High Thermoelectric Performance for n-Type K ₂ Bi ₈ S ₁₃ with Low Thermal Conductivity

The complex structure of bismuth chalcogenides shows promising prospects as a type of thermoelectric material with intrinsic low lattice thermal conductivity ( κ lat ). The n-type K 2 Bi 8 S 13 has the low κ lat of 0.56–0.42 W m –1 K –1 from 300 to 823 K because of the large unit cell, complex structure, and rattling of K in tunnels composed of the NaCl-type and CdI 2 -type rod-shaped fragments. The power factor (PF) of K 2 Bi 8 S 13 reaches a maximum value of 1.97 μW cm –1 K –2 at 823 K. With the low thermal conductivity of 0.50 W m –1 K –1, the pristine K 2 Bi 8 S 13 achieves a dimensionless figure of merit ZT of ∼0.33 at 823 K. With a heavy conduction band minimum for K 2 Bi 8 S 13, the extra Bi is introduced to enhance the carrier concentration, electrical conductivity ( σ ), and PF. Consequently, the σ and PF reveal an evident promotion from 34.85 S cm –1 and 1.97 μW cm –1 K –2 for K 2 Bi 8 S 13 to 73.47 S cm –1 and 3.09 μW cm –1 K –2 for K 2 Bi 8.003 S 13 at 823 K, respectively. Meanwhile, point defects introduced by extra Bi result in phonon scattering and decreased κ lat to 0.38 W m –1 K –1 at 823 K. Finally, a maximum ZT ∼ 0.53 of K 2 Bi 8.003 S 13 increases by 61% compared with 0.33 of intrinsic K 2 Bi 8 S 13 at 823 K.