Thermoelectric efficiency in graded indium-doped PbTe crystals

High efficiency thermoelectric conversion is achieved by using materials with a maximum figure of merit Z=S ²σ/k, where S is the Seebeck coefficient, σ and k, the electrical and thermal conductivities, respectively. High quality homogeneous thermoelectric materials, based on PbTe crystals, usually display an elevated value of Z over a narrow temperature range. A maximal value of figure of merit Z, as a function of electron density, is attained only for one specific location of the Fermi level, E _F, with respect to the conduction band edge, E _C. In order to maintain this optimal Z value, namely, maintain a constant location of the Fermi level, the electron density, which is determined by the dopant concentration, must increase with increasing temperature. We present a method for the generation of a dopant (indium) concentration profile in n-type PbTe crystals that gives rise to a constant location of the Fermi level, and hence, to an optimal value of Z over a wide temperature range. The resulting functionally graded material, based on PbTe〈In〉, displays a practically constant value of the Seebeck coefficient, over the 50-600°C temperature range.