Parity-Forbidden Transitions and Their Impacts on the Optical Absorption Properties of Lead-Free Metal Halide Perovskites and Double Perovskites

摘要

Using density-functional theory calculations, we analyze the opticalabsorption properties of lead (Pb)-free metal halide perovskites(AB$^{2+}$X$_3$) and double perovskites (AB$^+$B$^{3+}$X$_6$) (A = Cs ormonovalent organic ion, B$^{2+}$ = non-Pb divalent metal, B$^+$ = monovalentmetal, B$^{3+}$ = trivalent metal, X = halogen). We show that, if B$^{2+}$ isnot Sn or Ge, Pb-free metal halide perovskites exhibit poor optical absorptionsbecause of their indirect bandgap nature. Among the nine possible types ofPb-free metal halide double perovskites, six have direct bandgaps. Of these sixtypes, four show inversion symmetry-induced parity-forbidden or weaktransitions between band edges, making them not ideal for thin-film solar cellapplication. Only one type of Pb-free double perovskite shows opticalabsorption and electronic properties suitable for solar cell applications,namely those with B$^+$ = In, Tl and B$^{3+}$ = Sb, Bi. Our results provideimportant insights for designing new metal halide perovskites and doubleperovskites for optoelectronic applications.