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40 atmosphere in space) from the bare cell, and a minimum of 1300 W/kg once the cell is encapsulated and packaged. A metal organic vapour deposition reactor, similar to those for manufacturing consumer device components such as LEDs for computer monitors and power amplifier chips for mobile phones, is used to grow each layer of junction material one by one, atop a substrate material such as GaAs. Depending on the exact use-case, these materials can include indium gallium arsenide (InGaAs), indium gallium phosphide (InGaP) or other materials that are not lattice-matched to the GaAs substrate. Once the cell material has been grown, a backing metal is applied over it for structural integrity. The product is then peeled off, enabling the substrate to be reused to lower manufacturing costs. Anti- reflective coatings, busbars and so on can then be applied to the cells. Another triple-junction product (aimed at space vehicles) uses Ge for both its substrate and bottom solar layers, with InGaAs as its middle layer and InGaP on top. It has a minimum average conversion efficiency of 29.5%. While multi-junction cells grown on Ge are less flexible than those grown on GaAs, and potentially more expensive as Ge substrates cannot be reused (as a consequence of how they are manufactured), these are less of a concern for the satellite market where aerodynamic shapes are generally not vital. Conversion efficiency and longevity are the most critical factors for solar vehicles operating in the Earth’s orbit, say, where technicians will probably never visit or repair them. Also, the bespoke nature of much of the demand for multi-junction cells is such that their suppliers have comprehensive in-house data and software for modelling different types of cells and combinations thereof for performance metrics across different spectral, thermal and other conditions. And by simulating the thermomechanical aspects of solar module construction, system designs can be optimised to reduce manufacturing costs or increase specific power. Improvements in CAD simulations can also enable developers to model how to integrate multi-junction PV panels more easily into complex carbon or fibreglass composite structures, as well as into unconventional spacecraft and aircraft architectures. Future multi-junction solar panels are expected to continue advancing ahead of other materials in terms of power-to- weight and power-to-area ratios, with conversion efficiency targets upwards of 42% anticipated in the next few years. Perovskite and organic PV cells For space users from low-Earth orbit to deep space, perovskite and organic solar cells are expected to become highly viable alternatives to thin-film cell architectures. The former are constructed primarily using hybrid perovskite structure compounds such as methylammonium lead trihalide, one of the most widely studied examples for solar applications, as the solar absorber. The latter, as the name suggests, are based on organic polymers. Collectively, hybrid perovskite and organic photovoltaic cells (HOPVs) are also generally regarded as the fastest- advancing group of solar technologies. The approximate solar efficiencies of perovskites have improved from around 3-4% in 2009 to 25-29% now, while organic cells are reaching 17-20%. More important though, HOPVs can be manufactured with greater simplicity and at lower cost than even silicon modules, with vapour deposition and inkjet printing processes (as well as other related methods) showing a lot of promise for their cost-effective bulk production in the future. These cells are being investigated October/November 2020 | Unmanned Systems Technology Thin-film monocrystalline silicon solar cells such as the Solbian SX panels on XOcean’s XO-450 USV must be robustly encapsulated against moisture ingress (Courtesy of XOcean) Solar cells with three or more junction material layers are being used increasingly in spacecraft and HALE UAVs to capture a broader spectrum of sunlight (Courtesy of SolAero Technologies)