involves hydrophilic counter ions, which makes it difficult to use them for practical purposes.[13] Here, novel hybrid organic thermoelectric materials were developed without conducting polymers such as PEDOT-PSS. The new system contained nanoparticles of a polymer complex that served as a charge transporting dopant in addition to CNTs and insulating PVC. Previously CNTs were thought not to be very good thermoelectric materials because of their high thermal conductivity.[10] However, CNTs with high Seebeck coefficients were reported to have a good thermoelectric performance.[14] In addition, CNTs with organic ligands, such as tetraphenylphosphine, were recently demonstrated to have a relatively high negative Seebeck coefficient and low thermal conductivity, resulting in ZT = 0.07.[15] Nevertheless, these CNT buckypapers have disadvantages as practical thermoelectric materials, because they could not be obtained by a printing process. Independently, we have planned to develop the organic–inorganic hybrid thermoelectric materials with high performance, by using an approach in which thermoelectric devices with designed patterns can easily be produced by printing. In our method the mixed CNT dispersions and the usual insulating vinyl polymers such as PVC (which has a low thermal conductivity) were drop-cast to produce hybrid films. The obtained two-component hybrid organic films had a relatively high Seebeck coefficient and low thermal conductivity, but the electrical conductivity was never high enough. The low electrical conductivity of the hybrid films could
be understood by assuming that the insulating polymers located in between CNTs decreased electrical conductivity as well as the thermal conductivity. Since the presence of a small amount of metal nanoparticles between conducting polymer chains could enhance the electrical conductivity of the hybrid polymer films,[16] charge transporting dopants, instead of metal nanoparticles, were considered to assist the increase of electrical conductivity of the hybrid films of CNTs and insulating polymers. After many trials, we discovered that nanoparticles of the thermoelectric polymer complex, n-PETT, can enhance the electrical conductivity of the hybrid films. In other words, a three-component hybrid organic film, composed of n-PETT, CNTs and insulating PVC, showed a high thermoelectric performance. The nanodispersed PETT (n-PETT), composed of nanoparticles having diameters from 10 to 50 nm, was prepared in the presence of surfactant by modification of a previously reported method[17] and used as a charge-transporting dopant in the hybrid materials that consisted of CNTs and insulating polymers. Although a highly negative Seebeck coefficient (−121.6 µV K−1) has been reported for solid PETT at room temperature,[17] the absolute value of the Seebeck coefficient for the pressed block of n-PETT was moderately large, i.e., –43 µV K−1 at 340 K. Casting dispersed solutions of n-PETT and PVC provided smooth hybrid films. However, the thermoelectric performance, especially the electrical conductivity of the hybrid films, was very low. Recently, similar findings were reported for the composites of solid PETT particles, which were prepared by a ball-milling method (average diameter = 456–850 nm), and poly(vinylidene fluoride) in DMSO.[18] In our study CNTs were used to increase the electrical conductivity of n-PETT/PVC hybrid films. Addition of CNTs to the hybrid films to produce three-component hybrids surprisingly increased the electrical conductivity as well as the Seebeck coefficient, as shown in Table 1, although the polarity of the Seebeck coefficient was altered from negative to positive values by addition of only a small amount of CNTs. This is probably because the used CNT was a p-type semiconductor. Recently, Maniwa and co-workers reported that purified semiconducting SWCNTs had a very high Seebeck coefficient of 170 µV K−1 at 350 K, which would make them suitable for use in flexible thermoelectric materials.[14] The thermoelectric properties of the sheet of our CNTs used in this research are shown in Table 1, indicating that the CNTs themselves had a high enough electrical conductivity of ca. 690 S cm−1. When they are dispersed in PVC, however, the electrical conductivity
Table 1. Thermoelectric properties of three-component hybrid films and related samples at 340 K.a) Sample n-PETT
S [µV K−1]
σ [S cm−1]
Block
−39.6 ± 0.7
3.6 × 10−2 ± 0.7 × 10−2 −3
5.7 × 10−3 ± 1.4 × 10−3 4.7 × 10−4 ± 0.6 × 10−4
−32.9 ± 1.0
CNT
Sheet
38.2 ± 0.4
CNT/PVCc)
Film
31.9 ± 2.8
23.2 ± 5.2
2.3 ± 0.4
n-PETT/CNT/PVCd)
Film
30.2 ± 0.6
429.3 ± 7.1
39.1 ± 1.6
n-PETT/CNT/PVCd) (MeOH treatment)
Film
30.5 ± 0.5
629.9 ± 28.6
58.6 ± 1.5
a)Measured
b)
4.4 ×
10−3
PF [µW m−1 K−2]
Film
n-PETT/PVC
2
Form
± 0.9 × 10
690.6 ± 16.2
100.7 ± 1.9
with ULVAC ZEM-3. b)Mass ratio: 10/3; c) Mass ratio: 8/13; d)Mass ratio: 10/8/3.
