Fundamental Knowledge on Thermoelectric Materials

Thermoelectric materials are electronic materials that can exhibit noticeable voltage under temperature gradient and high electrical conductivity. The conventional thermoelectric materials are inorganic semiconductors or semimetals. Recently, flexible thermoelectric materials including conducting polymers and polymer composites gained great attention. The thermoelectric performance is usually characterized by the dimensionless figure of merit ( ZT ), ZT = S 2 σT / κ , where S being the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. S 2 σ is called the power factor. To achieve high thermoelectric performance, it is important to possess the fundamental knowledge of thermoelectric materials, particularly the physics of the Seebeck coefficient, Peltier coefficient, electrical conductivity, and thermal conductivity. The Seebeck coefficient is related to the dependence of charge carrier density on temperature. The Seebeck coefficient of conducting polymers is usually lower than that of their inorganic counterparts by about one to two orders of magnitude. The electrical conductivity depends on the charge carrier density and charge carrier mobility. But the Seebeck coefficient and electrical conductivity are interdependent. Decreasing the charge carrier density can increase the Seebeck coefficient while decreasing the electrical conductivity. Thus, there is an optimal power factor in terms of the charge carrier density. Low thermal conductivity is required for high thermoelectric performance. The thermal conductivity includes the contributions by phonons and charge carriers. The thermal conductivity of conducting polymers is usually significantly lower than the inorganic thermoelectric materials. The thermoelectric materials have important application in harvesting heat, cooling, or sensing. The efficiency of thermoelectric generators depends not only on the ZT values of the thermoelectric materials but also their electrical and thermal contact resistances. The operation mechanism of thermoelectric coolers is basically the opposite process of thermoelectric generators. The temperature-sensitive voltage due to the Seebeck effect of thermoelectric materials is the basic principle of their application in sensors.