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

Using density functional theory calculations, we analyze the optical absorption properties of lead (Pb)-free metal halide perovskites (ABX) and double perovskites (ABBX) (A = Cs or monovalent organic ion, B = non-Pb divalent metal, B = monovalent metal, B = trivalent metal, X = halogen). We show that if B is not Sn or Ge, Pb-free metal halide perovskites exhibit poor optical absorptions because of their indirect band gap nature. Among the nine possible types of Pb-free metal halide double perovskites, six have direct band gaps. 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 applications. 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 = Sb, Bi. Our results provide important insights for designing new metal halide perovskites and double perovskites for optoelectronic applications.