Electronic, optical and thermoelectric properties of halide double perovskites Rb₂AgInX₆ (X = Cl, Br, I)

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 Rb2AgIn(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 Rb2AgIn(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 Rb2AgIn(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 Rb2AgIn(Cl/Br/I)6. The optical bandgaps calculated using the Tauc plot were 2.59 eV, 1.7 eV, and 0.49 eV for Rb2AgIn(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 x 1020 cm-3, 1.72 x 1020 cm-3, and 8.37 x 1019 cm-3 for Rb2AgInCl6, Rb2AgInBr6 and Rb2AgInI6 respectively. Large Seebeck coefficients with high thermoelectric efficiencies (greater than unity at room temperature) were observed for these halide double perovskites compounds.