The development of multi-junction solar cells that exhibit a broad absorption band is crucial in order to harvest sunlight efficiently. Although the progress of the lead halide perovskite solar cells is remarkable, and high conversion efficiency of more than 20% has been reached, their absorption limit on the long wavelength side is around 800 nm. To further enhance the conversion efficiency of perovskite-based photovoltaics, a multi-junction solar cell with near-infrared (NIR) photovoltaics is a useful approach. We have carried out several studies regarding NIR sensitizer for dye-sensitized solar cells (DSSCs), such as ruthenium sensitizer DX1 and DX3 , which exhibits the spin-forbidden singlet-to-triplet direct transition in NIR region. Here, we report photophysical properties of phosphine-coordinated ruthenium sensitizers (DX1, DX2, DX3) and photoelectric characteristics of multi-junction solar cells with these dyes. As a result of the optimization of the ligand field strength of the phosphine ligand on the dyes, absorption spectrum of the sensitizers shifted to the longer wavelength. The DSSC employing DX1-DX3 exhibited a wideband photoresponse up to 1100nm and they showed high durability over 1000h under ambient light. Theses results are suitable for multi-junction solar cells. We developed a 4-junctionspectral splitting tandem DSSC achieved energy conversion efficiency exceeding 15% without perovskite cells. Using the DX3-based dye-sensitized solar cell in conjunction with a perovskite cell that harvests visible light, the multi-junction solar cell achieved a conversion efficiency of 21.5% by using a system of spectral splitting.