First-principles investigation of Na2TlSbY6 (Y=Cl, Br) compounds for high-performance photovoltaic applications

초록

The double perovskite halides Na2TlSbY6 (Y=Cl, Br) have emerged as promising materials for optoelectronic and photovoltaic applications owing to their high structural symmetry, thermodynamic stability and environmentally benign elemental composition. These compounds crystallize in a face-centered cubic (FCC) structure with the space group Fm-3m, where Tl and Sb cations are ordered in a rock-salt-type arrangement within a BX6 octahedral framework, forming a three-dimensional network of corner-sharing TlY6 and SbY6 octahedral stabilized by Na+ interstitials. Density functional theory (DFT) calculations show that the optimized lattice constants for Na2TlSbCl6 and Na2TlSbBr6 are 10.628 & Aring; and 11.012 & Aring;, respectively, with the larger lattice constant for the bromide variant attributed to the larger ionic radius of Br-. The compounds exhibit negative formation energies (-3.54 eV for Na2TlSbCl6 and -3.21 eV for Na2TlSbBr6), confirming their thermodynamic stability. Mechanical properties indicate that Na2TlSbBr6 is slightly stiffer, with higher bulk and shear moduli compared to Na2TlSbCl6. The calculated band gaps of 2.71eV for Na2TlSbCl6 and 2.43eV for Na2TlSbBr6 suggest that the bromide compound has extended absorption into the visible range, making it more suitable for solar energy applications. The compounds exhibit indirect band gaps, with a favorable electronic structure that suggests high carrier mobility, supported by low effective masses of electrons and holes. Optical properties reveal strong dielectric response and refractive index behavior, with both materials showing potential for applications in photodetectors, light-emitting diodes and solar cells. These findings highlight the potential of Na2TlSbY6 halide perovskites as environmentally friendly and efficient materials for next-generation optoelectronic and photovoltaic technologies.

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