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.