Oxygen And Carbon Behaviors In Multi-Crystalline Silicon And Their Effect On Solar Cell Conversion Efficiency

Solar cells have been paid extensive attention around the world as a green energy. Multi-crystalline silicon occupies more than 50% market of commercial crystalline silicon solar cells for its low cost. Most of them are grown by the directional solidification technology for its high production rate and low cost. The conversion efficiency of multi-crystalline solar cells is generally limited by defects, such as dislocations, grain boundaries, etc, and impurities, such as oxygen, carbon, etc. Hence understanding and controlling the impurity behavior are important for low-cost and high-efficiency of multi-crystalline silicon solar cells.According to the carbon enhancement effect on oxygen precipitation, we have amended the modified Becker-D?ring equations and proposed an oxygen precipitation growth model considering the influence of carbon to simulate the size distribution and concentration of oxygen precipitation after the thermal treatment. The result showed the carbon impact on oxygen precipitation: the higher carbon concentration in silicon from the middle part of the ingot accounts for the larger size and higher quantity of oxygen precipitation; the lower carbon concentration in silicon from the bottom part of the ingot results in the smaller size and lower quantity of oxygen precipitation in spite of the higher oxygen precipitation. In correlation with the solar cell conversion efficiency, we found the influence of oxygen precipitation on conversion efficiency: the effect of oxygen precipitation is caused by the size of precipitation, while the degree of the effect is caused by the concentration of the oxygen precipitation; i.e. when its size is below a certain value, the oxygen precipitation can enhance the conversion efficiency through gettering impurities, and the conversion efficiency will be enhanced with the increase of the oxygen precipitation concentration; when its size is above a certain value, the defect induced by the oxygen precipitation can deteriorate the conversion efficiency, and the conversion efficiency will decrease with the enhancement of the oxygen precipitation concentrationIn the further experiment, we have employed the infrared spectroscopy to study the change of oxygen and carbon concentration after thermal treatment in different parts of multi-crystalline silicon ingots grown by directional solidification technology. In correlation with the solar cell performances such as the minority carrier lifetime, photoelectric conversion efficiency and internal quantum efficiency, we investigate the physical mechanism of the effect of various concentrations of oxygen and carbon on the cell performances. Considering the simulation result, it is found that carbon is the main reason for the different performance of solar cell produced by different part of the directional solidification multi-crystalline silicon ingot: on one hand, huge amount of carbon precipitation deteriorates the multi-crystalline silicon solar cell performance directly; on the other hand, carbon concentration plays more important role than the initial oxygen concentration in the size and quantity of oxygen precipitation, which leads to the large and small size of oxygen precipitation in the middle and bottom part of the ingot respectively.We have further demonstrated the complex behavior of oxygen and carbon in the different part of the directional solidification multi-crystalline silicon ingot by a two-step thermal treatment technique, from which we point out that the two-step thermal treatment is only applicable to improve the efficiency of solar cells produced by the silicon from the bottom part of multi-crystalline silicon ingots.This work is supported by the National Basic Research Program of China (Grant No. 2010CB933702).