Triggering the onset of thermoelectric performance in BiCuSeO oxychalcogenides: Activation from a low intrinsic carrier concentration via Cd doping

BiCuSeO oxychalcogenides are promising p-type thermoelectric (TE) materials, yet their TE efficiency is limited by an extremely low intrinsic carrier concentration of ∼ 1018 cm-3. In this study, it is demonstrated that systematic Cd doping triggers the onset of TE performance of BiCuSeO. The substitution of Bi3+ with Cd2+ in a series of Bi1-xCdxCuSeO (x = 0, 0.02, 0.04, 0.06, and 0.08) polycrystalline alloys induces a predictable, near-linear increase in carrier concentration, reaching 3.19 × 1020 cm-3 at x = 0.08 with doping efficiency between 0.27 and 0.41 per dopant. The influence was particularly dramatic at the initial doping (x = 0.02), which boosted the carrier concentration to 1.08 × 1020 cm-3 from an extremely low carrier concentration of 2.82 × 1018 of the pristine sample. This substantial increase successfully activated a high effective mass and power factor. The Boltzmann transport calculations confirm that the electrical transport properties are radically optimized in this high carrier concentration regime. Lattice thermal conductivity decreased from 1.47 W/mK at x = 0 to 1.14 W/mK for x = 0.02 at 300 K, with the reduction being more pronounced gradually at higher doping levels. Consequently, the thermoelectric figure of merit zT showed a significant improvement by 70%, reaching 0.29 for x = 0.02 at 650 K. The optimal TE performance was reached for x ≥ 0.04, as zT values of 0.38-0.43 were attained, representing up to 150% enhancement over the pristine sample. This study demonstrated a facile doping strategy to activate the TE performance of compounds often dismissed for their low intrinsic carrier concentration.