Measurement of an InGaAsP/InGaAs tandem solar cell under GaAs

We have developed a low band gap tandem (two-junction) solar cell lattice-matched to InP, which is designed to work under a InGaP/GaAs tandem in a four-junction configuration. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new tunnel junction was used to connect the subcells, including thin layers of n-type InGaAs and p-type GaAsSb. The delicate critical interfaces were prepared employing metal organic vapor phase epitaxy (MOVPE) and were monitored with optical in-situ spectroscopy (reflectance anisotropy spectroscopy, RAS). After a contamination-free transfer, the in-situ signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) 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.