Low-Cost, Manufacturable, 6-Inch Wafer Bonding Process for Next-Generation 5-Junction IMM+Ge Photovoltaic Devices

We propose the development of a 6-inch wafer bonding process to allow bonding of a multi-junction inverted metamorphic (IMM) tandem solar cell structure to an upright single-junction Ge solar cell. The process will use SU8 epoxy to provide the mechanical connection between the substrates with an embedded metallic grid to provide electrical conductivity across the bonded interface. This process will enable the manufacture of next-generation, five-junction cells with the potential to achieve efficiency >37% under AM0 illumination in high-volume production. The proposed process will be designed for 6-inch substrates and will be low-cost, scalable, and high-yield. Compared to conventional wafer fusion approaches, this technique will be much more tolerant of the rough surfaces typical of metamorphic materials and eliminate the need for any pre-bonding surface polishing or preparation.

We anticipate that the proposed effort will lead to devices with AM0 efficiency >37%. These high-efficiency, multi-junction cells will be applicable to a wide variety of space platforms, including high-power applications such as solar electric propulsion (SEP). They will also be suitable as a replacement for the existing type of solar cells. The cells will be useful for powering high-altitude, long-endurance unmanned aerial vehicles (UAVs).

The high-efficiency, five-junction cells resulting from the proposed effort will be useful in a wide variety of applications. The cells five-junction will be suitable as a replacement for the types of solar cells that are currently used on military and civilian platforms. Other applications that would benefit from increased efficiency solar cells include high-altitude, long-endurance (HALE) solar-powered aircraft, such as the DARPA Vulture, or the Astrium Zephyr. Similarly, this type of solar cell could be used as an additional source of power to increase the endurance of short-range unmanned aerial vehicles (UAVs), such as the Raven or Puma. Another potential market for high-efficiency solar cells is terrestrial energy production. The five-junction cells are also suitable for use in terrestrial applications, such as solar sheets for power generation off the electrical grid. There is a current demand by the US military for such energy harvesting technology. There exist corresponding civilian applications in the camping and outdoor market. Similarly, this type of solar cell could be used for recharging electronic devices such as mobile phones or computers.

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