DOI: https://doi.org/10.55373/mjchem.v28i4.55

Keywords: Cs₃Bi₂I₉, Cs₃Bi₂Br₉, Cs₃Bi₂Cl₉, lead-free perovskite, CASTEP

Abstract

In recent years, increasing environmental and stability concerns surrounding lead-based perovskites have driven extensive research toward the development of lead-free alternatives. Among the promising candidates, caesium bismuth halides (Cs₃Bi₂X₉, X = I, Br, Cl) have attracted significant attention due to their excellent chemical stability, favourable electronic properties, and tuneable band gap energies. In this work, a comprehensive density functional theory (DFT) study was conducted within the CASTEP framework to investigate the structural, optical, electronic, and mechanical strain-dependent properties of Cs₃Bi₂I₉, Cs₃Bi₂Br₉, and Cs₃Bi₂Cl₉. The optimized crystal structures confirmed their hexagonal and trigonal layered phases with stable lattice configurations. Optical absorption spectra revealed strong absorptions with coefficients on the order of 10⁵ cm⁻¹, with the absorption edge systematically shifting towards shorter wavelengths from Cs₃Bi₂I₉ to Cs₃Bi₂Br₉ to Cs₃Bi₂Cl₉, corresponding to increasing band gap values. The calculated indirect band gaps were 1.96 eV (Cs₃Bi₂I₉), 2.46 eV (Cs₃Bi₂Br₉), and 3.05 eV (Cs₃Bi₂Cl₉). Under mechanical strain (±5 %), Cs₃Bi₂I₉ exhibited significant band gap tunability, increasing to 2.39 eV (+5 %) and 2.37 eV (−5 %), whereas Cs₃Bi₂Br₉ showed moderate reductions with 2.43 eV (+5 %) and 2.20 eV (-5 %), and Cs₃Bi₂Cl₉ with 3.00 eV (+5 %) and 2.98 eV (-5 %). These findings suggest that strain engineering effectively modulated the optoelectronic response, particularly in iodine-based compounds. Overall, this study provides valuable insights into the intrinsic and strain-dependent properties of Cs₃Bi₂X₉ perovskites and underscores their potential as sustainable, lead-free materials for next-generation photovoltaic and optoelectronic applications.