Exploring the stability, optoelectronic, and thermoelectric properties of Sc-based double perovskites X2ScAgI6 (X = K, Rb, Cs) for renewable energy applications

Double perovskites have garnered momentous attention for their possible application in renewable energy sources and photovoltaic cells. This study investigates the physical, optoelectronics, and transport properties of X2ScAgI6 (X = K, Rb, and Cs) using TB-mBJ calculations based on Density Functional Theory (DFT). The stability of the compounds was investigated using the tolerance factor ( tau f), formation energy ( Delta Hf), cohesive energy (ECoh), and Born-stability criteria determined by analyzing the elastic constants. The band-gaps of the three compounds are calculated as 2.38, 2.59, and 2.57 eV. The hybridization of cationic and anionic sub-states increases the band gap when K is replaced with Rb and Cs. The origin of optical nature is described in terms of different optical parameters. The computed values of the optical conductivity (OC) for K2ScAgI6, Rb2ScAgI6, and Cs2ScAgI6, which yield optimal results, are 5568 Omega- 1cm- 1 at 9.5 eV, 5821 Omega- 1cm- 1 at 9.6 eV, and 5730 Omega- 1cm- 1 at 9.1 eV, respectively. The maximum values of alpha (w) for K2ScAgI6, Rb2ScAgI6, and Cs2ScAgI6 are 147.44 x 104 cm- 1 (9.5 eV), 152.74 x 104 cm- 1 (9.4 eV), and 146.42 x 104 cm- 1 (10.2 eV), respectively. The redshift of the absorption spectra from visible to UV-region enhances optoelectronic device potential. Calculations of spectroscopic limited maximum efficiency (SLME) (%) vs thickness (mu m) and current density vs voltage are also reported. The calculated values of the power factor (PF) at 300 K are 5.1 x 10-4 W m-1 K-2, 5.0 x 10-4 W m-1 K-2, and 5.1 x 10-4 W m-1 K-2, respectively, for K2ScAgI6, Rb2ScAgI6, and Cs2ScAgI6. The transport characteristics discussed by the high ZT-values up to 0.68, 0.67, and 0.68, respectively, are closer to unity. The high ZT-values for the compounds make it appropriate for their potential use in thermoelectric device fabrication. The prediction of the negative values and declining trend of the Gibbs energy G* (V: T, P) calculated using Gibbs software for X2ScAgI6 (X = K, Rb, and Cs) at temperature (0-1000) K and pressure (0, and 10) GPa indicates the thermodynamic stability using the Debye quasi-harmonic model.