Promoted thermoelectric performance of (Ag, Na) co-doped polycrystalline BiSe by optimizing the thermal and electrical transports simultaneously

BiSe has recently emerged as a new promising Te-free thermoelectric material due to its superlattice structure and intrinsic low thermal conductivity. However, the inherent high carrier concentration of BiSe limits its thermoelectric performance. In this paper, Ag and Na were doped in polycrystalline BiSe to tune its thermoelectric transport properties. A series of Ag-doped and (Ag, Na) co-doped samples were synthesized via solid-phase sintering method and spark plasma sintering (SPS). It is found that Ag doping can effectively reduce the lattice thermal conductivity of BiSe, thereby reducing the total thermal conductivity, which increases the ZT value of Bi0.97Ag0.03Se from 0.24 to 0.28. Along with the co-doping of 1% Na, the carrier concentration of the (Ag, Na) co-doped sample effectively reduces from 32 × 1019 cm−3 to 6 × 1019 cm−3, while the mobility greatly increases, so that the electrical conductivities are maintained in a high range, and the Seebeck coefficients are slightly increased. A maximum ZT value of 0.31 at 523 K is achieved in Bi0.96Ag0.03Na0.01Se along the SPS pressing direction as a result of the higher power factor and lower total thermal conductivity, which is about 30% higher than that of undoped BiSe.