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

## TL;DR

This study uses density-functional theory to analyze the optical absorption of lead-free metal halide perovskites and double perovskites, revealing the impact of parity-forbidden transitions on their suitability for solar cells.

## Contribution

It identifies specific compositions of lead-free double perovskites with favorable optical properties for optoelectronic applications, highlighting the role of parity-forbidden transitions.

## Key findings

- Most Pb-free perovskites have indirect bandgaps and poor absorption.
- Six of nine double perovskite types have direct bandgaps.
- Only one type shows suitable absorption for solar cells.

## 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.

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Source: https://tomesphere.com/paper/1702.03593