University of Surreyepubs.surrey.ac.uk/851809/1/ThermoelectricWearable... · Web viewThe higher molecular weight of PEDOT:PSS for Clevios PH1000 against Clevios P, raised both the

IOP Publishing Journal TitleJournal XX (XXXX) XXXXXX https://doi.org/XXXX/XXXX

Thermoelectric polymer composite yarns and an energy harvesting wearable textile

Jonathan Pope1and Constantina Lekakou1

1 Department of Mechanical Engineering Sciences, University of Surrey, Guildford, UK

E-mail: [email protected]

Received xxxxxxAccepted for publication xxxxxxPublished xxxxxx

Abstract

A thermoelectric wearable is proposed based on an innovative 3d wearable design, with p- type polymer impregnated cotton yarns and n-type polymer impregnated cotton yarns, where the ratio of the p-/n-type yarn cross-sections is optimised to maximise the thermoelectric conversion efficiency. The thermoelectric yarns are embroidered through a double-layer cotton fabric, using plain-stitch, and connected via painted silver electrodag patches. The first stage of this study involved the characterisation of P3HT-, PEDOT:PSS- and PCBM-cotton fabric composite samples, in terms of their fibre volume fraction, specific heat capacity, thermal conductivity, electrical conductivity, Seebeck coefficient, Z and power factors. In this initial assessment, P3HT was selected as the p-type polymer compared to PEDOT:PSS, and PCBM was selected as the n-type polymer. A small, 3d wearable prototype was fabricated and tested: it exhibited a Seebeck coefficient of 380 V K-1 but a relatively high resistance. It was estimated that scaling up this thermoelectric wearable to body coverage would produce about 1.5-2 W for a temperature difference of 40 oC applied on the thermoelectric device.

Keywords: thermoelectric, fabric, cloth, organic semiconductors, P3HT, PCBM, PEDOT:PSS, cotton

1. Introduction

The selection of thermoelectric materials for a certain application of thermoelectric energy conversion is generally based on obtaining a maximum figure of merit, ZT, which is the product of the Z factor and absolute temperature T [1-6]:

ZT=α 2 σk

T (1)

where is the Seebeck coefficient, is the electrical conductivity, k is the thermal conductivity and Z = 2/k. The efficiency of the thermoelectric device, , is then given by:

η=(T H−T C

T H )( √1+ZT avg−1

√1+ZT avg+(TC /T H )) (2)

where TH and TC are the absolute temperatures at the hot and cold side, respectively, and ZTavg is the average figure of merit based on the average properties avg, avg and kavg in the temperature range of use. Another key performance indicator is the power factor, PF, which is not related to the device efficiency [7] and is given by the relation:

PF=α 2σ (3)

Hence, good thermoelectric materials have high Seebeck coefficient and electrical conductivity but low thermal conductivity.

Wearable thermoelectric generators (TEGs) with rigid substrates [8-9] may be able to easily incorporate high performance inorganic thermoelectric materials but are generally suitable for small area applications, e.g. a watch, as large area wearables would be preferred to be compliant in order to conform with the skin and maximise heat transfer.

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The next step has been to develop flexible wearable TEGs (review in [9]), comprising multiple rigid segments or wavy rigid TEGs on film substrates, typically of single curvature, although double curvature has also been achieved if using elastomeric substrates. Wearable thermoelectric textiles represent the best candidate materials to achieve forming over the body contours and close contact with the skin, and are also compatible with the usual textile format of clothes. They may comprise yarns of 100% thermoelectric material or a composite of thermoelectric and other material. Such other materials may be cotton, wool or synthetic materials used in cloths that might be employed as part of the yarn or in the form of a substrate textile. n-/p- Bi2Te3/Sb2Te3

thermoelectric materials were screen-printed on a textile substrate [10] but their required high temperature treatment to 530 oC excludes usual natural fibre or other polymeric substrates. Alternatively, an embroidery may be created using thermoelectric yarns over any type of substrate, including textiles [11-12], with additive manufacturing of the electrodes and electrical connections [9,12]. Very recently, electrospinning of thermoelectric materials has been suggested [13], which offers the ability to combine inorganic and organic materials in the same fibres [14] and produce “fabrics” of multilayers of unidirectional fibres at alternating 0/90o orientation [15-16]. Textiles are also suitable to integrate energy storage devices connected to the TEG, such as supercapacitors, with woven carbon electrodes [17] or woven separator in the role of porous insulator [18].

