**1. Introduction**

The considerable growth of research studies in energy-harvesting technologies, such as solar energy harvesting [1], RF power harvesting [2], thermoelectricgenerator-based electrolyte [3], thermoelectric-generator-based solid thermoelectric materials [4], associated with the Internet of Things (IoT) leads to more demands in the development of the high performance of a micro-thermoelectric generator (TEG). Micro-TEG keeps a role as a charger to the rechargeable battery of IoT sensing systems or even replaces the battery if micro-TEG with high performance is employed. The TEG utilizes the Seebeck effect that can convert thermal energy into electricity. The

TEG has many advantages, including small size, without moving parts, free from noise, greenhouse gases, and long-term operation time [5, 6]. A voltage will be generated once a temperature difference across the micro-TEG is provided.

To enhance the performance of the micro-TEG, high-performance thermoelectric materials and increasing the number of thermoelectric elements are vital factors. Regarding thermoelectric materials, until now, several thermoelectric materials have been studied, including organic materials (metalloporphyrin/single-walled carbon nanotube composite films [7], Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate [8], and compositions of conducting polymers and metal nanoparticles [9]) and inorganic materials (nanoporous silicon [10], cobalt triantimonide [11], bismuth telluride and antimony telluride [12], tin selenide [13], electrodeposited bismuth telluride [14]). Among them, thermoelectric-materials-based BiTe are widely investigated because of their high performance for applications at near room temperature. For synthesis of thermoelectric-materials-based BiTe, several methods have been reported, including thermally evaporated method [15], metal organic chemical vapor deposition method [16], and pulsed laser melting method [17]. Electrochemical deposition is one of the preferred ways to enable the deposited film with high-quality morphology and compactness. Moreover, the electrodeposition method is capable of modifying the morphology, composition, and crystal structure of the synthesized film, which would result in the high performance of the deposited materials. Concerning enhancing the integration density, hundreds of thermoelectric elements could be produced on a small footprint by utilizing micro/nano fabrication technologies; however, some issues still remain. For instance, a complex process is required to create the air bridge between two thermoelectric elements. High contact resistance between thermoelectric elements and substrate results in low-performance micro-TEG. The performance of thermoelectric materials is degraded during their fabrication of the micro-TEG. The height of the thermoelectric element is limited by micro/ nano fabrication technology. Thus, it makes micro-TEG low performance and against the practical applications.

In this work, we review the recent progress in the micro-TEG, including material synthesis, device fabrication, and application demonstration. Various highperformance thermoelectric materials synthesized by the electrodeposition method, including thick bulk-like thermoelectric material, Pt nanoparticles embedded in a thermoelectric material, and Ni-doped thermoelectric material, are presented. In addition, the fabrication of micro-TEGs based on micro/nano fabrication technology as well as assembly technology is demonstrated. The performance of the fabricated micro-TEG is compared with other related works. Moreover, the fabricated micro-TEG as a power source for a calculator and a twist watch has been investigated.

### **2. Basic principles of thermoelectric generator**

#### **2.1 Properties of thermoelectric material**

#### *2.1.1 Seebeck coefficient*

The Seebeck coefficient is defined as the harvested voltage from the temperature difference across the thermoelectric materials. Its standard unit is microvolts per kelvin (μV/K). The Seebeck coefficient may exhibit positive or negative signs, which represents p-type or n-type thermoelectric materials, respectively. The p-type

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*

thermoelectric material shows an excess of holes, while the n-type thermoelectric material possesses an excess of free electrons. When a temperature difference appears at the ends of the thermoelectric material block, the charge carriers (electrons or holes) move from the hot side to the cold side, causing a thermoelectric voltage. The following equation depicts the Seebeck coefficient *S* of thermoelectric materials:

$$\mathbf{S} = \frac{\Delta V}{\Delta T},\tag{1}$$

where Δ*V* is the voltage gradient between the hot and cold sides of the thermoelectric material, and Δ*T* is the temperature difference between two sides.

