Continuous Wafer Production for Solar Cells by Horizontal Ribbon Growth

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1438231 - YdstieThe technology for the direct conversion of sunlight to electricity using solar cells has not yet advanced to a point where it can have a significant impact in markets that do not benefit from government subsidies. The cost of solar cells is still very high and the technologies used to produce solar cells do not easily scale up. At the current rate of growth (20-30% per year) it will take many decades to build enough plants and infra-structure for solar electricity to displace fossil energy sources. Solar energy has so far not measured up to its potential due to the high cost of producing high purity silicon and mono-crystalline silicon wafers. Significant progress has been made in developing cheaper processes for making high purity poly-silicon in fluid bed reactors. Very limited progress has been made in finding alternatives to the expensive band-saw process for wafering. The ideas described in this proposal may contribute towards solving these important problems. The proposed process draws inspiration from the Pilkington glass process, which revolutionized the glass industry. The major objective is to reduce the cost of solar electricity by theoretical development and experimental verification of a continuous process for making silicon wafers. The proposed process could reduce the cost of a solar module by a factor of two relative to current technology and it can be rapidly scaled up since it is continuous. Sub-objectives include the development of models and control theory for coupled fluid flow, heat transfer and solidification in the context of horizontal ribbon growth.Intellectual Merit:This project aims to develop scientific foundations for a continuous process to produce crystalline silicon wafers from high purity poly-silicon. In this process the PIs plan to freeze a very thin (less than 0.3 mm) silicon sheet on top of a silicon melt in a float chamber. Solid silicon floats like ice on water and by careful control it is possible to produce mono-crystalline silicon sheets suitable for solar cells. The sheet can then be withdrawn continuously while silicon raw material is continuously fed to the process at the same rate. The production cost will be low relative to expensive wire saw processes since the new process is continuous and does not incur silicon loss. The PIs plan to study the stabilization and control of a freezing front on a molten substrate and how to design and control systems with several interfaces (gas,solid, liquid). Systems of this type may exhibit several types of instabilities and active control is needed to stabilize the system. These instabilities include the dynamics of the crystallization front, how impurities segregate and the dynamics of the exit meniscus. They will develop multi-scale process models capable and a theory capable of analyzing stability properties for process design and control. The models will be matched to a physical system using experimental data obtained from a large pilot plant at CMU. The simultaneous goal is to develop a theory for how to model, analyze and control complex flow and solidification problems.Broader Impacts:The most important broader impacts of this research are in the area of alternative energy - producing solar panels more economically. The research could lead to new methods for multi-scale modeling and stabilization and control of solidification fronts and multi-phase flow problems. These problems turn up in a number of application areas, including the drying of paints, film processing and coating.