The performance of thermoelectric materials may be classified on the basis of their ZT dependence on temperature into high temperature, > 900 K (e.g. SiGe-based materials), middle temperature (> 600 K and < 900 K, e.g. PbTe-based materials) and low temperature range (< 400 K, e.g. Bi2Te3-based materials) [5]. The temperature range for thermoelectric wearables is around the body temperature of 37 oC, against the environmental temperature spanning from sub-zero (-50 oC) to high 40s degrees Celcius. First strategies of designing thermoelectric materials with high Z factor started with electrically conductive materials of high Seebeck coefficient, in which defects or vacancies were created to lower their thermal conductivity, as in alloys or doped semiconductors, with semiconductors generally displaying higher values of Seebeck coefficient than metals due to the respective band structures [19]. Bismuth telluride alloys may be p- or n- type semiconductors [20], such as p-type (Bi1-

xSbx)2Te3 and n-type (Bi1-xSbx)2(Te1-ySey)3 that may reach a maximum Z of 3-3.3 x10-3 K-1 at room temperature, [8, 19, 21-22]. Investigations to reduce the thermal conductivity of these materials include alloying to increase their entropy (Z=1.4x10-3 K-1 for BiSbTe1.5Se1.5 with 0.9 at% Ag [23]), creating point defects (Z=3.4x10-3 [24]), nanostructuring (Z=3.75x10-3 K-1 [25-27], hierarchical nanostructuring (Z=3.59x10-3 [28]) and introducing dense dislocation arrays

at grain boundaries (Z=5.8x10-3 [29]). Growing metallic Ag nanoparticles onto the edges of antisymmetric chiral Sb2Te3

macrostructures raised both the electrical conductivity and the Seebeck coefficient, yielding a PF of 371x10-6 W K-2 m-

1[30].Implementing inorganic thermoelectric materials in the

fabrication of wearable textiles faces serious problems, due to the need of high temperature processing of the inorganic materials to “fuse” or anneal their structure, above the maximum temperature that a typical cloth material might withstand [9, 31]. Past works included printed n-/p- Bi2Te3/Sb2Te3-based thermoelectric devices on glass fibre fabrics to be able to apply heat treatments above 150 oC [32] to 530 oC [33]. Alternatively, synthesising these thermoelectric materials in the form of nanomaterials, Bi2Te3

nanotubes and Sb2Te3 nanoplates, and coating their paste with binder on a silk fabric [34] to form thermoelectric devices at low temperature treatment of 120 oC, to just evaporate the solvent and dry the fabric, yielded = 13.3 V K-1 and a power output of 0.035 nW K-1 for each p-n device at 310 K [35]; both of these performance indicators are lower than those expected from a typical Sb2Te3/ Bi2Te3 device, for example = 270 V K-1 and PF = 25x10-4 W K-2 m-1 in [36].

Hence, the next avenue of research is to consider organic p- and n- type semiconductors for wearable textiles, where such organic materials are easy to process in the liquid form of solutions and require relatively low temperature treatment for solvent evaporation and any annealing for grain coarsening [37-39]. In particular, polymer semiconductors have the additional advantage of low thermal conductivity that would benefit the figure of merit, ZT, while their operating temperatures are compatible with wearables and atmospheric conditions. However, they have a low power factor in the range of 10-10 to 10-6 W K-2 m-1, which is about 3 orders of magnitude lower than the PF of inorganic thermoelectric materials [40-41]. There are many p-type polymer semiconductors which have been studied as thermoelectric films. After controlling the oxidation level of PEDOT, a Z factor of 8.x10-4 K-1 was obtained at room temperature [42]. Poly(3,4-ethylenedioxythiophene) (PEDOT) thin films, doped with p-toluene sulfonate and fabricated by spin coating [43], exhibited higher electrical conductivity after doping but only in the in-plane direction, which is consistent with the formation of in-plane “pie”-type of grains in spin coating [38-39]. The higher molecular weight of PEDOT:PSS for Clevios PH1000 against Clevios P, raised both the Seebeck coefficient, = 24.8 V K-1

against 14.5 V K-1, and the electrical conductivity after post-treatment, raising the power factor to PF = 108 W K-2

m-1 against PF = 5 W K-2 m-1 [44]. A module of 48 elements of p-type PEDOT:PSS and n-type PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) co-doped with acridine orange base (3,6-bis (dimethylamino)acridine)



(AOB) and 1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (N-DMBI) (for n-leg: = -500 V K-1), resulted in a total = 0.011 V K-1 (per element: = 227 V K-1) and maximum power Pmax = 945 W [32].

Measurements of the thermoelectric properties and performance of poly(3-hexylthiophene) (P3HT) films and nanofibre mats yielded PF = 5.2 W K-2 m-1 and 3.7 W K-2

m-1, and Z = 3.2x10-5 K-1 and 5.3x10-5 K-1, respectively [45]. Furthermore, p-/n- type devices were optimised with P3HT-rich P3HT:PCBM p-type and PCBM-rich P3HT:PCBM n-type, with an optimised combined performance under illumination of = 34.8 V K-1, PF = 34.8x10-4 W K-2 m-1

and Z = 3.4x10-3 K-1 [46]: it was concluded that the doping with opposite-type semiconductor greatly enhances the device’s electrical conductivity due to separated charge carrier channels, but the Seebeck coefficient was reduced due to the increase of charge density; it was also observed that illumination raises performance which was attributed to an increase of the number of minority carriers.