One factor affecting the Seebeck coefficient is charge carrier concentration *n*. The relationship between the charge carrier and the Seebeck coefficient is proven experimentally and theoretically by published works [18–20].

$$S = \frac{8\pi^2 k\_B^2 T}{3eh^2} m^\* \left(\frac{\pi}{3n}\right)^2,\tag{2}$$

where *kB* is Boltzmann constant,*T* is temperature, *e* is the electron charge, *h* is Planck constant, and *m\** is effective mass.

#### *2.1.2 Electrical conductivity*

Electrical conductivity is an essential electrical property for thermoelectric material to conduct an electrical current. Electrical conductivity and electrical resistivity are the reciprocals of each other. Macroscopically, electrical conductivity is related to the dimensions and resistance of the measured thermoelectric material, which can be calculated by the following equation:

$$
\sigma = \frac{L}{RA},
\tag{3}
$$

where *L* is the length of the material, *R* is the resistance of the material, *A* is the contact area perpendicular to the current direction.

In principle, the electrical resistivity of a material characterizes the ability of the material to interrupt electricity flow. Therefore, it is strongly related to the flow of electrons and holes in a material. Those two factors influence the value of electrical conductivity, as shown in the following equation,

$$
\sigma = e \left( \mu\_\epsilon n + \mu\_h p \right),
\tag{4}
$$

where *μe*, *n*, *μh*, and *p* symbolize electron mobility, the carrier density of electron, hole mobility, and carrier density of hole, respectively.

#### *2.1.3 Thermal conductivity*

The thermal conductivity *k* of thermoelectric material is dependent on the charge carriers and the phonon's movement. Generally, the total thermal conductivity of metal increases when the electrical conductivity is high due to the directly proportional relation of electrical conductivity with carrier-charge thermal conductivity.

Therefore, the only option to reduce the thermal conductivity is by scrutinizing the value of lattice thermal conductivity [21, 22]. A lower lattice thermal conductivity results in a smaller value of total thermal conductivity. Introducing the nanoparticles in the metal might reduce the lattice thermal conductivity by blocking the excitation stream of lattice vibration, also known as phonons flow. The interrupted phonons flow increases the phonon scattering and elongates the phonon wavelength. Therefore, the time taken for the heat to transfer will be increased. The total thermal conductivity can be expressed by considering those two factors (charge carriers and lattice), as the following equation,

$$k = k\_l + k\_e,\tag{5}$$

where *kl* and *ke* are lattice and charge carrier thermal conductivity, respectively. Equation of lattice thermal conductivity can be referred to the following relationship.

$$k\_l = D \mathbf{C}\_p \rho,\tag{6}$$

where *D*, *Cp*, and *ρ* signify thermal diffusivity, specific heat, and material density, respectively.

Equation of charge-carriers thermal conductivity is estimated by

$$k\_{\varepsilon} = \nu \underline{\omega} L\_f T,\tag{7}$$

where *n* is carrier concentration, *e* is the electron charge, *μ* is carrier mobility, *Lf* is Lorenz factor (2.44 � <sup>10</sup>�<sup>8</sup> <sup>W</sup>ΩK�<sup>2</sup> ), and *T* is temperature.

#### *2.1.4 Figure of merit*

The figure of merit *ZT* is an instrument to evaluate the performance of thermoelectric materials, which encompassed the factor of the Seebeck coefficient *S*, electrical conductivity *σ*, thermal conductivity *k,* and absolute temperature *T* of the thermoelectric material. The ZT is defined as follows:

$$ZT = \frac{\sigma \mathbf{S}^2 T}{k} \tag{8}$$

To obtain high ZT values of thermoelectric materials, high *S* and large *σ* are desired; however, there is a trade-off between *S* and *σ*, as shown in Eqs. (2) and (4). Therefore, adjusting the coefficient between *S* and *σ* is a critical technique to achieve the highest ZT. Lowering thermal conductivity is also an important point to enhance the ZT, which can avoid the thermal shortcut problem and maintain a large temperature difference between the two sides.

