In-doping induced resonant level and thermoelectric performance enhancement in n-type GeBi2Te4 single crystals with intrinsically low lattice thermal conductivity

Resonant level, the impurity level located inside the valence/conduction band, is an effective strategy for decoupling the electrical conductivity and Seebeck coefficient and enhancing the thermoelectric power factor. In this work, we show that In doping induces resonant level and enhances the thermoelectric performance of GeBi2Te4, a promising n-type thermoelectric material with layered structure. Single crystals grown by Bridgman method were used in this study to take the advantage of its anisotropic transport properties. An ultralow lattice thermal conductivity of -0.51 W m-1 K-1 at 323 K was found along the c-axis of GeBi2Te4, driven by intrinsic cation disorder and the low phonon group velocity from the relatively weak chemical bonds as well as the strong lattice anharmonicity caused by hierarchical bond orders. Both enhanced electrical conductivity and Seebeck coefficient were observed in the In-doped samples, indicating that In-doping not only increases the carrier concentration, but also introduces the resonant level that was further confirmed by the first-principles calculation. A maximum zT of -0.42 at 523 K and an average zT of 0.36 over 323 to 573 K were achieved in Ge0.97In0.03Bi2Te4 crystal sample, -100% and -89% enhancement compared to those of pristine single crystal sample, respectively. This study demonstrates another example of resonant level for thermoelectric performance enhancement, which may be applied to other related materials.

Automated summary

In this study, it was shown that the introduction of resonant level through In-doping can enhance the thermoelectric performance of GeBi2Te4. The Ge0.97In0.03Bi2Te4 crystal sample exhibited significantly improved electrical conductivity, Seebeck coefficient, and thermoelectric figure of merit compared to the pristine single crystal sample.