InAs quantum dot development for enhanced InGaAs space solar cells

The metamorphic or lattice mis-matched triple junction cell under development by ERI and its partners has an InGaAs junction (bottom cell) of the three-cell stack. This junction is the current limiting, and therefore efficiency limiting, junction due to current matching which must be maintained through the device. This situation is further exacerbated when these devices are used in space, due to the bottom junction being the most affected by radiation degradation. This limitation may be addressed through the incorporation of InAs quantum dot array into the depletion region of an InGaAs cell. The InAs quantum dots in the InGaAs cell will provide sub-gap absorption and thus improve its short circuit current. This cell could then be integrated into the three-cell stack to achieve a space solar cell whose efficiency exceeds current state-of-the-art standards. A theoretical estimate predicts that a InGaAlP(1.95 eV)/InGaAsP(1.35 eV)/InGaAs(1.2 eV) triple junction cell with an improved bottom cell current could have an efficiency exceeding 40%. In addition, there is now a growing body of work that theoretically and experimentally indicates that the use of quantum dot structures may also hold ancillary benefits such as improved temperature coefficients and better radiation tolerance. These benefits are extremely important considering the intended space utilization of these devices. In this study, InAs quantum dots have been grown on lattice-mismatched InGaAs (1.2 eV bandgap) grown epitaxially on GaAs by metalorganic chemical vapor deposition (MOCVD) and characterized via photoluminescence (PL) and atomic force microscopy (AFM). Arrays of these InAs Quantum dots have been incorporated into prototype InGaAs devices. The photovoltaic efficiency under simulated 1 sun intensity and air mass zero (AM0) illumination was measured. The spectral response demonstrated that sub-gap photoconversion in InGaAs cells is possible through the incorporation of the InAs quantum dots.