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 (Cij = 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.