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UBC Theses and Dissertations

Thermo-mechanical simulation of Si processing for photovoltaic applications Sinha, Ambrish Chandra


Wafers for solar cell applications are increasingly being produced via wire sawing of directional solidified multi-crystalline silicon (mc-Si) ingots. During these two processes, defects such as dislocations arise in the material which reduce the final electrical efficiency of the solar cells. Furthermore, processing conditions during these processes can also influence the surface unevenness and warpage in the final wafers. Previously, several researchers have developed finite element models of mc-Si directional solidification process to understand the growth of residual stress and dislocation in the ingots under different cooling conditions, boundary conditions and geometry. Researchers have also developed analytical and finite element models to understand the temperature variation and the material removal behaviour during wire sawing process. However, there has not been much work done in understanding the influence of pre-existing residual stress in the work piece (directionally cooled mc-Si ingots) on the wafer quality during wire sawing.To investigate this, in this work, thermal-stress finite element models were developed for directional solidification and wire sawing process, with later taking the results from former as the stress/strain and dislocation initial conditions. Dislocation creep behaviour of Si was used as the material constitutive behaviour in both the models. The input parameters for the casting model were 3 different cooling rates. The wire sawing simulation was done in the top, middle and bottom section of these ingots. The warpage was, therefore, studied in 9 wafers. These models also simulated the spacial/temporal variation of in-elastic deformation and dislocation density in the ingot and the wafers. The predicted results from the casting model were in good agreement with the past models. The predicted warpage from the wire sawing simulation, however, was less compared to the warpage observed in the industry. This variation in the results may be due to the several assumptions and simplifications taken in the model in-terms of the constitutive behaviour, material properties and boundary conditions.

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