High near-room temperature figure of merit of n-type Bi₂GeTe₄-based thermoelectric materials via a stepwise optimization of carrier concentration

Bi₂GeTe₄ is a promising near room-temperature thermoelectric candidate with a low lattice thermal conductivity. Carrier concentration of intrinsic Bi₂GeTe₄ changes dramatically with tiny Ge content adjustment, leading to a challenge in carrier concentration optimization. To overcome this challenge, we firstly introduce excessive Ge into Bi₂GeTe₄ to shift the Fermi level deep into the conduction band and transfer Bi₂GeTe₄ into a highly degenerate n-type semiconductor. Secondly, the embedded p-type Bi₂Ge₂Te₅ secondary phase induces further optimization of the Fermi level and carrier concentration. Finally, the power factor of the as-synthesized Bi₂GeTe₄-based material is significantly increased from ∼ 0.08 μW cm⁻¹ K⁻² to ∼ 4.2 μW cm⁻¹ K⁻² at 423 K when increasing the nominal Ge content (x) of Bi₂Ge_xTe₄ from 1 to 1.45. Correspondingly, a high figure-of-merit of ∼ 0.22 at 423 K is achieved in Bi₂GeTe₄-based thermoelectric materials. This result indicates our viable stepwise strategy can be used to optimize carrier concentration and achieve high thermoelectric performance of the n-type Bi₂GeTe₄.