Polyimide substrate for thin multijunction solar cell

This SBIR project concerns the fabrication of thin high efficiency multijunction solar cells starting from lattice mismatched GaInP/GaInAs/Ge cells that are grown epitaxially inverted on germanium wafers. These 3-junction cells have achieved efficiencies up to 40.7% recently. The germanium wafers with the epi-layers are bonded to polyimide substrates and thinned by grinding and chemical etching. Different bonding techniques will be investigated including adhesiveless fusion, liquid based adhesives and dry film lamination. The sequence of thinning will be determined. The strength of the bond will be characterized. The thin epi-layer is transferred to the polyimide wafer. The polyimide substrate serves as a permanent lightweight and flexible carrier for the thin multijunction solar cell for space applications. The polyimide substrate has the advantage of thermal dimensional stability at high temperatures. It can support the thin epi-layer during subsequent front side processing. There is possibility of saving the expensive Ge wafer for reuse. The Phase I project will focus on optimizing the bonding between the III-V materials and the polyimide and on thinning the Ge and polyimide wafers to achieve specific power ratio of 1000 W/kg at the array level. BENEFIT: This project concerns flexible multijunction solar cells for military and commercial space applications. The ultimate customers are the satellite prime contractors and service operators who are very interested in high specific power, high efficiency solar arrays that can be stowed in small volume. In space on military and commercial satellites volume and payload are at a premium. The polyimide can meet the target specific power of 1000 W/kg at the array level cost effectively. Satellite manufacturers will be motivated to take advantage of the lower mass in order to utilize smaller less expensive launchers. Also, a smaller array stowage volume and mass means that the satellite payloads size and capability can be increased, which will result in higher revenues for the satellite service operators. This technology will enable solar arrays capable of providing 200 kW, which are contemplated for a large US government military program. Also this technology will be useful for high altitude airships.