Lead halide perovskites have emerged as promising optoelectronic materials; however, concerns regarding the toxicity of lead and the instability of organic cations necessitate the development of environmentally friendly and stable alternatives. Vacancy-ordered double perovskites (VODPs) present a viable solution to this challenge. However, due to the significant computational cost, theoretical studies focusing on excitonic and polaronic properties remain underexplored for these materials, which are crucial for exploring their applications in optoelectronics. In this study, we present the electronic, optical, excitonic, and polaronic properties of a series of VODPs with the chemical formula ${\mathrm{Rb}}_2{BX}_6$ $(B=\mathrm{Si}, \mathrm{Ge}, \mathrm{Sn}, \mathrm{Pt}; X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$ using state-of-the-art first-principles calculations. Our analysis reveals that these materials exhibit intriguing electronic and optical properties. The electronic structure calculations indicate that most compounds possess a direct band gap (except ${\mathrm{Rb}}_2{\mathrm{PtBr}}_6$ and ${\mathrm{Rb}}_2{\mathrm{PtI}}_6$), and the band-gap values computed using the $G_0{W}_0$@PBE approach, span a wide range from 0.56 to 6.12 eV. Optical properties, determined via the Bethe-Salpeter equation (BSE), indicate strong infrared to ultraviolet light absorption across most systems. Additionally, the analysis of excitonic properties shows low to moderate exciton binding energies and variable exciton lifetimes, implying higher quantum yield and conversion efficiency. Furthermore, utilizing the Feynman polaron model, polaronic parameters are evaluated, which reveal high polaron mobility for electrons $(3.33-85.11{\mathrm{cm}}^2{\mathrm{V}}^{-1}{s}^{-1})$ compared to recently studied Cs-based VODP materials. Our findings suggest that these materials exhibit favorable electronic, optical, excitonic, and polaronic characteristics, positioning them as viable candidates for a wide range of next-generation optoelectronic devices.

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• Dielectric properties
• Electronic structure
• Perovskites
• First-principles calculations
• GW method