N-type Bi2Se2S materials with high ZT > 1 engineered by multi-scale second phases designing

Bulk materials composed of n-type Bi₂Se₂S were synthesized through a solid-state reaction using spark plasma sintering technology. To improve the electrical conductivity of Bi₂Se₂S, SbCl₃ was employed as a dopant to increase the electron concentration. The power factor of the Bi₂Se₂S + 0.75 wt% SbCl₃ sample reached 659 μWm⁻¹K⁻², three times higher than that of the pristine sample. Surprisingly, their thermoelectric properties were considerably enhanced by designing multi-scale multiphase engineering, the addition of SbCl₃ resulted in the in situ precipitation of mesoscopic and nanoscale second phases, intermingling with the matrix. These multiscale second phases effectively scatter phonons across the full wavelength range, thereby reducing the lattice thermal conductivity. The lowest lattice thermal conductivity, 0.27 Wm⁻¹K⁻¹ at 773 K, was achieved for the Bi₂Se₂S + 0.75 wt% SbCl₃ sample. Due to the synergistic optimization of electrical and thermal properties, the figure of merit (ZT) for the SbCl₃-doped Bi₂Se₂S sample reached 1.13 at 773 K. This value is a record high in the Bi–Se–S system and is the first time a ZT value beyond 1 has been achieved for the Bi₂Se₂S system, indicating that Bi₂Se₂S is a highly promising thermoelectric material for medium-temperature applications.