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of the hybrids of CNT/PVC (8/13 in weight ratio) was as low as 23 S cm−1. Nevertheless, addition of n-PETT enhanced the electrical conductivity to 430 S cm−1 as shown in Table 1. The Seebeck coefficient of our three-component films was 30.2 µV K−1, which is acceptably high. Thus, the thermoelectric power factor (PF = σS2) was as high as 39.1 µW m−1 K−2. Figure 2a,b show the effect of the CNT concentration on the Seebeck coefficient and electrical conductivity, respectively, of the three-component hybrid organic films composed of n-PETT, CNT, and PVC. The Seebeck coefficient does not depend on the CNT concentration and nearly equals that of CNT/PVC hybrid films. In contrast, the electrical conductivity increases with increasing concentration. It should be emphasized that the Seebeck coefficient did not increase that much with increasing CNT concentration in the case of the twocomponent CNT/PVC hybrid films. Thus, it is striking that n-PETT is very effective to obtain a high electrical conductivity for the hybrid films. Because PVC is insulating and CNT in contrast is conducting, n-PETT should assist the electrical
Adv. Mater. 2015, DOI: 10.1002/adma.201405463
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Figure 2. CNT concentration dependence of the Seebeck coefficient (a, S) and electrical conductivity (b, σ) of the three-component hybrid films. All films have a thickness of about 6–7 µm, and the mass ratio of the components varied as n-PETT/CNT/PVC = 10/(2–8)/3. The Seebeck coefficient of the film without CNTs is negative but changes to positive by addition of the CNTs, while the electrical conductivity increases linearly with increasing CNT concentration until 38%.
conduction between the CNT chains in the three-component hybrid organic films. Although we have no direct evidence of this, the charge transfer interaction between p-type CNT and n-PETT with negative Seebeck coefficient could provide the strong contact, which might enhance the electrical conductivity between the CNT bundles via n-PETT. In fact, it was observed by eye that the addition of n-PETT to the suspensions of CNT bundles in NMP did improve the dispersions of CNTs in NMP. This dispersion was confirmed by a decrease in the average diameter of the CNT bundles (about 6.1 ± 3.6 nm in Figure 3d) by SEM in the n-PETT-containing system, compared with CNTs without n-PETT (about 10.9 ± 5.4 nm in Figure S1a, Supporting Information). In addition, the CNTs could not be well dispersed if Cu-containing PETT, a p-type semiconductor,[17] was used instead of the n-PETT containing Ni. This observation suggested that the electronic interaction between p-type CNT and n-type n-PETT could provide the good contact, which might result in the improved electrical conductivity. In addition we found that treatment of the three-component hybrid films with methanol could enhance the electrical conductivity, as shown in Figure 2b. A similar enhancement in electrical conductivity by treatment with a solvent such as DMSO and EG has been reported in the case of PEDOT films, where the solvent treatment was considered to enhance the alignment of the conducting polymer chains[19] and/or to remove the insulating materials from the surface of the films.