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

Abstract

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–x Br x 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–x Br x . 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.

Band Gap Tuning Through Cation and Halide Alloying in Mechanochemical Synthesized Cs3(Sb1-xBix)2Br9 and Cs3Sb2(I1-xBrx)9 Solid Solutions

“…The values of direct and indirect band gaps for Cs3Sb2Br9 and Cs3Bi2Br9 are in agreement with those previously reported in the current literature. 34,36,39,51 By looking at Figure 5b, a significant bowing of the band gap for mixed compositions is observed. This result is unexpected since, in general, metal ion replacement in solid solution provides a scaling of the band gap obeying the Vegard's law.…”

“…Among these systems, bismuth-and antimony-based perovskite derivatives of general formula Cs3M2X9 (M=Bi, Sb; X=Br, I) have shown a strong technological potential in particular in the area of photocatalysis (both for solar fuel generation and organic synthesis) and photodetection. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Bismuth-based defect-ordered perovskites have been object of more intense studies with respect to the corresponding Sb systems, notwithstanding their appealing optoelectronic properties such as high carrier mobility, low trap density, and long diffusion length. As a matter of fact, Cs3Sb2Br9 millimeter-sized single crystals and nanoflakes have been applied in the fabrication of photodetectors with excellent performance in terms of responsivity and detectvity.…”

Solvent-mediated crystallization of (TMS)2BiBr5·DMSO: a new 0D hybrid halide perovskite

“…Various types of perovskite structures such as three-dimensional (3D), 27,28 two-dimensional (2D), [29][30][31][32][33] one-dimensional (1D), 34,35 and zero-dimensional (0D) 36 can be realized by choosing appropriate components of the organometal halide perovskites owing to the structural tunability of these HHPs. For 0D hybrid perovskites, individual metal halide octahedra anions or metal halide clusters are completely surrounded and isolated by the organic cations.…”

DMAI-driven all-inorganic antimony-based perovskite-inspired solar cells with record open-circuit voltage

“…Cs 3 Sb 2 X 9 (X = I, Br, Cl) type perovskite-inspired compounds are another promising alternative to inorganic Pb-free perovskites. [15][16][17] Due to the different positions of the octahedron, Sb-based perovskiteinspired compounds can have either a zero-dimensional (0D) dimer or a two-dimensional (2D) layered structure, while the 2D layered structure has a smaller optical band gap and higher carrier mobility. 18 Unfortunately, Cs 3 Sb 2 I 9 lms prepared by the solution method have a 0D dimer structure, which is detrimental to their photovoltaic properties.…”