Increasing the thermoelectric power generated by composite films using chemically functionalized single-walled carbon nanotubes

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Increasing the thermoelectric power generated by composite films using chemically functionalized single-walled carbon nanotubes Mingxing Piao a, Junhong Na a, Jaewan Choi a, Jaesung Kim a, Gary P. Kennedy a, Gyutae Kim a, Siegmar Roth a,b, Urszula Dettlaff-Weglikowska a,* a b

School of Electrical Engineering, WCU Flexible Nano-systems, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-713, Republic of Korea Sineurop Nanotech GmbH, Muenchner Freiheit 6, 80802 Muenchen, Germany

A R T I C L E I N F O

A B S T R A C T

Article history:

We prepared and characterized flexible thermoelectric (TE) materials based on thin films of

Received 21 March 2013

single-walled carbon nanotube (SWCNT) composites with polyvinylalcohol. While pristine

Accepted 12 June 2013

SWCNTs incorporated in a polymer matrix generated a p-type TE material, chemical func-

Available online 20 June 2013

tionalization of SWCNTs by using polyethyleneimine produced an n-type TE material. TE modules made of both p- and n-type composite were fabricated to demonstrate TE voltage and power generation. A single p–n junction made of two composite strips containing 20 wt.% of SWCNTs generated a high TE voltage of 92 lV per 1 K temperature gradient (DT). By combining five electrically connected p–n junctions an output voltage of 25 mV was obtained upon the applying DT = 50 K. Furthermore, this module generated a power of 4.5 nW when a load resistance matched the internal module resistance of 30 kX. These promising results show the potential of TE energy conversion provided by the SWCNT composite films connected in scalable modules for applications that require light weight and mechanical flexibility.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Thermoelectric (TE) materials are capable of harvesting electric energy in environments where great quantities of waste heat are generated; this heat can be put to good use by producing a temperature gradient [1,2] across TE materials, thereby, generating an electrical output. Since the combustion of fossil fuel has caused alarming environmental problems, such TE materials have attracted much attention due to their potential application in solid state power generation from waste heat [3,4]. The performance of TE materials is evaluated by a figure of merit, ZT, defined by ZT = S2rT/j, where S, r, T, and j are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity,

respectively [5]. The term S2r, comprising the inverse relationship between the Seebeck coefficient and electrical conductivity described as the power factor, is directly related to the usable power attainable from TE materials. Hence, it is commonly used as a simpler measure of TE efficiency [2]. Traditional TE materials are fabricated from low band gap inorganic semiconductors, such as Bi2Te3, PbTe, and Sb2Te3 which are toxic and expensive to mass produce [6]. Theory predicts that a significantly higher TE efficiency can be obtained from low-dimensional TE materials by comparison with bulk materials due to quantum size effects and favorable carrier scattering mechanisms. Carbon nanotube (CNT)/polymer composites are of particular interest because their heterogeneous structure allows

* Corresponding author. E-mail address: [email protected] (U. Dettlaff-Weglikowska). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.06.028

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decoupling of TE parameters which produce higher ZT values [7–12]. Even though, the current best CNT/polymer TE materials have a ZT  0.02, while Bi2Te3 has a ZT  1, there are some significant potential benefits to CNT/polymer TE materials. For example, the low thermal conductivities of polymers allow for a sustained temperature difference across the composite. Additional benefits, such as, mechanical flexibility, durability, light weight and low manufacturing cost, if produced on a large scale, occur when CNTs and polymers are combined in thin films [10]. Moreover, chemical treatment of single-walled carbon nanotubes (SWCNTs) can improve the Seebeck coefficient as a result of a Fermi energy shift relative to its initial energy state [13,14]. Recently, efforts have been made to improve the TE properties of CNTs utilizing semiconducting stabilizers [2], composed of CNTs with conductive polymers [4,9,11], treated with a plasma [15]. To produce sufficient power, TE materials are combined into modules containing many alternating p–n junctions that are connected electrically in series and thermally in parallel [13,16]. While p-type CNT materials are very common due to ambient oxygen doping effects, n-type CNT materials are unstable in air, and, thus, are rare. Hence, a protective coating is required to preserve the n-doped nature of CNT samples [17]. This paper investigates the incorporation of n-doped SWCNTs in a polymer matrix as a practical option for preserving n-type TE characteristics. A variety of flexible, stable in air composite films based on polyvinylalcohol (PVA) and n-doped SWCNTs were prepared by adding polyethyleneimine (PEI) to the dispersion of SWCNTs in polymer. Varying the SWCNT concentration and the quantity of PEI in the polymer matrix, the desired high electrical conductivity and improved n-type TE properties were obtained. Following this, modules made with p- and n-type SWCNT/PVA composite films were assembled and electrically connected in series to demonstrate the TE energy conversion.

