Understanding bipolar thermal conductivity in terms of concentration ratio of minority to majority carriers

Bi₂Te₃ is a good candidate to be used in thermoelectric generators. For a higher efficiency of the generators, shifting the temperature at which Bi₂Te₃ performs best to higher temperatures is required. Bipolar thermal conductivity suppression is the most effective approach to improve high-temperature thermoelectric performance. However, characterization of the bipolar thermal conductivity is challenging because it is related to individual contribution to Seebeck coefficient and electrical conductivity from the majority and minority carriers. Two-band model calculations are performed using reported band parameters of n-type Bi₂Te₃ to estimate the effects of band gap and minority to majority carrier concentration ratio in the bipolar thermal conductivity suppression. Individual Seebeck coefficient, electrical conductivity, and carrier concentrations from the majority and minority carriers are evaluated while varying chemical potential with a different band gap. It was demonstrated that increasing the band gap and chemical potential increased the individual Seebeck coefficient from minority carriers while decreasing the individual electrical conductivity and concentration from minority carriers. As a result, it was shown that the band gap increase and especially the magnitude of the minority to majority carrier concentration ratio decrease was effective in the bipolar thermal conductivity suppression.