InGaP/InGaAs-on-Ge Concentrator Solar Cells for Space Power Generation

Essential Research, Inc. proposed to develop an ultra-high efficiency, dislocation-free, monolithic two-terminal, two-junction InGaP/InGaAs on Ge solar cells for space applications. The energy bandgap (Eg) values for the top and bottom cells will be 1.7 eV and 1.1 eV, respectively. These bandgaps are theoretically optimal for a two-junction tandem cell configuration. This solar cell will have a projected air-mass 1.5 (AM1.5), one-sun efficiency in excess of 36%. Under concentrated sunlight, conversion efficiency is projected to exceed 40%. A novel buffering technique will be used to grow dislocation-free InGaAs (Eg=l.l eV) layers on Ge substrates via organometallic vapor phase epitaxy (OMVPE), even though this layer will be lattice-mismatched to Ge. In Phase I, n/p InGaAs-on-Ge solar cells will be fabricated and tested. Subsequently, a passivating layer of lattice-matched InGaP (Eg=1.7 eV) will be grown on top of the InGaAs-on-Ge cell. InGaAs-on-Ge solar cells with InGaP window layers will also be fabricated and tested. Finally, the InGaAs-on-Ge cells will be mechanically stacked under commercially available InGaP2/GaAs monolithic, two-junction solar cells. This triple-junction cell configuration will be tested under AM0 and AM1.5 conditions. In Phase II, an optimized monolithic, two-terminal tandem cell will be developed. It will consist of InGaP/InGaAs on a Ge substrate with a lattice-matched AlInP front passivating window. Benefits include more cost-effective solar arrays and concentrator solar cells for space power applications. The use of Ge as a substrate substantially increases the specific power. The successful growth of a dislocation-free InGaAs layer on Ge, will also enable the development of next generation, ultra-high efficiency triple-junctions cells, and cost-effective cells for thermophotovoltaic applications.