1, The University of Queensland, Brisbane, Queensland, Australia
3, The University of Queensland, Brisbane, Queensland, Australia
Thermoelectric performance, gauged by figure-of-merit, is proportional to power-factor and the reciprocal of thermal conductivity. In materials with multi-electronic bands close to the band gap region, power-factor can be increased by enlarging the contribution from the extra band(s). However, the effective mass of the extra band studied so far is commonly much heavier than the primary band, leading to a significantly reduced carrier mobility, which in turn limits the net increase in power-factor. On the other hand, the thermal conductivity of a given material can be decreased by introducing different phonon scattering mechanisms. To further decrease thermal conductivity, additional new phonon scattering sources need to be explored.
In this study, we employ a light extra band in p-type AgSbTe2-xSex alloys to increase power-factor and introduce a high density of stacking faults to achieve an ultra-low thermal conductivity. Synergistically, we achieve a figure-of-merit over 2.0, which is the record for AgSbTe2-based systems. Our density functional theory calculations confirmed the existence of such a light band and the power-factor enhancement. Using transmission electron microscopy techniques, we find dense stacking faults. Based on the modeling study on phonon transport by considering various scattering sources, we attribute the obtained ultra-low thermal conductivity to the co-existence of grain boundaries, stacking faults, and point defects, in which the stacking faults account for the larger proportion of the reduced thermal conductivity. The strategy of enhancing the power-factor by engineering extra light band(s) and enhancing phonon scattering through introducing stacking faults opens up a robust pathway to tailor thermoelectric performance.
 M. Hong, Z. C. Chen,* L. Yang, Z. M. Liao, Y. C. Zou, Y. H. Chen, S. Matsumur, and J. Zou* Adv. Energy Mater. 2017 (Accepted)