Abstract:
Here we present the optoelectronic properties of pure inorganic lead-free halide perovskites in the
form of Cs2AgBiX6 (X=Br, Cl, F, I) using the density functional theory calculations on cubic phase
(Fm 3̅m) and tetragonal phase (I4/m). First, all the structures of the two phases were optimized at the
PBE level. Structural, electronic, optical properties, phonon, and thermal properties of Cs2AgBiX6 in
cubic (Fm 3̅m) and tetragonal phases (I4/m) were obtained using the VASP code. Tetragonal phases
of all compounds of the form Cs2AgBiX6, except Cs2AgBiBr6, are reported here for the very first time.
Among all the Cs2AgBiX6 (X=F, Cl, Br, I) structures, the cubic phase of Cs2AgBiBr6 was seen to have
the highest absorption coefficient along with prominent electronic features that are favorable for
optoelectronic applications. Thus, the cubic phase of Cs2AgBiBr6 was selected as the host lattice and
bromine atoms were partly replaced with chlorine and iodine atoms. Electronic and optical properties
of these mixed halide compounds of Cs2AgBiBr6−xFx, Cs2AgBiBr6−xClx, and Cs2AgBiBr6−xIx where
x=1, 2, 3, 4, 5 are investigated with hybrid functional HSE06 level. The electronic structure revealed
that these mixed compounds exhibited indirect band gap nature regardless of the halide substitution
(different x concentration) and the band gap of Cs2AgBiBr6 could be varied with the substitutions of
fluorine, chlorine, and iodine atoms. Our in-depth analysis shows that Cs2AgBiBr6 and their mixed
halides have the potential to become active double perovskite materials for photovoltaic applications
and as photocatalysts for water splitting.
