Graphite cloth cuts solar cell weight

Newark, DE-Conventional solar cells consist of flat panels supported by a rigid structure. Although this type of assembly satisfies many requirements, it imposes a rigid, clumsy envelope on engineers who might otherwise find solar cells an attractive power source.

Breaking away from conventional thinking about solar cells, AstroPower Inc. has developed a new thin-film, light, polycrystalline silicon solar cell that uses graphite cloth as a support substrate. The cells can be molded into rigid shapes. Work now underway aims at developing folding models.

The solar cells exhibit a confirmed efficiency of 13.4%--a record for a silicon cell on graphite, according to James Rand, Manager of Product Technology, and co-developer of the cells. Because they grow directly on the graphite cloth and do not require wafering, the cells may become available at a fraction of the cost of conventional lightweight solar cells.

Lightweight solar panels for spacecraft were the original product application. Because their efficiency approximately equals that of conventional cells, they require about the same panel area. But they will consume substantially less of a spacecraft's payload weight.

Prior to coming up with the new cells, engineers at AstroPower developed and manufactured a thin-film, polycrystalline solar cell grown on a ceramic-like substrate (see Design News, 7-6-92). "We could easily make the new structure. The beauty of it is, it's very thin," says Rand. "We looked at other backings for the light cells--molybdenum, for instance. But graphite was an almost instant success."

In the new solar cell, silicon entirely encases the graphite cloth, so AstroPower designs the unit like a conventional polycrystalline silicon solar cell. AstroPower engineers say they believe the graphite-cloth solar cells could be grown on a form, and thus take on the shape of, say, an ultralight aircraft's skin.

Further, the engineering team has been working on ways to fold the solar cell. Simply depositing the silicon in squares with bendable spaces between segments didn't work. The graphite proved too brittle for folding. They decided to try quartz reinforcement, because quartz adds flexibility while resisting the high temperatures needed for silicon deposition. In this variation on the basic design, spun quartz thread runs in one direction and graphite in the other. Engineers then double up the quartz at the bendable border. The resulting assembly folds nicely.

AstroPower's graphite-substrate solar cells are reasonably fragile. "The army considered them for backpacks, but you had to be able to walk on them. That's a very tough test. And if we make them so you can walk on them, they lose all their weight advantage. But recreational backpackers could handle them more gingerly," said Rand.

Additional details...James Rand, Manager of Product Technology, AstroPower Inc., Solar Park, Newark, DE 19716-2000.