Dual-site mixed layer-structured FA x Cs 3- x Sb 2 I 6 Cl 3 Pb-free metal halide perovskite solar cells.

Structure engineering of trivalent metal halide perovskites (MHPs) such as A 3 Sb 2 X 9 (A = a monovalent cation such as methyl ammonium (MA), cesium (Cs), and formamidinium (FA) and X = a halogen such as I, Br, and Cl) is of great interest because a two dimensional (2D) layer structure with direct bandgap has narrower bandgap energy than a zero dimensional (0D) dimer structure with indirect bandgap. Here, we demonstrated 2D layer structured FACs 2 Sb 2 I 6 Cl 3 MHP by dual-site (A and X site) mixing. Thanks to the lattice-symmetry change by I-Cl mixed halide, the shortest ionic radius of Cs, and the lower solution energy due to dual-site mixing, the FACs 2 Sb 2 I 6 Cl 3 MHP had 2D layer structure and thereby the MHP solar cells exhibited improved short-circuit current density.