Origins of the enhanced thermoelectric performance for p-type Ge_1-xPb_xTe alloys

GeTe crystallizes with a strong deviation from the stoichiometric composition (in the direction of Te excess), which results in a growth of free carrier concentration (holes) up to p ~ 10²¹ cm⁻³. Such concentration is too high from the viewpoint of thermoelectric applications (maximum value of the figure of merit Z). The work demonstrates the possibility of increasing the thermoelectric efficiency of the GeTe-based material, used for the p-type leg of the thermoelectric couple over the temperature range of 600–825 K. The samples were prepared using Spark Plasma Sintering (SPS) technique. A systematic analysis of thermoelectric properties for Ge_1-xPb_xTe alloys (x up to 5 at.%) has been performed. An increase in the lead content in Ge_1-xPb_xTe alloys creates two parallel processes. The first process is a decrease of the lattice thermal conductivity κ_lat, which acts up to T ≈ 550 K, when the lead content in Ge_1-xPb_xTe alloys increases up to 5 at. %. However, the main process is to reduce deviation from the stoichiometric composition, which consequently leads to lower hole concentration p. Therefore, at a high temperature of T ≥ 550 K Fermi level E_F is approaching the edge of the valence band E_v. As a result, the thermoelectric dimensionless figure of merit ZT increases up to 1.1 at the temperature of 825 K.