Solar-grade silicon substrates by a powder-to-ribbon process

Abstract

A new technique for producing silicon ribbons for solar-cell substrates is described. The process begins with inexpensive, 98% pure silicon that is crushed and acid leached to raise the purity to 99.9%. This powder is spread on a graphite plate and electron-beam annealed to form a flat, self-supporting "preribbon." After removal of the graphite and unmelted powder, the preribbon is given a second electron-beam scan that recrystallizes the silicon into a smooth polycrystalline ribbon. This zone melting further improves the purity to over 99.99%.Ribbons 0.4 mm thick and up to 16 mm wide were produced in this initial work. The ribbons are p-type, 0.07 Ω∙cm, and have long crystals about 1 mm wide. Electron-diffusion lengths of 20–30 μm were measured. Calculations indicate that solar-cell efficiencies up to 13% should be possible. If the process can be scaled up and automated, the cost of volume production could be as low as 43 cents/W. It is concluded that the process has the potential for achieving low-cost "solar-grade" substrates and has advantages over other processes. Further work is planned.