Layered structures of organic/inorganic hybrid halide perovskites

Organic-inorganic hybrid halide perovskites, in which the A cations of an ABX₃ 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)₂SnI₃ 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)₂SnI₃ are found to be rather similar to those of CH₃NH₃SnI₃, formamidnium tin iodide may also be promising for the applications of methylammonium tin iodide.