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 (epsilon(1), epsilon(2)), 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.