Full analysis for the optical properties of the Cs₂XInBr₆ (X = Cu or Ag) double perovskites using both GW and...

Exploration for renewable and environmentally friendly energy sources has become a major challenge to overcome the depletion of fossil fuels and their environmental hazards. Therefore, solar cell technology, as an alternative solution, has attracted the interest of many researchers. In the present work, the Cs₂XInBr₆ (X = Cu or Ag) compounds as lead-free halide perovskites have been studied due to their direct energy gap in the range of solar energy, thermodynamic stability, low effective mass of electrons, and high absorption coefficient. The calculated optical gap and static refractive index about 1.35 (1.51) eV and 1.47 (1.41), respectively using BS (GW) approach for the Cs₂AgInBr₆ compound were in good agreement with experimental data. The threshold absorption was estimated about 1.03 (1.22) using BS (GW) approach (which correspond to the optical gap) for the Cs₂CuInBr₆ compound. Both compounds have small (< 0.35) reflection coefficient in the infrared, visible and UV regions and high absorption coefficient (10⁵ cm^-1). In the infrared and visible regions, the absorption coefficient of the Cu-based compound is much larger than the other, therefore this material can be more useful as an absorbent layer in solar cells (SC). Due to the fact that the spectrum of sunlight that reaches the earth includes 47% infrared, 46% visible and 7% ultraviolet, the Cu-based compound is more efficient for the SCs and the Cs₂AgInBr₆ compound is more suitable in the design of detectors.

The electronic, structural and optical properties of Cs₂XInBr₆ (X = Cu or Ag) compounds have been calculated and analyzed using the Abinit computational package based on density functional theory (DFT). The ultrasoft pseudopotentials within the framework of the generalized gradient approximation (GGA) are adopted for the electron exchange-correlation potential and are employed for the calculations of the electronic, and optical properties as well. The wave functions have expanded on a plane-wave basis set to cutoff energy of 950 eV and 64 k-points with a k-mesh of 4 × 4 × 4 are considered for the integrations over the Brillouin zone. The behavior of the real and imaginary parts of the dielectric function and other optical quantities have been simulated using both RPA-GW (random phase approximation with GW energies) and Bethe-Salpeter formalisms.