Abstract: Bi₂S₂Se has emerged as a promising tellurium-free thermoelectric material for medium-temperature applications due to its low cost, environmental friendliness, and intrinsically low lattice thermal conductivity. However, its relatively poor electrical conductivity has restricted the thermoelectric capabilities of this material. In this study, a novel strategy to enhance the electronic transport properties of Bi₂S₂Se through ZrCl₄ doping is presented. Experimental results reveal that Zr ions predominantly occupy interstitial sites, introducing defect states near the Fermi level that effectively reduce the band gap. These high-charge gap zirconium ions compensate for the negative charge associated with the intrinsic selenium vacancy and maintain high mobility (∼200 cm² V^-1 s^-1) while increasing the carrier concentration (from 0.21 × 10¹⁹ cm^-3 to 2.46 × 10¹⁹ cm^-3) at room temperature. Moreover, the lattice distortion and point defects induced by ZrCl₄ doping support a low lattice thermal conductivity of 0.35 W/(m K) at 773 K. Consequently, an optimized ZT value of approximately 0.8 is achieved at 773 K in polycrystalline Bi_{2-x %}S₂Se_{1-4x %}+x mol.% ZrCl₄ (x = 0.3), which is 2.5 times higher than that of the pristine sample. This work not only demonstrates a viable approach for enhancing the thermoelectric performance of Bi₂S₂Se but also provides new insights into defects in thermoelectric materials.

Key words: Thermoelectric material, Bi₂S₂Se, Sulfide compound, ZrCl₄ doping