Poly(3-octylthiophene)/carbon fibre composites yielded = 135 V K-1 and PF = 7.05 W K-2 m-1 [47]. Studies on hybrid polymer/inorganic materials include PEDOT:PSS/Te nanorod composite films of = 15 to 51 V K-1, maximum PF = 51.4 W K-2 m-1 and Z = 3.4x10-4 K-1 [48], and PEDOT:PSS/0.5% (Ca0.85Ag0.15)3Co4O9, with H2SO4

posttreatment, of = 18.5 V K-1 and PF = 75.2 W K-2 m-1

[49]. Solutions of these polymers have low viscosity and can

easily impregnate usual textiles [50-52]: a PEDOT:PSS impregnated textile exhibited = 16-17 V K-1 and PF = 0.04 W K-2 m-1 [53]; a thermoelectric module of 38 elements consisting of p-type PEDOT:PSS dyed silk yarns and n-type yarns coated with a trilayer of poly(N-vinylpyrrolidone) (PVP)/ 0.25wt% MWCNT:PVP nanocomposite/ protection elastomer exhibited = 1.23 mV K-1 (or per p-/n- element: = 32 V K-1) and a maximum power of 7.1 nW for T = 80 oC, where the module resistance was 334 k [54]; yarns were coated with p-type nonionic waterborne polyurethane (NWPU)/PEDOT:PSS/MWCNT or n-type NWPU/N-doped MWCNT, were embroidered on a fabric substrate and connected to copper wires with silver paint: a 10-device thermoelectric module exhibited = 11.5 V K-1. Although there are recent studies of organic thermoelectrics combined with typical wearable fabric substrates, such as silk, the studies do not contain detailed characterisation of the composite polymer-fabrics.

The present study focusses on a cotton fabric-based wearable modified with thermoelectric organic polymers, PEDOT:PSS and P3HT as p-type and PCBM as n-type. First investigations concentrate on the characterisation of regions of composite cotton fabric-polymer for each of these polymers, where a drop deposition method was used for the

composite sample preparation. On the basis of these results, a p-/n- thermoelectric element is designed with optimised ratio of p-/n-leg cross-sections to maximise thermoelectric conversion efficiency. p- and n-type yarns are then manufactured via impregnation of cotton yarns with the respective polymers, and these yarns are embroidered on cotton fabric substrates to create a thermoelectric wearable module, the performance of which is assessed.

2. Methods

2.1 Materials

The cloth substrate and the cotton yarns were from a Thai cotton poplin, plain weave fabric (C2654 from Closs & Hamblin, UK) of a nominal thickness of 0.24 mm, areal density of 13.7 mg cm-2, 60 ends/cm and 30 picks/cm, with warp and weft yarns being of 25 tex (Figure 1(a)). PEDOT:PSS was supplied (Sigma Aldrich, UK) and used as an emulsion of 1.3 wt% dispersion in water, of polymer density of 1011.5 kg m-3. P3HT (poly(3-hexylthiophene-2,5-diyl), regioregular, electronic grade, 99.995% trace metals basis, average Mn = 15,000-45,000, density: 1150 kg m-3 (Plexcore® OS 1100, supplier: Sigma Aldrich, UK) was dissolved in dichlorobenzene at a concentration of 10mg ml-

1, for liquid processing. PCBM ([6,6]-phenyl C61 butyric acid methyl ester), density: 1500 kg m-3 (Ossila, UK), was dissolved in dichlorobenzene at a concentration of 8 mg ml-1, for liquid processing. Silver electrodag 1415M (Agar Scientific, UK) was used for electrical contacts.

2.2 Fabrication methods

Samples of the cotton fabric (Figure 1(a)) were impregnated with a polymer solution (containing PCBM or P3HT or PEDOT:PSS) by drop deposition using a micropipette to create the following three types of composite polymer-cotton fabric samples: PCBM-cotton fabric; P3HT-cotton fabric; and PEDOT:PSS-cotton fabric. Each composite sample was left to dry on a hot plate at 50 oC. After drying, the PEDOT:PSS-fabric samples were further annealed on a hot plate at 150 oC for 10 minutes.

All samples were examined under optical microscopy (M) in transmission mode and polarised light microscopy (PM) in reflection mode; the images obtained are presented in Figure 1. The M image of the cotton fabric (a) shows the pattern of the woven yarns and the lighter, generally rectangular-shaped macropores between the yarns. The PM image of the cotton fabric (b) has a very light bluish hue, indicating lack of orientation in the material. The drop-deposition of PEDOT:PSS yields a dark composite PEDOT:PSS-fabric area in the M image of the front surface (c), in full contrast with any pure fabric beyond the area of the dried, deposited drop in image (c), where the macropores are visible in the



pure fabric area but totally invisible in the dark PEDOT:PSS-fabric area. The PM image of the front surface of the PEDOT:PSS-fabric composite (d) has a reddish hue, indicating orientation attributed to grain or other molecular orientation in PEDOT:PSS. On the other hand, the PM image of the back surface of the PEDOT:PSS-fabric composite (e) has a yellowish hue, which might be considered as the colour-blend of the bluish hue of the cotton fabric (image (b)) and some light pink from PEDOT:PSS, indicating that little PEDOT:PSS has impregnated the cotton fabric through the whole of its thickness, from the front all the way to the back surface. The images of the composite cotton fabric-P3HT or cotton fabric-PCBM were the same for the front and back surfaces, indicating good through-thickness impregnation, therefore only the images of the front surface are shown in Figure 1. The M image of the composite P3HT-fabric (f) shows large, empty macropores between the well-impregnated yarns, indicating that the impregnation

