  We report $^{59}$Co NMR and transport measurements on $n$-type filled
skutterudites Ba$_x$Yb$_y$Co$_4$Sb$_{12}$ and $A$$_x$Co$_4$Sb$_{12}$ ($A$= Ba,
Sr), promising thermoelectric materials. The results demonstrate consistently
that a shallow defect level near the conduction band minimum dominates the
electronic behavior, in contrast to the behavior of unfilled CoSb$_3$. To
analyze the results, we modeled the defect as having a single peak in the
density of states, occupied at low temperatures due to donated charges from
filler atoms. We fitted the NMR shifts and spin-lattice relaxation rates
allowing for arbitrary carrier densities and degeneracies. The results provide
a consistent picture for the Hall data, explaining the temperature dependence
of the carrier concentration. Furthermore, without adjusting model parameters,
we calculated Seebeck coefficient curves, which also provide good consistency.
In agreement with recently reported computational results, it appears that
composite native defects induced by the presence of filler atoms can explain
this behavior. These results provide a better understanding of the balance of
charge carriers, of crucial importance for designing improved thermoelectric
materials.
