Understanding of the Band Gap Transition in Cs₃Sb₂Cl_9–xBr_x: Anion Site Preference-Induced Structural Distortion

Cs3Sb2X9 (X = Cl, Br, and I) perovskites containing less toxic elements, known as two-dimensional (2D) materials, generate enormous research interest due to their inherent photovoltaic properties. Tuning the band gap and understanding the change in the band type in these materials are essential for practical applications in photovoltaics. In this article, we have studied an indirect to direct band gap transition in Cs3Sb2Cl9–xBrx with Br substitution, and a possible explanation is provided from both experimental and theoretical studies. Incorporation of Br in Cs3Sb2Cl9 is confirmed from powder X-ray diffraction, scanning electron microscopy–energy dispersive X-ray spectroscopy, and Raman studies. Rietveld refinement of powder X-ray diffraction data revealed that Br prefers the terminal position over the bridging position with initial substitution and induces a distortion in the Sb(Cl/Br)6 polyhedra. Further higher substitution of Br results in occupation of both terminal and bridging positions. Optical study shows that trigonal Cs3Sb2Cl9 has an indirect band gap of 2.88 eV, while the Br analogue, Cs3Sb2Br9, has a direct band gap of 2.43 eV. Theoretical study also confirms that Cs3Sb2Cl9 is an indirect band gap material, which undergoes a transition to a direct band gap type with minimal (two moles) substitution of Br in Cs3Sb2Cl9–xBrx. However, in these compounds, it is observed that with Br substitution, the valence band maximum remains unaltered, whereas the conduction band minimum changes from the A-point to the Γ-point. Analysis of the density of states of the halide and Sb revealed that the conduction band is contributed from Sb p, halide p (terminal), and halide s (bridging) states. The splitting of p-states of halides and Sb just above the Fermi level induced by the change in the terminal Cl/Br–Sb–Cl/Br bond angle is observed to be the primary reason for the transition of the band from an indirect to direct type with Br substitution. Understanding of the underlying relationship among the structural distortion, electronic properties, and band gap tuning will help in designing suitable materials with desired optoelectronic properties.