Inorganic double perovskites have garnered significant attention due to their desirable characteristics, such as low-toxicity, stability and long charge-carrier lifetimes. However, most double perovskites, especially Cs2AgBiBr6, have wide bandgaps, which limits power conversion efficiencies. In this work, through the first principles method corrected by self-energy, we investigate the mechanical, electric and optical properties of Cs2B'B''Br6 (B' = Ag, Au, Cu; B'' = Bi, Al, Sb, In). Based on performance screening, three kinds of materials with good toughness, high carrier mobility and wide visible-light absorption (around 105 cm-1) are obtained, which are compared with Cs2AgBiBr6. Meanwhile, we use a SACPS-1D simulation to design lead-free double perovskites with excellent properties suitable for photovoltaic solar cell devices, which are made into a planar perovskite heterojunction. Ultimately, the optimal structure is determined to be FTO/WS2/Cs2CuBiBr6/spiro-OMeTAD/Ag, which achieves a power conversion efficiency of 14.08%, surpassing the conventional structure efficiency of 6.1%. It provides valuable guidance for the structure design of a lead-free double perovskite device and offers new insights into the development of optoelectronic devices for solar energy utilization.