Synergistically enhanced thermoelectric properties in n-type Bi₆Cu₂Se₄O₆ through inducing resonant levels

Oxygen-containing semiconductors are considered to be promising thermoelectric materials because of the high physicochemical stability. New layered oxyselenide Bi₆Cu₂Se₄O₆ possesses intrinsic low lattice thermal conductivity and can be modulated to n-type semiconductor through halogen doping, which is considered as a potential n-type thermoelectric oxide. In this work, first-principles calculations are utilized to obtain the electronic and phonon band structures for deeply recognizing the transport features of intrinsic Bi₆Cu₂Se₄O₆. Furthermore, Nb is selected as donor dopant in Bi₆Cu₂Se_3.6Cl_0.4O₆, realizing a significantly improved power factor of ∼ 4.3 μV cm⁻¹ K⁻², which originates from the simultaneously optimized carrier concentration by effective doping and strengthened effective mass by inducing resonant levels around Fermi level. Meanwhile, the enhancement of phonon scattering are introduced, leading to a low lattice thermal conductivity of ∼ 0.68 W m⁻¹ K⁻¹. Finally, thanks to the effective doping of Nb and the introduction of resonant levels, a maximum ZT ∼ 0.4 at 873 K and an average ZT ∼ 0.21 from 303 to 873 K can be obtained in n-type Bi_5.91Nb_0.09Cu₂Se_3.6Cl_0.4O₆. This study broadens the applicability of resonant levels to optimize performances of thermoelectric oxides, and promotes the potential application of n-type Bi₆Cu₂Se₄O₆-based thermoelectric materials in medium temperature range.