A low band gap InP-based double junction (2 J) solar cell was designed, realized and measured. The cell is supposed to work in a 4 junction configuration below an InGaP/GaAs 2 J solar cell. For the lower and higher band gap absorbers InGaAs (Eg=0.73 eV) and InGaAsP (Eg= 1.03 eV) were utilized, respectively. These two subcells were connected via a tunnel junction including thin, highly doped n-type InGaAs and p-type GaAsSb layers. The growth of GaAsSb layers on different InGaAs surface reconstructions (4 x 3), (2 x 4) and (4 x 2) was investigated with in-situ reflectance anisotropy spectroscopy (RAS). The optical signals were benchmarked in ultrahigh vacuum (UHV) with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The influence of different preparation procedures on the InGaAs/GaAsSb heterointerface and on the cell performance was investigated. 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, i.e., (2 x 4)- or (4 x 2)-reconstructed InGaAs (100) surfaces. This was also confirmed by solar power conversion efficiency measurements of the solar cells, where slightly lower efficiencies were observed for the cells with the V-rich-prepared tunnel junction interfaces (4 x 3). (c) 2008 Published by Elsevier B.