Producing Poly-Silicon from Silane in a Fluidized Bed Reactor

Abstract

The accumulated world solar cell capacity was 2.54 GW in 2006, 89.9% based on monoor multi-crystalline silicon wafer technology, 7.4% thin film silicon, and 2.6% direct wafering (Neuhaus & Munzer, 2007). The rapidly expanding market and high cost of silicon led to the development of thin-film technologies such as the Cadmium Telluride (CdTe), Copper-Indium-Gallium Selenide (CIGS), Dye Sensitized Solar Cells, amorphous Si on steel and many other. The market share for thin-film technology jumped to nearly 20% of the total 7.7 GW of solar cells production in 2009 (Cavallaro, 2010). There are more than 25 types of solar cells and modules in current use (Green & Emery, 1993). Technology based on mono-crystalline and multi-crystalline silicon wafers presently dominate and will probably continue to dominate since raw material availability is not a problem given that silicon is abundant and cheap. Solar cells based on rare-earth metals pose a challenge since the cost of the raw materials tend to fluctuate and availability is limited. However the cost of silicon solar cells and the raw material, solar grade poly-silicon is too high and this technology will be displaced unless cost effective alternatives are found to make silicon solar cells. Figure 1 shows the approximate distributions for the different costs in producing a silicon based solar module (Muller et al., 2006). The figure shows where there is significant incentive to reduce costs. The areas of solar grade silicon (SOG) production and wafer manufacture stand out. These processes are presently not well optimized and many opportunities exist to improve the manufacturing technology through process innovation, retro-fit, optimization and process control. Poly-silicon, the feedstock for the semiconductor and photovoltaic industries, was in short supply during the beginning of the last decade due to the expansion of the photovoltaic (PV) industry and limited recovery of reject silicon from the semiconductor industry. The relative market share of silicon for the electronic and solar industries is depicted in Figure 2. This figure shows the growing importance of the the solar cell industry in the poly-silicon market. Take last year as an example, a total amount of 170,000 metric tons of poly-silicon was produced and 85% was consumed by solar industry while only 15% was consumed by the semiconductor industry. This represents a complete reversal of the situation less than two decades ago. During the last decade, the total PV industry demand for feedstock grew by more than 20% annually. The forecasted growth rate for the next decade is a conservative 15% per year. The available silicon capacities for both semiconductor and PV industry are limited to 220,000 metric tons for the time being. Producing Poly-Silicon from Silane in a Fluidized Bed Reactor 7