2.

Experimental

2.1.

Materials and methods

The SWCNTs used in this study were synthesized by the chemical vapor deposition method and purchased from Thomas Swan & Co. Ltd., Crockhall, Consett, UK (Product Ref: PR0920) in the form of a ‘‘wet cake’’ which improved SWCNT dispersibility in water solutions. According to the manufacturer’s specifications, the diameter of SWCNTs was in the range 1–2 nm, and the length > 1 lm. Our detailed analysis based on the Raman spectra measured using three laser excitations (514, 633, and 785 nm) revealed that the purchased sample was a heterogeneous mixture of several metallic and semiconducting SWCNTs. The following semiconducting SWCNTs, (8, 4) (9, 5), (10, 5), (11, 3), (11, 4), (11, 6), (12, 2), (12, 4), (13, 5), (13, 6), and (14, 1) having diameters within the range 0.83–1.33 nm were indentified. On the other hand, the following metallic SWCNTs (9, 9), (12, 0), (12, 2), (13, 4), (15, 6), (16, 4), (17, 5), (18, 3), and (19, 1) with diameter between 0.94 and 1.56 nm were found. As the SWCNTs were applied as conductive fillers in polymer composites, their electrical properties in networks were of particular interest. Therefore, films known

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as ‘‘bucky papers’’ were prepared by vacuum-filtration of the suspended SWCNTs before the composite were fabricated. The electrical conductivity of these free-standing papers was measured to be 104 S/m with a corresponding Seebeck coefficient of 40 lV/K. PVA was selected as a basic bulk material for the polymer matrix. Its good solubility in water enabled preparation of thin flexible foils by simple casting and drying. The estimated molecular weight of the PVA was in the range 89,000–98,000. PEI (capped with amine groups at the ends of the polymer chain, average molecular weight of 600) was used for the chemical functionalization of SWCNTs incorporated in the polymer. Both chemicals were purchased from Aldrich and applied directly without any further purification. P-type composite films were prepared by casting an aqueous dispersion of pristine SWCNTs in polymer. Here, PVA was first dissolved in deionized water and an appropriate quantity of SWCNTs was suspended using an ultrasonic probe homogenizer for 15–30 min. The resulting viscous homogeneous slurry was poured onto a pre-cleaned glass plate. After drying for more than 24 h at ambient conditions, the composite films were easily peeled off from the glass surface. The doping effect of PEI on the SWCNT properties in composite films was investigated by adding PEI to the SWCNT dispersion and changing the weight ratio of PEI to SWCNTs from 0.25 to 20 weight% (wt.%). An optimal SWCNT concentration in composite films to produce good electrical transport properties was found by adjusting the weight ratio of PEI to SWCNTs at 2 wt.%. The resulting single film thicknesses were in the range 150–200 lm. The SWCNT composite films were freezefractured for micro-structural analysis using a scanning electron microscope (SEM, Hitachi S-4800). An Ar+ ion laser (514 nm) was used as a source in the Jobin–Yvon HR800UV Raman spectrometer to probe SWCNTs present in the samples. The X-ray induced photoelectron spectra (XPS) was measured using a PHI5000 Versa Probe (Ulvac PHI) with a monochromator Al Ka (1486 eV) operating at 15 kV.

