Thermoelectric Performance of the 2D Bi₂Si₂Te₆ Semiconductor

Bi₂Si₂Te₆, a 2D compound, is a direct band gap semiconductor with an optical band gap of ∼0.25 eV, and is a promising thermoelectric material. Single-phase Bi₂Si₂Te₆ is prepared by a scalable ball-milling and annealing process, and the highly densified polycrystalline samples are prepared by spark plasma sintering. Bi₂Si₂Te₆ shows a p-type semiconductor transport behavior and exhibits an intrinsically low lattice thermal conductivity of ∼0.48 W m^-1 K^-1 (cross-plane) at 573 K. The first-principles density functional theory calculations indicate that such low lattice thermal conductivity is derived from the interactions between acoustic phonons and low-lying optical phonons, local vibrations of Bi, the low Debye temperature, and strong anharmonicity result from the unique 2D crystal structure and metavalent bonding of Bi₂Si₂Te₆. The Bi₂Si₂Te₆ exhibits an optimal figure of merit ZT of ∼0.51 at 623 K, which can be further enhanced by the substitution of Bi with Pb. Pb doping leads to a large increase in power factor S²σ, from ∼3.9 μW cm^-1 K^-2 of Bi₂Si₂Te₆ to ∼8.0 μW cm^-1 K^-2 of Bi_1.98Pb_0.02Si₂Te₆ at 773 K, owing to the increase in carrier concentration. Moreover, Pb doping induces a further reduction in the lattice thermal conductivity to ∼0.38 W m^-1 K^-1 (cross-plane) at 623 K in Bi_1.98Pb_0.02Si₂Te₆, due to strengthened point defect (Pb_Bi') scattering. The simultaneous optimization of the power factor and lattice thermal conductivity achieves a peak ZT of ∼0.90 at 723 K and a high average ZT of ∼0.66 at 400-773 K in Bi_1.98Pb_0.02Si₂Te₆.