**3. Textile thermoelectric generators**

Generally, a traditional inorganic TEG possesses a sandwich structure. Several n-type and p-type thermoelectric bulk materials are alternatively arranged and connected in series with conductors in the middle part of TEG. Ceramics are used at the top and bottom layers to avoid short-circuit between the metal interconnects. This sandwich structure helps to maintain an excellent heat exchange with surroundings. Temperature difference will be generated along the thickness direction of sandwich TEG, which is necessary for wearable thermoelectrics to some extent. However, it is difficult to apply this sandwich structure in flexible film materials. The structure of a typical organic film TEG is a two-dimensional plane. Although p-type and n-type film thermoelectric legs are still alternatively connected in series by flexible conductors, the temperature difference in this structure will only exists along the film length direction rather than along the thickness direction, since the dimension limitation in film thickness direction cannot sustain the temperature difference. Thus, the flexible 2D film generator will be challenged in wearable application. Textiles TEG with various structure design flexibility may provide one solution for this problem.

#### **3.1. Two-dimensional textile TEGs**

The studies of yarn and fiber thermoelectric materials are just beginning. The same coating process of fabrics can be further used to prepare fiber and yarn thermoelectric materials. Additionally, more complicated fabrication technique such as electrospinning and twisting process can also be adopted to prepare fiber and yarn materials with better performance. It can be imagined that there will be more ingenious methods in future to fabricate high performance thermoelectric fiber or yarn materials, so that a fully textile-based thermoelectric

**Figure 5.** Schematic illustration of the conversion of thermoelectric sheet into a yarn.

Generally, a traditional inorganic TEG possesses a sandwich structure. Several n-type and p-type thermoelectric bulk materials are alternatively arranged and connected in series with conductors in the middle part of TEG. Ceramics are used at the top and bottom layers to avoid short-circuit between the metal interconnects. This sandwich structure helps to maintain an excellent heat exchange with surroundings. Temperature difference will be generated along the thickness direction of sandwich TEG, which is necessary for wearable thermoelectrics to some extent. However, it is difficult to apply this sandwich structure in flexible film materials. The structure of a typical organic film TEG is a two-dimensional plane. Although p-type and n-type film thermoelectric legs are still alternatively connected in series by flexible conductors, the temperature difference in this structure will only exists along the film length direction rather than along the thickness direction, since the dimension limitation in film thickness direction cannot sustain the temperature difference. Thus, the flexible 2D film generator will be challenged in wearable application. Textiles TEG with various structure design flexibility may provide one solution for this problem.

generator can be achieved.

28 Bringing Thermoelectricity into Reality

**3. Textile thermoelectric generators**

A 2D flat TEG structure is also adopted in the first generation of textile TEGs. In 2012, C. A. Hewitt et al. developed a PVDF/CNT composite-based fabric TEG. Several n-type and p-type CNT composite films were alternatively arranged between the insulated PVDF films. The PVDF films are shorter than the CNT composite films. Thus, n-type and p-type CNT composites could form p-n junctions to connect the generator legs by hot press the stacked films, and resemble a felt fabric TEG [17]. The schematic structure of they prepared fabric TEG is shown in **Figure 6**. The prepared TEG composed of 72 layers fabric could generate power about 137 nW with an internal resistance of 1270 Ω.

In 2015, Yong Du et al. prepared a 2D fabric TEG by using PEDOT:PSS coated polyester fabric strips [12]. To fabricate the TEG, a whole PEDOT:PSS coated polyester fabric was cut into several strips first. Then, these strips were further adhered on a polyester fabric substrate by silver paint and connected in series with silver wires. Thus, a fabric TEG only composed of p-type materials can be prepared. The TEG arrangement is shown in **Figure 7**. Temperature difference will generate along the fabric length direction. The fabric TEG prototype consisting of five fabric strips could generate 4.3 mV when temperature difference ΔT is 75.2 K. The maximum output power Pmax could achieve 12.29 nW.

