High thermoelectric performance in Te-free (Bi,Sb)2Se3via structural transition induced band convergence and chemical bond softening

Abstract

Semiconductors with converging multiple electronic valleys and soft chemical bonds are ideal for high-performance thermoelectrics. Narrow gap Bi₂Se₃ is a well-known three-dimensional topological insulator with non-trivial surface states, while possessing low thermoelectric properties due to its single-degenerate band conduction, despite being an important constituent of highly efficient n-type thermoelectric Bi₂(Te,Se)₃. Here we demonstrate that in Te-free Bi_2−xSb_xSe₃ converging multiple electronic band valleys and strengthening phonon scattering can be realized simultaneously via a composition-induced (Sb-alloying) structural transition from a rhombohedral phase to an orthorhombic phase. The accompanying chemical bond softening and structural distortion cause significant modifications to the electronic band structure and phonon dispersion. The convergence of heavy bands realized in the orthorhombic phase (x ≥ 1.0) largely increases the electron density of states effective mass, and thus gives rise to a high Seebeck coefficient of ∼−280 μV K⁻¹ at 800 K. Meanwhile, phonon softening and substantial lattice anharmonicity pertain to weak interchain interactions considerably block the heat-carrying acoustic phonons, resulting in ultralow lattice thermal conductivities of ∼0.6 W m⁻¹ K⁻¹ at 300 K and ∼0.3 W m⁻¹ K⁻¹ at 800 K. Consequently, a maximum thermoelectric figure of merit ZT of ∼1.0 can be achieved for n-type BiSbSe₃, about three times higher than that of the optimized Bi₂Se₃. The moderately high ZT of Te-free BiSbSe₃ makes it a promising candidate for low-mid temperature power generations. Furthermore, the concept of structural transition driven band convergence and chemical bond softening can be applied to improve the thermoelectric properties of other materials and may also shed light on identifying new materials.