Fig. 1. (a) Crystal structure of a perovskite ABX₃, where A = MA⁺ or FA⁺; B = Pb²⁺, Sn²⁺, or Ge²⁺; and X = Cl^–, Br^–, I^– or mixtures thereof in organic–inorganic halide perovskites. (b) Electronic band structures of MAPbI₃, the left panel corresponds to the projected density of states (PDOS) and the middle and right panels correspond to band structures without and with spin–orbit coupling (SOC), respectively. (c) UV-vis absorption spectra and (d) photoluminescence (PL) spectra of the mixed halide perovskites MAPbI_3–x Br _x . (b) is reproduced with permission from ref. 72, copyright 2015, American Chemical Society. (c and d) are reproduced with permission from ref. 73, copyright 2016, American Chemical Society.