Band Flattening and Strain Field Assists an Excellent Thermoelectric Performance of <i>n</i>‐type Bi₂Se₃ for Room to Mid‐Temperature Application

Bismuth selenide, (Bi₂Se₃) is a typical n-type V-VI chalcogenide-based thermoelectric (TE) material with high lattice thermal conductivity, which limits its TE performance. Single element doping is generally adopted to enhance the TE performance, since it is not affect significantly. Herein, it is revealed that the impact of the co-doping strategy boost the TE performance of Bi₂Se₃ through tuning the band structure and phonon transport properties. The synergy of isovalent Ga (gallium) and aliovalent Ge (germanium) co-doping produces a noteworthy improvement in carrier tuning from -1.1 × 10¹⁸ to -3.56 × 10¹⁸ cm^-3. Bi_1.95Ga_0.05Ge_0.033Se₃ sample reached the enhanced power factor of 1403 µWm^-1K^-2 obtained at 303 K. The reduced low lattice thermal conductivity of 0.24 Wm^-1K^-1 is occurred by the multi-scale phonon scattering mechanisms through the dissemination of strain field raised from various defects. The combined strategy of flat band structure and low lattice thermal conductivity significantly optimized the thermoelectric zT of 0.7 is achieved at 513 K and the highest zT_average of 0.68 is obtained. The maximum zT values leads to attain a theoretical efficiency of 5.9% in the temperature range between 333-573 K. This work shows a practical approach to improve the zT of n-type Bi₂Se₃ via co-doping.