  Transparent conducting oxides (TCOs) are essential to many technologies.
These materials are doped (\emph{n}- or \emph{p}-type) oxides with a large
enough band gap (ideally $>$3~eV) to ensure transparency. However, the high
carrier concentration present in TCOs lead additionally to the possibility for
optical transitions from the occupied conduction bands to higher states for
\emph{n}-type materials and from lower states to the unoccupied valence bands
for \emph{p}-type TCOs. The "second gap" formed by these transitions might
limit transparency and a large second gap has been sometimes proposed as a
design criteria for high performance TCOs. Here, we study the influence of this
second gap on optical absorption using \emph{ab initio} computations for
several well-known \emph{n}- and \emph{p}-type TCOs. Our work demonstrates that
most known \emph{n}-type TCOs do not suffer from second gap absorption in the
visible even at very high carrier concentrations. On the contrary,
\emph{p}-type oxides show lowering of their optical transmission for high
carrier concentrations due to second gap effects. We link this dissimilarity to
the different chemistries involved in \emph{n}- versus typical \emph{p}-type
TCOs. Quantitatively, we show that second gap effects lead to only moderate
loss of transmission (even in p-type TCOs) and suggest that a wide second gap,
while beneficial, should not be considered as a needed criteria for a working
TCO.
