Multi-scale Modeling for Scale-up and Control of a New Solar Cell Wafering Process

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0932556YdstieThis project aims to develop scientific foundations for a continuous process to produce crystalline Silicon wafers from high purity poly-Silicon. In this process Silicon is floated over a two-layered molten substrate to form a very thin (less than 0.3 mm) Silicon sheet, which solidifies to produce crystalline Silicon wafers suitable for solar cells. The production cost will be low relative to expensive wire saw processes since this process is continuous and does not incur Silicon loss. Intellectual Merit: In this research the PIs will study the stabilization and control of a freezing front on a molten substrate. Systems of this type may exhibit Mullins-Sekerka instability and active control is needed to stabilize the process. They will develop multi-scale process models capable of representing the instabilities present in the freezing front. The models will be matched to a physical system using experimental data. Control methods will be developed to stabilize the freezing front using a thermodynamics based approach to passivity based control. The focus is on the application of the methods to Silicon for making solar cells. A number of new measurement techniques will be tested for the Silicon solidification problem and process parameters needed for process design, scale-up and control will be determined. A thin layer of molten Silicon will be slowly poured on a high-density liquid used as a substrate in micro scale experiments. The physical properties of the substrate will be tuned so that the molten Silicon can be continually cooled and withdrawn in the form a continuous sheet of single-/multi-crystalline Silicon. They will show that the use of molten (liquid) substrate forming three liquid-liquid layer prevents the crystal imperfections, dislocations and grain boundaries present in current continuous, horizontal wafering processes. They will also demonstrate that limited purification can be experienced. The simultaneous goal is to develop multi-scale mathematical models to compare the micro-scale experimental results on movement and flow of the molten Silicon over the liquid substrate. Model predictions will be compared with macro-scale experiments. Broad Impact: Solar energy has so far not measured up to its potential due to the high cost of producing high purity Silicon and 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, however. The ideas described here may contribute towards solving this problem. This three layer process draws inspiration from the Pilkington glass process which revolutionized the glass industry. Preliminary experiments show that it is feasible to produce silicon wafers in small scale using a similar idea. The most important broader impacts of this research are expected to be found in the area of alternative energy. It is also expected that the research will lead to new methods for multi-scale modeling and stabilization and control of solidification fronts. These problems turn up in a number of application areas, including the drying of paints, film processing and coating.