#### **2.2 Thermoelectric generator structure**

A TEG is a solid device, which is able to convert thermal energy into electricity or vice versa. It consists of n and p-type thermoelectric elements arranged electrically in series and thermally in parallel. A cross-sectional view and titled view of the TEG structure are shown in **Figure 1(a)** and **(b)**, respectively. It mainly consists of n- and p-type thermoelectric elements, a metal bar, and a substrate.

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*

**Figure 1.** *TEG structure. (a) Cross-sectional view. (b) Titled view.*

As mentioned previously, the p-type element has a positive Seebeck coefficient and an excess of holes *h*<sup>+</sup> . The n-type element has a negative Seebeck coefficient and an excess of free electrons *e* �. The two elements are connected by an electrical conductor forming a junction, usually a copper strip. When a load resistor *R*<sup>L</sup> is connected in the output terminal of the micro-TEG, an electrical circuit is created. A potential voltage across the resistor is generated once the electrical current flows. The micro-TEG will create the current when a temperature difference across the micro-TEG appears. Higher temperature difference Δ*T* results in the larger electric output power.

The resistance of the thermoelectric elements is estimated by:

$$R = n\left(\rho\_n \frac{L\_n}{A\_n} + \rho\_p \frac{L\_p}{A\_p}\right) \tag{9}$$

where *ρ<sup>n</sup>* and *ρ<sup>p</sup>* are the electrical resistivity of n and p-type thermoelectric material, respectively, *Ln* and *Lp* are the height of n- and p-type thermoelectric elements, respectively, and *An* and *Ap* are the cross-sectional area of n and p-type thermoelectric elements, respectively.

In the above Eq. (10), the electrical contact resistance is eliminated. However, this resistance is typically quite difficult to be negligible due to the fabrication process. Therefore, the electrical contact resistance *R*<sup>a</sup> should be counted.

$$R = n\left(\rho\_n \frac{L\_n}{A\_n} + \rho\_p \frac{L\_p}{A\_p}\right) + \mathbf{R\_a} \tag{10}$$

The generated voltage *V*TEG could be estimated by the following equation:

$$V\_{T \to G} = n \left( \mathbb{S}\_p - \mathbb{S}\_n \right) \Delta T \tag{11}$$

where n is the number of thermoelectric elements, *S*<sup>p</sup> and *S*<sup>n</sup> are the Seebeck coefficient of p and n types thermoelectric materials, respectively, and *ΔT* is a temperature difference across the thermoelectric elements.

The maximum electrical output power of the TEG can be calculated by using Eq. (13), which is obtained if a load resistance *R*<sup>L</sup> is equal to the equivalent internal resistance of thermoelectric elements in series [23].

$$P\_{\max} = \mathbf{n} \frac{\left(\mathbf{S}\_p - \mathbf{S}\_n\right)^2 \Delta T^2}{4R\_L} = \frac{\mathbf{n} \mathbf{A}}{h} \frac{\left(\mathbf{S}\_p - \mathbf{S}\_n\right)^2 \Delta T^2}{4\left(\rho\_p + \rho\_n\right)}\tag{12}$$

where *A* and *h* are a cross-sectional surface area and height of thermoelectric elements, respectively. *ρ*<sup>p</sup> and *ρ*<sup>n</sup> are the electrical resistivities of p-type and n-type thermoelectric materials.

Several factors could affect the performance of the TEGs. Thermoelectric materials with excellent characteristics, including a high Seebeck coefficient, a small electrical resistivity, and a low thermal conductivity, are always desired for enhancing the TEG's performance. Many novel approaches, including utilizing metal nanoparticles [24], nanoporous materials [25], carbon black particles [26], and metal doping [27, 28], have been investigated to improve thermoelectric material's properties. Besides the effects of material properties, selecting proper physical dimensions of thermoelectric elements, such as the width and height of thermoelectric elements, could also contribute to better performance of the TEG [28]. Also, increasing the number of thermoelectric elements would be a valuable method for improving the performance of the TEG, as shown in Eq. (13).