2.2. Measurement of electrical conductivity and the Seebeck coefficient In order to measure the electrical conductivity, the composite films were cut into strips, typically 10 · 30 mm. Each film sample was pressure-contacted with four parallel metallic wires to perform the standard four probe measurement of electrical conductivity at room temperature. Current–voltage (I–V) sweeps were carried out using a Keithley 238 as the current source and a 34401A multimeter from Hewlett–Packard (HP) for the voltage measurement. Electrical conductance was then obtained by taking the slope of the I–V curve and converting it into electrical conductivity by multiplying it by a geometric factor. To determine the Seebeck coefficient, a temperature gradient (DT) was generated along the film strip by heating one end of the strip and leaving the other end exposed to air. Two platinum (Pt 100 X) resistors were clamped with electrical contacts at the ends of the sample strips to measure the sample temperature. The Seebeck coefficient was calculated from the voltage DV generated by DT using S = DV/DT.

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2.3.

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Fabrication of TE modules

The electrical p–n junctions were formed by mechanically compressing the ends of the sample strips. TE modules made of one two, three, four and five p–n junctions were arranged producing an alternating assembly of pristine p-type SWCNT composite films and PEI-doped n-type composite films with a pure PVA film acting as an insulating layer in between them. In order to test the module composed of five p–n junctions, DTs were applied across the module, while the total output voltage (VTEP), current and power generated for several load resistances were determined.

3.

Results and discussion

3.1.

Charcterization of SWCNT/PVA composites

Fig 1a demonstrates the mechanical stability and flexibility of a SWCNT/PVA composite film that contains 20 wt.% of pristine SWCNTs. The dispersion of SWCNTs in the PVA matrix was observed using an SEM. A typical image of a composite film cross-section (Fig. 1b) shows long, flexible, thin and uniform SWCNT bundles throughout the volume of the polymer matrix, demonstrating that PVA stabilizes the liquid dispersion and prevents excessive aggregation of SWCNTs in solid composite. In addition, a randomly interconnected, conductive CNT network embedded in the polymer was clearly established. Hence, upon incorporating 20 wt.% of pristine SWCNTs into a PVA matrix resulted in an electrical conductive composite with conductivity of 52 S/m. A TE power of +42 lV/K indicated a p-type electrical nature due to the doping effects from ambient oxygen in the pristine SWCNTs. These p-type SWCNTs conductive films were used in the fabrication and investigation of the p–n TE modules.

3.2.

TE properties of SWCNT/polymer composites

The origin of TE power in SWCNT composites is in their heterogeneous composition, where materials with different thermal and electrical conductivities like SWCNTs, polymeric matrices and chemical agents, used as doping materials, are present and interacting with each other. As demonstrated in the experimental part, the pristine SWCNT samples were a mixture of a variety of SWCNT chiralities which determined the SWCNT behavior as either metallic or semiconducting. The ratio between metallic and semiconducting SWCNTs

was not estimated here. However, the value of the Seebeck coefficient of ‘‘bucky papers’’ (40 lV/K) appeared to be consistent with that from a typical SWCNT material consisting of a mixture of 1/3 metallic and 2/3 semiconducting SWCNTs. The population of individual chiralities, in fact, can affect the thermoelectric performance. Semiconducting SWCNTs would be expected to have high Seebeck coefficients (100 lV/K), while metallic SWCNTs would be expected to have a low thermopower (10 lV/K), similar to metals and graphite. In addition, in networks consisting of mixed metallic and semiconducting SWCNTs Schottky barriers always appear at metal–semiconductor junctions. These barriers are also considered to contribute to the TE power in SWCNT composites. Thus, the TE effect observed in SWCNT networks is the result of the interplay between metallic and semiconducting SWCNTs. SWCNTs dispersed within the polymers form highly electrically conductive networks but their thermal conductivity is low. Polymers and chemical molecules are electrical and thermal insulators. Insertion of an insulating material into SWCNT–SWCNT junctions acts as a phonon scattering interface, while simultaneously allowing charge carriers to hop across energy barriers at the interface, and, consequently, increasing TE power. Here, we demonstrate how the interactions between SWCNTs, PEI dopant and PVA matrix affect the Seebeck coefficient and electrical conductivities of the resulting thin composite films.

