metals can have high electrical conductivity, their Seebeck coefficient is usually quite low. Intrinsic semiconductors can have high Seebeck coefficient but low electrical conductivity. Hence, both of them are not good thermoelectric materials. The conventional thermoelectric materials are inorganic semiconductors or semimetals such as BiTe. They can have high Seebeck coefficient and high electrical conductivity. But their high thermal conductivity is a big concern. Thus, effort in improving the thermoelectric performance of inorganic materials has been focused on lowering the thermal conductivity. Because of low intrinsic thermal conductivity, high mechanical flexibility, and low fabrication cost, organic thermoelectric materials have been gaining increased attention [1–4]. The organic thermoelectric materials are usually conducting polymers or doped conjugated organic molecules. Figure 1.2 presents the chemical structure of some representative conducting polymers. The Seebeck coefficient of organic materials is usually lower than their inorganic counterparts by about one to two orders of magnitude. To achieve high thermoelectric performance, it is important to improve the Seebeck coefficient and/or the electrical conductivity of organic thermoelectric materials.
To develop materials with high thermoelectric properties, good understanding in the Seebeck coefficient, electrical conductivity, and thermal conductivity is needed. Although the knowledge acquired from the study of inorganic thermoelectric materials is often used, it is not always true for organic thermoelectric materials.
1.1.1 Thermoelectric Effect
There are three thermoelectric effects, that is, Seebeck effect, Peltier effect, and Thomson effect. Correspondingly, there are Seebeck coefficient, Peltier coefficient, and Thomson coefficient. The Seebeck and Peltier coefficients are usually used for the thermoelectric generation and cooling, respectively. The Thomson coefficient is used because of the temperature dependence of the Seebeck coefficient. The three coefficients are related, and the Seebeck coefficient is the most popular term used in literature.
1.1.2 Seebeck Effect
Thermoelectric materials are usually referred to electronic materials with high Seebeck coefficient. When there is a temperature gradient between the two ends of a thermoelectric material, a voltage will be generated between the two ends. This effect was first discovered by Alessandro Volta in 1794 and independently rediscovered by Thomas J. Seebeck in 1821. The Seebeck coefficient (S) is determined by the voltage (ΔV) and the temperature gradient (ΔT),
Figure 1.2 Chemical structures of some representative conducting polymers: (a) poly(3,4‐ethylenedioxythiophene):tosylate (PEDOT:Tos), (b) poly(3,4‐ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS), (c) poly(3,4‐ethylenedioxythiophene): trifluoromethanesulfonate (PEDOT:OTf), (d) polyaniline (PANI), (e) polythiophene (PTh), (f) poly(3‐hexylthipohene) (P3HT), (g) polypyrrole (PPy), and (h) poly(nickel‐ethylenetetrathiolate) [poly(Ni‐ett)].
(1.1)
The Seebeck effect can be understood in terms of the temperature effect on the charge carrier distribution. For example, the electrons of a metal follow the Fermi–Dirac distribution,
(1.2)
where E is the electron energy and k B the Boltzmann constant. The electron distribution near the Fermi level is affected by temperature. The f(E) value at E F is 1 at 0 K and less than one at a temperature above 0 K. Consider two different temperatures, T H and T C, at the two ends of a metal, the Fermi–Dirac distributions at the two temperatures are shown in Figure 1.3. When the temperature (T C) at the cold side is 0 K, the states above the Fermi level is completely empty, while all the states below the Fermi level is fully occupied. At the hot side, the temperature (T H) is greater than 0 K. Thus, in terms of the Fermi–Dirac distribution, some states at the energy above E F is occupied, while some states below the Fermi level are vacant. As a result, electrons with the energy higher than E F will diffuse from the hot side to the cold side, while the electrons with the energy lower than E F will migrate in the opposite direction. But the electron diffusions along the two directions are not equivalent. At equilibrium, some electrons accumulate at the hot or cold side, this leads to an internal electric field and a voltage between the two sides. If the electrons accumulate at the cold side, the potential of the cold side will be lower than the hot side. This will lead to a negative Seebeck coefficient, and the material is an n‐type thermoelectric material. Instead, if the electrons accumulate at the hot side, the cold side has a potential higher than the hot side. The Seebeck coefficient is positive, and it is a p‐type material.
Figure 1.3 (a) A metal with different temperatures at the two sides. (b) Voltage between the two sides induced by temperature gradient. (c) Fermi–Dirac distributions at two different temperatures of T H and T C and their difference.
By using the Fermi gas model, the Seebeck coefficient of metals is
(1.3)
where T F is the Fermi temperature. This is the Mott formula in metals. In terms of this formula, metals usually have small Seebeck coefficient because of their large T F value of 104−105 K. The absolute Seebeck coefficient values of metals are usually less than 10 μV K−1 (Table 1.1).
The Seebeck coefficient of semiconductors depends on the doping level. For nondegenerated semiconductors, Eqs. (1.4) and (1.5) are the Mott formulae for the Seebeck coefficients in the conduction band and valence band, respectively,