Carrier distribution in multi-band materials and its effect on thermoelectric properties

Band convergence is one of the most interesting topics in recent studies of thermoelectrics. However, its effect on thermoelectric properties is only simply stated as improving band degeneracy. In this paper, the enhanced thermoelectric performance due to band convergence is clarified from the viewpoint of distribution of carriers in the electronic bands. The n-type Mg2Sn0.75Ge0.25 is used as a case study, and the effect of band offset E on its thermoelectric properties is investigated based on the three-band model, i.e., one light conduction band, one heavy conduction band, and one valence band. The results show that E has a decisive effect on controlling the distribution of carriers in the two conduction bands, thus affecting the thermoelectric properties. Since the optimal carrier concentration nopt is related to the density of state effective mass m∗ at a given temperature, an appropriate distribution of carriers should be a higher carrier concentration in the heavy band (with larger m∗) and a lower carrier concentration in the light band (with smaller m∗). In order to achieve a proper distribution of carriers, E should be as small as possible at any temperature, which explains the reason why band convergence could lead to the enhanced thermoelectric performance.

The effect of band offset E on the thermoelectric properties of Mg2Sn0.75Ge0.25 is investigated based upon the three-band model, i.e., one light conduction band, one heavy conduction band, and one valence band. An appropriate distribution of carriers should be a higher carrier concentration in the heavy conduction band (with larger m∗) while a lower carrier concentration in the light conduction band (with smaller m∗). In order to achieve a proper distribution of carriers, E should be as small as possible at any temperature, which explains the reason why band convergence could lead to enhanced thermoelectric performance.Figure optionsDownload as PowerPoint slide