The formula of an electrical energy conversion efficiency *η*TEG of the TEG [29] is defined by Eq. (14), which indicates that high electrical efficiency of the TEG could be achieved by a high figure of merit ZT as well as a large temperature difference ΔT.

$$\eta\_{\rm TEG} = \frac{\Delta T}{T\_H} \frac{\sqrt{\mathbf{1} + Z\overline{T}} - \mathbf{1}}{\sqrt{\mathbf{1} + Z\overline{T}} + \frac{T\_C}{T\_H}},\tag{13}$$

where *T*<sup>H</sup> and *T*<sup>C</sup> are the hot and cold temperatures of TEG, respectively.

### **3. Material synthesis**

#### **3.1 Electrodeposition method**

Thermoelectric materials presented in this work are synthesized by the conventional three-electrode system, which is controlled electrochemically by a potentiostat. The system involves a working electrode, a counter electrode, and a reference electrode. A silicon wafer with Cr-Au layers on the top insulated by SiO2 layer is employed as a working electrode, while a Pt strip and Ag/AgCl with 3 M KCl solution are utilized as counter and reference electrodes. The synthesized material is formed on the working electrode caused by the oxidation–reduction (redox) reaction. The electrochemical deposition mechanism is quite complicated and has been presented in many publications [30, 31]. It can be summarized as follows. In the electrolyte, the absorbed atom is in the form of the hydrated matter, which is stripped at the interface between the solution and the cathode. Then, it combines with other absorbed atoms to form a new nucleus. This process continues and contributes to the further growth of the deposited material.

One of the benefits of the electrodeposition method is the ability to change the morphology, composition, and crystal structure of deposited film by adjusting certain parameters in the electrodeposition system. All the changes might influence the alteration of the electronic or/and thermal properties of the deposited film. The effectively applied potential on the working electrode is one of the important parameters in the

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*

electrodeposition system that reflect on the variation of the current density. A change of the over potential on the electrode normally affects the current density and a chance to change the morphology.

#### **3.2 Thick bulk-like thermoelectric material**

As mentioned in the introduction section, thermoelectric materials could be synthesized by several methods. Although high performance of thin-film thermoelectric materials has been achieved, the TEG produced by thin-film thermoelectric materials possesses a low output power. Once the height of thermoelectric elements is low (a few micrometer heights), it is hard to create a large temperature difference across the TEG device. Thus, its output voltage, as well as output power, is in small value. The evidence could be easily seen via Eqs. (12) and (13). Although an output power of the TEG-utilized thin films could be enhanced by a novel design for heat transfer in a lateral direction, TEG's output power is still not enough for realistic applications. Therefore, a thick film of thermoelectric material with high Seebeck, large electrical conductivity, and low thermal conductivity are always desired to achieve highperformance micro-thermoelectric generators. Typically, thick thermoelectric material films could be formed by a screen printing method, a powder synthesis and sintering method, and a mechanical alloying and spark plasma sintering method; nevertheless, these methods have at least the following disadvantages, such as poor mechanical strength, a high fabrication cost, and low material performance. Herein, we present the thick and stable thermoelectric films synthesized by electrodeposition.

**Figure 2** shows the sample preparation process for material synthesis and material evaluation. It starts from a silicon substrate with a thickness of 300 μm (**Figure 2(a)**).

#### **Figure 2.**

*Sample preparation process. (a) Silicon. (b) SiO2 deposition. (c) Cr-Au deposition. (d) Thermoelectric material formed by electrodeposition. (d) Epoxy coating. (f) Sample for evaluation.*

On top of this substrate, a SiO2 layer with a thickness of 200 nm is deposited by a plasma-enhanced chemical vapor deposition (PECVD) employing TEOS (TetraEthOxySilan Si (OC2H5)4), as shown in **Figure 2(b)**. Next, Cr-Au layers with a thickness of 20 nm and 150 nm are formed on the SiO2 layer by the sputtering method, respectively (**Figure 2(c)**). The thermoelectric material is subsequently deposited by the electrodeposition method, as discussed in Section 3.1 (**Figure 2(d)**). Because a material property evaluation needs to be conducted on an insulating substrate to avoid short-circuiting, the synthesized films are peeled off from the substrate by epoxy resin, as shown in **Figure 2(e)** and **(f)**. **Figure 2(g)** and **(h)** show the electrodeposited thermoelectric material (Bi2Te3) on the silicon substrate and transferred thermoelectric material on epoxy, respectively.

