Stepwise Ge vacancy manipulation enhances the thermoelectric performance of cubic GeSe

In thermoelectrics, the native point defect enables the delicate balance between carrier concentration and carrier mobility. The proper native point defect is prerequisite for the carrier concentration optimization. However, the carrier mobility is inevitable to be affected in case of the carrier mean free path approaches to the size of point defects. Herein, we reported the stepwise Ge vacancy manipulation as a point defect engineering to realize the zT enhancement in cubic GeSe. The pristine orthorhombic GeSe has the ultralow zT of 0.05 due to the inferior carrier concentration arising from the low Ge vacancy concentration, as well as the low carrier mean free path. Hence, we adopted InSnTe2 alloying to stabilize the cubic phase at ambient conditions and significantly increase the Ge vacancy concentration due to the generation of Ge precipitates, thereby increasing the carrier concentration by several orders of magnitude but damaging the carrier mobility. Subsequently, the moderate Te doping was adopted to make the Ge-precipitations to be dissolved into the matrix, thus mitigating the strong carrier scattering deriving from the Ge vacancy and enhancing the carrier mobility to some extent. Combined with the restrained thermal conductivity originating from the multiscale defects and the reduced sound velocity, the maximum zT of 0.62 at 573 K was achieved in cubic GeSeTe0.15(InSnTe2)0.2. These results demonstrated that the stepwise point defect manipulation is an effective strategy for enhancing the figure of merit in thermoelectric materials with intrinsic low carrier mobility.