Fabrication of Transparent Paper-Based Flexible Thermoelectric

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Applications of Polymer, Composite, and Coating Materials

Fabrication of Transparent Paper-Based Flexible Thermoelectric Generator for Wearable Energy Harvester using Modified Distributor Printing Technology Xuan Zhao, Wenjia Han, Chuanshan Zhao, Sha Wang, Fangong Kong, Xingxiang Ji, Ziyuan Li, and Xiaoan Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21716 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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ACS Applied Materials & Interfaces

Fabrication of Transparent Paper-Based Flexible Thermoelectric Generator for Wearable Energy Harvester using Modified Distributor Printing Technology Xuan Zhao, Wenjia Han*, Chuanshan Zhao, Sha Wang, Fangong Kong, Xingxiang Ji, Ziyuan Li, Xiaoan Shen State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353, China

Abstract: Paper-based substrates have been increasingly attractive in flexible electronics technology as flexible support substrates due to their advantages of availability, environmental friendliness (as disposable, degradable, and renewable materials), and foldability. Hereby, a facile method for installation of p-type and n-type semiconductor legs in the thickness direction of a paper substrate was established. A transparent paper-based thermoelectric generator prototype by impregnating the paper with resin was then fabricated. The resulting transparent paper-based thermoelectric generator with 10 thermocouples showed excellent mechanical flexibility. The generator maintained a maximum voltage and an output power of ~8.3 mV and ~10 nW, respectively, at a temperature difference of 35 K after 1000 bending cycles. This work offers a promising strategy for the development of paper-based thermoelectric generators which is adaptable to a wide variety of complex curved surface heat source. Therefore, the heat recovery efficiency in both human and natural environments can be greatly improved. Keywords: transparent paper; thermoelectric; wearable; energy harvesting; flexible electronics

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1. Introduction Flexible substrate-based wearable devices including solar cells, biosensors, and multifunctional wearable devices in the use of disease diagnosis and rehabilitation have received widespread research attention. The functions of wearable devices cover a range of areas in daily life, including entertainment, fitness and exercise, medical care, navigation, and numerous other aspects.1-3 Wearable electronic devices commonly require a reliable, stable, and durable power source to enable them to function well. At present, lithium batteries are largely used as power sources. However, some critical issues such as being environmental pollution, explosive, and with short battery lives of lithium batteries need to be addressed. Most importantly, a micro-sized lithium battery with limited power cannot provide a long-lasting power for wearable electronic devices. Moreover, frequent disassembly and replacement of batteries would be inconvenient for the wearers.4-6 Therefore, the development of a clean, safe, stable, and long-lasting new power source has become a research hotspot worldwide. Recently, the proposed self-powered modes based on energy collection have been mainly including micro-solar generators, flexible piezoelectric generators, and flexible thermoelectric generators.7-9 Thermoelectric generators (TEGs) primarily convert thermal energy into electrical energy based on the thermoelectric (TE) effect using thermoelectric materials.10 The Seebeck effect provides a unique solution for a continuous power to wearable devices due to the direct conversion of thermal energy into electrical energy. However, the rigidity of conventional TEGs resulting in limited contact area with heat sources greatly reduces heat recovery efficiency.11-13 Therefore, a variety of flexible composite thermoelectric materials and devices


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have been developed based on conductive polymers, carbon-based materials, and inorganic nanomaterials. For example, Qu et al. compounded poly (3-hexylthiophene) and cotton to fabricate a flexible TEG, and their coated thermoelectric cotton thread can be sewn onto a flexible fabric to form a thermoelectric device. The device with 13 p-n junctions produced a maximum output power of 1.15 μW at a temperature gradient of 50 K.14 Juntunen et al. reported an inkjet-printed large-area flexible graphene film with excellent thermoelectric properties. The thermoelectric power factor of all-graphene film was 18.7 μW/ (m· K2) at room temperature.15 Hussain et al. used a simple micromachining and microfabrication-based technique to combine inorganic materials with paper substrates to develop a highly efficient thermoelectric nanoscale generator with output power of 0.5 nW at a 50 K temperature difference.16 To date, most of the flexible substrates that have been used in wearable thermoelectric devices have been based on polymer substrate materials such as polydimethylsiloxane (PDMS),17,

