Atomic and Electronic Structure of Two-Dimensional Inorganic Halide Perovskites An+1MnX3n+1 (n=1-6, A = Cs, M = Pb and Sn, and X = Cl, Br, and I) from ab Initio Calculations

Thin layers of inorganic halide perovskites A(n+1)M(n)X(3n+1) (n = 1-6, A = Cs, M = Pb and Sn, and X = Cl, Br, and I) have been studied in orthorhombic and cubic phases along with layers of monoclinic CsSnCl3. It is found that one-unit-cell-thick layers have low stability except the monoclinic CsSnCl3 for which formation energy is slighly less than the bulk value. However, Cs2PbI4 is unstable in both cubic and orthorhombic phases. The formation energy for n > 3 becomes comparable to bulk, but the inclusion of spin orbit coupling is found to be important for the stabilization particularly for layers with Pb. Importantly, layers of environmentally friendly Sn-based systems have similar values of formation energy in the orthorhombic and cubic phases as well as similar band gaps which make them good materials for solar cell applications as temperature range changes during their operation. The studied 66 cubic and orthorhombic nanosystems have direct band gap (0.6-2.9 eV) using generalized gradient approximation for the exchange correlation functional, but the use of the HSE06 method increases the band gap. The reduced dimensionality leads to elongation (contraction) of MX6 octahedra perpendicular (parallel) to the plane of the layers and an increase in the band gap. The presence of surface makes the hybridization between M s and X p orbitals near the valence band maximum stronger than in bulk which is good for light absorption. The effective mass of the electrons and holes is very light which augers well for the transport properties. Lead-based systems have larger band gap, and these can be useful in applications such as light-emitting diodes.