  Structural stability, electronic structure and optical properties of
CH3NH3BaI3 hybrid perovskite is examined both from theory as well as
experiment. Solution-processed thin films of CH3NH3BaI3 exhibited a high band
gap of approximately 3.87 eV, which is in excellent agreement with the
theoretical estimate of 4 eV. Also, the XRD patterns of the thin films match
well with the l-peaks of the simulated pattern obtained from the relaxed unit
cell of CH3NH3BaI3, crystallizing in the I4/mcm space group, with lattice
parameters, a = 9.30 A, c = 13.94 A. Atom projected density of state and band
structure calculations reveal the conduction and valence band edges to be
comprised primarily of Barium d-orbitals and Iodine p-orbitals, respectively.
The larger band gap of CH3NH3BaI3 compared to CH3NH3PbI3 can be attributed to
the lower electro-negativity coupled with the lack of d-orbitals in the valence
band of Ba{2+}. A more detailed analysis reveals the excellent chemical and
mechanical stability of CH3NH3BaI3 against humidity, unlike its lead halide
counterpart, which degrades under such conditions. The dopability of the
CH3NH3BaI3 compound e.g. by doping La on the Ba site combined with its
structural and mechanical stability under the ambient conditions, suggests this
compound as a promising candidate for transparent conductor applications,
especially for all perovskite solar cells.
