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7 MicroLink Devices has developed a lightweight, flexible, high-efficiency solar sheet to enable UAV applications calling for extended flight endurance, the company’s director of solar sheet products, Ray Chan, has reported. MicroLink manufactures the individual solar cells that are incorporated into the solar sheet using a proprietary epitaxial lift-off process resulting in cells that operate at greater than 30% AM1.5 efficiency at room temperature. The efficiency rises further at low temperatures, unlike some solar cell technologies which struggle to operate efficiently under similar conditions, Chan noted. Completed solar sheets can be placed over curved surfaces such as aircraft wings and can generate more than 250 W/m 2 and over 500 W/kg under AM1.5 illumination. The cells are produced using metal organic chemical vapour deposition. A release layer is first deposited directly on top of a gallium arsenide (GaAs) substrate, and multi-junction solar cells are then deposited on the release layer. A thin, flexible metal backing layer is then applied to the solar cell structure to provide mechanical support for the solar cell after epitaxial lift-off, which allows the solar cell structure to be removed from the GaAs substrate without damage. Conventional solar cell fabrication is then performed to create the multi-junction solar cells which generate the power. This approach brings significant cost reductions, Chan reported, since the GaAs substrate can be re-used to grow other solar cells. MicroLink’s solar sheet technology has been applied to the Raven and Puma small, electric-powered UAVs made by AeroVironment. Using a lithium-ion battery the Raven has a range of up to 6.2 miles and can reach speeds of up to 60 mph with a flight time of an hour, while the larger Puma has a range of 9.0 miles and an approximate endurance of two hours. Chan remarked that in collaboration with the US Air Force Research Lab’s Materials and Manufacturing Directorate, MicroLink’s solar sheets were integrated with the existing power systems of both UAV platforms and demonstrated a doubling of flight endurance. MicroLink has also been working with Imperial College London, Rochester Institute of Technology, and the US Naval Research Laboratory on a new triple-junction solar cell that could provide an unprecedented 50% conversion efficiency. At present, the world record for triple-junction solar cell efficiency is 44%, and Chan added that it is generally accepted that a major technology breakthrough will be required for the efficiency of these cells to increase much further. The three-year programme is to develop multi-junction solar cells where each junction is ‘tuned’ to different wavelength bands in the solar spectrum to increase efficiency. High- bandgap semiconductor material is used to absorb the short-wavelength radiation, with longer wavelength parts being transmitted to subsequent semiconductors. The group is using a combination of indium, aluminium, arsenic and antimony as a high-bandgap material layer that can be grown and matched to a lattice of indium phosphide. As a result, the team has created a solar cell design that is a potential route to 50%-plus power conversion efficiency under concentrated solar illumination. Platform one New-generation solar cells from MicroLink Devices are extending UAV flight times Cool approach to power Solar energy Unmanned Systems Technology | Spring 2015 The cells operate at more than 30% efficiency, which rises further at low temperatures, unlike some cell technologies that struggle under such conditions