Strain-balanced Quantum Wells as an Efficiency Booster for III-V Solar Cells

This paper will summarize our approach toward efficiency enhancement using InGaAs/GaAsP quantum wells with both the quasi-lattice-match relationship with GaAs host and the extended absorption edge to a longer wavelength than the value of GaAs. The optimized structure for efficient collection of photo-generated carriers in the InGaAs wells employed both thin (<= 3 nm) barriers for tunneling-assisted carrier transport and a stepwise potential to assist thermionic carrier escape. The growth of such a structure on a 6 degrees-off substrate, which is an industrial standard for a multi-junction cell on Ge, necessitated elaborate low-temperature condition as optimized with in situ monitoring. The drop in open-circuit voltage from the effective bandgap energy was smaller for quantum well cells than a GaAs bulk cell even when the absorption-edge extension was large. The drop got even smaller under sunlight concentration, although further confirmation is necessary.