Unmanned Systems Technology 020 | Alpha 800 I Additive Manufacturing focus I USVs insight I Pegasus GE70 I GuardBot I AUVSI Xponential 2018 show report I Solar Power focus I CUAV Expo Europe 2018 show report

86 Focus | Solar power absorption properties, making them ideal for installation on aircraft wings and fuselages. They are made from materials such as gallium arsenide (GaAs) and other III-V semiconductors, so-called because they are compounds of elements from the third and fifth columns of the periodic table. Materials from these columns tend to crystallise well with each other, in a similar way to how silicon atoms bond with each other to make a crystal structure. They can achieve far higher solar energy conversion efficiencies than silicon. The present world record of 28.8% efficiency for single-junction (single-layer) solar cells was achieved by a GaAs module. Thin-film cells for UAVs can also be made from materials such as copper indium gallium selenide (CIGS), a semiconductor with conversion efficiencies of 13-16% in commercial form. While lower than those of GaAs, which tend to exceed 23%, CIGS products can still be produced as extremely thin and lightweight cells – down to 0.052 mm. That means they can be well-suited where UAV weight is constrained. Beyond single-junction cells, unmanned systems designers can also opt for a multi-junction solar cell, as having multiple layers gives the ability to capture light across multiple wavelengths. Single-junction semiconductors will lose solar energy outside of the wavelength corresponding to their material properties, and cannot achieve the efficiencies of multi-junction UAV solar cells, which can capture 30- 32% of solar energy and provide more than 3 kW of power per kilogramme. Multi-junction materials often use GaAs among their layers, combined with complementary materials such as indium gallium arsenide (InGaAs), germanium (Ge) or indium gallium phosphide (InGaP). The high efficiency and coverage of dual-junction, triple-junction and even quad-junction cells can make them ideal for air and space vehicles in particularly demanding applications. However, the relative difficulty of manufacturing multi- junction cells compared with single- junction PV systems means they tend to be produced in only small volumes, and at high cost, for projects that can bear the expense in exchange for the wide photon energy spectrum captured. That may be justified for UAVs operating at low altitudes or for space operations as high as geosynchronous orbit, where the thicknesses of triple- junction solar wafers are reported to be as thin as 5 µm. For UAVs designed to be produced in high volumes though, thin-film cells might present a more cost- effective and time-saving option. Manufacturing While there is a range of techniques for producing PV modules, they all tend to share some common approaches. Often the process begins with a semiconductor wafer, which acts as a substrate for the layer to be grown on top of it. Single-junction GaAs cells will typically use a GaAs substrate, while multi-junction UAV cells might be grown on GaAs or Ge wafers, for example. A common initial process used is MOCVD (metal organic chemical vapour deposition), which essentially means generating very thin layers on top of the substrate wafer using reactant gases consisting of atoms of selected materials. The deposition depends on chemical reactions caused by the combination of those gases at elevated temperatures in an MOCVD reactor. The reactor must be able to withstand and contain the high temperatures required for the process, while also not reacting to the gases themselves. Quartz or June/July 2018 | Unmanned Systems Technology Multi-junction solar cells can be produced as thin as 5 µm, and absorb light from multiple wavelengths (Courtesy of MicroLink Devices) Single-junction UAV cells cannot achieve the efficiencies of multi-junction cells, which can capture 30-32% of solar energy