Structural and optoelectronic properties of 2D halide perovskites Cs₂MBr₄ (M = Zn, Cd, Hg): a first principle study

Electronic band structures, density of states, optical parameters, and structure properties of 2D layered halide perovskites Cs₂MBr₄ (M = Zn, Cd, Hg) are investigated using density functional theory (DFT). The structure optimization/relaxation was executed by generalized gradient approximation (GGA) and to handle band gap dependent properties (optical and electronic) modified Becke Johnson (GGA-mBJ) exchange potential was operated. In the structure composition of Cs₂MBr₄, the metal cations (M = Zn, Cd, Hg) build an isolated tetrahedra with bromine atoms i.e. [MBr₄]²⁻. Eleven Cs atoms confine the tetrahedra and each Cs is surrounded by six tetrahedra, making an octahedral geometry [MBr₄]₆²⁻. The calculated tolerance factors are greater than unity, confirming their orthorhombic crystal symmetry because perovskites having τ > 1 and large A-site cation destroy the 3D crystal structure. The bonds between octahedral layers are open and A-site (large-sized) cations connect two adjacent octahedra through intermolecular forces to form 2D- perovskite. The understudied 2D layered halide perovskites possess direct band gap nature and their band gap lies between 2.5–3.80 eV. The valence band edge is mainly attributed to M-d, M-s, and Br-4p orbitals, whereas the conduction band is dominated by M-s and Br-4p orbitals. The optical properties tell that these 2D perovskites are excellent dielectric and are prospective materials for optoelectronic devices.