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Silicon Info: Si PV - Future Trends

The PV industry is expected to continue to grow at an annual rate of more than 30%.  The well-established technology base and ready availability, proven performance, and salubrity of silicon, coupled with economies of scale in larger factories, will likely allow Si to remain the dominant PV material for the foreseeable future. The remarkable cost reductions in Siemens-process polycrystalline silicon feedstock production for PV use is a key contributor to this.

In ingot growth, the trend for single crystals will be away from the smaller 100- and 125-mm-dia. sizes with more focus on larger diameters.  Despite the potential advantages of FZ material, it is unlikely that its role in PV will increase significantly because of higher costs for the crack-free, long cylindrical feedstock it requires and the difficulty in producing the larger FZ diameters.  In CZ growth, we are likely to see an increased effort to make hot zones more energy efficient, to grow larger diameters, and to achieve continuously melt-replenished long growth runs.  There will be a continuing effort to achieve more wafers per length of ingot, and to take advantage of potentially higher cell efficiencies afforded by thinner wafers when back surface fields are used in the cell design.  Multicrystalline casting, directional solidification, and electromagnetic casting, while playing a dominant role in the Si PV market during the late 1990's (53% of all ingot-based modules sold in 1998 were multicrystalline) will likely continue to give way to the higher quality CZ growth which allows higher cell efficiencies.

In the ribbon- and sheet-growth technologies, a challenge for dendritic web growth and edge-supported pulling was to increase areal throughput via wider ribbons, multiple ribbons, or other approaches.  Even though these methods have the advantage of minimal silicon consumption and elimination of wafering, it is unlikely that they can effectively compete due to their current low throughput of 1-2 m2/day.  This is because the effective areal throughputs of CZ growth are >30 m2/day.  While capillary die growth of octagons produces about 20 m2/day, the lower material quality and fragility of handling limit their applicability and the technique has largely been phased out. We may see continued progress in large-area solid/liquid interface sheets by some variant of the methods discussed in Silicon Ribbon and Sheet Growth because the throughput potential is enormous and one growth furnace could easily generate material for 35 MWp/year or more of solar cell production. However, enormous improvement in material quality and minority charge carrier lifetime wil be required.

The future is expected to bring continued exploration of thin-layer Si growth approaches, in search of ones that have significant economic advantages over the best ingot and sheet techniques.  Successful ones will have fast deposition rates, large grain sizes, high cell efficiencies, compatibility with low-cost substrates, and amenability to low-cost cell-fabrication schemes.  It is not likely that production of thin-layer Si PV modules will be a significant fraction of the mainstream PV market in the near term, although they, like the ingot and sheet approaches, would have substantial advantages over many other thin-film PV approaches.  These include the simple chemistry and relative abundance of the Si starting material. 

The Earth's crust contains 27.7% Si, in contrast to 0.00002% Cd, 0.00001% In, 0.000009% Se, and 0.0000002% Te (commonly used thin-film elements).  In addition, crystalline Si benefits from an extremely well-established technology base, compatibility with SiO2 surface passivation, relative salubrity with respect to toxicity, and stability under light exposure.  These facts coupled with the amazing cost reductions achieved in polycrystalline feedstock production and Czochralski crystal growth are likely to allow it to be the dominant PV technology for the foreseeable future.
Roy, A., Chen, Q.S., Zhang, H., Prasad, V., Mackintosh, B., and Kalejs, J.P. (1999) Presentation at the 11th American Conference on Crystal Growth & Epitaxy, Tucson August 1-6.  To be published in J. Crystal Growth.


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