  Organic-inorganic hybrid halide perovskites, in which the A cations of an
ABX$_3$ perovskite are replaced by organic cations, may be used for
photovoltaic and solar thermoelectric applications. In this contribution, we
systematically study three lead-free hybrid perovskites, i.e., methylammonium
tin iodide CH$_3$NH$_3$SnI$_3$, ammonium tin iodide NH$_4$SnI$_3$, and
formamidnium tin iodide HC(NH$_2$)$_2$SnI$_3$, by first-principles
calculations. We find that in addition to the commonly known motif in which the
corner-shared SnI$_6$ octahedra form a three-dimensional network, these
materials may also favor a two-dimensional (layered) motif formed by
alternating layers of the SnI$_6$ octahedra and the organic cations. These two
motifs are nearly equal in free energy and are separated by low barriers. These
layered structures features many flat electronic bands near the band edges,
making their electronic structures significantly different from those of the
structural phases composed of three-dimension networks of SnI$_6$ octahedra.
Furthermore, because the electronic structures of HC(NH$_2$)$_2$SnI$_3$ are
found to be rather similar to those of CH$_3$NH$_3$SnI$_3$, formamidnium tin
iodide may also be promising for the applications of methylammonium tin iodide.

Materials Science