Enhanced thermoelectric performance of p-type Bi2Si2Te6 enabled via synergistically optimizing carrier concentration and suppressing bipolar effect

Bipolar diffusion is regarded as an obstacle to improving thermoelectric performance because it dramatically deteriorates the thermopower and increases the thermal conductivity at high temperatures. Here, we propose two effective strategies, either substitution of Bi with Mn, or replacement of Si with Ge, to optimize the carrier concentration and suppress the bipolar effect in Bi2Si2Te6. The electrical resistivity is largely reduced and a compensation of the Seebeck coefficient by minority charge carriers is eliminated because of increased carrier concentration. Meanwhile, both Mn doping at Bi site and Ge alloying at Si site can achieve a larger DOS effective mass which is favorable to the Seebeck coefficient. Accordingly, the maximum power factor is considerably improved from 4.4 μW cm−1 K−2 for pristine Bi2Si2Te6 to 9.6 μW cm−1 K−2 for Bi1.97Mn0.03Si2Te6 and 9.2 μW cm−1 K−2 for Bi2Si1.8Ge0.2Te6. Furthermore, the lattice thermal conductivity is largely diminished by ∼40% at elevated temperatures, which is ascribed to the suppressed bipolar thermal conductivity and strengthened phonon scattering by extra substitutional point defects. As a result, a peak zT value of unity is achieved in both Bi1.98Mn0.02Si2Te6 and Bi2Si1.8Ge0.2Te6 at 773 K.