  It is well known that the efficiency of a good thermoelectric material should
be optimized with respect to doping concentration. However, much less attention
has been paid to the optimization of the dopant's energy level. Thermoelectric
materials doped with shallow levels may experience a dramatic reduction in
their figures of merit at high temperatures due to the excitation of minority
carriers that reduces the Seebeck coefficient and increases bipolar heat
conduction. Doping with deep level impurities can delay the excitation of
minority carriers as it requires a higher temperature to ionize all dopants. We
find through modeling that, depending on the material type and temperature
range of operation, different impurity levels (shallow or deep) will be desired
to optimize the efficiency of a thermoelectric material. For different
materials, we further clarify where the most preferable position of the
impurity level within the band gap falls. Our research provides insights in
choosing the most appropriate dopants for a thermoelectric material in order to
maximize the device efficiency.
