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

88 Focus | Solar power stainless steel are typically used for the reactor walls, ensuring safe conditions for crystalline growth, or ‘epitaxy’ as the process is known. In the case of single-junction GaAs systems suitable for UAVs, the MOCVD process might first involve setting down a ‘release layer’ (also known as a sacrificial layer) before epitaxy of the desired compound semiconductor takes place. This can come as another III-V compound material on top of the GaAs substrate, such as aluminium arsenide (AlAs). After this layer is formed, the GaAs intended for use in the solar product can be deposited on top of it. For a multi-junction cell, the deposition process might also make use of metamorphic buffer layers. These help to enable the growth of junction semiconductors with desirable band gaps for energy conversion but that are not lattice-matched to the substrate being used – for example, between an InGaAs junction and a GaAs substrate. This kind of solar cell might also incorporate a release layer into its semiconductor stack, before going through the wafer fabrication process, during which a back-metal would be applied to the exposed surface of the cell’s structure. If a sacrificial layer has been deposited in the course of manufacturing thus far, then after the successful formation of the junction(s) of the intended PV cell, the manufacturer might then use a process known as ‘epitaxial lift-off’ (ELO). This involves ‘etching’ away the release layer in order to peel off the thin semiconductor material from the substrate while leaving the substrate intact. That means the substrate can be re- used multiple times, rather than having to expend an entire wafer each time. As a result, the cost of producing solar cells is far lower when a release layer and ELO are incorporated into the manufacturing process. Crucially, it also ensures that solar materials can be grown with markedly different properties to the substrate, otherwise the product resulting from the use of a rigid wafer substrate would simply be another rigid wafer, which would then require chopping into smaller, inflexible cells. By keeping the compound semiconductor separate from the substrate with a release layer, a thin, flexible solar cell can be epitaxied. So, for unmanned vehicle developers seeking to incorporate solar modules cost-effectively (be they single- or multi-junction) onto significant volumes of autonomous craft while maintaining their desired aerodynamic profile and take-off weight, the use of ELO can be vital. Naturally, variations on these methods are used by different solar suppliers using different materials. Production of thin-film CIGS solar cells, for example, can involve epitaxy on a film-type substrate, in the course of a roll-to-roll deposition process. This fast method of fabricating PV cells typically involves unspooling a roll of thin foil or film such as polyimide, aluminium or steel through a series of chambers. This acts as the substrate for the subsequent epitaxy. In each chamber, a different semiconductor junction, buffer, coating or back-metal contact can then be deposited. Additional features of roll-to-roll set- ups can include mechanisms for drying the deposited material, and ‘gas gates’ between chambers to prevent leaks or contamination of the different deposition vapours. After roll-to-roll deposition or MOCVD, the PV sheets are then divided into cells for interconnection and arrangement according to the end-user’s desired shape and surface area. Configurations Most solar arrays feature some kind of tab, or interconnect, to allow the cells to be arranged next to one another, and wiring is needed of course to connect them together. The precise way in June/July 2018 | Unmanned Systems Technology This CIGS solar cell is manufactured using roll- to-roll deposition (Courtesy of Ascent Solar) ELO allows the substrate to be re-used multiple times, rather than expending an entire wafer each time, resulting in far lower costs