  The sodium cobaltate system Na$_{x}$CoO$_2$ is a prominent representant of
strongly correlated materials with promising thermoelectric response. In a
combined theoretical and experimental study we show that by doping the Co site
of the compound at $x$=0.7 with iron, a further increase of the Seebeck
coefficient is achieved. The Fe defects give rise to effective hole doping in
the high-thermopower region of larger sodium content $x$. Originally filled
hole pockets in the angular-resolved spectral function of the Fe-free material
shift to low energy when introducing Fe, leading to a multi-sheet interacting
Fermi surface. Because of the higher sensitivity of correlated materials to
doping, introducing adequate substitutional defects is thus a promising route
to manipulate their thermopower.
