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38 Focus | Solar cells modules. That is a critical improvement for long-term power supply, as well as it being generally thinner and lighter than poly-c-Si for the same power output, making mono-c-Si widely preferred over poly-c-Si as a single-junction PV material for UAVs and some spacecraft. However, as other materials such as gallium arsenide (GaAs) and multi- junction PVs offer higher efficiencies, suppliers of silicon-based solar cells have introduced features to give their modules other advantages. These include using advanced polymer encapsulants, which will add weight but enhance protection against mechanical, thermal and moisture-related degradation to enhance the longevity of their solar modules beyond 10 or even 20 years. Lifespan has become a vital metric for solar module performance, as electric powertrain designers in aerospace and automotive engineering increasingly aim to get at least 10 years of desirable performance from each battery pack – that is, to have them provide 10 years of operation without their storage capacity falling to 80% of their factory value. Accordingly they will expect at least the same lifespan from their solar panels in harsh environments, and potentially 20 or more years from solar cells in more benign applications. High-quality encapsulants are particularly critical for USVs, as thin-film solar cells are highly sensitive to moisture. While all unmanned vehicles therefore need anti-moisture protection on their solar modules, constant exposure to seawater and rain means using more advanced or larger quantities of protective polymers on seagoing vehicles. Using steel or aluminium foils as backing materials also goes a long way towards strengthening PV modules, although these will of course add weight and expense. Vapour deposition of polymers is increasingly used on some monocrystalline silicon modules (in combination with ion beams for sputter- cleaning the substrate surfaces) to apply protective coatings with a high-quality distribution and finish of the material, while retaining high transparency and longevity for solar-powered USVs and other unmanned vehicle types. Multi-junction cells: high-power solar GaAs remains the record-holder for conversion efficiency among single- junction cell materials, being capable of reaching around 29% conversion efficiency in lab conditions and typically providing higher than 20% in most environmental conditions. Indeed, direct bandgap materials such as GaAs and copper indium gallium selenide naturally provide higher carrier mobility and stronger light absorption than indirect bandgap materials such as silicon, albeit at higher prices. GaAs is also far more UV-resistant, thermally stable and thinner than c-Si cells (and often more flexible), giving it great applicability in space, aviation and automotive systems. However, the benefits of GaAs are most often realised these days in multi-junction solar cells, as part of a module with two or more layers of active solar material, each one being chosen to capture light from a different portion of the solar spectrum. Multi-junction panels (along with c-Si panels) have taken the lead in PV systems for unmanned vehicles in air and space operations. While c-Si generally leads in terms of power-to- price ratio, multi-junction cells provide the highest power-to-weight and conversion efficiencies for applications where consistently having the highest possible energy – upwards of 30% conversion efficiency – is essential. The past two years have seen production quantities of multi-junction panels increase greatly. While they are still not manufactured at the scale of c-Si cells, that can be attributed partly to the more bespoke requirements of multi- junction end-users. Many varieties are now available for different vehicles, satellites and pseudosatellite-type vehicles, although putting exact figures on efficiency varies with the active solar materials incorporated. As a general rule, the multi-junction cells manufactured for unmanned systems routinely achieve conversion efficiencies of 30-35%. Most are built as triple-junction products, with the active material layers fabricated atop a semiconductor substrate. The substrate is most often GaAs or germanium (Ge), although indium gallium nitride and indium phosphide are also used. As an example, one of the most popular cells in aerospace applications – used in Airbus’ Zephyr UAS and BAE’s PHASA-35 – is an inverted metamorphic multi-junction type, which is made by depositing thin layers of semiconductor onto a GaAs substrate. The technology provides a minimum of 3200 W/kg specific power at AM0 (the sunlight intensity above the October/November 2020 | Unmanned Systems Technology Solar charging systems for self-driving cars and UAVs can be programmed to turn and face the sun, to provide grid-independent power all year round (Courtesy of Envision Solar)