The same arrangement can also be achieved by thermoelectric yarns. In 2017, Ryan et al. reported a highly durable thermoelectric silk yarn made by dyeing with PEDOT:PSS, which could be produced with a length of up to 40 m and keep steady after repeated machine washing and drying [18]. Then, they embroidered these yarns into a felted wool fabric substrate and connected them end-to-end with silver wires to form a fabric TEG. The structure is illustrated in **Figure 8**. In an in-plane fabric TEG prototype composed of 26 yarn legs, the internal resistance is about 8.7 KΩ, and the output voltage of Vout/ΔT is about 313 μV K−1 when temperature difference ΔT is about 120°C. An output current of 1.25 μA can be observed when ΔT is 66°C, and resulted a maximum power output Pmax of ~12 nW.

**Figure 6.** (a) Alternative arranged multilayer fabric TEG structure and (b) fabric film TEG prepared by Hewitt et al. [17].

**3.2. Three-dimensional textile TEGs**

endure repeated bending for 120 cycles.

100 cycles of bending and twisting.

Zhisong Lu et al. deposited nanostructured Bi<sup>2</sup>

Te3

and Sb<sup>2</sup>

realize textile TEG that suitable for fabric thickness power generation.

Te3

**Figure 9**. The inorganic Bi2

In addition to the initial 2D textile TEG structures, researchers also developed several 3D textile TEGs similar to the classical sandwich bulk TEG. In 2014, Kim et al. fabricated a wearable TEG on a glass fabric by screen-printing technology [19]. The device structure and is shown in

ric substrate first. Then, the fabric containing an array of eight thermocouples was connected by several cooper foils, and finally encapsulated with PDMS. The fabricated TEG exhibits a high output power density of 3.8 mWcm−2 and 28 mWg−1 at ΔT = 50 K. Besides, the TEG could

Another research reported a silk fabric-based TEG similar to the Kim's structure in 2016.

method on two sides of a commercial available silk fabric [20]. The deposited p-type and n-type thermoelectric materials were further connected with silver foils to form a flexible TEG using a similar arrangement of Kim et al. [19]. The prototype containing 12 thermocouples could generate a maximum thermos-voltage of ~10 mV and output power of ~15 nw under a temperature difference of 35 K. The power output performance can sustain stable even during

Jae Ah Lee et al. using textile structure designed a new type of fabric TEG, which can utilize thermal energy along fabric thickness direction. Both knitting and weaving technology can be employed to fabricate this fabric TEG [16]. Several p-type and n-type yarns prepared by electro-spinning were arranged in the fabric according to the predesigned patterns. In knitted structure, p-type and n-type yarns were alternatively arranged and connected in series. In a plain weave structure, several single yarns containing metallic connected n-type and p-type components were woven into fabrics with insulating yarns. In addition, these single yarns should be carefully placed in correct way to ensure the right contact between p-n junctions and hot/cold surfaces. **Figure 10** illustrates the fabric TEG in knitted and woven structures respectively. The best output power of the prepared fabric TEG could achieve 8.56 Wm−2 at a temperature difference of 200°C. This study is the first attempt to utilize fabric structures to

**Figure 9.** Arrangement of fabric TEG that allowing generate temperature difference along fabric thickness direction.

Te3

and Sb<sup>2</sup>

Te3

thermoelectric materials were printed on a glass fab-

synthesized by hydrothermal

Thermoelectric Textile Materials

31

http://dx.doi.org/10.5772/intechopen.75474

**Figure 7.** Arrangement of 2D fabric TEG composed of only p-type materials.

**Figure 8.** Thermoelectric yarn arrangement in a fabric substrate.

In general, 2D fabric TEG is a first attempt to apply textile structures to the design of flexible TEGs. Although these initial 2D fabric TEGs have almost the same in-plane structure with film TEGs, fabric TEGs can be easily rolled up, bent, twisted, and are permeable to air and moisture, making them more flexible and comfort to wear.
