Predictive analysis of the physical characteristics of mechanically stable Cs₂Rb B′ I₆ (B′ = Al, Ge, Tl) for photovoltaics and optoelectronic applications: a DFT study

Halide double perovskites are desirable for optoelectronics and photovoltaics because they have tunable bandgap. The physical properties of Cs₂Rb B′ I₆ (B′ = Al, Ge, Tl) are investigated through the FP-LAPW approach. Structural analysis for Cs₂Rb B′ I₆ (B′ = Al, Ge, Tl) revealed complete cubic symmetry at room temperature. Elastic constants (C_ij = C₁₁, C₁₂ and C₄₄) are evaluated to check mechanical stability. Mechanical properties revealed that Cs₂RbAlI₆ possesses high crystal rigidity and is stiffer than Cs₂RbGeI₆ and Cs₂RbTlI₆. Furthermore, Cs₂Rb B′ I₆ (B′ = Al, Ge, Tl) is brittle and anisotropic. Thermodynamically traits confirm that studied double perovskites can endure high temperatures and are efficient for industrial and commercial usage. The direct bandgaps of 2.69 eV, 3.33 eV and 1.01 eV are reported for Cs₂RbAlI₆, Cs₂RbGeI₆, and Cs₂RbTlI₆, respectively. Moreover, optical properties revealed that Cs₂RbTlI₆ and Cs₂RbAlI₆ have high visible absorption suited for solar cell applications, while Cs₂RbGeI₆ has strong UV absorption, making it promising candidates for UV-based optoelectronic devices.