dc.description.abstract | The silicon (Si) solar cell is still the highest market share nowadays. To accomplish the goal of grid parity, for the solar cell, the wafer quality has to improve, and the production cost has to reduce. The high concentration of carbon and oxygen impurity cause the dislocation and thermal donor in the mc-Si wafer, respectively. The carbon and oxygen are the main impurity for affecting the efficiency of solar cell.
The simulations of thermal flow field, carbon concentration, oxygen concentration and silicon oxide concentration in directional solidification system (DSS) are carried out by the finite volume method (FVM) in this study. The distributions of carbon and oxygen concentration in the grown ingot were measured by the SAS Company and the measurement results are compared with that of the simulation predictions. The simulation results are in good agreement with the experimental ones. The simulation shows that the melt convection is induced by buoyancy force. The flow pattern in the melt changes during the growth process. In order to improve the uniformity of carbon distribution in the melt, a heat insulation of crucible is used to increase the temperature gradient and vortex intensity of melt. Using wafers of the whole ingot obtained from the modified case, the average conversion efficiency of solar cells can be improved up to 1.8% of the one of standard case.
As the solidification fraction enlarges, the oxygen concentration in the melt diminishes, because of the reduction in the amount of crucible surface immersed below the silicon melt. When the solidification fraction is small, the oxygen concentration is higher with a higher furnace pressure than with a lower one due to there being less SiO evaporation at the free surface. When the solidification fraction increases, because of the cooling effect of the argon gas, the oxygen concentration is smaller when the furnace pressure is higher rather than lower. Hence, to adjust variably the furnace pressure during the mc-Si ingot growth is good method for reducing the oxygen concentration.
To increase the argon flow rate can bring more evaporated SiO gas above the free surface outwards the furnace; hence the oxygen concentration in the melt is decrease. In the present study, the gas guiding plate is used to increase the argon velocity above the free surface. The effect of reducing oxygen concentration for the gas guiding plat is similar to the one of increasing the 25% amount of original argon flow rate. The gas guiding plate can decrease the oxygen concentration without increase the argon flow rate, the furnace enhancement can be used on advancing the wafer’s quality.
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