Ultrahigh average zT realized in polycrystalline SnSe_0.95 materials through Sn stabilizing and carrier modulation

The average zT determines the conversion efficiency, and the power factor plays an important role in average zT value. However, the inadequate electrical conductivity of SnSe materials seriously limits its application. Herein, the TaCl₅-doped in polycrystalline SnSe_0.95 materials synthesized using the melting method and combined with spark plasma sintering technology achieves a zT value of 1.64 at 773 K and a record zT_ave of 0.62 from 323 K to 773 K. The electrical conductivity increases due to the released electron carrier induced by effective TaCl₅ doping. According to the DFT calculation, the energy band of TaCl₅-doped samples is narrowed, which can enhance the electron transport. Besides, the Seebeck coefficient is maintained at an elevated level as a result of the incorporation of the heavy element Ta. Due to the significantly enhanced electrical conductivity and maintained high Seebeck coefficient, the power factor reaches to 622 μW·m⁻¹·K⁻² at 773 K for the SnSe_0.95 + 1.75% (in mass) TaCl₅ sample, which is almost 21 times higher than that of the pristine sample. Simultaneously, a high average power factor value of 334 μW·m⁻¹·K⁻² for the SnSe_0.95 + 1.75% (in mass) TaCl₅ sample from 323 to 773 K was obtained. It is surprisingly found that the Ta element plays another important role to improve the stability of SnSe_0.95 by forming Ta₂Sn₃ and removing the low melting point Sn, which usually existed in n-type SnSe samples, resulting in the decreased lattice thermal conductivity. A low lattice thermal conductivity value of 0.24 W·m⁻¹·K⁻¹ was also obtained for the SnSe_0.95 + 2.0% (in mass) TaCl₅ sample at 773 K due to the multiscale defects. Consequently, the SnSe_0.95 + 2.0% (in mass) TaCl₅ sample obtains a peak zT value of 1.64 at 773 K and a record zT_ave of 0.62 from 323 to 773 K, and the theoretically calculated conversion efficiency reaches 11.2%, it can be utilized for power generation and/or cooling at a broad temperature range. This strategy of introducing high-valence halides with heavy element can optimize the thermoelectric performance for other material systems.