U. Seidel

Lifetime and performance of InGaAsP and InGaAs absorbers for low bandgap tandem solar cells

Time resolved photoluminescence (TRPL) measurements were used to evaluate the lifetimes of the low bandgap absorber materials InGaAsP (1.03 eV) and InGaAs (0.73 eV) embedded between InP barriers. A low bandgap tandem solar cell based on these absorber materials has been developed. The cell is designed to work below an InGaP / GaAs high bandgap tandem solar cell. Tandem solar cells grown with these absorber materials reached efficiencies above 10% (in-house) below a 4-¿m-thick GaAs filter under 35 suns concentration.

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.