Abstract In the last decades, lead-based perovskites have been substantially investigated due to their superior efficiency in photovoltaics. However, owing to the harmful effect of lead, the search for other environmentally friendly materials is vital. Recently, halide double perovskites have gained attention due to their environmental friendliness and potential applications in solar cells and thermoelectric devices. The energy gap, optical and thermoelectric parameters of Rb 2 AgIn(Cl/Br/I) 6 halide double perovskites compounds were investigated using Wien2k and BoltzTraP2 code based on DFT. The energy gap was 2.62 eV, 1.74, and 0.56 eV for Rb 2 AgIn(Cl/Br/I) 6 at the Γ-points, which were direct bandgaps. More states were localized near the upper valence band, closer to the fermi energy, than near the lower conduction band, suggesting that Rb 2 AgIn(Cl/Br/I) 6 was p-type. The fundamental edge of absorption arises at 2.48 eV, 1.67 eV, and 0.45 eV for Rb 2 AgIn(Cl/Br/I) 6 . The optical bandgaps calculated using the Tauc plot were 2.59 eV, 1.7 eV, and 0.49 eV for Rb 2 AgIn(Cl/Br/I) 6 . The first absorption band illustrates that these halide double perovskite compounds show high α ω in the visible spectrum, with maximum absorption in the UV region. The Seebeck coefficient was positive, indicating the presence of p-type carriers. The calculated hole concentrations at room temperature were 3.08 × 10 20 cm −3 , 1.72 × 10 20 cm −3 , and 8.37 × 10 19 cm −3 for Rb 2 AgInCl 6, Rb 2 AgInBr 6 and Rb 2 AgInI 6 respectively. Large Seebeck coefficients with high thermoelectric efficiencies (greater than unity at room temperature) were observed for these halide double perovskites compounds.

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