18, 19

polyethylene

terephthalate (PET),15 and polyimide (PI).13, 20 The assembled devices based on these materials showed featured flexibility. However, the slow degradability and poor biocompatibility of these polymeric materials came up as side-effect limiting the heat harvesting applications in or on the human body. Paper is made from the main consistent of cellulose fiber, which is considered as most abundant natural polymer on Earth.21, 22 Unlike petroleum-based products such as PI, paper offers the advantages of degradability, renewability, biocompatibility, and recyclability. Moreover, the orientations of the fibers and the surface morphology of paper can be tuned to fulfill the requirements of different types of paper products.18 As a product for use in writing and drawing with excellent penetration performance, Xuan paper always has a highly isotropic



structure that contains randomly oriented cellulose fibers.21 In light of the unique fiber orientation and porosity properties of Xuan paper, it was speculated that such an isotropic paper can be an ideal substrate for fabrication of TEGs. Additionally, there is a temperature difference between human body and environment, which determines that the temperature gradient of the wearable TEG must be perpendicular to the wearer’s skin.14,23-25 However, most two-dimensional (2D) thermoelectric devices that have been fabricated by methods including sputtering,26-27 evaporation,30, 31 inkjet printing,15,20,32 and screen printing12, 33 produce temperature gradients in the plane direction, which thus limits the TE devices use in human body applications. In recent years, some researchers have used new technologies such as 3D printing to fabricate TEGs, which has completely changed the TEG geometry of the plane.34-36 In this work, we describe a transparent paper-based flexible and wearable TEG structure that allows temperature difference generation along the paper thickness direction. The paper-based TEG was fabricated by alternately infiltrating p- and ntype inorganic thermoelectric composites into a porous paper substrate, and then connecting these composites electrically in series using conductive silver paste. The work demonstrated the feasibility of a paper-based TEG, and, to the best of our knowledge, is the first example of a transparent paper-based TE device for human waste heat recovery.

2. Experimental 2.1. Chemicals Bi0.5Sb1.5Te3 (powder, 99.99%) and Bi2Se0.3Te2.7 (powder, 99.99%) were purchased from Kaiyada (Sichuan, China). Glycerol (analytical reagent (AR) grade, 99%), ethylene glycol (AR, 99%), and polyvinyl pyrrolidone (PVP; molecular weight (MW): 58000) were purchased from


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Aladdin Chemical (Shanghai, China). Epoxy resin and the associated curing agent were purchased from Huisheng (Anhui, China). 2.2. Fabrication of paper substrate Paper substrates were fabricated using small laboratory papermaking equipment (Kaiser Method paper sheet former, Frank-PTI, Germany) without addition of any chemicals. Briefly, 2.512 g of softwood pulp was mixed with 2000 mL of deionized water, and was then stirred at 3000 rpm in a defiberer to form a well-dispersed pulp suspension. A wet paper sheet was formed after vacuum dehydration and was then transferred to a flat dryer after the excess water was removed by pressing. A paper sheet with a thickness of ~200 μm was prepared after drying at 95°C for 10 min. 2.3. Fabrication of transparent paper-based TEG A TE paste that was composed of 80 wt.% TE powder, 5 wt.% glycerol, 5 wt.% ethylene glycol, and 10 wt.% PVP in an aqueous solution was mixed thoroughly by grinding in agate. The viscosity and evaporation temperatures were controlled by varying the amounts of glycerol and ethylene glycol used. An organic binder was used to adhere the TE particles on the surface and the interior of the paper. The fabrication of the transparent paper-based flexible TEG is illustrated schematically in Figure 1.The prepared p- and n-type TE pastes were dripped on a PTFE film alternately using a pipette. Since the PTFE film is hydrophobic, the paste forms round droplets on the film due to surface tention, and the weight of each droplet is ~80 mg (Figure 1a). Then, the paper prepared in 2.2 was used to cover the PTFE film and the droplets were allowed to penetrate into the paper under pressure of 1.2 MPa. Cylindrical TE legs with a thickness of ~200 μm, a