[2] In the three-component hybrid systems, the films were dried at 80 °C for 30 min after the treatment with methanol, and were found to have higher electrical conductivity than the corresponding pristine films (Figure 2b). The Seebeck coefficient of the threecomponent hybrid films was at a level similar to the pristine ones, resulting in power factors as high as 58.6 µW m−1 K−1. We found that methanol can slightly solve n-PETT and PVC under the present conditions. In fact, EDX (energy dispersive X-ray spectroscopy) measurements in SEM of the three-component hybrid films before and after the methanol treatment indicated that both atomic ratios of sulfur and nickel (both present in n-PETT), and chlorine (in PVC) against carbon (mainly in CNT) decreased from 5.47 ± 0.31 to 2.58 ± 0.31 for S/C, from 1.75 ± 0.11 to 0.88 ± 0.15 for Ni/C, and from 2.77 ± 0.17 to 1.16 ± 0.18 for Cl/C, respectively, with the methanol treatment, suggesting the removal of n-PETT and PVC from the films. The SEM pictures (Figure 3 and Figure S1, Supporting Information), current mode AFM images (Figure S2b, Supporting Information) as well as XRD patterns (Figure S4, Supporting Information) suggest the enhancement of smooth contact between the CNT bundles by methanol treatment and addition of n-PETT, although the measurements could not provide direct evidence. The thermal conductivity was as low as ≈0.06 m−1 K−1, as shown in Table 2 in the Experimental section, although the thermal diffusivity α was based on the measurement through the film. Thus, the maximum thermoelectric figure-of-merit was anticipated to be ca. 0.31 at 340 K for the present threecomponent hybrid films, although the thermal conductivity through plane was used for the calculation in spite of the presence of the anisotropy in the film. Temperature dependence of ZT values is also shown in Figure S3, Supporting Information. In order to understand the low thermal conductivity and the effect of the methanol treatment, scanning electron microscopy
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Figure 3. SEM images of the films of CNT / PVC (a,b) and n-PETT/CNT/PVC (c,d) before (a,c) and after (b,d) the methanol treatment. Mass ratios are CNT/PVC = 8/13 and n-PETT/ CNT/PVC = 10/8/3. The CNT bundles are clearly observed after the methanol treatment, especially in the case of the three-component hybrid organic films.
thermoelectric elements, Ro = NL/(σwt), and Pmax = Vo2/(4Ro). In these calculations, we used S = 38 µV K−1, σ = 425 S cm−1, ΔT = 100 K, N = 5, L = 5 mm, w = 4 mm, and t = 5.68 µm, and neglected the contact resistance and silver electrode resistance. Assuming an optimum packing density of the single legs, defined as the direction of heat transport, we estimated the expected electrical power density from such the devices to be ≈3400 µW cm−2 at ΔT = 100 K, by dividing the measured Pmax values by the crosssectional area of legs. The lateral geometry used here has the advantage that large temperature gradient can be achieved. However, from the point of view of conversion efficiency, the heat flow through the polyimide substrate will lead to a loss in efficiency; this is a drawback of the lateral architecture. In this study, we were not able to measure the heat flow in our homemade measurement system, thus the efficiency was not known. In the lateral geometry, a substrate with very low thermal conductivity should be used for obtaining high efficiency. On the other hand,
(a)
Table 2. Thermal conductivity, κ, and related data of hybrid films at 290 K. α [mm2 s−1]
Film
ρ [g cm−3]
κ [W m−1 K−1]
n-PETT/ CNT/ PVCa)
0.09 ± 0.01
0.85 ± 0.03
0.89 ± 0.19
0.07 ± 0.025
CNT/ PVCb)
0.08 ± 0.01
0.88 ± 0.03
0.83 ± 0.12
0.06 ± 0.003
a)Mass
4
Cp [J g−1 K−1]
ratio: 10/8/3; b)Mass ratio: 8/13
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(b) 20
Voltage (mV)
4 15 3 10
2
5 0
1
0
Power (µW)
