MarketWatch: Prices of polysilicon, already pressured by a coming supply glut, are tumbling fast as the credit crisis drives down demand for the solar panels that are the biggest users of the raw material. Polysilicon — a sand-derived crystalline material that helps turn sunlight into electricity — has fallen swiftly on the spot market in past months, dropping to about $200 per kilogram from $450 to $500 earlier this year, say analysts and solar-module makers. And they are likely to fall …