This Small Business Innovation Research Phase I project develops a method of lifting-off an epitaxially grown high-efficiency (> 30%) triple-junction III-V solar cell from a Ge or GaAs wafer onto a polyimide substrate. A high-coefficient of thermal expansion (CTE) polyimide wafer is used to induce a crack that propagates parallel to the surface at the epi/wafer interface, due to the mismatch between the coefficients of thermal expansion, as the wafer is cooled down below room temperature. The lift-off happens in a fraction of a second and no expensive ion implantation or slow chemical etching of a sacrificial layer is needed. The epi-layer is attached to a low-cost flexible polyimide substrate having a thickness between 25 and 100 microns which serves as the permanent carrier of the solar cell. Inverted and non-inverted cells can be lifted off using this technique. The base substrate can be reused to grow another epi-layer and the cycle repeated. For these devices, the cost of substrate materials is about 40% of the cost of the finished cell. This process will result in savings of raw materials and grinding and etching costs, up to a total savings of 30% of the cost of the cell. The broader impact/commercial potential of this project will be to develop a method to transfer epitaxially-grown device layers from semiconductor wafers to inexpensive polymeric substrates to create high-performance flexible circuits. The application of this technology goes beyond photovoltaics (PV). Space-grade as well as terrestrial high-efficiency PV cells and modules can be made lighter, flexible, and less expensive when this process is integrated into the manufacturing sequence of established suppliers. Additionally, when the thinned cell is transferred to a metallic substrate, the reduced thickness improves performance of terrestrial concentrating PV systems. Once this method is demonstrated, we will leverage our relationships with major high-efficiency solar cell manufacturers to negotiate licensing of the technology for their applications
