  Bipolar carrier transport is often a limiting factor in the thermoelectric
efficiency of narrow bandgap materials (such as Bi2Te3 and PbTe) at high
temperatures due to the introduction of an additional term to the thermal
conductivity and a reduction in the Seebeck coefficient. In this work, we
present a theoretical investigation into the ability of nanoinclusions to
reduce the detrimental effect of bipolar transport. Using the quantum
mechanical non equilibrium Greens function (NEGF) transport formalism, we
simulate electronic transport through two-dimensional systems containing
densely packed nanoinclusions, separated by distances similar to the electron
mean free path. Specifically, considering an n type material, where the bipolar
effect comes from the valence band, we insert nanoinclusions that impose
potential barriers only for the minority holes. We then extract the materials
electrical conductivity, Seebeck coefficient, and electronic thermal
conductivity including its bipolar contribution. We show that nanoinclusions
can indeed have some success in reducing the minority carrier transport and the
bipolar effect on both the electronic thermal conductivity and the Seebeck
coefficient. The benefits from reducing the bipolar conductivity are larger the
more conductive the minority band is to begin with (larger hole mean free path
in particular), as expected. Interestingly, however, the benefits on the
Seebeck coefficient and the power factor are even more pronounced not only when
the minority mean free path is large, but when it is larger compared to the
majority conduction band mean free path. Finally, we extract an overall
estimate for the benefits that nanoinclusions can have on the ZT figure of
merit.
