4. Materials for Thermoelectric Devices

MATERIAL RESEARCH AND ENGINEERING is one of the cornerstones of efficient thermoelectric devices. This chapter first gives an overview on characteristic material properties as well as their dependence on temperature and carrier concentration. A good thermoelectric material is identified by high Seebeck coefficients and good electric transport properties, while thermal transport is held on the short side [107].

Usually, these material properties are subject to a pronounced temperature dependence, leading to the definition of ideal thermal operation conditions for a certain material. Furthermore, the influence of the doping on several relevant parameters is investigated and an approximation for the ideal material doping is given.

In the following, technologically important thermoelectric materials are introduced and their operational range is outlined. On the example of silicon-germanium, the important feature of lowered thermal conductivity within semiconductor alloys compared to their pure constituents is illustrated. Furthermore, SiGe plays an important role in modeling and simulation due to its elaborate available physical description driven by mainstream microelectronics. On the temperature scale, SiGe is located at relatively high operational values. In contrast to some other materials, it is available for both p- and n-doped samples.

Lead telluride (PbTe) is an interesting candidate for the intermediate temperature range with a maximum operating temperature of about $900\,\ensuremath{\mathrm{K}}$ . Beside its application in thermoelectrics, it is also used for optical devices in the infrared wavelength regime. Its material description is carried out for device modeling in detail in Chapter 5. In addition to doping by foreign atoms, the material type can be adjusted by deviation of its stoichiometric composition. Beside pure lead telluride, several ternary alloys exist, which are subject to ongoing research. Generally, lead telluride is applicable for both n-type and p-type samples. However, in contrast to n-type samples, p-type samples suffer under low stability under high temperatures, difficult bonding, as well as poorer mechanical properties depending on their dopants [108]. Thus, the p-doped leg is often replaced by alloys consisting of silver antimony telluride and germanium telluride, often referred to as TAGS.

The lower end of the temperature scale is covered by bismuth telluride. Due to its good thermoelectric properties at room temperature, devices made of bismuth telluride are often used for cooling applications. As in lead telluride, the material type and number of access carriers can be adjusted by a deviation of stoichiometry.

Besides these "classical" thermoelectric materials, which are well established in several generation and cooling applications, ongoing research focuses on novel materials [109,110,111] as well as nanostructures [112,113,114,115,116,117,118,119,120] especially designed for thermoelectric needs.

Subsections

• 4.1 Characterization of Materials
• 4.2 Optimization of Device Performance
• 4.3 Silicon-Germanium
• 4.4 Lead Telluride and its Alloys
• 4.5 Bismuth Telluride and its Alloys

M. Wagner: Simulation of Thermoelectric Devices