Optimizing thermoelectric properties of BiSe through Cu additive enhanced effective mass and phonon scattering

Known as a weak topological insulator (TI), BiSe structurally exhibits alternating stacks of quantum spin Hall bilayer (“Bi₂”) and three-dimensional TI layer (“Bi₂Se₃”). The low lattice thermal conductivity of BiSe due to the presence of Bi₂ bilayers promises potentially good thermoelectric performance. Herein, the thermoelectric properties of nominal Bi_1−xCu_xSe samples were studied as the functions of the content of Cu additive and temperature. It is found that Cu additives in BiSe (1) profoundly affect the texture of densified polycrystalline samples by inclining the crystallographic c-axis parallel toward the pressure direction in the densification process, (2) increase considerably the effective mass and thus the Seebeck coefficient, and (3) yield point defects and Cu–Se secondary phases that effectively scatter heat-carrying phonons. As a result, the optimized electrical and thermal properties yield a thermoelectric figure of merit of zT ~ 0.29 in Bi_1−xCu_xSe (x = 0.03) sample at 467 K in parallel to the pressure direction and a zT ~ 0.20 at 468 K in the perpendicular direction.