Realizing High Thermoelectric Performance in GeTe through Optimizing Ge Vacancies and Manipulating Ge Precipitates

Recent vast reinvestigations on GeTe, a promising thermoelectric material at medium temperature, have triggered enormous enthusiasm in the thermoelectric community again. Intrinsically, GeTe, as a p-type semiconductor, possesses plenty of Ge vacancies, resulting in extremely high carrier concentration. Meanwhile, because of the vast off-stoichiometric Ge, both concomitant Ge precipitates and Ge vacancies will influence the electrical and thermal transport properties caused by phonons and carriers scattering. Herein, we report one effective way to optimize the thermoelectric performance of GeTe through manipulating Ge vacancies and Ge precipitates via successive continuous steps. First, the optimization of carrier concentration through Bi counter-doping led to an optimum carrier concentration of ∼2.79 × 10²⁰ cm^-3. Second, the Ge phase aggregation was successfully suppressed by heat treatment, which enhanced the electrical properties due to improved carrier mobility. Additionally, the Ge precipitates were uniformly distributed by heat treatment, which provided all-scale phonons scattering and effectively reduced the lattice thermal conductivity. Finally, combined with the significantly increased power factor (PF) and the decreased thermal conductivity, the maximum ZT of ∼1.9 at 723 K was eventually obtained in Ge_0.94Bi_0.06Te just through optimizing Ge vacancies and manipulating Ge precipitates by means of heat treatment.