1. Technical Field
This invention relates to seed layers and a process of manufacturing seed layers for casting silicon suitable for use in solar cells or solar modules.
2. Discussion of Related Art
Known steps to prepare seed material for silicon casting use a monocrystalline boule of silicon prepared by the Czochralski (CZ) technique or the float zone (FZ) technique. Both CZ and FZ techniques produce round or circular boules of material. The round boule is cut down to a square or rectangular cross-section and then sliced into pieces for the seed material. The conventional CZ and FZ techniques produce boules of up to a few hundred millimeters with very large boules being about 300-450 millimeters in diameter. Squaring the boule and slicing the seed material results in a square of about 210 millimeters per side from a 300 millimeter diameter boule. The resulting seed material covers less than 50 percent of a bottom of a crucible used in directional solidification.
Without excessive care, loosely abutted seed tiles will tend to form highly defective crystal areas both in the fused seed layer and in the grown ingot above. These highly defective areas grow and become problematic for the solar cells produced from the material, typically resulting in a lower solar cell efficiency.
There is a need and a desire to produce seed layers corresponding in size to larger crucibles used for directional solidification. There is a need and a desire to produce monolithic seed layers that do not break during normal handling and can be used during several casting cycles. There is a need and a desire to form substantially defect free seed layers at significantly larger scales. There is also a need and a desire to lower the cost per unit volume of cast crystalline silicon by increasing a volume of cast silicon produced.