Uncrewed Systems Technology 048 | Kodiak Driver | 5G focus | Tiburon USV | Skypersonic Skycopter and Skyrover | CES 2023 | Limbach L 2400 DX and L 550 EFG | NXInnovation NX 100 Enviro | Solar power focus | Protegimus Protection

102 Focus | Solar power Single and dual junction At the present absolute limit, the world record for solar conversion efficiency is 47.1%, which was set in 2019 by scientists using a six-junction cell at the National Renewable Energy Laboratory, in Golden, Colorado. However, given that the number of junctions directly increases the cost of solar cells, innovations in dual and single- junction cells have been key to widening accessibility to solar power across the uncrewed market. Crystalline silicon PV cells generally remain the most affordable option, by virtue of being easy to work with and mass production by several suppliers. They range in solar conversion efficiency from sub-10% in commercial products to 25-27% in laboratories, and remain popular among USV manufacturers for their greater moisture resistance than some more expensive materials based on GaAs or CdTe (cadmium tellurium). The more sensitive a cell is to moisture, the thicker and heavier its encapsulation has to be, countering any weight advantages. Also, perovskites (a range of calcium titaniumoxideminerals) are on the cusp of being used in some commercial pilot solar product runs, partly as they are the solar cell technology that is advancing the fastest. While single-junction perovskite cells exhibited only 3-4% efficiencies in 2009, now they are capable of around 26% in laboratories, and dual-junction perovskite-silicon PV cells have come very close to 30% under test conditions. They also have desirable properties in terms of light absorption, long carrier diffusion lengths, a bandgap that can be tuned, and tolerance of any defects. They are also able to match the efficiency of crystalline silicon cells while being three orders of magnitude thinner, making them particularly viable for installing on aerodynamic surfaces. Just as interesting are tests in ageing chambers indicating that perovskites could maintain their intrinsic stability for around 20 years. In essence, there is so much r&d going into perovskites that the principal challenge they pose to solar manufacturers is which of the innovations coming from laboratories they would like to commercialise first. Perovskites themselves are a very broad category of material combinations. Dozens of them are being researched – too many to be listed here – but they include methylammonium lead trihalide, formamidinium lead trihalide and germanium tin alloy-based perovskites. Typically, the structure of a perovskite cell consists of an electron transporting layer (ETL), a hole transporting layer (HTL), an anode and a cathode. Various sub-categories of the cells are defined by the ordering and architecture of these layers, as well as further variations without an ETL, without an HTL, and with the aforementioned perovskite-silicon tandem structure. In any case, nothing used in perovskites is especially scarce (apart from their trace amounts of lead), and this abundance is highly appealing from a supply chain perspective. Meanwhile, the thin, flexible, efficient nature of perovskite cells makes them increasingly promising for powering uncrewed vehicles as well as IoT and other applications; the first perovskite PV-powered building came online in Lublin, Poland, in 2021. Production processes In the uncrewed sector, there are a few solar manufacturers investing in research and machinery for integrating perovskites into their production chains. One of them is an organisation that also uses a highly flexible (physically and electrically) microgroove-based structural design that, on top of enabling an angular surface to solar cells for better absorption of light from different February/March 2023 | Uncrewed Systems Technology Productionised triple-junction solar cells made from InGaP, GaAs and InGaAs can currently achieve 30% power conversion efficiency in AM1.5 conditions (Courtesy of Microlink Devices) Crystalline silicon cells are increasingly affordable, and hence cost-effective for UGVs where maximising efficiency to weight is not required (Courtesy of Solbian)