GOALI: Thermal-Capillary Analysis of the Horizontal Ribbon Growth of Solar Silicon via Finite-Element Process Models

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CBET-0755030Derby Solar-grade, crystalline silicon is produced by a variety of techniques which melt and carefully re-solidify silicon via thermal transport processes. These processes typically involve an interesting trade-off: Methods that grow single crystals of silicon are expensive, but such material achieves the highest cell efficiencies. Lower-cost growth methods typically produce multi-crystalline material that results in cells of lower efficiency. A promising technology to grow single crystals of silicon at much lower costs, known as the horizontal ribbon growth (HRG) process, was put forth in the 70's and early 80's but was abandoned in favor of more traditional methods that were easier to develop. Accumulated advances in crystal growth modeling and understanding make it propitious to reconsider the HRG method for growing crystalline silicon for solar cells. If this process were successfully deployed, it could significantly reduce production costs for silicon substrate by dramatically increasing growth rates and by avoiding the costly kerf losses associated with cutting wafers from silicon ingots. In addition, HRG methods promise the growth of single-crystal material needed for solar cells of the highest efficiency, rather than multi-crystalline silicon currently produced by casting and vertical ribbon growth methods. A successful HRG process would be transformative by promoting simultaneous cost reductions and efficiency increases for silicon solar cells. The premise of the work proposed here is that the great potential of the horizontal ribbon growth method will not be realized until a more fundamental understanding of its mechanistic workings enables new progress in process development. Theoretical, thermal-capillary models will be developed and applied in conjunction with process experiments to assess the feasibility of the HRG process for the production of crystalline silicon for photovoltaic applications. These studies will allow for fundamental investigations of coupled heat transfer and interfacial processes (solidification and capillarity) in this crystal growth system.Intellectual merit of the proposed activity This work will develop and apply rigorous theoretical models to study the nonlinear interactions between thermal transport processes and interfacial phenomena that control silicon crystal production via the horizontal ribbon growth process. Of particular intellectual merit will be understanding the stability and dynamics of this system. Unlike vertical, meniscus-defined crystal growth processes that are inherently stable, the HRG process may be unstable, and its successful operation will likely rely on a thorough understanding of system design and control.Broader impacts resulting from the proposed activity This effort will advance discovery and understanding by training a graduate student in a multi-disciplinary and industrially relevant project. It will enhance infrastructure by promoting collaborations with industry. This work will directly impact process development at Ribbon Technology International, a company founded by Bleil, the originator of the horizontal ribbon growth (HRG) process. This interaction will leverage experimental observation with the theory conducted in this work. From a broader perspective, a successful HRG process could trigger a transformative enabling of current technology by simultaneously reducing silicon crystal production costs while improving solar cell efficiency. Dissemination activities will include an outreach program for the general public involving the Science Museum of Minnesota. This project is jointly funded by the Thermal Transport Processes (TTP) Program, of the Chemical, Bioengineering, Environmental, and Transport Systems (CBET) Division, by the Materials Processing & Manufacturing (MPM) Program, of the Civil, Mechanical, and Manufacturing Innovation (CMMI) Division, and by the Grant Opportunities for Academic Liaison with Industry (GOALI) Program, of the Industrial Innovation & Partnerships (IIP) Division, all within the Directorate for Engineering (ENG).