Realizing high thermoelectric performance in GeTe by defect engineering on cation sites

Abstract

GeTe-based materials have attracted a lot of attention due to their excellent thermoelectric performance. In this paper, the thermoelectric properties are enhanced via simultaneous Ti/Bi co-doping and Ge self-doping on cation sites of GeTe. Firstly, the equivalent replacement of Ge by Ti atoms leads to band convergence and the increase of the density of states. As a result, 1.5 mol% Ti doping in GeTe increases the effective mass from ∼1.13 m₀ to ∼1.71 m₀ and enhances the quality factor B from ∼0.69 to 1.33. Subsequently, Ti/Bi co-doping successfully reduces the carrier concentration (n_H) from 8.21 × 10²⁰ cm⁻³ (pristine GeTe) to 3.25 × 10²⁰ cm⁻³. Based on the Ti/Bi co-doping, the effects of Ge-deficient and Ge-excess strategies are further investigated. The positive modulation of Ge deficiencies on lattice thermal conductivity (κ_L) is not enough to balance its negative influence on the electronic transport properties. Moreover, excessive Ge can further decrease n_H, increase carrier mobility, and reduce κ_L due to the strong phonon scattering. Finally, a high maximum ZT of 1.89 at ∼735 K and an average ZT of 1.07 in the temperature range of 300–785 K are achieved in Ge_0.945Ti_0.015Bi_0.06Te. The results demonstrate that defect engineering on cation sites is an effective way of optimizing the performance of GeTe-based thermoelectrics.