New lead-free double perovskites X₂GeI₆ (X = K, Rb, Cs) for solar cells, and renewable energy as an alternate of hybrid perovskites

Recently, the unique double perovskites are continuously being used for solar cells and thermoelectric applications because of stable structures and high energy conversion efficiency. We addressed the optical and transport characteristics of X₂GeI₆ (X = K, Rb, Cs) by first-principle calculations. The tolerance factor confirms the structural stability. The computed band gaps 1.27 eV (K₂GeI₆), 1.24 eV (Rb₂GeI₆), and 1.13 eV (Cs₂GeI₆) plunge the maximum absorption in visible light zone. The first absorption bands 459 to 826 nm for K₂GeI₆, 477 to 885 nm for Rb₂GeI₆, and 653 to 992 nm for Cs₂GeI₆ are outstanding for visible light solar cells. While the second absorption bands are for other optoelectronic devices like surgical equipment. The optical characteristics are debated by frequency-dependent dielectric constants (ε₁, ε₂), refractive index, and absorption coefficient. Thermoelectric performance is evaluated by large values of power factor and ultralow values of thermal conductivity for both n- and p-type of charge carriers. Highlights: An alternative of organic-inorganic and Pb based perovskites Ideal band gaps 1.27 eV, 1.24 eV, and 1.13 eV of K₂GeI₆, Rb₂GeI₆, and Cs₂GeI₆ for solar cells Absorption in the visible region increases their potential for solar cells. Observation of ultralow lattice thermal conductivity.