  We explore the interface properties of perovskite heterostructure
CsPbBr$_3$/CsPbI$_3$ through first-principles calculations. The structural
interface is formed by the bonding of Cs-Br and Cs-I with bond length of
$\sim$4.106 and 3.922 \AA. The upshift of Goldsmith tolerance factor in the
range $0.8<t<1$ from $t<0.8$ is revealed for the bi-layer interface, from bulk,
reflecting the structural rearrangement from anisotropy to isotropy in
confinement. The band gap arises mainly due to the energy difference of I-5p
orbital than that of Br-4p at the valence band and Pb-6p at the conduction
band. Heavier halide shows the red shift in the absorption spectra, for the
pristine monolayer component. For the bilayer geometry, iodine contribution is
more observed than that of bromine and the underlying interface properties may
be useful for solar cell devices application.
