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41 Solar cells | Focus across different architectures that use perovskite and organic materials in varying ways, as well as using different electrode and substrate materials. For example, a recent experiment aimed at testing their potential in space applications used two perovskite cell architectures and two organic ones. The perovskites – a tin dioxide-based planar perovskite, and one based on mesoscopic titanium dioxide – both exceeded 14 mW/cm 2 and a power conversion efficiency of 20%. The organic PVs reached 4 and 7 mW/cm 2 , while exceeding 8% conversion efficiency. Commercialisation of HOPVs remains some way off, however, as optimum configurations for module architecture and production scaling are yet to be identified. Also, new coatings of some kind are needed to prevent HOPV materials from suffering degradation and efficiency losses in standard environmental conditions. Moisture, oxygen and warmth can cause these issues to arise more rapidly than in commercially established cell materials, and mitigation must be achieved cost-effectively so as not to outweigh their touted advantages. In the meantime though, unmanned space vehicles certainly stand to derive major reductions in manufacturing costs from advances in HOPVs, and a dual- junction perovskite-silicon solar cell capable of 28% conversion efficiency (with a theoretical limit of 43%) is due to become commercially available by the end of this year. Microstructural grooves In addition to using a plethora of different materials, the thin-film solar cell industry is also producing a few interesting new ways of constructing and shaping the outer layers of the modules, to enable greater energy efficiency, performance and cost-effectiveness. One of these is a new architecture for semiconductor material to generate and store energy from sunlight. Most cells are constructed as a series of horizontal layers from top to bottom, but constructing thin and highly flexible cells based on this architecture in large quantities can be challenging. One company has developed a solution that essentially rotates the architecture by 90 º , to enable the production of roll-to-roll solar material using a new way of shaping the combined material layers. To do this, its manufacturing technique embosses microstructures – shaped as long, running grooves several microns thick – into a plastic base layer such as PET. Each ‘microgroove’ is, much like those in traditional solar cells, typically made by combining an n-type semiconductor, a p-type semiconductor, a conductor contact layer, and a (possibly conductive) adhesive for bonding the layers together on top of the microgrooves in the plastic. Each cell groove in these types of modules is typically 1-2 µm wide and deep, and each sub-module of grooves has a similar width to a human hair (around 100 µm on average). The sawtooth-like cross-section of the groove enables light to be funnelled in from different angles (with a metallised backing to reflect light passing between the grooves back into one of the solar-absorbing structures). It also means gaps of air or some other preferred insulator can be kept between the grooves to prevent shunting or other sources of parasitic losses. The semiconductor and conductive materials in this architecture can largely be any of those already in use. The processes behind this approach are potentially less expensive, simpler and quicker than conventional methods, eliminating key steps that other module architectures rely on (such as not needing a transparent conductive film layer). The approach also enables the modules’ integration to be customised to a great extent, as they can be designed with whatever voltages the end-user may need purely by altering the pattern of the microgrooves in the plastic. Each microgroove corresponds to roughly 1 V, and a sheet can easily consist of thousands of them. However, to prevent voltages from running far higher than what end-users actually need, the grooves are chopped into what the company calls ‘cascades’ during moulding, to combine and string grooves together in series cost-effectively. Cascading the sub-modules avoids excessive wiring and connectors as well as the various process stages. Each sheet is calculated, designed and manufactured accordingly, whether it be for a 48 V system or a 1500 V system. And as each sheet produced in this way consists of hundreds of Unmanned Systems Technology | October/November 2020 Constructing solar modules as rows of microgrooves greatly reduces PV manufacturing costs relative to conventional designs (Courtesy of Power Roll)