The device fabrication process is illustrated schematically in Figures a thru d. First, a reusable GaAs growth substrate is introduced into a metal-organic chemical vapor deposition (MOCVD) chamber. A buffer layer of GaAs material is grown, followed by a thin release layer, on top of which the PV device structure was grown. What later becomes the sunfacing side of the device is grown first, and the back side of the device is grown last (Figure a). A back contact metal is deposited and this metal-on-semiconductor stack is then attached to a flexible handle, using an adhesive (Figure b).
The handle-metal-semiconductor stack is then introduced to a heated bath of aqueous acid. The acid etches the release layer, but leaves the remainder of the device intact. The etch completes leaving a semiconductor thin-film supported by the back metal composite and flexible handle ready for device processing (Figure c).
Front metallization is deposited using a plating process. An anti-reflective coating (ARC) is applied (Figure d) and the device is separated into cells to complete the process.
By interconnecting these cells, Alta Devices can generate large area assemblies with efficiencies greater than 24% and packing densities of over 90% on most existing UAV layouts. The cell material can be directly integrated into existing layup materials, eliminating additional encapsulation materials and resulting in a minimal weight gain of ~240g/m2. Combined, these characteristics provide a power to weight ratio of roughly 1g/W under optimal illumination conditions (typically 4 to 5 times higher than alternative solutions) that can be used for preliminary energy balance calculations.
There are additional advantages to the Alta Devices technology. Due to the high voltage and small size of the cells, voltage can be built in small areas, allowing the solar system voltage to quickly reach the bus voltage of the UAV and minimize demands on voltage matching electronics. The use of shingling to electrically interconnect the cells minimizes the impact of partial shading without significant additional electrical circuitry. Finally, the GaAs material has a significantly lower thermal coefficient than silicon (0.1% degradation /°C vs 0.4%/°C for silicon), maintaining maximum output even when in extremely hot operating environments.
Solar powered UAVs promise significant advantages to the operational theater. Real world flight times can be increased from one to over six hours, providing significant increases in loiter time and range. This additional power can also be used to enable new, high-power sensors or communication abilities without degrading existing flight times. While improving ROI on existing UAVs and missions, these capability improvements can fundamentally change how UAVs are used in the field, increasing soldier safety and effectiveness.