Rationale for the Silicon Wafer Replacement Tool
This page provides background on the rationale for developing the Silicon Wafer Replacement (SWR) Tool in the Process Development and Integration Laboratory.
The photovoltaics industry consists of many different technologies, but the dominant technology is based on silicon wafers. Silicon wafers are made using high-temperature processing to turn silicon-containing gases into the desired form of crystalline silicon. The solid silicon is then sawn into wafers, losing about half of the material (kerf) to the sawing process. The wafers are then processed into solar cells using a variety of device structures. These wafers are actually much thicker (150–220 micrometers) than necessary to be a solar cell, but thinner wafers are difficult to process because the wafers are brittle and prone to breakage. Because the cost of high-purity silicon can account for up to 60% of the cost of the module, there have been a variety of approaches investigated to significantly reduce the amount of silicon used in a module. The SWR tool was designed to investigate one of these approaches—namely, growing high-quality crystal silicon film from a gas directly onto a substrate. This is a lower-temperature process, to make a significantly thinner device (less than 20 micrometers), and completely avoids the kerf loss that occurs during wafering. Thus, this approach has the potential to significantly lower manufacturing costs.
To realize these cost savings, significant technological barriers must be understood and overcome. These barriers include the development of the following:
- Low-cost seed layers on which to grow the high-quality epitaxial silicon
- Deposition of high-quality epitaxial silicon on foreign substrates that is uniform, free of pin-holes, and low in impurities
- Manufacturable device structures using thin epitaxial silicon films
- Processing steps to passivate the grain boundaries in the multicrystalline silicon films
- Optical enhancements to maximize light trapping in the thin crystalline silicon films.