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Halide-based double perovskites have recently been promoted as high-performing semiconductors for photovoltaic and thermoelectricity applications owing to their outstanding efficiency, non-toxicity and ecological stability. In the framework of this research, we have systematically investigated the structural, mechanical, electronic, optical, and thermoelectric properties of Rb2InSb(Cl,Br)6 double halide perovskites. Based on Born stability and tolerance factor criteria, we have found that Rb2InSb(Cl,Br)6 are mechanically and structurally stable. Furthermore, we have performed a comprehensive evaluation of the electronic, optoelectronic, and thermoelectric characteristics. From the electronic band structure results, Rb2InSbCl6 and Rb2InSbBr6 exhibit direct semiconducting band gaps of 1.41 eV and 0.53 eV, respectively. The optical parameters of Rb2InSb(Cl,Br)6 reveal that our active structures have a higher dielectric constant, with maximum absorption in the visible range reaching over 5.68 = 105 cm1 and high optical conductivity (2.19 fs1 for Rb2InSbCl6 and 2.14 fs1 for Rb2InSbCl6). Moreover, the maximum limited spectroscopic efficiency reaches an impressive value of approximately 28.0% for Rb2InSbBr6 and 33.7% for Rb2InSbCl6. The thermoelectric properties were accurately calculated using the BoltzTraP simulation package. The obtained results reveal a significant electrical conductivity, a strong Seebeck coefficient (S 2756 mu VK1 at 300 K), and an average figure of merit close to one for both structures (ZT 1). Our findings suggest the versatility of these materials and could be used for a wide range of applications, including commercial solar cells and thermoelectricity.