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A supplier may classify the polysilicon produced into 50 or more different grades with different specifications specific to the user and the application. In general, the supplier may offer:
Within each product grade are detailed specifications for rod size (diameter, length), chunk size (large, medium, small), acceptor/donor/carbon purity, bulk and surface metals purity, surface texture, and appearance. Sales prices are based on industry demand, product quality, and cost of production.
When supply and demand have been in balance, prices range from ~$100/kg for high purity fabricated Float Zone poly rods to ~$ 5/kg for waste/scrap polysilicon. In general, polysilicon products are identified by form (rod, chunks, chips) and purity. Purity specifications for solar-grade (SOG) polysilicon are much lower than for Electronic Grade (EG) poly. Table 1 lists general purity specifications for SOG and EG poly.
User A, B, and C are summaries from dozens of user specifications for solar cell applications. Standard Cz and Advanced IC are summaries from dozens of user specifications for Electronic Grade polysilicon. Acceptor elements, primarily boron, and donor elements, primarily phosphorus, are measured in parts per billion, atomic. Carbon and oxygen are measured in parts per million, atomic. Bulk metals and surface metals are measured in parts per billion, by weight.
Most solar cells are fabricated on silicon wafers that have been cut from ingots at about 0.1 ohm-cm p-type, so the polysilicon can have relatively large amounts of boron, compared to EG poly. An important difference between SOG and EG poly is the metals content. Reduction of metals content increases production costs. Metals content is not usually specified for SOG.