# High-Performance Wearable Bi\({}_{2}\)Te\({}_{3}\)-Based Thermoelectric Generator
## 1 Introduction  
With the improvement of people's living standards, wearable devices have become an indispensable part of people's lives, including smart watches, smart glasses, smart wristbands, etc. [1; 2; 3; 4]. However, the battery capacity and energy density of wearable devices limit their service life and stability, which has become a major obstacle limiting their development and application [5; 6; 7; 8]. Therefore, it has become a global consensus to develop and apply green energy technologies [9; 10]. As an emerging green energy technology, thermoelectric (TE) power generation can harvest energy from temperature differences in the environment to drive electronic devices by using the Seebeck effect (Figure 1A), which effectively solves problems such as the insufficient battery capacity of traditional wearable devices [11; 12; 13]. A thermoelectric generator(TEG) is a solid-state device that contains no mechanical moving parts, thus requiring no maintenance, high reliability, and noiseless operation, while being lightweight, compact, and taking up little space [14; 15; 16; 17]. Furthermore, wearable thermoelectric generators (w-TEGs) have been studied by many scholars [18; 19; 20; 21; 22]. [PERSON] et al. [23] used a simple cutting and gluing method to fabricate a w-TEG containing 48 pairs of P/N thermoelectric legs without a ceramic substrate. The w-TEG can be easily attached to the human body, and the maximum power density of the w-TEG was 7.9 \(\upmu\)Wcm\({}^{-2}\) and 43.6 \(\upmu\)Wcm\({}^{-2}\) under windless and normal walking conditions, respectively. [PERSON] et al. [24] used a radiative cooling coating on the w-TEG. Compared with the original device without the radiation cooling coating, its output power was increased by 128% in an exposed environment and 96% in a non-exposed environment. Its output power density was about 5.5 \(\upmu\)Wcm\({}^{-2}\) when the room temperature was 295 K. [PERSON] et al. [4] prepared a flexible micro-TEG with high power density and light mass using a pulse plating technique and effectively reduced the contact resistance between the thermoelectric leg and the electrode by adjusting the plating conditions. The optimized flexible micro-TEG had a maximum area power density and mass power density as high as 14.3 mWcm\({}^{-2}\) and 189 mWg\({}^{-1}\), respectively, at a temperature difference of 29.9 \({}^{\circ}\)C. [PERSON] et al. [6] designed a non-planar \(\pi\)-type flexible TEG with passive radiation cooling and a wave-shaped heat sink instead of a metal heat sink. The output power density of 12.36 \(\upmu\)Wcm\({}^{-2}\) and voltage density of 4.04 mVcm\({}^{-2}\) at 23 \({}^{\circ}\)C when worn on the human body is sufficient to drive some microwatt and sub-microwatt wearable electronics. W-TEGs can be flexible using three approaches: fabrication of thin-film type TEGs, TEGs using conductive polymers as thermoelectric units, and TEGs using flexible organic substrates connected to rigid inorganic thermoelectric units [25; 26; 27; 28; 29; 30; 31]. However, thin-film type thermoelectric units have difficulty in establishing a large enough temperature difference, and their power generation performance is usually poor [32; 33; 34; 35]. Although conductive polymers are flexible, and thus fit the skin well, their low power factor largely limits the power generation performance of the corresponding TEG [36; 37; 38; 39]. While the TE properties of inorganic semiconductors are excellent, by using flexible connections, the resulting TEGs not only have high \(P_{\text{out}}\), but also good flexibility [40; 41; 42; 43; 44]. By using rigid thermoelectric legs  
Figure 1: Flexible design of wearable thermoelectric generator. (**A**) Schematic diagram of Seebeck effect; (**B**) model of w-TEG on the arm and its equivalent thermal resistance; (**C**) physical image of the w-TEG modules 1# and 2#. in the flexible design of the \"strap\", such a TEG can generate enough power and still fit well on the human body.