Layered structures of organic/inorganic hybrid halide perovskites

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 CH3NH3SnI3, ammonium tin iodide NH4SnI3, and formamidnium tin iodide HC(NH2)(2)SnI3 by first-principles calculations. We find that in addition to the commonly known motif in which the corner-shared SnI6 octahedra form a three-dimensional network, these materials may also favor a two-dimensional (layered) motif formed by alternating layers of the SnI6 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 SnI6 octahedra. Furthermore, because the electronic structures of HC(NH2)(2)SnI3 are found to be rather similar to those of CH3NH3SnI3, formamidnium tin iodide may also be promising for the applications of methylammonium tin iodide.