3.3.

N-type doping of SWCNTs embedded in PVA

One of the chemical approaches to obtain n-type doped SWCNTs is to treat them with reducing agents [13,18] or cause them to adsorb organic molecules, which act like electron donors [19]. Recently, efforts have been made to utilize aminerich polymers such as PEI for preparation of the n-type TE materials [14,20]. In this study, the effectiveness of PEI as an n-type dopant for SWCNTs embedded in PVA for TE applications was investigated. Firstly, the weight ratio of PEI to the SWCNTs was fixed at 2 wt.% for each SWNT loading of 10, 20, or 30 wt.%. The Seebeck coefficient, electrical conductivity and the related power factor determined for the three types of composite films are shown in Fig. 2. The negative sign of the Seebeck coefficient observed for all samples indicated that the PEI ratio of 2 wt.% was sufficient to convert the SWCNTs from p- to ntype. Generally, the TE properties were determined by the

Fig. 1 – Photograph (a) and SEM image (b) of a cross-section of a composite film containing 20 wt.% of pristine SWCNTs.

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Fig. 2 – Electrical conductivity (a), Seebeck coefficient (b) and power factor (c) of SWCNT/PVA composite films doped with 2 wt.% of PEI as a function of SWCNT loading.

Fig. 3 – Seebeck coefficient (a), electrical conductivity (b) and power factor (c) of SWCNT /PVA composite films as a function of the PEI/SWCNT weight ratio.

concentration of SWCNTs. The conductivity increased as a function of increasing SWCNT concentration, because more conducting pathways were generated. However, the absolute value of the Seebeck coefficients gradually decreased with increasing SWCNT loading for the same fixed ratio of PEI to SWCNTs available for doping in the composite films. The greatest Seebeck coefficient reached a maximum of 35 lV/ K for a SWCNT loading of 10 wt.%. As the SWCNT concentration increased, the obvious bundling and aggregation of tubes in the dense dispersion precluded free adsorption of PEI molecules onto the sidewalls of the SWCNTs and resulted in reduced n-type characteristics. Under these conditions, the highest power factor was achieved for the composite with 20 wt.% of SWCNTs as shown in Fig. 2c. It should be noted that the linear trend of decreasing TE power with increasing conductivity is a general property of the heterogeneous conducting systems [10] – inclusively the SWCNT/PVA/PEI composites. Secondly, the effect of PEI doping level on the TE properties of the composite films was systematically investigated by fixing the concentration of SWCNTs to 20 wt.%. Then, the weight ratio of PEI to the SWCNTs dispersed in the polymer was gradually increased from 0.25 to 20 wt.%. Fig. 3 presents the Seebeck coefficient, electrical conductivity and the power factor for a series of SWCNTs/PVA composite films with increasing quantities of PEI up to a maximum of 20 wt.%. The observed changes

in these parameters are thought to be due to the n-type doping of PEI molecules adsorbed on the surface of SWCNTs. The molecular structure of PEI is shown in the inset of Fig. 3a. Many imine functional groups included in the PEI polymer act as electron donors through the lone-pair of electrons in nitrogen atoms. Thus, electron donation to SWCNTs caused the Fermi level to shift into the conduction band relative to the initial energetic state affecting both the Seebeck coefficient and the electrical conductivity of the incorporated SWCNT networks. As a consequence, the TE power of composite films converted from p- to n-type and was accompanied by strong reductions in electrical conductivity. Fig 3a reveals that a small amount of PEI (<1 wt.%) simultaneously compensated the holes in the p-type doped pristine SWCNTs but the samples still exhibited a positive TE power until charge equilibrium at 0 lV/K was reached. Also, consequently, the electrical conductivities reduce sharply due to the reduced carrier concentration and the scattering of PEI molecules (impurity species) contributing to a decreased carrier mobility [21]. In other words, 1 wt.% of PEI was required in the composite to de-dope the naturally doped SWCNTs. Considering the molecular structure of PEI shown in the inset of Fig. 3a, a molecular weight of 1 unit containing 13 nitrogen atoms is 505, which is close to the specification from the manufacturer (600). Thus, the concentration of 1 wt.% of PEI corresponds to a carbon to PEI molar ratio of 1:2.3 · 10 4.