diameter of ~12 mm were thus fabricated (Figure 1b, c). The spacing of two TE legs is 25 mm (Figure 2a). After this, the paper substrate containing the TE paste was cured in a vacuum oven at 105 °C for 30 min. The p-type and n-type TE legs were then connected in series using conductive silver paste. The fabricated TEG prototype was then used in performance measurements. The paper-based TE device prototype was cut into a 150×120 mm rectangle and immersed in liquid epoxy resin (an epoxy resin/curing agent mixture with a mass ratio of 1:1) at room temperature for 5 min (Figure 1d). The samples were then degassed by pressing and subsequently cured at room temperature for 24 h. 2.4. Characterization of transparent paper-based thermoelectric generator The sizes of the Bi0.5Sb1.5Te3 and Bi2Se0.3Te2.7 particles were measured using a nanometer particle size analyzer (Zetasizer Nano S90, Malvern Instruments, UK). The pore size of the paper substrate was measured using a membrane aperture analyzer (Porometer 3G, Quantachrome, USA). The morphologies of the paper and the thermoelectric materials were observed under a scanning electron microscope (SEM; Regulus-8220, Hitachi, Japan) operating at an acceleration voltage of 5 kV. Energy dispersive X-ray spectrometry (EDX) was performed via scanning electron microscopy using an EM 30 Plus (COXEM, Korea). The light transmittance of the paper was measured using a near-infrared ultraviolet spectrophotometer (Cary 5000, Agilent, USA). Tensile stress measurements were performed using an electronic universal material testing machine (Instron 5940, Instron, USA). A multi-scale X-ray nanocomputed tomography (nano-CT) system (Skyscan 2211, Bruker, Germany) was used to obtain three-dimensional (3D) images of the material’s interior. The output voltage data were collected using homemade apparatus combined with a Keithley 2400 measurement unit. The


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Seebeck coefficient was determined as the slope of the voltage versus temperature difference (∆T) curves. A typical four-probe technique was used to measure the electrical conductivity of the paper. The thermal conductivity of paper-based thermoelectric materials was tested using a laser flash method (LFA 467, Netzsch, Germany).

3. Results and discussion 3.1. Structural characterization of paper-based flexible TEG Paper is well known to be composed of an interconnecting 3D porous network of cellulose fibers.37 Commercially available papers largely consist of highly aligned, tightly packed, and closely bound cellulose fibers. In this work, a paper substrate with randomly orientated cellulose fibers and micron-sized pores was prepared by the imitation of the process used for hand-made Xuan paper. After that, Bi2Se0.3Te2.7 (n-type) and Bi0.5Sb1.5Te3 (p-type) pastes were prepared and infiltrated into both sides of the paper using a modified dispenser printing method (Figure 1). The liquid-like paste infiltrates easily into the paper pores due to capillary action, which is benifited by the modified dispenser printing technique. The TE paste can penetrate uniformly from one side of the paper to the other to form a cylindrical leg duo to the random orientation. The silver paste does not penetrate into the internal structure of the paper base, which is due to the poor fluidity of the silver paste we use. The 66% solids content of the silver paste and the viscosity of 260 ±50 poise cause it to form a film on the surface of the paper without penetrating into the paper. Therefore, the silver paste does not destroy the alternating interconnection structure of p-n junction. In contrast to the prevous work showing millimetersized holes in a polyester fabric, the paper substrate can provide sufficient support for attachment of the TE materials.38 In this work, the paper substrates are made of wood fibers.



When the temperature exceeds 180 °C, paper will be burned in air. TE materials commonly need to be sintered at a high temperature above 500 °C to be densified. However, high temperature sintering is not feasible for paper-based TEGs. Therefore, the TE material was densified by pressing. The organic binder (PVP) in TE paste can produce bonding effect at low temperature. Finally, the TE particles are further densified by pressing. Afterwards, TE ink deposited on paper substrates will be dried at 105 °C. It is of vital importance that the dense packing and contact of the TE materials on both sides of the paper during the fabrication of a paper-based TE power generator without use of external supports.

Figure 1. Schematic diagram showing the fabrication process for a transparent paper-based flexible TEG. (a) Alternating arrangement of p- and n-type pastes on a PTFE film. (b), (c) Infiltration of the TE paste inside the base paper. (d) P-type and n-type legs connected in series using silver paste. And the paper-based TEG was made transparent by impregnation with epoxy resin.


