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Flexible Thermoelectric Polymers and Systems. Группа авторовЧитать онлайн книгу.

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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

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)upper S equals StartFraction upper Delta upper V Over upper Delta upper T EndFraction period

      (1.2)f left-parenthesis upper E right-parenthesis equals StartStartFraction 1 OverOver 1 plus exp left-parenthesis StartFraction upper E minus upper E Subscript normal upper F Baseline Over k Subscript upper B Baseline upper T EndFraction right-parenthesis EndEndFraction comma

Schematic illustration of (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 TH and TC and their difference.

      By using the Fermi gas model, the Seebeck coefficient of metals is

      (1.3)upper S equivalent-to minus StartFraction pi squared k Subscript upper B Baseline Over 3 e EndFraction StartFraction upper T Over upper T Subscript normal upper F Baseline EndFraction comma

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