advanced in the micropores within the yarns (in preference to the impregnation of macropores), controlled by a high capillary pressure [50-51], and after solvent evaporation, the impregnated yarns shrank further. The M image of the composite PCBM-fabric (h) illustrates that impregnation of the cotton fabric by PCBM seems to also be controlled by capillary forces [50-51], but after solvent evaporation the yarns have not shrunk to homogeneous bodies as in image (f). The PM images of the P3HT- and PCBM-cotton fabric composites ((g) and (i) respectively) have an orange and reddish hue, respectively, indicating orientation attributed to the orientation of P3HT and PCBM, respectively. Images (j) and (k) are the M and PM images, respectively, of the front surface of the silver electrodag electrode printed on the cotton fabric; the PM image of the back surface of the fabric (l) shows the cotton fabric with some particles of the silver electrodag that have permeated the fabric and reached its back surface.

Figure 1. Images of composite samples from optical microscopy (M) in transmission mode and polarised light microscopy (PM) in reflection mode; scale for all M images: 500 m, scale for all PM images: 100 m: (a) cotton fabric (M); (b) cotton fabric (PM); (c) PEDOT:PSS-fabric composite, front surface (dark area), and plain cotton fabric area beyond the boundary of the deposited drop (M); (d) PEDOT:PSS-fabric composite, front surface (PM); (e) PEDOT:PSS-fabric composite, back surface (PM); (f) P3HT-fabric composite (M); (g) P3HT-fabric composite (PM); (h) PCBM-fabric composite (M); (i) PCBM-



fabric composite (PM); (j) silver dag printed electrode on cotton fabric, front surface (M); (k) silver dag printed electrode on cotton fabric, front surface (PM); (l) silver dag printed electrode on cotton fabric, back surface (PM).

The three types of composite samples (PEDOT:PSS-cotton fabric, P3HT-cotton fabric, PCBM-cotton fabric) and the cotton fabric (for comparison) were used directly in DSC analysis to determine their specific heat capacity. PEDOT:PSS, P3HT and PCBM films were fabricated on glass slides, using the same drop deposition technique, drying and annealing procedure. The films were peeled off and also subjected to DSC analysis, and had their specific heat capacity determined, for comparison with the composite samples.

Three types of single-leg thermoelectric devices were fabricated using the corresponding type of composite sample, i.e. cotton fabric impregnated with PEDOT:PSS or P3HT or PCBM. Silver electrodag was painted on each surface of the PEDOT:PSS-cotton fabric to improve electrical contact with the current collector, outer electrodes which were aluminium tape: Figure 2(a) presents a schematic of such single-leg thermoelectric device and Figure 2(b) displays a photograph of a PEDOT:PSS-cotton fabric composite-based thermoelectric. However, the silver electrodag permeated from one side to the bottom side of the P3HT-fabric and PCBM-fabric composite samples, as P3HT and PCBM do not cover the macropores of the cotton fabric (Figures 1(f) and (h)). In these cases, to avoid short-circuiting, no silver electrodag was used; instead, the aluminium tape electrodes were adhered on either side of the composite specimen, before the polymer was completely dry to ensure good contact (Figure 2(c) and (d)).

Figure 2. Schematic and single-leg thermoelectric specimens, used in tests of characterisation of thermoelectric properties of composite polymer-cotton fabric active materials. (a) schematic of a single-leg device; (b) a PEDOT:PSS-cotton fabric thermoelectric device; (c) a PCBM-cotton fabric thermoelectric device; (d) a P3HT-cotton fabric thermoelectric device.

After the above single-leg composite specimens were fabricated and tested, the findings and test results were used

to design a p-/n- thermoelectric wearable with details of this design to be reported in the results section 3.2. According to the thermoelectric properties data of the p- and n- composite samples, the optimum design is based on the ratio of n- and p- leg cross-sections An/Ap = 5.5, for the same length of legs. Furthermore, due to the wide spread of the P3HT and PCBM solutions across the cotton fabric when deposited dropwise using a micropipette, a different design and, subsequently, fabrication technique have been devised for the p-/n- thermoelectric wearable, involving whole composite P3HT- and PCBM-cotton yarns.



Figure 3. Design and specimen of thermoelectric wearable device based on a double-layer cotton fabric substrate and embroidered p-type and n-type composite yarns: (a) p-type and n-type composite yarns, starting with the first step of their preparation and a final image using a 3D optical microscope; (b) design of the thermoelectric wearable; (c) specimen of the thermoelectric wearable ready for testing.

Long cotton yarns were removed from the cotton fabric and were used as threads in a needle. Three cotton yarns were threaded through the needle eye, as shown on the left hand side of the diagram in Figure 3(a), the three yarns were then twisted and brought together to form a 6-thread twisted yarn, which was dipped in the PCBM-dichlorobenzene solution that impregnated the cotton yarn; after evaporation of the dichlorobenzene, an n-type PCBM-cotton composite yarn was manufactured. A single yarn was threaded through the needle eye, as shown on the right hand side of the diagram in Figure 3(a), and dipped in P3HT-dichlorobenzene solution which impregnated the cotton yarn; after evaporation of the dichlorobenzene, a p-type P3HT-cotton composite yarn was manufactured. In this manner, the two types of composite yarns had a cross-section ratio of An/Ap = 6 (microscopy image in Figure 3(a)), which was close to the optimum design proposed in section 3.2.