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The uniformity of TE paste distributed in the network of paper substrate can be controlled easily by adjusting the paper density (Figure 2c). The diameter of patterned TE legs is regulated by adjusting paper thickness, weight of deposited ink, and pressure of pressing plate, as shown in Figure 3. Figure 2c shows TE ink penetration on paper substrates with densities of 0.4 g/cm3, 0.6 g/cm3 and 0.8 g/cm3, respectively. When the paper density is 0.4 g/cm3, the TE paste can thoroughly infiltrated across the thickness of paper, giving rise to a uniform TE leg. However, when the paper density is more than 0.6 g/cm3, the TE paste cannot be completely infiltrated to form a regular pattern. The illustration of controlling size of the TE legs by different means i.e. adjusting the thickness of the paper substrate, the weight of the ink deposited on the paper and the pressure of the platen, respectively is shown in Figure 3. Measuring the diameter of the TE leg can determine the law of TE leg size control. The paper substrate with thickness of ~200 μm was selected for the TEG prototype and ~75 mg of TE ink was further deposited on the paper substrate at a pressure of 1.2 MPa. Cylindrical TE legs with a thickness of ~200 μm and a diameter of ~12 mm were fabricated. Due to the uneven thickness of the hand-made paper, the size of the actually fabricated TE leg fluctuates around 12 mm.



Figure 2. (a) Digital photograph of paper-based thermoelectric composites with a p-n junction. The spacing of two TE legs is 25 mm. (b) Cross-sectional SEM image of a paper substrate printed with silver paste. The yellow part is the silver paste printed on the surface of the paper. (c) Permeability and uniformity of ink on paper substrates with different densities.


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Figure 3. (a) The thickness of the paper substrate, (b) the weight of the ink deposited on the paper and (c) the pressure of the platen affect the size of the TE leg, respectively. Figure 4a shows the pore size distribution between the base paper fibers and an illustration of the fabricated base paper. Generally speaking, the width of a wood pulp fiber is ~30 μm, and the fiber width determines the size of the large pores that are generated during formation of the base paper. As shown in Figure 4a, there are micron-sized pores between the fibers, where the pore sizes are concentrated in the range between 26 μm and 33 μm with the average pore diameter of 28 μm. The size distribution of TE particle are shown in Figure 4b, where the inset shows the prepared TE paste. These three test results show that the TE particles have an average size of ~600 nm. The TE particles can be penetrated easily into the paper substrate because the nano-scale sizes are much smaller than the pores of paper substrate. As shown in Figure 5a and 5d, the cellulose fibers are randomly oriented and form a 3D network structure, thus exhibiting



the isotropy of the paper. The pores allow the TE paste to penetrate uniformly, and these multiscale pores can significantly reduce the thermal conductivity of the material by phonon scattering over a wide wavelength range at the pore location.39, 22, 23 Figure 5b and 5c show pand n-type TE pastes deposited in paper, respectively. The TE paste forms interconnect structures on both sides of the paper through the pores between the fibers and adheres to the entire fiber surface (Figure 5e and 5f), thus indicating the efficient deposition of the TE material. Figure 5g and 5h show the EDX spectrum of the cross-section of the paper-based TE material. The existence of the Bi, Sb, and Te elements in the p-type material and the Bi, Se, and Te elements in the n-type TE material were confirmed. The atomic ratio of Bi:Sb:Te in the p-type TE material is approximately 1:3:6, while the Bi:Se:Te ratio in the n-type TE material is approximately 20:3:27. These results are consistent with the elemental compositions of Bi0.5Sb1.5Te3 and Bi2Se0.3Te2.7, respectively.

Figure 4. (a) Pore size distribution of the base paper. Inset: the base paper. (b) Size distribution of the TE particles. Inset: the TE paste.