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Fig. 4 – G-band region of the Raman spectra measured for SWCNT/PVA and SWCNT/PVA – doped with 20 wt.% of PEI – composite films (a); the inset shows an enlarged spectral region around the G-band peak to demonstrate the resulting band shift; XPS core level spectra of C 1s measured for pristine and PEI doped SWCNT films (b); the inset shows schematically the electronic density of states of a representative pristine metallic and semiconducting SWCNT with EF = 0 and the arrow indicating the shift of the Fermi level caused by the PEI doping.

The number of nitrogen donor atoms per carbon is calculated to be 3 · 10 3. In fact, this value is within the range 1–6 · 10 3 suggested by experiment as the doping fraction of nitrogen for each carbon in functionalized SWCNTs in order to convert the naturally p-type gate response into an n-type response in a field effect transistor [22]. This could be the reason why 1 wt.% of PEI can transform the composite film properties from p- to n-type. Further increases in the PEI ratio produced a negative Seebeck coefficient which could be attributed to further electron transfer with increasing PEI coating on the surface of SWCNTs. Thus, the majority of charge carriers were changed from holes to electrons [23]. When the quantity of PEI > 5 wt.%, the absolute value of the TE power slightly increased but saturation was still not achieved, thus implying, the higher the quantity of PEI, the better the n-type characteristics. A TE power of 57 lV/K was obtained for 20 wt.% of PEI. Furthermore, PEI, which acts as an electrical insulator coated on the SWCNT walls, is thought to behave as a phonon scattering interface, while simultaneously allowing charge carriers to hop across the energy barrier at the interface resulting in an improvement of the TE power. The low thermal conductivity of the PVA matrix strengthens the electron energy filtering by blocking or scattering the transport of some carriers in the doped composite films creating a synergistic effect [24]. However, when electrons were established as majority carriers at higher PEI concentrations (>5 wt.%), the electrical conductivity fluctuated strongly as a function of PEI concentration. Similar behavior for the electrical conductivity of n-doped SWCNTs was reported earlier for doping with aromatic amines [19] and can be associated with a varying population in the density of states in SWCNTs as result of the Fermi level shift toward empty van Hove singularities. The maximum value of the power factor was obtained for the composite with a PEI/ SWCNT ratio of 10 wt.% as shown in Fig. 3c. Raman spectroscopy is particularly useful in the characterization of doped SWCNTs, since the transfer of charges to and from SWCNTs alters the C–C bonds and causes changes in most vibrational modes. Because Raman scattering in SWCNTs is a resonant process, it is possible to measure well-defined vibrational modes for SWCNTs embedded in polymers [18]. Fig. 4a shows an example of a typical G-band

