  Thermoelectric performance is of interest for numerous applications such as
waste heat recovery and solid state energy conversion, and will be seen to be
closely connected to topological insulator behavior. In this context we here
report first principles transport and defect calculations for
Bi$_{2}$Te$_{2}$Se in relation to Bi$_{2}$Te$_{3}$. The two compounds are found
to contain remarkably different electronic structures in spite of being
isostructural and isoelectronic. We discuss these results in terms of the
topological insulator characteristics of these compounds.
