Enhancing thermoelectric performance by Fermi level tuning and thermal conductivity degradation in (Ge_1−xBi_x)Te crystals

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge_1−xBi_xTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (E_F) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 10²⁰ cm⁻³ to 3–5 × 10²⁰ cm⁻³ and the Seebeck coefficient is boosted three times to 75 μVK⁻¹. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level E_F starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m⁻¹K⁻¹ at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge_1−xBi_xTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.