region of the Raman spectra for two SWCNTs/PVA composite films containing pristine and PEI doped CNTs. The transfer of negative charges to SWCNTs causing an increase in electron density in the C–C bonds was identified by a tangential mode downshift from 1582 cm 1 to 1578 cm 1 for pristine SWCNTs in the presence of PEI. This confirms that n-type doping occurs after treating the SWCNTs with PEI. Comparable spectral changes have been reported by other authors when they doped SWCNT networks with viologene [18], piperidine [25] and other aromatic molecular amines [19]. Our Raman results confirm that SWCNTs, dispersed in a polymer matrix, can be functionalized by the use of organic chemicals, in particular, the SWCNTs can be n-type doped by organic chemicals in a similar way to the doping of pure SWCNT films, known as bucky papers [14]. Electronic and structural information about carbon atoms and their local chemical environment can be deduced from XPS spectra. Fig. 4b shows changes in the core level spectra of SWCNT networks following treatment with PEI. The carbon C 1s, observed at 284.5 eV, in pristine SWCNT film is related to the binding energy of carbon in the sp2 hybridized systems (284.3 eV). When some carbon atoms in the SWCNT films accept electrons donated by nitrogen atoms in PEI, their binding energy increases which results in a shoulder appearing at the higher binding energy side of the C 1s peak. This new feature in the XPS spectrum corresponds to an upshift of the C 1s peak and is characteristic of the enhancement of electron density in carbon atoms in the n-type doped SWCNTs [19]. The effect of charge donation on the electronic structure of SWCNTs is addressed in the inset of Fig. 4b, which depicts the schematic electronic density of states of a representative pristine metallic and semiconducting SWCNT. The adsorbate-induced charge transfer shifts the Fermi level into the region of empty van Hove singularities which modifies the density of states at the Fermi level and consequently change the electrical conductivity and the Seebeck coefficient of functionalized SWCNTs.

3.4. Effect of the n-type doping level on the TE voltage of a p–n junction device To investigate the efficiency of a p–n junction made of SWCNT/PVA composites, two strips of composite films were

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Fig. 5 – A/B junction made of the SWCNT/PVA (A) and SWCNT/PVA composite films doped with different quantities of PEI (B) (a), Voltage generated by the A/B junction per 1 K DT as a function of the dopant ratio in B; the TE voltages generated by the individual composite films A and B are also plotted for comparison (b), VTEP produced by the A/B junction device when DT = 50 K was applied between the hot and the cold side as a function of the dopant ratio in B (c).

3.5.

(a)

(b) 25 20 15 10

40

ΔT=50K

VTEP (mV)

VTEP (mV)

electrically contacted [26]. Voltages in open electric circuits formed by joining a p-type SWCNT/PVA film and a n-type SWCNT/PVA film doped with PEI were measured, while the contact points between the films and the free ends were exposed to temperature gradients as shown in Fig. 5a. A composite SWCNT/PVA film containing 20 wt.% of pristine ptype SWCNTs was used as the p-type component. On the other side, three SWCNT/PVA composite films doped with 0.75, 1 and 10 wt.% of PEI were used as the n-type components in the p–n junction device. The corresponding TE voltage generated per 1 K (DV/DT) and the VTEP produced when the junction was subject to a temperature difference of 50 K parallel to the film surface are plotted in Fig. 4b and c. Since the film doped with 0.75 wt.% of PEI was a p-type conductor with holes as the dominant charge carriers, the holes were accumulated at the two free ends of the device. As a result, the TE voltage generated per 1 K is the difference between the two Seebeck coefficients obtained for individual films. However, for the devices with films doped with 1 and 10 wt.% of PEI, both the holes and electrons were accumulated at the cold side of the device leading to direct addition of the TE voltage contribution from each strip. Thus, the TE voltage generated per 1 K was the sum of the individual Seebeck coefficients. Furthermore, the Fig. 5b and c show that devices with higher PEI concentrations resulted in better n-type TE characteristics in the n-type strip of the device and, thus, determined the VTEP generated by this p–n junction.

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ΔT (K)

4

Number of junctions (N)

5

(c)

Characterization of the TE module devices

TE modules composed of one, two, three, four and five p–n junction devices were assembled by electrically connecting the p-type composite films containing 20 wt.% of pristine SWCNTs and the n-type composite films containing the same SWCNT concentration but doped with 10 wt.% of PEI. Schematic of the TE circuit involving five p–n junction devices is demostrated in Fig. 6a. The photograph shown in the inset of Fig. 6b presents a typical sandwich structure of 10 flexible composite films forming a flexible and mechanically stable TE module. The measured VTEP versus the number of junctions in the module devices is presented in Fig. 6b. For example, a temperature gradient of 50 K provided an output voltage

Fig. 6 – Schematic of the TE circuit involving five p–n junction devices composed of p- and n-doped SWCNT composite films (a), VTEP generated by a multi p–n junction module as a function of the number of junctions stacked in a sandwich with DT = 50 K, the inset in the top left and bottom right show the a photograph of the five junction TE module and VTEP generated by this module with an increasing DT, respectively (b), a plot of the power and voltage produced by this module for varying loaded resistances when DT = 50 K was applied (c).

