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37 Additional upper layers include sheets of anti-reflective and transparent conductive materials. Protective materials can vary as widely as the active solar materials, but plastics such as ETFE and FEP are commonly used, while transparent polymers such as polybutylene terephthalate or ethylene-vinyl acetate provide high resistance to scratching, UV radiation and other sources of degradation. Beneath these will be layers of active solar material – either single- or multi- junction cells. Single-junction cells consist of a single pn-junction created via doping to separate the electrons and holes produced by the absorption of light in the semiconductor materials, to create voltage and current at the module output. Additional layers and junctions typically enable better absorption of light across a wider part of the solar spectrum, with the upper solar material optimised for light at the UV end of the spectrum, and lower layers oriented towards the IR end. At the bottom typically sits a substrate layer, upon which the solar cell will have been fabricated (especially in the case of thin-film cells), with the fabrication approach varying according to the material and architecture. Notably, heavy protective layers such as the tempered glass and aluminium used in stationary solar installations and some spacecraft PV modules are absent in the vast majority of products aimed at the unmanned market, such is the primacy of power-to-weight optimisation in all things unmanned. Exceptions here of course are charging stations for ground and airborne vehicles, and automated battery swapping, where it is vital to protect the modules from dust and adverse weather. Silicon cells: low-cost solar Crystalline silicon (c-Si), particularly its polycrystalline variant, remains the most widely used PV material. Its popularity comes largely from having the highest power-to-cost ratio of any solar material (with that figure improving all the time), and the knowledge that supply chains of PV silicon are secure and widespread, even for unmanned systems needing highly customised modules. Polycrystalline Si is also easy to work with, and is cost-effective relative to other solar materials, but it lags behind in conversion efficiency (around 22% for the highest recorded efficiency, and around 18% at best for high-volume, low- cost production modules), flexibility and power-to-weight ratio. Monocrystalline Si is more expensive but has achieved roughly 24.5% conversion efficiency in some Solar cells | Focus Unmanned Systems Technology | October/November 2020 BAE Systems’ PHASA-35 UAV wing, with MicroLink Devices’ ELO IMM flexible solar arrays installed (Courtesy of Prismatic)