40% Efficiency Concentrator Space Solar Cell

Currently, tandem solar cells are used for high-performance space power generation and achieve conversion efficiencies approaching 25%. Relative to single-cell technology, the higher production costs of tandem cells are more than offset by their higher efficiency, and consequential reduction in array size and weight. The traditional tandem cell design is far from optimal, however. The bandgaps for the top and bottom cells are chosen to avoid growing lattice-mismatched epitaxial layers, instead of for achieving ultimate conversion efficiencies. We have demonstrated a buffer-layer scheme that enables InGaAs mismatched layers to be grown on GaAs without electrical degradation. By so doing, we can for the first time, grow dislocation-free tandem cells with optimized bandgaps. We propose the development of an ultra-high efficiency triple-junction concentrator cell. The structure of this new tandem solar cell will be: InAlP top cell, InGaP middle cell, and InGaAs bottom cell. It will be a monolithic, two terminal device fabricated on an inexpensive and rugged Ge substrate. The AMO efficiency of this cell under 10-100X ratios is projected to be approximately 40%. Because the growth and fabrication procedures for this cell will be identical to the commercially available InGaP2/GaAs-on-Ge tandem cells, it will be a commercially viable product.