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of 5 mV for one p–n junction device. When more p–n junction devices were added in series, the generated voltage increased linearly as the sum of the voltage contributions from each film (determined by the Seebeck coefficient), up to 25 mV. Due to the Seebeck effect, the VTEP is proportional to the DT – enlarging DT resulted in a linear increase of the VTEP, which is demonstrated in the inset of Fig. 6b. Hence, a DT of 25 K applied to the module, consisting of 5 p–n junctions generated a VTEP of 14 mV, whereas, a DT of 75 K provided a VTEP as high as 36 mV. Here, it should be noted that the single p–n junction composed of two electrically contacted composite strips containing 20 wt.% of SWCNTs, p-type pristine SWCNTs, and n-type PEI doped SWCNTs generated VTEP values as large as 92 lV per 1 K DT. By comparison, a recently reported [16] single p–n junction fabricated from two polyvinylidene fluoride films with 95 or 20 wt.% of CNTs produced a VTEP of 15 lV per 1 K DT. This improvement in the TE voltage, by factor of 6, is attributed to a large Seebeck coefficient ( 50 lV/K) produced by the chemical functionalization of SWCNTs with PEI present in the polymer composite. These results are consistent with those previously reported for the SWCNT fabrics treated with PEI [13]. Power measurements on the testing module composed of 5 junctions were performed for several different load resistances and the results are shown in Fig. 6c. The module was exposed to a DT of 50 K. Maximum power generation (4.5 nW) occurred when the load resistance (30 kX) matched the internal module resistance. At this load resistance, the VTEP was 11.6 mV compared to an open circuit VTEP of 25 mV, at the same DT. Due to the relatively high internal module resistance, most of the generated power was consumed at low load resistance. When the load resistance > 30 kX, VTEP increased continuously, approaching open circuit voltage, but the power decreased with an exponentially increasing load resistance. Considering that commercial TE modules consist of hundreds of p–n junctions with a correspondingly greater power output, TE modules fabricated from SWCNT/polymer composites have significant potential in applications, such as, electronic devices with local hot spots or in wearable electronics, where power generation from temperature gradients can be incorporated in flexible textiles.

4.

Conclusions

We investigated the interaction of materials exhibiting diverse electrical and thermal properties: PVA as an insulating polymer matrix, SWCNTs forming electrical conductive networks, and PEI as an n-type doping agent for SWCNTs. Each of these materials interacting together as one material created a synergistic effect producing superior TE properties than any of the individual materials that compose the whole. In particular, a simultaneous change of majority charge carriers occurs, from holes to electrons, upon the addition of PEI, which caused the Seebeck coefficient to change sign from positive to negative. The resulting n-doped thin flexible SWCNT composite films were used for the fabrication of a device

which demonstrated TE energy conversion. A single p–n junction comprised of two composite strips containing 20 wt.% of SWCNTs generated a large TE voltage of 92 lV per 1 K DT. The VTEP increased linearly with additional p–n junctions connected in series and with increased DT. A TE module composed of five electrically connected p–n junctions demonstrated a high VTEP of 25 mV upon the application of a 50 K DT. Also, this module generated a power of 4.5 nW when the load resistance matched the internal module resistance (30 kX). Our simple, inexpensive and scalable fabrication method is suitable for TE applications requiring light weight and mechanical flexibility.

Acknowledgements This work was supported by World Class University Project (WCU, R32-2008-000-10082-0) and by Global Frontier Research Program (2011-0031638) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.

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