Comprehensive study of the physical and optoelectronic properties of A ₂ AgIrF ₆ (A = Cs, Rb, and K) double perovskites for energy harvesting applications: a DFT approach

In this study, we performed a comprehensive theoretical investigation of lead-free A₂AgIrF₆ (A = Cs, Rb, and K) double perovskites to examine their structural, electronic, optical, mechanical and thermodynamic properties using first-principles calculations. Known parameters, such as the Goldschmidt tolerance factor, octahedral factor and a novel tolerance factor (all indicating that these compounds are expected to form), were applied to validate the stability of the perovskite structure. Additional thermodynamic and dynamic stability checks, such as negative formation energies, positive phonon frequencies, and <i>ab initio</i> molecular dynamics (AIMD) simulations, strongly imply that these compounds can be synthesized experimentally. The band gaps derived from the Perdew-Burke-Ernzerhof Generalized Gradient Approximation (PBE-GGA) method for Cs₂AgIrF₆, Rb₂AgIrF₆, and K₂AgIrF₆ are 1.07 eV, 1.13 eV and 1.16 eV, respectively. We employed the Tran-Blaha modified Becke-Johnson (TB-mBJ) functional to adjust the band gaps. It yielded electronic band gaps of 1.65 eV, 1.76 eV and 1.83 eV for A₂AgIrF₆ (A = Cs, Rb, and K, respectively). The calculated band gaps are indirect and within the appropriate range for solar cell applications, as evidenced by density of states (DOS) studies. Optical property characterization showed that the solar cells based on all three perovskites could efficiently absorb visible light and achieve a peak absorption coefficient greater than 10⁵ cm^-1, designating them as good absorbers. Mechanical behavior is determined through the analysis of elastic constants, and the compounds are found to be stable, ductile and anisotropic. In conclusion, A₂AgIrF₆ perovskites are highly promising, environmentally friendly candidates for use in optoelectronics and solar energy technologies. This theoretical research offers a crucial foundation for future experimental work and practical development.