Contraintes thermomécaniques et dislocations dans les lingots de silicium pour applications photovoltaïques

摘要

SIMaP-EPM laboratory of Grenoble and INES institute of Chambery have both financed this thesis which investigates the effect of thermo-mechanical stresses on the crystal quality during production of silicon ingots for photovoltaic applications. This work begins by showing how photovoltaic industry makes solar panels and the influence of dislocations (defects induced by stresses) on the conversion efficiency. Bibliographic review is also performed in order to describe physical and numerical models of dislocation motion and their multiplication in silicon. Several characterization methods of the dislocation density at the surface of a sample are also presented in the first part of this work.In the second part of this manuscript, comparative study of different quick characterization methods is done in order to show their strength and weaknesses. Therefore, a sample, which is wide, not containing grain boundaries, and having areas of high and low dislocation density, is used as reference sample for the comparison. The first characterization technique studied in this work is the “accurate method” consisting in manually counting the dislocations at the surface of the sample in order to have a precise characterization of dislocation density. The “INES method” uses numerical treatment of SEM pictures to count dislocations. The “Ganapati method” links the grey scale of a sample picture taken with a scanner and the dislocation density. Finally, the “PVScan method”, using the eponymous device, uses diffusion of a laser beam on the surface of the sample for characterization. This comparative study underlines the best applications for each method and which questions should be thought about before performing dislocation characterization.The third part of this work is intended to build two numerical simulations using Comsol commercial software in order to predict dislocation density in silicon ingot at the end of its production. Therefore, Alexander and Haasen model, describing dislocation density and plastic relaxation rate, is implemented into the software and coupled with the thermo-mechanical stress calculation. In the first model, named “continuous evolution”, the entire ingot is taken into account (liquid and solid parts) and, during solving of this numerical simulation, temperature changes continuously. In the second model, named “step by step” only the solid part of the ingot is taken into account with new geometry and new temperature at each step. Both of these models are compared to numerical simulations performed by Japanese and Norwegian teams. Results of the first one are also compared to the experimental characterization of a sample. Thus, this part shows the pertinence of using commercial software for the prediction of dislocation density in a silicon ingot at the end of its production. Its use is simple and shows good adaptability to different furnace geometries and thermal fields.In the last part, ingot/crucible attachment is studied because it creates high stresses and then dislocations in the crystal. This problem is also solved by numerical simulation using Comsol software. Therefore, a physical model is created: the J-integral is used to estimate elastic energy at the attachment area and then this value is compared to ingot/crucible adhesion energy. This model is implemented into the software and the results are compared to an experiment realized during a previous thesis. This numerical simulation is also applied to two attachment configurations of a silicon ingot in order to study the attachment duration, the localization and the size of crystal area impacted by plasticity.