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8 Researchers in China have developed a lightweight foldable solar cell intended for UAVs (writes Nick Flaherty). The team, at the Research Centre for New Energy Technology, Shanghai Institute of Microsystem and Information Technology, say the cell is based on c-Si wafers. Reducing the thickness of a wafer can improve its flexibility but reduce its lightharvesting efficiency. Reducing a 160 µm-thick silicon wafer to 60µm provides a flexibility similar to that of a sheet of paper, but it has proven unsuitable for solar cell fabrication because more than 30% of the incident sunlight is reflected by its glossy surface. Chemically texturing microscale pyramids on the surface of the silicon has been widely used as an efficient strategy to reduce the reflectivity to less than 10%. However, when bending forces are applied to the texture, the maximum stress is located in the sharp channels between the pyramids, causing cracks. The team used an ultra high-speed video camera to investigate the cracking process of the wafer. This recorded a long fracture with three silicon particles being ejected from the edge of the wafer, their initial positions coinciding with the point at which the cracking initiated. The fracture behaviour of the wafers can be manipulated by tuning the sharpness of the channels between the pyramids, which modifies the stress state and deformation mechanism under bending loads. This reduces the intrinsic brittleness of the c-Si wafer. The team then built the foldable wafers into solar cells. Rather than using the more common passivated emitter and rear cells and tunnelling oxide passivated contact solar cells – which have an asymmetric structure design and are fired at a temperature of 800±20oC – a technology called superheterojunction (SHJ) solar cells with a symmetric structural design is fired at 180±5oC. That makes the technology more suitable for manufacturing flexible solar cells, because it is free from edge warping caused by inner stresses during the firing process. The resulting cell has a certified efficiency of 24.50% for a 244.3 cm2 wafer. Although this value was lower than that of a thick cell (25.83%), because it was affected by the inferior light-harvesting ability of the thinner wafer, it is higher than current flexible solar cells fabricated from other cost-effective materials. The efficiency should be further improved through better surface passivation. The team assembled the cells into a 10,000 cm2 flexible module and attached it to an inflated bag of gas. They then used a fan to model the effect of wind at a speed of 30m/s to simulate a violent storm. After 20minutes, the relative power loss was only 3.07%, suggesting that the module can operate well under such conditions. The lightweight nature of the flexible SHJ modules makes them suitable for charging near-space aerial vehicles, where the temperature can dip to −70oC at 20-75km. To model this, the researchers cycled the flexible modules between −70oC for an hour and 85oC for another hour. After continuous temperature cycling for 120hours, the average relative power loss was only 0.32%, showing that the modules can safely be operated in cold near-space conditions or at the South or North poles. Solar power Flexible cells advance Platform one August/September 2023 | Uncrewed Systems Technology Researchers have found a way to make thin, flexible cells with a comparable efficiency to thick cells