**Figure 3(a)** and **(b)** show the electrodeposited thermoelectric materials by constant and pulsed conditions, respectively. As can be seen that, the constant electrodeposited film (**Figure 3(a)**) exhibits an initial 4 μm-thick compact layer while the top layer includes pillar structures. Although the thick-film thermoelectric material can be achieved by further deposition, its mechanical strength is very weak due to its porous structure. The thick electrodeposited film by the constant condition is easily peeled off for substrate. To overcome this problem, pulsed electrodeposition has been conducted. Compared with the constant electrochemical deposition, the pulsed electrodeposition with a pulse delay time for the recovery of the ion concentration always leads to a crystalline structure with high orientation and good uniformity [32]. This is proven in **Figure 3(b)**. The deposited surface under pulsed conditions is more uniform and smoother than that under constant conditions. **Figure 3(c)** shows a representative cross-sectional SEM image of the 600 μm-thick Bi2Te3 electrodeposited film, which is comparable to the bulk Bi2Te3 material. Consequently, by using simple and

**Figure 3.** *Thermoelectric material. (a) Constant deposition. (b) Pulsed deposition. (c) A 600 μm-thick Bi2Te3 electrodeposited film.*


#### **Table 1.**

*Electrodeposited thermoelectric material properties.*

low-cost electrochemical deposition technique, thick bulk-like thermoelectric material posing a highly compact and uniform appearance could be achieved.

Thermoelectric material properties, including Seebeck coefficient and electrical resistivity, are evaluated, as shown in **Table 1**. The pulsed deposited film has a higher Seebeck coefficient as well as lower electrical resistivity than those of the constant deposited film. The power factor for pulsed deposited material is 3.2 <sup>10</sup><sup>4</sup> W/mK<sup>2</sup> while it is 0.5 <sup>10</sup><sup>4</sup> W/mK<sup>2</sup> for constant deposited material. Moreover, an annealing process has been performed to enhance the characteristics of the electrodeposited thermoelectric materials. The highest Seebeck coefficient is found at the annealing temperature of 250°C. The details of measurement setup and evaluation results can be found in [33].

In summary, thick bulk-like thermoelectric material based on the electrochemical deposition technique has been demonstrated. The electrodeposited film possesses a highly compact and uniform surface. The electrodeposited material properties by pulsed deposition are much higher than those by constant deposition. Also, thermoelectric performances of the electrodeposited film enhanced by the annealing process have been investigated.

#### **3.3 Platinum nanoparticles embedded in thermoelectric material**

Metal nanoparticle inclusion in the nanocomposite process is one of the promising methods to enhance the figure of merit *ZT*. However, there are a limited number of research studies on metal nanoparticle inclusion to improve thermoelectric material in film condition, especially through the synthesis of the electrochemical deposition. Au nanoparticle-Bi2Te3 nanocomposite has been demonstrated in [34], which is synthesized by a chemical-solution-based bottom-up method at low temperature. The ZT reaches up to 0.95 at 450 K [34]. A similar technique has been applied successfully for the Ag nanoparticle-Bi2Te3 nanocomposite, as shown in [35]. Nevertheless, its performance only improved significantly at a high-temperature region while at room temperature, its performance is just a half that of the pure Bi2Te3 because of the lower value of the Seebeck coefficient resulting in a smaller the ZT value. Herein, we select the Pt nanoparticles for embedding to Bi2Te3 because it has been proven by [36]. In this reference, the Pt nanoparticles have been embedded in Sb2Te3, which can enhance the Seebeck coefficient by filtering the low-energy carriers caused by band-bending potential formation, thus improving the power factor. Moreover, the Pt nanoparticles can help reduce the thermal conductivity due to scattering the mid- to longwavelength phonons. Therefore, the ZT of nanocomposite thermoelectric material is much higher than that of pure thermoelectric material.

