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Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today's most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two-junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs/GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.