Investigation of the bipolar effect in the thermoelectric material CaMg2Bi2 using a first-principles study.

The bipolar effect in relatively narrow band-gap thermoelectric (TE) compounds is a negative process deteriorating the TE properties particularly at higher temperatures. In this work, we investigate the TE performance of the compound CaMg2Bi2 using the first-principles calculation and semi-classical Boltzmann transport theory in combination with our experimental data. It is revealed that this compound exhibits a remarkable bipolar effect and temperature-dependent carrier concentration. The bipolar effect imposes remarkable influence on all the electron-transport related TE parameters. An effective carrier concentration neff as a function of temperature is proposed to account for the bipolar effect induced carrier excitations. The as-evaluated TE parameters then show good consistency with measured results. This work may shed light on our understanding of the bipolar effect in TE compounds.