  High Seebeck coefficient by creating large density of state (DOS) around the
Fermi level through either electronic structure modification or manipulating
nanostructures, is commonly considered as a route to advanced thermoelectrics.
However, large density of state due to flat bands leads to large effective
mass, which results in a simultaneous decrease of mobility. In fact, the net
effect of high effective mass is a lower thermoelectric figure of merit when
the carriers are predominantly scattered by acoustic phonons according to the
deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial
effect of light effective mass leading to high power factor in n-type
thermoelectric PbTe, where doping and temperature can be used to tune the
effective mass. This clear demonstration of the deformation potential theory to
thermoelectrics shows that the guiding principle for band structure engineering
should be low effective mass along the transport direction.
