  We address the role of Sn-substitution and Pb-vacancy (Pb-$\Box$) in
regulating stability and carrier concentration of
CH$_3$NH$_3$Pb$_{1-X-Y}$Sn$_X$$\Box_Y$I$_3$ perovskite using density functional
theory, where the performance of the exchange-correlation functional is
carefully analyzed, and validated w.r.t. available experimental results. We
find the most stable configuration does not prefer any Pb at 50\% concentration
of Sn. However, the Pb-$\Box$s become unfavourable above 250K due to the
reduced linearity of Sn-I bonds. For n-type host the Sn substitution is more
preferable than Pb-$\Box$ formation, while for p-type host the trend is exactly
opposite. The charge states of both Sn and Pb-$\Box$ are found to be dependent
on the Sn concentration, which in turn alters the perovskite from n-type to
p-type with increasing $X$ ($>$0.5).