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Figure 5. (a)–(c) SEM images of the base paper and the infiltrated n-type and p-type TE material papers, respectively. (d) SEM image of single cellulose fiber surface. (e)–(f) The fiber surfaces of the n-type and p-type TE particles were deposited, respectively. (g) and (h) EDX maps of paper cross sections of Bi0.5Sb1.5Te3 and Bi2Se0.3Te2.7 were deposited, respectively. Figure 6 shows a nano-CT image of a paper-based TE material, where the green portion represents the TE particles and the brown colored part represents the cellulose fibers. The microscopic 3D structure of the paper-based TE material is shown in Figure 6a. The figure clearly shows that the cellulose fibers in the paper-based TE material are interwoven to form a skeleton structure. The TE particles are clearly remained within the interior of the paper. The retention of the TE particles in paper is related to the synergistic effects of mechanical interception by the fibers and the colloidal adsorption mechanism. The larger TE particles are intercepted by a fiber interlacing layer when passing through the pores of the paper. Simultaneously, a floc was firstly formed by TE fine particles and the organic binder (PVP) and further used as a bridge to connect the TE particles with fibers resulting in the adsorption on the surface of the fibers. As shown in Figure 6b and 6c, the TE particles are uniform in the plane and in the vertical direction in the paper. It is indicating that the formation of effective TE contact on both sides of the paper substrate using the modified dispenser printing method, which is highly advantageous for fabrication of paper-based TE generators.


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Figure 6. Nano-CT images of the paper-based TE materials. (a) Microscopic 3D structure of paper-based TE materials. (b) Surface and (c) cross-section of paper-based TE materials. (The green part represents the TE particles and the brown part represents the cellulose fibers). 3.2. Characterization of transparent paper-based flexible TEG Since the cellulose fibers are much larger in size than the wavelength of the light, most of the light that is incident on paper is scattered. The refractive index difference between cellulose and air causes the light to refract within the paper. These scattering and refraction actions cause a shift in the angle of most of the incident light, which eventually will fail to pass through the paper, resulting in the paper to be opaque.40,

41

The transparency of the paper substrate is

therefore improved the by impregnating it with resin having a similar refractive index to reduce light scattering. Figure 7a shows the change of the paper substrate with before and after epoxy resin impregnation. The untreated paper substrate showed low transparency with all the invisible letters behind the paper substrate. The resin-impregnated paper has high transparency with obviously visible letters through this paper. Figure 7b shows the change in the light



transmittance of the paper, where the light transmittance of the paper increased after the immersion treatment from 35% to 89%. It is indicated that the resin fills the pores inside the paper substrate resulting in remarkable improvement in the transparency, where the air that was distributed in the paper is removed. Excellent flexibility, waterproofing and wear resistance are desirable for wearable TEGs. However, the paper substrate is weak because of its loosely bonded fibrous matrix. Additionally, the plant fibers are rich in hydroxyl groups, which means that the fibers are hydrophilic. When the paper encounters water, the hydrogen bond is then destroyed. Afterwards, the paper substrate lose its original strength, which is highly disadvantageous for paper-based TEGs. However, the impregnated resin can increase the strength of the base paper and also make it waterproof. The tensile properties of both the base paper and the transparent paper are shown in Figure 7c. The maximum tensile stress of the base paper shown in the inset is ~0.006 MPa. However, the tensile stress of the transparent paper was increased to ~0.3 MPa. This robust strength, excellent waterproofing and transparency therefore endow the transparent paper-based TEG with great potential for use in human waste heat recovery.


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Figure 7. (a) Images showing that the paper-based TEG becomes highly transparent after epoxy resin impregnation. (b) Transmittance of the paper-based TEG before and after resin impregnation. (c) Tensile stress-strain image of paper-based TEG after resin impregnation. Inset: Tensile stress-strain of base paper. 3.3 Performance of transparent paper-based TEG One side of the transparent paper-based TEG is in close contact with the heating plate, while the other side is in contact with the Peltier refrigeration module to enable measurement of the output voltage and power. The paper strip (20 mm×10 mm) was infiltrated with the TE pastes and tested the resulting strips’ Seebeck coefficients. Figure 8a shows the temperaturedependent Seebeck coefficients of the Bi2Se0.3Te2.7 and Bi0.5Sb1.5Te3 paper strips obtained over



