Effect of Silver and Iodine Co-doping on the Thermoelectric Properties of n-Type Bi₂S₃

Bi₂S₃ is a promising thermoelectric material due to its low thermal conductivity. However, the low electrical conductivity has become an obstacle for its higher thermoelectric properties. To overcome this, on the basis of 1 mol.% BiI₃ doping, Bi₂S₃ polycrystalline samples doped with various silver (Ag) concentrations were successfully prepared by melting combined with spark plasma sintering (SPS) to improve thermoelectric properties. The variation of lattice parameters were analyzed using x-ray diffraction, and electrical and thermal properties were investigated in the temperature range from 300 K to 723 K. The Ag and iodine (I) co-doping plays an important role in enhancing the electrical conductivity and maintaining a high Seebeck coefficient simultaneously. An ideal power factor of 3.95 μW cm⁻¹ K⁻² is obtained for the Ag_0.0075Bi₂S₃ sample at 723 K. Meanwhile, the thermal conductivity is also reduced. As a result, a high thermoelectric figure-of-merit (ZT) of 0.62 is achieved, which is four times higher than that of pristine Bi₂S₃ at the same temperature.