Entropy engineering for superior performance in BiCuSeO by band flattening and all-scale hierarchical microstructures

Thermoelectric technology, which gains attention as a sustainable and eco-friendly energy source, has blocked application by low conversion efficiency. To address this, we utilized entropy engineering to create advanced Bi1-x-2yPbxCayYbyCuSeO (x = y = 0–0.05; x = 0.06–0.08 (x + 2y = 0.12)) thermoelectric materials. Our method involved optimizing energy bands and tailoring all-scale hierarchical microstructures. By incorporating Yb, we reduced the gap between the light band and Fermi level, resulting in high mobility. Combined with a high carrier concentration, Bi0.85Pb0.05Ca0.05Yb0.05CuSeO achieved exceptional conductivity of 180 S/cm at 873 K, surpassing pristine BiCuSeO by 12 times. The addition of Pb flattened the energy band, enabling Bi0.88Pb0.06Ca0.03Yb0.03CuSeO to maintain a high Seebeck coefficient and achieve a power factor of ∼ 700 μWm-1K−2. Furthermore, the inclusion of CaO2 impeded BiCuSeO growth and facilitated the formation of multiscale nanograin boundaries. This, combined with increased entropy causing significant lattice distortion, led to extremely low thermal conductivity in Bi0.88Pb0.06Ca0.03Yb0.03CuSeO sample (κlat ∼ 0.244 Wm-1K−1 and κtot ∼ 0.461 Wm-1K−1 at 873 K). Consequently, Bi0.88Pb0.06Ca0.03Yb0.03CuSeO achieved a remarkable ZT of 1.2, outperforming pristine BiCuSeO by a factor of 2.5. In addition, we evaluated the thermoelectric conversion performance of the BP6CY3/n-type PbTe multi-legged device, achieving a maximum efficiency of ∼ 10% at ΔT = 561 K.