Title: Parity-Forbidden Transitions and Their Impacts on the Optical Absorption Properties of Lead-Free Metal Halide Perovskites and Double Perovskites,
Abstract: Using density-functional theory calculations, we analyze the optical
absorption properties of lead (Pb)-free metal halide perovskites
(AB$^{2+}$X$_3$) and double perovskites (AB$^+$B$^{3+}$X$_6$) (A = Cs or
monovalent organic ion, B$^{2+}$ = non-Pb divalent metal, B$^+$ = monovalent
metal, B$^{3+}$ = trivalent metal, X = halogen). We show that, if B$^{2+}$ is
not Sn or Ge, Pb-free metal halide perovskites exhibit poor optical absorptions
because of their indirect bandgap nature. Among the nine possible types of
Pb-free metal halide double perovskites, six have direct bandgaps. Of these six
types, four show inversion symmetry-induced parity-forbidden or weak
transitions between band edges, making them not ideal for thin-film solar cell
application. Only one type of Pb-free double perovskite shows optical
absorption and electronic properties suitable for solar cell applications,
namely those with B$^+$ = In, Tl and B$^{3+}$ = Sb, Bi. Our results provide
important insights for designing new metal halide perovskites and double
perovskites for optoelectronic applications.