These p- and n-type composite yarns were used to embroider a 3d thermoelectric wearable using two layers of cotton fabric as substrates. Figure 3(b) presents a sketch of the 3d thermoelectric wearable. Two layers of cotton fabric of the same dimensions were brought together. Straight, parallel lines of plain stitch were sewn manually through the double-layer fabric substrate, using alternating p- and n- type composite yarns, with the pattern as well aligned as possible (Figure 3(c)). The yarn ends at one edge were secured with insulating adhesive tape, whereas at the opposite edge the yarn ends were connected to aluminium tape electrodes using silver electrodag paste for improving the electrical contacts. Thus, five p-/n-type yarn pairs were embroidered. Along each pair, five rectangular patches of silver electrodag were painted over the p- and n- “dashes” of each p-/n- type yarn pair on the top surface of the wearable, to create parallel electrical thermoelectric devices and reduce the resistance. In a staggered pattern, rectangular patches of silver electrodag were painted over the p- and n- “dashes” of adjacent p-/n- yarn pairs on the bottom surface of the wearable, to create in series connections between the p-/n- thermoelectric pairs. The final fabricated wearable device is presented in Figure 3(c) (top and bottom surface). The reason for using a double-layer fabric substrate in the 3d wearable is to insulate the top and bottom surface silver electrodag connection patches.

2.3 Characterisation and Testing

The specific heat capacity, cp, of each type of material (fabric, PEDOT:PSS, P3HT, PCBM, and their composites with the fabric) was determined in differential scanning calorimetry (DSC) experiments, using TA Instruments DSC Q1000, in the temperature range of 20-80 oC at a rate of 5 oC/min.

In the next set of tests, single-leg thermoelectric devices (Figure 2) were sandwiched between glass slides and characterised while heated when placed on a hot plate up to 80 oC or cooled after placing on top a container with a mixture of water and ice (0 oC). During heating or cooling the open circuit voltage was continuously recorded with a voltmeter while an automated thermal camera FLIR A300 (with a high magnification lens to 25 m resolution) was taking a video of the temperature map of the device front face in Figure 2(a), with an image frequency of 30 Hz. The FLIR Tools+ software was used for the thermal data analysis and postprocessing. The electrical resistance of these devices was also measured.

The temperature maps within the thermoelectric material layer are used in the heat transfer analysis to determine the thermal diffusivity, k/cp. The heat transfer equation in the transverse direction, y, of the thermoelectric specimens in Figure 2 is as follows:

ρ c p∂ T∂ t

=k ∂2T∂ y2+

J 2

σ+T ∂ α

∂ TJ ∂T

∂ y+αT ∂ J

∂ y (4)

where t is the time and J is the current density. In equation (4), the Peltier term T(∂J/∂y) can be ignored as ∂J/∂y=0 in a thermoelectric leg, and the Thomson effect term T(∂/∂T)J(∂T/∂y) is neglected on the assumption of negligible variation of the Seebeck coefficient in the temperature difference experienced by an organic thermoelectric in a wearable. An estimate of the different terms also concluded that the Joule heating effect term J2/ is also negligible.



Figure 4. (a) Equipment set up for the measurement of thermoelectric properties: thermal diffusivity (video thermal camera controlled by laptop), Seebeck coefficient (voltmeter) and electrical conductivity; (b) temperature map image analysis of video frames for the determination of thermal diffusivity.

Considering the whole system, including the two glass slides, perfect contact between the thermoelectric layer and the glass slides was assumed. After adding a heat loss or gain term due to natural heat convection from the front face of the sample to the air, equation (4) was discretised following a finite volume/finite difference time implicit method [55] to yield the following equation:

ρ C pT i

n+1−T in

∆ t= k( y i+1 /2− y i−1/2) ((T i+1

n+1−T in+1)

( y i+1− y i)−(T 1

n+1−T i−1n+1)

( y i− y i−1) )− hLbl

¿

) (5)

The natural convection term in equation (5) includes the heat transfer coefficient, h, and a boundary layer length, Lbl. Superscripts n and n+1 denote two consecutive times,

subscripts i-1, i, i+1 denote three consecutive positions in the discretised y axis or the variable values at those positions, with the corresponding midpoints i-1/2 and i+1/2. Identifying consecutive yi-1, yi, yi+1 positions within the transverse face of the active thermoelectric material in the thermal map in Figure 4(b), the temperatures at these points were read at many different times. Given that cp is known as a function of temperature from the experimental data of the DSC analysis, (X, Y) pairs of values of the following equation (6) were recorded and plotted, so that the gradient of the data linear fit would yield the value of thermal conductivity, k:

Y=kX+constant (6)

In most heat transfer experiments in this study, small temperature variations were studied, so the natural convection term was represented by the constant in equation (6). Two samples were tested from each type of polymer-fabric composite.