the range from 300 to 400 K. As the temperature increased, both the Bi2Se0.3Te2.7 and Bi0.5Sb1.5Te3 paper strips showed increases in their Seebeck coefficients. The Seebeck coefficient of the Bi2Se0.3Te2.7 paper strip increased from |−30.8| μV·K−1 at 300 K to |−36.9| μV·K−1 at 400 K. The Seebeck coefficient of the Bi0.5Sb1.5Te3 paper strip increased correspondingly from 60.4 μV·K−1 to 85.9 μV·K−1. The Seebeck coefficients of the Bi2Se0.3Te2.7/cellulose composites are all negative exhibiting n-type behavior. In contrast, the Bi0.5Se1.5Te3/cellulose composites exhibit p-type behavior. Figure 8b shows the electrical conductivity of the Bi2Te3/cellulose composite. Since paper is insulated, the deposition of Bi2Te3 on porous paper substrates leads to deterioration of electrical conductivity. In addition, organic adhesives can also lead to poor interconnection between TE particles. At room temperature, the electrical conductivities of the Bi2Se0.3Te2.7/cellulose composite and Bi0.5Sb1.5Te3/cellulose composite were 79 S/cm and 63 S/cm, respectively. The electrical conductivity of paper-based TE materials decreases by 1–2 orders of magnitude compared with pure TE materials. However, the thermal conductivity of the Bi2Te3/cellulose composite is optimized due to the porosity of the paper substrate. And the thermal conductivity of Bi2Te3/cellulose composites is 0.12 W/ (m·K). The low thermal conductivity can improve the thermoelectric properties of paper-based TE materials to some extent. Therefore, the ZT value of the paper-based TE material still reaches 0.07 at 300 K. In previous studies, the work done by researchers focused on TE fabrics, and we developed a paper-based TE composite (Table 1). The fabric filled with Bi2Te3 in its millimeter-scale pores to form TE legs. However, the flexibility of the fabric might be decreased due to the rigidity and brittleness of a large amount of filled inorganic TE material.


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1 2 3 4 Ultimately, it causes the stability of the thermoelectric fabric during bending process. Moreover, 5 6 high thermal conductivity of thermoelectric fabrics also reduces their thermoelectric properties 7 8 9 resulting in a low ZT value of ~0.01. The surface of the plant fiber is rough and contains a large 10 11 amount of hydroxyl groups, so that the TE particles adhere well to the fibers. And the 12 13 14 deposition of TE particles in the fibers does not destroy the flexibility of the paper. The micron 15 16 17 and nanoscale pores of the paper effectively reduce the thermal conductivity of the composite 18 19 (0.12 W/ (m·K)), increasing the ZT value to 0.07. In addition, to our best knowledge, the paper20 21 22 based thermoelectric devices with high transparency and strong mechanical properties were 23 24 first-ever fabricated in this study for energy harvesting in humans. Overall, our transparent 25 26 27 and flexible Bi2Te3/paper composite shows superior thermoelectric performance at room 28 29 30 temperature with lower thermal conductivity and higher ZT value. 31 32 33 34 35 Table 1. Comparison of the performance of reported paper-based thermoelectric materials and 36 37 composite thermoelectric fabrics at room temperature (~300 K). 38 39 Seebeck Electrical Thermal 40 41 Thermoelectric Materials coefficient conductivity conductivity ZT Transparency Ref 42 -1 -1 -1 -1 (μV·K ) (S·cm ) (W·m ·K ) 43 44 Bi2Te3/polymer fabric composite 176 20 1.73 0.01 No 12 45 260 15 2.1 0.01 No 9 46 Bi2Te3/polyester fabric composite 47 Bi2Te3/fiberglass fabric composite 98 150 1.48 0.03 No 19 48 Bi2Te3/paper composite 60 79 0.12 0.07 Yes This work 49 50 51 52 53 Under normal physiological conditions, human body temperature remains stable at 54 55 56 approximately 310 K. The normal temperature of the external environment can vary between 57 58 260 and 315 K. Therefore, the performance of the TEG over the ∆T range of 5−35 K was tested. 59 60



Figure 8c and 8d present the output voltage and the maximum output power of the transparent paper-based TEG when using different numbers of thermocouples at different temperature gradients. Both the voltage and the output power increase with increasing ΔT showed linear relationship between the voltage and the temperature difference. Additionally, the voltage and the output power of the TE prototype also increased when more thermocouples were incorporated into the paper. When ∆T increased from 5 to 35 K, the voltage of the transparent paper-based TEG that contained 10 thermocouples gradually increased from ~0.8 mV to ~8 mV, while the output power increased correspondingly from ~0.1 nW to ~10 nW. At a temperature difference of 35 K, the output power density was 0.53 nW·cm-2. These results confirmed that the transparent paper-based TEG can effectively convert thermal energy into electrical energy over the ∆T range of 5–35 K. To demonstrate the flexibility of the transparent paper-based TEG and the possibility of being designed for use as a power supply system for wearable devices, the stability of its internal resistance under bending stress in both the warp and weft directions was studied. Figure 8e shows that the internal resistance does not change significantly (less than 5%) between the flat state and a bend radius of 20 mm. Figure 8f shows the variation in the internal resistance during the bending cycle at a bending radius of 20 mm. The results show that after 1000 bending cycles, the internal resistance of the TEG is reduced by less than 10%, which is acceptable for practical applications.


