**1. Introduction**

Energy has a fundamental importance in the human civilization. Conventional methods are used for the production of energy use oil, gas, and coal. The reservoirs of oil and gas in the world are decreasing, and the burning of oil and gas causes a threat to the environment; therefore, people are searching for cheap methods for the production of clean energy. These renewable energy production methods include photovoltaic, nuclear energy, biogas, wind energy, and thermoelectricity. All these methods have their advantages and disadvantages. For example, solar cells can produce energy during the daylight and also need high technology for the fabrication of solar cells. Nuclear energy production needs nuclear power plants which

not only require high cost but also a high risk for the community. Similarly, wind energy can only be produced in the strong windy areas. On the other hand, thermoelectricity is very cheap and an easy method for the clean energy production. Thermoelectricity is based on the very famous Seebeck effect which was invented by Seebeck in 1821 and is stated as.

An emf is induced when a temperature difference is created between two metal junctions. Thermoelectricity needs only temperature difference between two metals; therefore, it is supposed to be the cheapest form of clean energy. It is reported that that 60% of heat produced during cooking process, in industry and in running vehicles, is wasted, but we are able to convert this wasted heat into electricity using thermoelectric power generators; we can save huge amount of money. Furthermore, thermoelectricity has other advantages over other sources of energy such as it has no moving parts, it is environment friendly, no specialized technology is required, and it is less maintenance.

#### **1.1 Physical interpretation**

The thermoelectric devices convert thermal energy into electrical energy, and the principle is based on the Seebeck effect invented by Seebeck in 1821. It states that a voltage is induced between two points of metal/semiconductor having a difference of temperature as shown in **Figure 1**. The charge carriers on the hot side can have more energy than the cold side; therefore, they form a potential difference. Suppose dT is the temperature between hot and cold side of sample, therefore according to Seebeck effect.

$$\mathbf{d}\mathbf{T} = \mathbf{S}\mathbf{V} \tag{1}$$

where S is the Seebeck coefficient.

Another term frequently used in thermoelectric is the power factor which is defined as.

$$\text{Power factor} = \mathbb{S}^2 \mathfrak{a} \tag{2}$$

**51**

*Thermoelectric Properties of Oxide Semiconductors DOI: http://dx.doi.org/10.5772/intechopen.88709*

of merit and can be written as [1].

*A brief history of materials used for thermoelectric applications.*

efforts to increase the figure of merit.

properties. The general techniques are as follows [3]:

1.Optimization using doping techniques

thermal conductivity.

**Figure 2.**

2.Substructuring

3.Nanostructuring

4.Compositing

**1.2 Band gap**

This equation shows that for a good thermoelectric material, high Seebeck coefficient, high electrical conductivity, and low thermal conductivity are essential. The thermoelectric conversion efficiency depends upon a quantity called figure

where S is the Seebeck coefficient, α is the electrical conductivity, and σ is the

The figure of merit for a material to be used for practical power generation system should be in the range of 2–3 [2]. But the best reported value of figure of merit for oxide semiconductor is not more than 0.1. **Figure 2** indicates the history of

Different strategies have been employed to tune and alter the thermoelectric

But for the semiconductors the governing parameters includes the following:

Band structure is a very important parameter to tune the thermoelectric properties of oxide semiconductors. One of the most important methods of band gap control is varying the carrier concentration by doping [4]. But the doping process itself required a high technology that increased the cost of thermoelectric devices very high. So if we tune the carrier concentration by controlling the density of intrinsic defects, this will cut short the cost of the final device. Interestingly, oxide semiconductors have rich the chemistry of intrinsic defects. Oxygen vacancy and zinc interstitials act as intrinsic

shallow donors and form electronic states near the conduction band [5].

ZT = S<sup>2</sup> α/σ (4)

where α is electrical conductivity.

The performance of thermoelectric material strongly depends upon a unit less quantity called figure of merit:

**Figure 1.** *Schematic diagram of the Seebeck effect.*

*Thermoelectric Properties of Oxide Semiconductors DOI: http://dx.doi.org/10.5772/intechopen.88709*

*Solid State Physics - Metastable, Spintronics Materials and Mechanics of Deformable...*

by Seebeck in 1821 and is stated as.

**1.1 Physical interpretation**

according to Seebeck effect.

defined as.

where S is the Seebeck coefficient.

where α is electrical conductivity.

quantity called figure of merit:

*Schematic diagram of the Seebeck effect.*

not only require high cost but also a high risk for the community. Similarly, wind energy can only be produced in the strong windy areas. On the other hand, thermoelectricity is very cheap and an easy method for the clean energy production. Thermoelectricity is based on the very famous Seebeck effect which was invented

An emf is induced when a temperature difference is created between two metal junctions. Thermoelectricity needs only temperature difference between two metals; therefore, it is supposed to be the cheapest form of clean energy. It is reported that that 60% of heat produced during cooking process, in industry and in running vehicles, is wasted, but we are able to convert this wasted heat into electricity using thermoelectric power generators; we can save huge amount of money. Furthermore, thermoelectricity has other advantages over other sources of energy such as it has no moving parts, it is environment friendly, no specialized technology is required, and it is less maintenance.

The thermoelectric devices convert thermal energy into electrical energy, and the principle is based on the Seebeck effect invented by Seebeck in 1821. It states that a voltage is induced between two points of metal/semiconductor having a difference of temperature as shown in **Figure 1**. The charge carriers on the hot side can have more energy than the cold side; therefore, they form a potential difference. Suppose dT is the temperature between hot and cold side of sample, therefore

Another term frequently used in thermoelectric is the power factor which is

The performance of thermoelectric material strongly depends upon a unit less

dT = SV (1)

Power factor = S<sup>2</sup>α (2)

(3)

**50**

**Figure 1.**

**Figure 2.** *A brief history of materials used for thermoelectric applications.*

This equation shows that for a good thermoelectric material, high Seebeck coefficient, high electrical conductivity, and low thermal conductivity are essential.

The thermoelectric conversion efficiency depends upon a quantity called figure of merit and can be written as [1].

$$\mathbf{Z}\mathbf{T} = \mathbf{S}^2 \,\mathrm{\alpha}/\sigma \tag{4}$$

where S is the Seebeck coefficient, α is the electrical conductivity, and σ is the thermal conductivity.

The figure of merit for a material to be used for practical power generation system should be in the range of 2–3 [2]. But the best reported value of figure of merit for oxide semiconductor is not more than 0.1. **Figure 2** indicates the history of efforts to increase the figure of merit.

Different strategies have been employed to tune and alter the thermoelectric properties. The general techniques are as follows [3]:


But for the semiconductors the governing parameters includes the following:

#### **1.2 Band gap**

Band structure is a very important parameter to tune the thermoelectric properties of oxide semiconductors. One of the most important methods of band gap control is varying the carrier concentration by doping [4]. But the doping process itself required a high technology that increased the cost of thermoelectric devices very high. So if we tune the carrier concentration by controlling the density of intrinsic defects, this will cut short the cost of the final device. Interestingly, oxide semiconductors have rich the chemistry of intrinsic defects. Oxygen vacancy and zinc interstitials act as intrinsic shallow donors and form electronic states near the conduction band [5].
