Strain-Balanced Quantum-Well Superlattice for Efficiency Enhancement of Multijunction Solar Cells(<Special Issue>Novel Solar Cells Utilizing Nano- and Hetero-Structure)

Abstract

This paper will summarize our approach toward efficiency enhancement of multijunction cells 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 (&le;3 nm) barriers for tunneling-assisted carrier transport and a stepwise potential to assist thermionic carrier escape. In spite of its small thickness, 100 stacks of InGaAs wells in GaAs pin junction evidenced up to 80% internal quantum efficiency in the sub-bandgap region of GaAs. The growth of such a structure necessitates elaborate control of strain accumulation, for which in situ monitoring of wafer curvature proved to be quite effective. The heterointerfaces between compressive InGaAs and tensile GaAsP imposes difficulty in keeping surface morphology up to >100 stacks. The interlayer should be inserted at such interfaces to mitigate straininduced crystal imperfection, as evidenced by in situ surface reflectance monitoring.