High thermoelectric performance in entropy-driven Ge1−2x−yPbxSnxSbyTe

Abstract

Germanium telluride (GeTe) is one of the most fascinating inorganic compounds in thermoelectrics due to its intriguing chemical bonding, crystal and electronic structure. However, thermoelectric performance of pristine GeTe is greatly affected by its high lattice thermal conductivity and intrinsically high p-type carrier concentration. Recently, innovative strategies have been applied to improve thermoelectric performance either by enhancing electronic transport or decreasing the lattice thermal conductivity by phonon scattering. Herein, in order to increase the configurational entropy of the system, we have doped Sn and Pb (2.5 mol% each) in GeTe, which reduces the lattice thermal conductivity (κ_L) from 2.8 W m⁻¹ K⁻¹ of pristine GeTe to ∼1.9 W m⁻¹ K⁻¹ of Ge_0.95Pb_0.025Sn_0.025Te sample at room temperature. Furthermore, we have doped donor dopant Sb on the Ge site in Ge_0.95Pb_0.025Sn_0.025Te which optimizes the p-type carrier concentration of the system and enhances the Seebeck coefficient due to valence band convergence. Finally, high energy ball-milling along with spark plasma sintering (BM + SPS) has been performed, which helps in further enhancement of the Seebeck coefficient due to extreme valence band convergence and reduction in lattice thermal conductivity to ∼0.63 W m⁻¹ K⁻¹. As a result, a high thermoelectric figure of merit, zT of ∼2.3 (with Dulong–Petit C_p, zT is 2.5) at 723 K and a high average zT_avg of 1.3 in the 300–723 K temperature range have been achieved in BM + SPS processed Ge_0.84Pb_0.025Sn_0.025Sb_0.11Te sample. Motivated by the ultra-high zT, we have constructed a double-leg TE device of Ge_0.84Pb_0.025Sn_0.025Sb_0.11Te (BM + SPS) sample as a p-type leg, where In and I doped PbTe is used as the n-type leg, which shows a high output power density (PD_max) of ∼590 mW cm⁻² at ΔT = 448 K.