**1. Introduction**

Recently, the rapid development of wearable electronic devices such as smart watches, wrist bands, smart glasses, and electronic skins has attracted extensive research interest of selfpowered and wearable technologies [1, 2]. Energy harvesting technologies such as solar cell, piezoelectric, triboelectric, and thermoelectric technologies can be utilized to meet the requirement of self-powered system [3–7]. As one of the most promising energy harvesting strategies, thermoelectric technology has received extensive research attentions in recent years. In a thermoelectric system, heat especially waste heat, such as human body heat, geothermal energy, solar thermal energy, and the residual heat of motor engines, can be directly converted into electrical energy without any pollution [8]. A typical thermoelectric generator (TEG) is composed of several p-type and n-type thermoelectric materials connected parallelly

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

with conductors. When temperature difference exists between the two sides of thermoelectric materials, the charge carriers in the materials will be driven to flow from the hot side to the cold side, and thus a thermo-voltage can be formed. As key components in a thermoelectric system, the energy conversion efficiency of a thermoelectric material can be evaluated by a thermoelectric figure of merit, also called ZT value [9]. As shown in Eq. (1).

$$\text{ZT} = \frac{\sigma S^2}{K} T \tag{1}$$

**2. Textile thermoelectric materials**

**2.1. Fabric-based thermoelectric materials**

given us inspirations for future wearable thermoelectric systems.

Textile is an ideal candidate to utilize our human body heat to generate power due to its excellent flexibility, wearability, comfort, and air-permeability properties, especially compared with those rigid bulk electronic materials and impermeable polymer-based flexible films. Thus, it is promising to develop textile material-based thermoelectric materials and generators to utilize our body heat. In the past few years, many research works have been done on both textile thermoelectric materials and fabric generators, which have paved the way and

Thermoelectric Textile Materials

25

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

Fabric finishing and coating are mature technologies in textile industry that can be easily used for large-scale production. Thus, it is a cost-effective and practical way to added effective thermoelectric materials on fabrics for both organic and inorganic materials. In 2015, Yong Du et al. prepared a textile thermoelectric material by coating commercial polyester fabrics with PEDOT:PSS [12]. Their adopted a simple dip coating process to fabricate textiles with thermoelectric functions. **Figure 1** shows the dip coating process the prepared thermoelectric textile strip by Tong Lin. P-type PEDOT:PSS doped with DMSO was acted as effective thermoelectric materials. Commercial available polyester fabric was used as a flexible substrate. Both advantages of textiles and PEDOT:PSS were utilized. The prepared textile materials exhibit electrical conductivities ranged from 0.5–3 S/cm and Seebeck coefficient of 15.3–16.3 μV/K at 300 K. The highest power factor of 0.045 μWm−1 K−2 was achieved at 390 K. Although the thermoelectric efficiency of this flexible textile thermoelectric material is not quite satisfied, this is

**Figure 1.** (a) A dip coating process and (b) PEDOT:PSS coated polyester fabric by Yong Du et al. [12].

In this equation, σ is electrical conductivity, S is Seebeck coefficient, *κ* is thermal conductivity, T is absolute temperature, and ZT is a dimensionless value without upper limit. The value of S2 σ is called power factor, which also can be used to represent the thermoelectric performance of materials where the impact of thermal conductivity is secondary. For example, in polymerbased thermoelectric materials, power factor is widely used, since most polymers have much lower thermal conductivity than inorganic semiconductors and then their thermoelectric performance will be mainly decided by the power factor.

Now, two main strategies can be adopted to improve the efficiency of thermoelectric system. One is to increase the ZT value of the thermoelectric materials, and the other is to adjust the structure of TEG for better utilization of heat energy. In past few years, the thermoelectric efficiency of bulk inorganic metal alloys has been improved significantly. A record-breaking high ZT of 2.6 achieved by SnSe single crystals at 923 K was reported in 2014, which gives thermoelectric generators a better application prospect in future [10]. Meanwhile, a new solar heat TEG system has been developed. By combining solar heat collection and thermoelectric conversion technologies, the new system could possess a peak efficiency of 7.4% [11].

Nowadays, with the rapid development of wearable and flexible electronics, the field of flexible thermoelectric materials and generators were growing dramatically. Various flexible thermoelectric materials were developed in form of films, papers, fabrics, and even fibers. Thermoelectric films are most studied among them, while the research about thermoelectric fabrics and fibers are just beginning very recently. Generally, these flexible thermoelectric materials are mostly made by organic polymers or polymer/inorganic semiconductor composites, so that they are easier to fabricate on large scales. However, the thermoelectric efficiency of these polymer-based flexible materials is much lower than the inorganic semiconductors. Hence, the methods to further improve the thermoelectric efficiency of these flexible materials will still be a hot research issue in future.

Although fabrics, fibers and polymer films are all flexible materials, textile thermoelectric materials have exhibited more competitive characteristics than simple films, especially on the wearable application. Textiles composed of different sets of fibers and yarns have excellent structure design flexibility, which allows them to meet the requirement of flexible, wearable, nontoxic, light weight, and even washable for wearable devices so that we can utilize our human body heat as power source. Until now, many pioneering research works have been conducted on this area and showed gratifying results. Therefore, in this chapter, the development of these textile thermoelectric materials and generators will be thoroughly described.
