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Flexible Thermoelectric Polymers and Systems. Группа авторовЧитать онлайн книгу.

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of the power factor on the charge carrier density is valid for conducting polymers as well. The Seebeck coefficient and electrical conductivity of conducting polymers strongly depend on the doping level. An undoped conducting polymer has a conductivity in the insulator range. The increase in the doping level can decrease the Seebeck coefficient while increase the electrical conductivity. Thus, doping engineering is an effective way to find the optimal power factor of conducting polymers as well. Chemical and electrochemical de‐dopings are popular methods to find the optimal doping level of conducting polymers in terms of the power factor. For example, Crispin et al. chemically de‐doped PEDOT:TsO with tetrakis(dimethylamino)ethylene (TDAE) and found the optimal doping level of ~0.22 (Figure 1.6) [5].

Schematic illustration of dependences of the electrical conductivity, Seebeck coefficient, and power factor of PEDOT:Tso on the doping level. Oxidation level is used for the doping level, and α is used for the Seebeck coefficient.

      Source: Bubnova et al. [5]. © Springer Nature.

Schematic illustration of (a) Conductivity, (b) Seebeck coefficient, and (c) power factor of PEDOT:OTf films as a function of doping levels. NaOH, glucose, and ascorbic acid were used to de-dope PEDOT:OTf.

      Source: Yao et al. [7]. © Royal Society of Chemistry.

      1.1.3 Peltier Effect

      Electrical current is generated at the presence of temperature gradient in terms of the Seebeck effect. There is a reverse process to take away heat by applying an electrical current to a thermoelectric material. This is called the Peltier effect. When a charge carrier transports from the cold side to the hot side, it will bring heat from the cold side to the hot side. Thus, this can lower the temperature of the cold side.

      The Peltier coefficient (Π) is related to the Seebeck coefficient by

      (1.10)upper Pi equals upper S upper T period

      In terms of this relationship, a material with a high Seebeck coefficient also has a high Peltier coefficient. Apart from the heat transfer by the Peltier effect, electrical current will generate Joule heat as well. Thus, the conductivity of the thermoelectric materials is also an important parameter for the Peltier cooling.

      1.1.4 Thomson Effect

      The Seebeck coefficient of materials is dependent on the temperature. A spatial gradient in temperature can generate a gradient in the Seebeck coefficient. When an electrical current flows through a material with temperature gradient, continuous Peltier effect can take place. This is called Thomson effect.

      When a current density (J) transports through a homogeneous conductor, the heat generation rate per unit volume will be generated in terms of the Thomson effect,

      (1.11)ModifyingAbove q With dot equals minus upper K Subscript upper T h Baseline upper J StartFraction d upper T Over d x EndFraction comma

      (1.12)upper K Subscript upper T h Baseline equals upper T StartFraction d upper S Over d upper T EndFraction period

      1.1.5 Electrical Conductivity

      In terms of the classic theory, the electrical conductivity of electronic materials depends on the charge carrier density (n) and charge carrier mobility (μ),

      (1.13)sigma equals n e mu comma

      where e is the elementary charge of an electron.

      1.1.5.1 Charge Carrier Density

      Metals have high charge carrier density and thus high electrical conductivity. Their charge carrier density depends on the density and valence electrons of the metal atoms. Intrinsic semiconductors have very low charge carrier density, and charge carriers can be generated by temperature excitation or light‐induced excitation. The conductivity of a semiconductors can be increased by several orders of magnitude by doping. The electrical conductivity of a semiconductor depends on the electrons in the conduction band and holes in the valence band,

      (1.14)sigma equals n e mu Subscript normal e Baseline plus p e mu Subscript normal p comma

      where


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