by Alyssa A. Lappen
Forbes | Aug. 15, 1983
Vol. 132, No. 15, p. 105
There are several types of solar cells, but they all work on the same principle: Semiconductorgrade silicon, formed into wafers, absorbs light, producing electrically charged particles and emitting a current that is collected through attached wires.
The most common production method is the Czochralski process, in which a large silicon crystal is “grown’ from a molten liquid in a furnace and then sliced into wafers. Raw polysilicon is melted to 2,200 degrees F, and a speck of silicon crystal is implanted. As the material is heated and turned, a sausagelike crystal emerges, up to 36 inches long and 150 pounds in weight. The wafers are sliced off by diamond blades and then processed into solar cells.
The Czochralski cells have over a 60% share of the market. The efficiency of panels of cells–the percentage of surface sunlight they turn into electricity–has reached a respectable 11%, even in mass production. But a prime disadvantage is the $45-per-kilogram cost of the raw material.
Semicrystalline cells make up about another 20% of the market. These are made chiefly by Solarex, a Rockville, Md. company that uses a “dirtier,’ less expensive polysilicon raw material. Instead of “growing’ crystals, Solarex casts the silicon into square ingots. The square wafers that are cut from the ingot have a better packing density and area efficiency on panels than their round Czochralski cousins. The semicrystalline cells get about a 10% to 12% efficiency when mass produced, but Solarex has produced cells in the lab that get about 18%. The company thinks it can reduce its cost to perhaps $5 per kilogram by the end of the decade by making its own silicon feedstock from quartz and sand.
Amorphous silicon solar cells have about 15% of the world market, but are only 5% to 6% efficient and are made primarily by the Japanese for consumer products like watches, calculators and battery chargers. American labs are working to improve this technology, too. Ribbon cells, made by Mobil Solar, resemble 2-inch steel ribbons and as yet account for less than 1% of the market.
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