Optical Properties of Lead and Lead-Free Halide Perovskites

Lead halide perovskites CsPbX3 are still being promoted as photovoltaic materials and much attention is now drawn towards this class of alloys in spite of their long-term instability and the lead toxicity. Our work aims to investigate the substitution of Pb by Sn or Ge. The structural, optical, and electronic properties of the perovskites CsBX3 (B = Pb, Sn, Ge; X = Cl, Br, I) are analyzed using density functional theory (DFT) through the software package Quantum ESPRESSO. The obtained optical and electronic properties are discussed as a function of the electronegativity of the B and X elements. The electronic study indicates that all these alloys are semiconductors with a direct band gap and reveals the dependence of the electronic band gap upon the nature of the cation B and the halogen X, which makes it easily adjustable by changing the B and/or X elements. Within the visible domain, the highest values of the dielectric constant, refractive index, and photoconductivity are obtained for the Ge halides. However, the lowest reflection and extinction coefficients are obtained for the Pb halides. In addition, the Ge halides are characterized by a wide absorption range from the infrared to the ultraviolet. The maximum absorption spectra are obtained for the chloride halides CsBCl3. These results shed some light on how to combine the positive characteristics of Pb and Ge halides to bring new possibilities of obtaining the best candidate for photovoltaic applications.