A high-efficiency GeTe-based thermoelectric module for low-grade heat recovery

Abstract

GeTe-based compounds are well-known thermoelectric materials at medium temperatures. However, their poor performance near the room-temperature region limits their practical application of heat recovery. In this work, we demonstrate that CuI–Bi codoping can effectively improve the ZT values of GeTe at mid-low temperatures. Comprised of our (Ge_0.96Bi_0.04Te)_0.96(CuI)_0.04 and n-type Bi₂Te_2.7Se_0.3, the constructed thermoelectric module obtains a conversion efficiency of 8.6% when the cold- and hot-side temperatures are 300 and 700 K, respectively. It is revealed that CuI–Bi codoping can significantly decrease the redundant hole carriers while retaining high carrier mobility. Density functional calculations indicate that both CuI and Bi doping can promote a symmetry change from rhombohedral to cubic, leading to a slight valence band convergence and Seebeck coefficient improvement. The lattice thermal conductivity is obviously suppressed due to the strengthened phonon scattering by emerged defects. Consequently, a peak ZT of 2.0 at 700 K and an average ZT (300–700 K) of 1.3 are realized. This study suggests that CuI–Bi codoped GeTe is very promising for low-grade heat recovery in the mid-low temperature range.