Toward viable horizontal ribbon growth of solar silicon: Understanding and ameliorating process instabilities

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CBET 1336164DerbyThe development of renewable energy may be the most important challenge for the future of human civilization. Low-cost and reliable solar-to-electric conversion methods are especially important toward this endeavor. Of these methods, by far the most established technology is based on the fabrication of solar cells from crystalline silicon. However, significant reduction in the cost per watt produced by these technologies is needed to make solar energy competitive with fossil fuels. For silicon-based solar cells, production costs must be decreased while maintaining or improving cell efficiency. Advances in silicon growth methods are needed to achieve these objectives. Solar-grade, crystalline silicon is produced by a variety of techniques that melt and carefully re-solidify silicon via thermal processes. Methods that grow single crystals of silicon, such as the Czochralski process, are expensive, but such material achieves the highest cell efficiencies. Lower-cost growth methods, such as casting and vertical ribbon growth, have been developed, but their multi-crystalline material results in cells of lower efficiency. A promising crystal growth technology, known as the horizontal ribbon growth (HRG) process, was put forth in the late 1960's and was subsequently advanced in the late 1970's and early 1980's in Japan and in the U.S. After encouraging early results, experimental advances and process development efforts stalled, and this technique was abandoned by the mid-1980's in favor of more traditional methods that were easier to develop. Recently, however, there has been renewed interest in the HRG method, since, if successful, it promises a win-win scenario of significantly reduced production costs coupled with the ability to grow high-quality, single-crystal material. However, significant advances in the understanding and optimization of this growth method are needed to make it viable as a large-scale production process for silicon solar cells.This research is aimed at making the horizontal ribbon growth of silicon viable, particularly by addressing failure modes associated with its unique geometry and operation. Specifically, this project will apply sophisticated computational models of the horizontal ribbon growth method to elucidate and ameliorate known process instabilities and to study the salient issues associated with increasing growth rates to further reduce production costs. This research will apply a computation-centric approach to probe these issues and ultimately optimize growth conditions. Thus, this research will address fundamental scientific issues of crystal growth together with a clear goal toward improving a material with important technological applications.