(SEM) of the films was carried out before and after the methanol treatment (Figure 3). The SEM images clearly show the removal of the rather insulating materials that cover the CNTs by the methanol treatment, suggesting a smooth contact and formation of gaps or voids between the CNT bundles, which could possibly be one of the reasons for the high electrical conductivity and low thermal conductivity. Since the present three-component hybrid materials had relatively good thermal properties, the thermoelectric devices with a uni-leg structure having five legs (Figure 4a and Figure S5, Supporting Information) were constructed by printing technology on a polyimide substrate. When the temperature difference of approximately 100 K was applied between the two edges of the devices in the in-plane direction (Figure S6, Supporting Information), the output voltage, V, and power, P, changed with electric current as shown in Figure 4b, and the maximum power, Pmax = 3.88 µW was obtained at a matching condition from the current–power curves. The open-circuit voltage Vo = 19.9 mV, internal resistance of the device R0 = 25.6 Ω, and maximum power Pmax = 3.88 µW was in reasonable agreement with the calculated values (Vo = 19.0 mV, R0 = 25.9 Ω, Pmax = 3.49 µW) using the following equations: Vo = NSΔT, where N is the number of
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Current (mA) Figure 4. Photograph (a) and voltage–current line and current–power curve (b) of a flexible five-leg thermoelectric device composed of threecomponent n-PETT/CNT/PVC hybrid films and Ag electrodes on a polyimide substrate. The open circuit voltage Vo = 19.9 mV and the internal resistance R0 = 25.6 Ω at the temperature gradient ΔT ≈ 100 K. The maximum power Pmax = 3.88 µW was obtained at a matching condition from the current–power curves.
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Experimental Section Preparation of Nanodispersable PETT (n-PETT): Dispersed nanoparticles of n-type thermoelectric polymer complex, PETT, were prepared from 1,3,4,6-tetrathiapentalene-2,5-dione and nickel(II) chloride in the presence of the surfactant dodecyltrimethylammonium bromide, by modifying the method reported by Sun et al. to produce a PETT polymer complex without surfactants.[18] Nevertheless, the nanoparticles were well dispersed in an NMP solution containing poly(vinyl chloride) (PVC). Fabrication of the Three-component Hybrid Films: The three-component n-PETT/CNT/PVC hybrid films were usually prepared by drop-casting the mixed dispersion of n-PETT, CNTs (PureTubes, NanoIntegris Inc., single-walled carbon nanotubes prepared by a HiPCO (high pressure carbon monoxide) method and mixtures of 30% metallic and 70% semiconducting ones) and PVC (Average degree of polymerization: ca. 1100, Wako Pure Chemical Ind., Ltd., Osaka, Japan) in NMP at the designed ratios on a quartz or polyimide substrate. The films were slowly dried in air on a hot plate at ca. 60 °C for 12 h. The film thickness, measured with a linear gage (model LGK-010, Resolution: 0.1 µm, Mitsutoyo corp., Kawasaki, Japan) by subtracting the substrate thickness, was typically 5–20 µm. Fabrication of the Thermoelectric Devices: The devices consisted of five thermocouples, a leg of the three-component hybrid organic film that was 5 mm long, 4 mm wide and ca. 10 µm thick, and an Ag electrode 5 mm long, 3 mm wide and ca. 15µm thick on a polyimide substrate (UBE Industries, Ltd., Yamaguchi, Japan). Measurements of the Thermal Conductivity: Thermal conductivity, κ, was calculated by the equation κ = αρCp, where α is the thermal diffusivity, ρ is the density calculated by measuring the weight and volume of the films, and Cp is the specific heat capacity at constant pressure measured using a Netzsch DSC 204 F1 Phoenix. The thermal diffusivity α was measured with a Netzsch LFA 447 Nanoflash in a through-plane direction of self-standing films at 290 K.
