Six new mixed-ligand bimetallic complexes [Eu2(bz)4(tta)2(phen)2] (1), [Gd2(bz)4(tta)2(phen)2] (2), [EuTb(bz)4(tta)2(phen)2] (3), [EuGd(bz)4(tta)2(phen)2] (4), [Eu1,2Gd0,8(bz)4(tta)2(phen)2] (5) and [Eu1,6Gd0,4(bz)4(tta)2(phen)2] (6) have been prepared with the Eu3+, Gd3+ and Tb3+ ions and the benzoate (bz-), 2-thenoyltrifluoroacetonate (tta-) and the 1,10-phenanthroline (phen) ligands. The compounds combine highly efficient antennas to obtain highly luminescent complexes to enhance solar cell efficiency. The benzoate ligand has been chosen to take its advantage as a bridging ligand to end up with bimetallic complexes to study the effect of combining two metal ions in the luminescent molecule. The structure of 1 was obtained by single-crystal X-ray diffraction, and 1-6 were found to be isostructural by powder X-ray diffraction analysis. The photophysical properties were studied by the absorbance and emission spectra and emission lifetimes. The magnetic properties of 2 were studied, and we found intramolecular antiferromagnetic interactions between the Gd3+ ions. We prepared luminescent down-shifting layers (LDSL) with the 1, 3-6 complexes embedded in ethylene-vinyl-acetate and studied their effect in the external quantum efficiency (EQE) and intensity-voltage (I-V) plots of a solar mini-module. We found that LDSL containing the bimetallic complexes 3 and 6 enhance the efficiency of the solar mini-module from 11.26(3)% to 11.76(4)% (+0.52%) and to 11.44(2)% (+0.21%), respectively.