Point Defect Scattering and Phonon Softening for Achieving High Thermoelectric Performance in p-Type ZnSb with Optimal Carrier Concentration.

Abstract

The thermoelectric material ZnSb has been intensively studied on account of its good thermodynamic stability and earth-abundant constituent elements, both of which make it feasible for mass production. However, the practical application of ZnSb is limited by its relatively poor thermoelectric performance, characterized by a low power factor and high lattice thermal conductivity. Herein, we demonstrate that there is a significant improvement in the thermoelectric figure of merit of ZnSb by combining Ge doping at the Sb site with Cd alloying at the Zn site. First, Ge doping at the Sb site can effectively optimize the carrier concentration, thereby resulting in an ∼82% increase in the peak power factor through a concentration of only 0.6%. Second, Cd alloying at the Zn site can bring about a strong point defect scattering to phonon propagation, leading to reduced phonon relaxation time. Meanwhile, the significant softening of acoustic phonons is also introduced by Cd alloying at the Zn site, and thus group velocities of acoustic phonon modes are suppressed. Consequently, a ∼44% reduction in the lattice thermal conductivity is achieved in Zn_0.7Cd_0.3Sb at room temperature. As a result of the optimized carrier concentration and suppressed lattice thermal conductivity, a peak <i>zT</i> value as high as ∼1.08 at 564 K is attained in Zn_0.7Cd_0.3Sb_0.96Ge_0.04.