  We study an inhomogeneous three-orbital Hubbard model for the Cu-substituted
iron pnictides using an extended real-space Green's function method combined
with density functional calculations. We find that the onsite interactions of
the Cu ions are the principal determinant of whether an electron dopant or a
hole dopant is caused by the Cu substitution. It is found that the Cu
substitution could lead to a hole doping when its onsite interactions are
smaller than a critical value, as opposed to an electron doping when the
interactions of Cu ions are larger than the critical value, which may explain
why the effects of Cu substitution on the carrier density are entirely
different in NaFe$_{1-x}$Cu$_x$As and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$. We also
find that the effect of a doping-induced disorder is considerable in the
Cu-substituted iron pnictides, and its cooperative effect with electron
correlations contributes to the orbital-selective insulating phases in
NaFe$_{1-x}$Cu$_x$As and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$.