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a vertical geometry has the advantage that the internal resistance can be lowered, resulting in higher output power than the in-plane device. However, in this case, a very small temperature gradient can be realistically obtained. In a vertical geometry, a very high thermally conductive substrate should be used for high efficiency to reduce the thermal resistance of the heat flow through the device. The direction of the design concept of the thin-film thermoelectric device should be different and should be optimized for its application. In summary, we have demonstrated the fabrication of novel three-component hybrid organic materials of n-PETT/CNT/PVC that show reasonably good thermoelectric properties, without the need for conducting polymers. The nanoparticles of the polymer complexes, n-PETT, can be well dispersed in PVC hybrids, although the powders of solid PETT could not be well dispersed in the polymer composites,[18] and played an important role for the smooth contact and good charge transport between CNT bundles in the films. This discovery could lead to the development of a new field of organic thermoelectric materials, which could have advantages in terms of flexibility and long lifetime. We envision that such materials could be applied for recovering electric energy from waste heat as well as providing electric resources for environmental sensors. The material used in the new system is stable and is likely to be better suited than PEDOT:PSS for practical applications in ambient atmosphere. In addition, this novel concept on the role of nanoparticles may provide a new field in organic electronics and photovoltaics.
The experimental results of the thermal conductivity measurements of the films are summarized in Table 2. Measurements of Thermoelectric Properties: The thermoelectric properties were measured by the following methods: a) Seebeck coefficient (S): Measured with a thin-film measurement system ULVAC ZEM-3M8 at 330–380 K under vacuum with He at least three times, as reported elsewhere.[20,21] The Seebeck coefficient was obtained from measuring thermo-electromotive force as a function of temperature difference within a range of 5 K at each temperature. The absolute Seebeck coefficient was obtained by subtracting the Seebeck coefficient of the Pt probe wire from the Seebeck coefficient, calculated from the measured thermo-electromotive force and temperature difference over the entire temperature range. b) Electrical conductivity (σ): Measured with a thin-film measurement system ULVAC ZEM3M8 at 330–380 K under vacuum with He at least three times. The electrical conductivity was measured with a standard four-point probes method with a direct-current power supply of 0.1–0.2 mA, and the thermo-electromoive force was subtracted by alternating the current direction. c) Power factor (PF): Calculated by the equation PF = S2σ. d) Thermal conductivity (κ): Measured as shown above. e) Thermoelectric figure-of-merit (ZT): Calculated by the equation ZT = (σS2/κ)T, where T is the absolute temperature of the measurement. It should be emphasized that although the electrical conductivity σ and Seebeck coefficient S were measured in an in-plane direction, thermal conductivity was measured in a through-plane direction. Since the organic films usually have anisotropic properties, the ZT value calculated by the factors with different directions is not exactly correct. Nevertheless, tentatively we used the above value because of difficulty in the evaluation of thermal diffusivity in an in-plane direction. Measurements of Power-Generation Characteristics: The powergeneration characteristics of an n-PETT/CNT/PVC film flexible device was measured by a homemade system (Figure S7), which has been reported previously.[20,21] We heated one side of the device using a cartridge heater (the temperature of cartridge heater block was up to 473 K); simultaneously, we cooled the other side by circulating water at constant temperature of 293 K. A polyimide sheet was inserted between the n-PETT/CNT/PVC device and cartridge heater block for insulation. Thermally conducting paste was used to reduce the thermal resistance between the interfaces. We set the temperature difference ΔT to ≈100 K in an in-plane direction between the two sides of the device. The temperature difference ΔT of the surface of the device was measured by a thermal imaging camera (Model T640, FILR Systems Japan K. K., Tokyo, Japan). The output power characteristics of the device were evaluated in air using a homemade system; this system measured the voltage–current and output power–current curves by varying a potentiometer (load resistance RL) from 0 to 100 Ω.
Supporting Information Supporting information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by “Yamaguchi Green Materials Cluster” project registered as Regional Innovation Strategy Support Program (Global Type) by the Ministry of Education, Culture, Sports, Science & Technology (MEXT), Japan. Polyimide films were provided by UBE Industries, Ltd.
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Received: November 29, 2014 Revised: January 7, 2015 Published online:
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