Ultralow Thermal Conductivity and Enhanced Thermoelectric Properties Realized in Polycrystalline Bi₂S₂Se via Carrier and Microstructure Modulation

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Bi2S2Se compounds have been investigated as the most promising ternary component in the Bi2S3–Bi2Se3 system. However, there is still some space to increase the thermoelectric (TE) performance. In this work, the TE properties of the Bi2S2Se system have been enhanced through the utilization of BiI3 as a highly effective n-type dopant, leveraging the advantageous high solubility of the iodine(I) within the material. The results showed that BiI3 could effectively improve the electrical conductivity of the Bi2S2Se matrix by shifting the Fermi level toward the conduction band. As the BiI3 content increases, the conductivity shows an increasing trend. When the BiI3 content is 1 wt %, it reaches its maximum value of 32.4 S cm–1, representing a 400% improvement compared to the undoped sample. Additionally, the point defects due to the I solid solution served as effective phonon scattering centers combined with pores left by sublimation of I. Both factors worked together to suppress lattice thermal conductivity effectively. Ultimately, benefiting from the simultaneous optimization of electrical and thermal properties, the best ZT value of 0.65 is achieved for the sample with 1 wt % BiI3 addition. The temperature range of 473–673 K exhibits an average ZT that approximates 0.55. This study confirmed that BiI3 could effectively enhance the TE performance of Bi2S2Se, and held promising prospects for further applications.