  Organic-inorganic hybrid double perovskites have attracted increasing
interest in the commercialization of lead-free and nontoxic perovskites due to
their unique optoelectronic properties compared with inorganic perovskites. In
this study, the structural, electronic, optical and properties of 3 Pb-free
compounds (CsMA)NaBiX$_6$ (MA= methylammonium; X= Cl, Br, I) were simulated
using first-principles density functional theory (DFT). Results show that the
investigated compounds are semiconductors with tunable bandgaps characteristics
that can be used in devices like light emitting diodes, and predict the
suitability of the (CsMA)NaBiI$_6$ organic-inorganic hybrid double perovskite
in this study for optoelectronic applications owing to its high absorption
coefficient (in the order of $10^6$cm$^{-1}$), dielectric constant ($\approx
3.24$), and refractive index (2.50) as well as its stability as revealed by its
high formation energy. Additionally, the high absorption coefficient, high
refractive indices and dielectric constants of the investigated materials
posits that they have a number of optoelectronic applications including
photovoltaic cells among others.
