Some science behind the scenes
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference.
In a typical demonstration of thermoelectricity, one side of the material is kept at one temperature and the other side at a different temperature, and the result is a permanent voltage across the crystal.
The Seebeck effect is the conversion of temperature differences directly into electricity and is named for German physicist Thomas Johann Seebeck, who, in 1821 discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This is how thermocouples work. Although the effect was first noticed in metals, any crystalline structure will demonstrate the effect.
The ‘ Seebeck coefficient’, = S, of a material measures the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. The material's temperature and crystal structure influence S; typically metals have small thermopower. In contrast, semiconductors can be ‘doped’ with excess electrons or holes, causing the magnitude of S to be large.
Thermoelectric materials show the thermoelectric effect in a strong and/or convenient form. While all materials have a nonzero thermoelectric effect, in most materials it is too small to be useful. The types of materials, however, which are useful are complex and include those with very complex crystalline structures. Bismuth chalcogenides form one important group as do skutterudites and certain oxides.