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

•Stepwise carrier concentration optimization of Bi2GeTe4 thermoelectric materials.•Ge vacancy-engineering transfer Bi2GeTe4 into highly degenerated semiconductor.•Carrier concentration optimization leads to high power factor of 4.2 μW cm−1 K−2.•Approaching a record-high zT value of ∼ 0.22 at 423 K in Bi2Ge1.45Te4. 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 Bi2GexTe4 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.