#### **Figure 4.**

*(a) Electrodeposited surface of Bi2Te3. (b) Electrodeposited surface of Pt-Bi2Te3. (c) High resolution of TEM image of Pt-Bi2Te3.*

**Figure 4(a)** shows the surface morphology of the electrodeposited pure Bi2Te3 with its crystal as plate-like structure. The surface morphology has been modified by the inclusion of Pt nanoparticles in the Bi2Te3, as shown in **Figure 4(b)**. The crystal grain size of Pt- Bi2Te3 composite is smaller than that of pure Bi2Te3, as can be seen in **Figure 4(a)** and **(b)**. Thus, the electrodeposited film with Pt nanoparticles tends to form lower porosity and denser surface structure in comparison to pure Bi2Te3. A high-resolution transmission electron microscopy image of Pt- Bi2Te3 composite is shown in **Figure 4(c)**, where black areas represent the Pt nanoparticles.

**Table 2** shows the average grain size calculated by identifying FWHM and Integral Breadth *β.* As can be seen, the crystal's grain size becomes smaller at higher Pt nanoparticle content. The smallest grain size of 7.9 nm is found at the 1.9 wt% of Pt nanoparticles in the composite, which is four times smaller compared with that of pure Bi2Te3.

The summary of characteristic of the synthesized films is shown in **Table 3**. Experimental results indicate that once the grain size decreases, the carrier


**Table 2.**

*Average grain size on Bi2Te3 and Pt-Bi2Te3 nanocomposite films at 2*θ *= 27.7°.*

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*


**Table 3.**

*Summary characteristics of the synthesized films.*

concentration becomes lower. The lowest carrier concentration is observed for 1.9 wt% Pt-Bi2Te3 composite in comparison with others, including Bi2Te3, 1.5 wt% Pt-Bi2Te3, and 1.0 wt% Pt-Bi2Te3. As mentioned in Section 2, the Seebeck coefficient and electrical conductivity are trade-off, and they strongly depend on the carrier concentration. Lower carrier concentration results in a higher Seebeck coefficient but causes the smaller electrical conductivity, which agrees with the observation in this work, as given in **Table 3**.

**Figure 5** shows the measurement result of the thermal conductivity of the electrodeposited film. The thermal conductivity decreases as the Pt nanoparticle concentration increases. The main reason is due to a reduction of the phonon mean free path caused by phonon grain boundary scattering [37]. The scattering mechanism of mid- to long-wavelength of phonons in the Pt-Bi2Te3 nanocomposite can be imagined via **Figure 5(b)**. Short-wavelength phonons are scattered by imperfections such as atomic defects and stacking defects while the Pt nanoparticles and grain boundaries are effective at scattering the mid-to long-wavelength phonon. A close adjacent between the Pt nanoparticles also contributed to the phonon scattering effect by reducing the phonon mean free path. Based on measurement results, including Seebeck coefficient, electrical conductivity, and thermal conductivity, the maximum ZT for Pt-Bi2Te3 nanocomposite is found at 0.61, which is 300% higher than that of the electrodeposited pure Bi2Te3. The details of evaluation setup, measurement results, and other discussions can be found in [24].

#### **Figure 5.**

*Thermal conductivity and ZT as a function of Pt nanoparticle concentration. (b) Illumination of phonon scattering mechanisms in the Pt-Bi2Te3 nanocomposite.*

In summary, Pt-Bi2Te3 nanocomposite has been synthesized successfully by the electrochemical deposition technique. It is found that as higher Pt nanoparticles are deposited in the nanocomposite film, the grain size becomes smaller and the nanostructure experienced significant defects. The change of grain size could be a help to adjust the trade-off between Seebeck coefficient and electrical conductivity, which results in the highest power factor. In addition, the defects caused by Pt nanoparticle benefit the phonon scattering enhancement, thus lowering the thermal conductivity. Consequently, the ZT can be improved.