The final p-/n- type thermoelectric wearable (Figure 3(b) and (c)) was also characterised in terms of measuring the open circuit voltage during heating or cooling from room temperature to 40 oC and also conducting resistance measurements. Such tests were carried out for 1, 2, 3, 4 and 5 p-/n- pairs of composite yarns connected in series.

3. Results

3.1 Results for the composite thermoelectric polymer-fabric samples

Figures 5(a), (b) and (c) present the results of specific heat capacity, cp, as a function of temperature from the DSC analysis in the range of 22-80 oC, where each figure includes the curve for the cotton fabric, the respective polymer, PEDOT:PSS, P3HT and PCBM, and the corresponding composite polymer-cotton fabric. It can be seen that the cp of P3HT and PCBM experiences small change with temperature. On the other hand, the cp of PEDOT:PSS and cotton fabric increases non-linearly with temperature. For PEDOT:PSS, cp reaches a maximum at about 71 oC, and thereafter decreases, which may be attributed to an increase of the material’s density because of rising crystallinity due to annealing effects.

Table 1. Thermoelectric properties of composite polymer-cotton fabric samples

Compositematerial

Vf k(W m-1 K-1)

(S m-1)

V K-1)

Z (K-1)

PF(W K-2 m-1)

PEDOT:PSS-Cotton fabric

0.89 0.0075 30.6 10.3 (p-type) 4.33x10-7 3.2x10-3



P3HT-Cotton fabric

0.87 0.0068 7 210 (p-type) 4.54x10-5 0.309

PCBM-Cotton fabric

0.93 0.0012 1.33 -283 (n-type) 8.88x10-5 0.107

The mass fraction of fabric in a polymer-fabric composite, Xf, is given by the following relation as a function of the specific heat capacities of the fabric, cp,f, polymer, cp,p and composite, cp,c:

X f=c p , p−c p ,c

c p , p−c p , f (7)

from which the fabric volume fraction, Vf, can be derived as follows:

V f=ρp X f

ρp X f+ ρf (1−X f) (8)

Figure 5(d) displays the linear fits of the cp data according to equation (7), from the gradient of which the mass fraction of fabric, Xf, is obtained and, consequently, the fabric volume fraction, Vf, using equation (8). The following results were obtained: PEDOT:PSS-cotton fabric: Xf = 0.82, Vf = 0.89, Vp

= 0.11; P3HT-cotton fabric: Xf = 0.77, Vf = 0.87, Vp = 0.13; PCBM-cotton fabric: Xf = 0.84, Vf = 0.93, Vp = 0.07.



Figure 5. Results of specific heat capacity for cotton fabric, PEDOT:PSS, P3HT, PCBM, and the composite samples of each of the three polymers with the cotton fabric: (a), (b), (c); (d) graphs for the determination of the mass fraction of each phase (fabric: Xf or polymer: Xp) according to equation (7), where Xp = 1-Xf: PEDOT:PSS-cotton fabric: Xf = 0.82; P3HT-cotton fabric: Xf = 0.77; PCBM-cotton fabric: Xf = 0.84.

Figure 6 presents the data fits following equation (6) to determine the transverse thermal conductivity of the polymer-cotton fabric specimens from the gradient of each line. The results for thermal conductivity for each type of sample are presented in Table 1. Table 1 also presents the results from the measurements of the Seebeck coefficient and electrical conductivity, from which the Z factor and power

factor were also calculated and are displayed in Table 1. It can be concluded that the PEDOT:PSS-cotton fabric and P3HT-cotton fabric behave as p-type thermoelectrics whereas the PCBM-cotton fabric behaves as an n-type thermoelectric. The PEDOT:PSS-cotton fabric has the lowest overall performance in terms of both Z and PF factors, due mainly to the low value of its Seebeck coefficient, .



Figure 6. Linear fits of the thermal maps data according to equation (6) to obtain the transverse thermal conductivity of the composite samples: (a) PEDOT:PSS-cotton fabric, (b) P3HT:cotton fabric and (c) PCBM-cotton fabric.

3.2 Design of the p-/n- thermoelectric wearable

As a result, a p-/n-type thermoelectric wearable could be designed on the basis of the better performing P3HT-cotton fabric and PCBM-cotton fabric composites. Assuming the same length of p- and n-type legs, their cross-sections, Ap

and An, respectively, may be optimised according to the following relation to maximise the device efficiency [56]:

Ap/An = [(kn n)/(kp p)]1/2 (9)

After substituting the respective data from Table 1 in equation (9), it yields: An/Ap = 5.5. This formed the basis of the design of the wearable thermoelectric p-/n- device fabricated as described in section 2.2 (Figure 3).

Furthermore, it was noted that the drops deposited into the cotton fabric from P3HT and PCBM solutions in dichlorobenzene spread quickly to large areas and could not be confined. Therefore, it was thought that the best method to fabricate p- and n- legs of controllable cross-section would be by using cotton yarns and impregnate them with the respective solution. A sketch of the 3d p-/n- thermoelectric wearable is presented in Figure 3(b), its fabrication is described in section 2.2 and the final device is presented in Figure 3(c).

