“…In 1839, Russian mineralogist L. A. Perovski firstly proposed the perovskite crystal structure when studying calcium titanate (CaTiO 3 ) in the metamorphic rocks of the Ural Mountains. Since then, CaTiO 3 and later any compound denoted by a general chemical formula of ABX 3 are named as perovskite . Among metal halide perovskite materials, symbol A generally represents a monovalent organic cation or inorganic metal cation (eg, methylammonium [MA + , ], formamidinium [FA + , ], Cs + , Rb + ), symbol B is a bivalent metal cation (eg, Pb 2+ , Sn 2+ , Ge 2+ , Cu 2+ , Eu 2+ , and Ni 2+ ), symbol X represents one or mixed halide anions (Cl − , Br − , and I − ) (Figure ) …”