Lightweight, flexible solar cells that are as efficient as conventional ones could greatly expand the use of solar energy.
Most solar cells in use today are based on crystalline silicon, thanks to their high efficiency and ease of fabrication, but they can only be placed on flat, static surfaces because they crack under stress. Existing flexible solar cells made of other materials offer an alternative, but at the cost of much lower energy efficiency.
Now Wenzhu Liu of the Chinese Academy of Sciences in Shanghai and his colleagues have developed a thin, lightweight silicon cell that is so flexible that it can wrap around itself without losing efficiency.
The team created the cell by first thinning a wafer of silicon photovoltaics by more than 60 percent, making it as flexible as paper, but also reflective and therefore less efficient. Chemical surface treatment to create microscopic pyramid-shaped bumps restores this efficiency, but makes it brittle when bent, until Liu had an idea to fix it.
“One day when I tore open a plastic food bag, the small nick on the edge of the bag inspired me to wonder if the tendency of silicon cells to fragment under mechanical stress is caused by sharp nicks on the edges of the bags. wafers,” he says.
After filming the treated cells breaking in slow motion, Liu and his team realized that the cracks would start at the edge of a cell in the channels between the pyramids. When they softened these channels and tried to bend a cell, they found that many small surface cracks had accumulated, but the cell did not break and maintained its efficiency.
The flexible cells can be easily fabricated using existing silicon wafer techniques, and because they are 95 percent lighter than rigid cells, they can be used in walls without compromising building integrity. Liu and his team tested the cells in extreme environments, such as at the South Pole, in a drone flying at an altitude of 20 kilometers, and in a balloon in strong wind, and found that they performed as well, and sometimes better than rigid solar cells.
Kyle Frohna of the University of Cambridge says the work is very impressive. “It opens up silicon to another class of applications, some of which we’ve thought of and some of which we probably haven’t even thought of yet.”
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