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Figure 8. Output performance of transparent paper-based flexible TEG. (a) Dependence of the Seebeck coefficient on the temperature. (b) Dependence of the electrical conductivity on the temperature. Profiles of (c) the output voltage and (d) the output power of the paper-based TEG with different thermocouple units. (e) Relationship between the internal resistance and the bending radius. (f) Relationship between the internal resistance and the number of bending cycles. The inset shows the flexibility of the transparent paper-based TEG.



3.4 Demonstration of human waste heat conversion To evaluate the possibility of use of the transparent paper-based TEG in wearable field applications, the manufactured TEG with 10 thermocouples to a human arm was connected, as shown in Figure 9a. The generated voltage by the temperature difference between the wearer’s skin and the environment was measured. During the initial stage of the testing, the human wearer’s skin temperature was 306 K and the indoor air temperature was 298 K, which produced a voltage of 2.3 mV. However, the voltage gradually decreased with prolonged time, and finally remained at a constant level when the temperature was balanced. This occurred because the temperature difference between the skin temperature and the ambient temperature is initially quite large. However, as the heat gradually transferred from the skin to the surrounding environment through the TEG, the temperature difference decreases leading to the voltage drop. Based on prior experience, it is anticipated that the side of the TEG that is exposed to the environment will produce a large amount of air convection when the wearer is walking. Meanwhile, the movement also causes the surface temperature of the wearer’s skin to rise, thereby generating more energy. The harvested energy from the human body was monitored separately during periods of stopping and walking, as shown in Figure 9b. The output voltage when walking is significantly higher than that output when standing still. Therefore, the feasibility of the use of the designed transparent paper-based TEG structure in body heat-based energy harvesting was thoroughly verified.


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Figure 9. (a) Photograph of the output voltage of a transparent paper-based flexible TEG attached to a human arm. (b) Output voltage characteristics when the human wearer is stationary and when they are walking at an ambient temperature of 298 K.

4. Conclusions In summary, a scalable transparent paper-based TEG was successfully fabricated using a modified dispenser printing method. By regulating the paper fabricating process, an isotropic paper with a loose porous structure allowing the p- and n-type TE materials to form excellent thermoelectric contacts in the thickness direction of the paper substrate was prepared. Adoption of paper as a substrate not only provides robust flexibility for the TEG but also offers wear resistance and waterproofing function after transparency treatment. These properties are necessary for wearable energy harvesters. In addition, the transparent paper TEG can effectively convert thermal energy into electrical energy. A fabricated TEG prototype with 10 thermocouples produced an output voltage of ~8 mV and a maximum output power of ~10 nW from a temperature difference of 35 K. When the TEG prototype was applied on human body, the TEG prototype produced an output voltage of 2.3 mV in the case where the temperature difference between the human skin and the ambient temperature was 10 K. It was also proved



that the thermal voltage generated by the human body is higher while exercising than is stationary. Human usage experiments have demonstrated the feasibility of using the transparent paper-based TEG for self-powered wearable electronics. While the energy conversion efficiency of the manufactured TEG prototype is low, this control substrate structure design provides a new concept for the study of flexible and wearable TEGs. The transparent paperbased TEG exhibits robust strength, excellent flexibility, and high transparency, demonstrating a promising potential for the use in the fields of self-powered wearable electronics, noninvasive medical testing, and waste body heat recovery.

Author Information Corresponding Authors *E-mail: [email protected] Author Contributions All of the authors discussed the results and reviewed the manuscript. Notes The authors declare no competing financial interest.

Acknowledgments This work is supported by the National Key R&D Program of China (2017YFB0304100), the National Natural Science Foundation of China (31670590) and the Science and Technology Projects of Shandong Province (2016GGX102014, ZR201702220356, J16LE15).


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Fabrication of Transparent Paper-Based Flexible Thermoelectric

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