3.3 Results for the p-/n- thermoelectric wearable

Figure 7(a) presents the test arrangements in measuring the open circuit voltage between 2, 3, 4 and 5 thermoelectric (TE) pairs connected in series. Figure 7(b) presents the test data for both the resistance and the open circuit voltage as a function of the applied temperature difference between the hot and cold side, for an increasing number of TE pairs connected in series.The open circuit voltage increases linearly as a function of the applied temperature difference and the gradient of the linear fit represents the Seebeck coefficient of the device. Figure 7(c) illustrates that both the Seebeck coefficient and

the resistance of the TE wearable increase approximately linearly as more TE pairs are added in series.

Figure 7. Testing and results for the p-/n- thermoelectric (TE) wearable: (a) schematic of testing 2, 3, 4 and 5 TE pairs in series; (b) experimental measurements of open circuit voltage as a function of the temperature difference between the hot and cold side and measurement data of the device resistance; (c) total Seebeck coefficient and resistance for wearable device as a function of number of TE pairs connected in series; (d) estimated maximum power as a



function of the applied temperature difference between the hot and cold side.

The potential power, P, to be delivered by the thermoelectric wearable as a function of the internal device resistance, R, and current, I, is given by:

P = a T I – R I2 (10)

Equation (10) has a maximum at an optimum current, Iopt:

I opt=α ∆T2 R

(11)

Using the values of the device resistance and Seebeck coefficient as a function of the number of TE pairs in series (Figure 7(c)) and equations (10) and (11), the maximum power, Pmax, was estimated as a function of the temperature difference between the TE wearable hot and cold side and the number of TE pairs in series, and the results are presented in Figure 7(d).

4. Discussion

Our results of specific heat capacity agree with the literature for P3HT (cp = 1.65 J g-1 K-1 for P3HT nanofibres [45]) as well as the low value of cp for PCBM ((cp = 0.6 J g-1

K-1 the lower limit for PCBM in [57]. Lee et al [58] present the cp value for PEDOT:PSS in the range of 1.2-1.3 J g-1 K-1

for different grades, which agrees with our value of 1.38 J g -1

K-1 at 22.8 oC, but we also present its significant dependence on temperature in Figure 2(a).

The level of the retained content of the thermoelectric polymer has not been reported in the literature before. The full coverage of macropores between yarns by a PEDOT:PSS film in Figure 1(c) justifies the relatively high value of Vp

(=0.11) in the PEDOT:PSS-cotton fabric composite, despite the low PEDOT:PSS concentration of 1.3 wt% in the initial emulsion; however, the cotton yarns were poorly impregnated by PEDOT:PSS which may be attributed to this low concentration and possibly poor wettability of the cotton fibres. The P3HT and PCBM solutions in dichlorobenzene fully wetted and impregnated the cotton yarns, indicating good capillary pressure to impregnate the micropores, but did not fill the macropores which may be attributed to flow controlled by the capillary pressure and been preferentially diverted to the micropores rather than macropores [50-51]. The higher Vp value for the P3HT-cotton fabric composite, Vp = 0.13, compared to Vp = 0.07 for the PCBM solution can be attributed to the higher solution concentration of the former.

Figure 6 displays large fluctuation in the X and Y values of equation (6), originating from the temperature data of the thermal maps, for timesteps t = 5 s. The main reason for these fluctuations is thought to be the fact that the i-1, i, and i+1 numerical location points of equation (5) were

distributed in the mesoscopic scale of the fabric composite, hence, each point could be in a macropore between yarns or inside a yarn or at other intermediate locations. So, the individual points did not represent the composite cross-section as a continuum but individual parts of the mesoscopic structure. Nevertheless, it is considered that the best fits through each dataset represented an average thermal conductivity for the continuum.

It has been reported that the thermal conductivity of cotton is within the range of 0.026-0.065 W m-1 K-1 [59-60] and its electrical conductivity is low of the order of 10-8 S m-

1. Table 1 demonstrates that all polymer-cotton fabric composite samples have improved thermoelectric properties in terms of lowering the cotton thermal conductivity to the range of 0.001-0.007 W m-1 K-1 and increasing the electrical conductivity to the range of 1-30 S m-1. This is a great improvement to previous studies, where for example, the electrical conductivity of cotton fabric was increased to 0.06 S m-1 at best by Güneşoğlu et al [61] by coating with PANI/Co@C-coated metal nanoparticles. There is a lack of thermal conductivity data in the literature for polymer-cotton fabric or other natural fabric composites.

The Seebeck coefficient of the PEDOT:PSS-cotton fabric in Table 1 is similar to findings in the literature (for example, = 15 V K-1 for PEDOT:PSS-silk yarn [54]) but the electrical conductivity in this study is lower than that of PEDOT:PSS-silk yarn = 1400 S m-1 [54]: the reason for this might be that (a) Ryan et al [54] measured the electrical conductivity along the yarn rather than transverse to the yarn as in our study; (b) the contact resistance at contact points was high in the present measurements.

The present study has achieved much improved Seebeck coefficient for the P3HT-cotton fabric, = 210 V K-1

compared to = 70 V K-1 at best for P3HT films [45], but a lower magnitude = -283 V K-1 for the PCBM-cotton fabric compared to = -550 V K-1 for AOB- and DMBI-POH-codoped PCBM [32]. Very low thermal conductivity has also been achieved in this study, k≈0.007 and 0.001 W m-

1 K-1 for P3HT-cotton fabric and PCBM-cotton fabric, respectively, compared to k=0.16 and 0.07 W m-1 K-1 for P3HT film and nanofibre mat, respectively [45], and 0.03-0.10 W m-1 K-1 for PCBM films [57,62]. However, the electrical conductivities reported in Table 1 are much lower to those of P3HT film, 300 S m-1 [45], and AOB- and N-DMBI-co-doped PCBM, 200 S m-1 [32], which may be attributed to the high volume fraction of insulating cotton fabric (Vf ≈ 0.9) in this study and the disruption of the P3HT and PCBM domains by the cotton fibres which would further increase the insulating properties of these polymers, both in terms of electrical and thermal transport. If the electrical conductivity could be increased, by improving the electrical contact points for example, the values of the Z and PF factors in Table 1 could be raised.



As reported in the experimental methods and results, it was not possible to print miniaturised p- and n-patterns in the cotton fabric using a drop deposition technique, due to the immediate, wide spreading of the P3HT and PCBM solutions in dichlorobenzene across the fabric. Instead, the design and realisation of the 3d wearable prototype consisting of P3HT- and PCBM-impregnated cotton yarns, embroidered across a double-layer cotton fabric, offer the possibility of miniaturising and upscaling the wearable at the mesoscopic scale of at least two yarn-period, with at least one blank cotton yarn between p- and n-type yarns to maintain the insulation.

The tested 3d wearable prototype seems to reach approximately the same power performance to that of a 3d wearable prototype with PEDOT:PSS/MWCNT (multiwall carbon nanotube)-coated polyester p-type yarns and polyurethane/MWCNT-coated polyester n-type yarns [63]: a more detailed comparison between the two 3d wearables demonstrates that the 3d wearable of the present study has about 4 times higher Seebeck coefficient but also 120 times higher electrical resistance than the device by Wu et al [63]. The advantage of the currently proposed wearable is that it does not contain nanomaterials, while the P3HT and PCBM materials are not classified as hazardous and may be considered in similar manner as dyes for cotton and other wearable fabrics. Furthermore, the proposed polymers are easily recyclable via solution-based techniques [64-65]. It is also suspected that our device has lower thermal conductivity than the MWCNT-containing fabric although no thermal conductivity values were reported in the paper by Wu et al [63]; in such case, our wearable is expected to exhibit higher thermoelectric conversion efficiency than a MWCNT-containing wearable.

Developing the thermoelectric wearable prototype proposed in this study into a tighter embroidery of 18 p-/n-type yarns per 5 cm width, for example, and scaling the wearable up to body coverage would produce about 1.5-2 W for a temperature difference of 40 oC applied on the thermoelectric device. While this is a small power source, there is currently extensive research to develop micro-watt powered sensor ciruits [66] to monitor heart rate, pulse rate and oxygen uptake for example [67].

5. Conclusion

The present study resulted in the design of a 3d p-/n-thermoelectric wearable consisting of composite, P3HT-cotton p-type yarns and PCBM-cotton n-type yarns, embroidered on a double-layer cotton fabric. In the innovative design of the 3d wearable, the cross-section area ratio of the p- and n-type yarns was optimised for maximum thermoelectric conversion efficiency according to data of the thermoelectric properties of composite P3HT- and PCBM-

cotton fabric samples, characterised in the first stage of this study. During this first stage, it was also found that the P3HT-cotton fabric had superior performance compared to PEDOT:PSS-cotton fabric (both composite samples being p-type thermoelectrics), in terms of both Z and PF factors, due mainly to 20-fold higher Seebeck coefficient of the former.

The proposed 3d wearable offers the advantages of non-hazardous p- and n-type dyes, compared to nanomaterial coatings of previous studies [35, 63]. Furthermore, these organic polymers can be processed and annealed at relatively low temperature which does not affect adversely natural fibre fabrics such as cotton or silk, compared to printed n-/p- Bi2Te3/Sb2Te3 thermoelectric materials that need high temperature treatments to 530 oC [10, 33]. The simple plain-stitch embroidery technique using dye-impregnated yarns across usual wearable fabrics and the additive manufacturing of electrical contacts via inkjet printing, for example, can be easily applied in large-scale production or fabrication of individualised wearables.

The tested 3d-wearable prototype of five p-/n- TE pairs in series, of 5x5 cm approximately and yarn spacing of about 5 mm, could reach a maximum power of 0.24 nW at T = 40 K which was based on a high device Seebeck coefficient, = 380 V K-1, but also high resistance R = 12 k. We believe that further improvements may be achieved in the future in terms of lowering the contact resistance. As it is, the proposed thermoelectric 3d-wearable may be used for energy harvesting and feed micro-power sensors or other ultralow power microelectronics.

Acknowledgements

We would like to thank Mrs Violeta Doukova, University of Surrey, Chemistry Department, for conducting the DSC tests.

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