| Currently, cast multicrystalline silicon has replaced monocrystalline silicon as the main photovoltaic materials. High density of impurities, such as oxygen, carbon and iron, and defects, such as dislocations and grain boundaries, play a crucial role on the degradation of mc-si solar cells performance. Understanding the properties of these impurities and defects as well as their impacts on the quality of mc-si materials could help us find the way to reduce the cost of mc-si solar cells and produce high quality mc-si ingots.In this thesis, the properties of as-grown impurities and defects in mc-si as well as their impacts on the minority carrier lifetime in mc-si ingots have been systematically studied by means of Fourier transform infrared spectroscopy (FTIR), Microwave Photo Conductive Decay (μ—PCD), Scanning Infrared Microscopy (SIRM), and Optical Microscopy(OM) techniques. The main work includes the distribution of the interstitial oxygen along the growth direction of mc-si ingots, the interactions between the distribution impurities and minority carrier lifetime and the defects in mc-si and their impacts to the electrical quality of mc-si materials. The main results are as follows:Oxygen concentration as well as its spatial distribution in mc-si is one of the most important factors governing crystal quality. It was found that the distribution of oxygen in mc-si ingots is mainly determined by oxygen segregation and evaporation during the casting processes. A model considering both of the segregation and evaporation of oxygen was proposed to treat the oxygen concentration distribution in growth direction. Good agreement between the simulation and experimental data on oxygen distribution has been established. The oxygen profile would be affected strongly by the exponent X. With the increase of the exponent X, the total oxygen content in the ingots would reduce, and the region near the bottom with higher oxygen concentration would be narrower.The minority carrier lifetime mapping along the multicrystalline ingot was obtained by μ-PCD. The lifetime measurements exhibit a degraded regions with thewidth of the order of 4-5 centimeters at the bottom and about 2cm at the top of the ingot, while a large uniform with high lifetime zone exist in the central position. The profiles of interstitial oxygen and substituted concentration were investigated by FTIR, the results show that the concentration of oxygen decreases from the bottom to the top of the ingot, while in the case of carbon, just the opposite is true. By measuring the τ before and after the sample annealing at 200 ℃ for 10 minutes, the interstitial iron concentration can be evaluated. The results show that there is a strong increase towards both the top and the bottom of the ingot, which is attributed to the solid-state diffusion from the crucible after crystallization and the segregation into the molten phase respectively. The high concentration of impurities such as iron and oxygen, which can induce electrically active recombination centers, is believed to be responsible for the lifetime reduction in the two sides of the ingot.Further, the defects in mc-si were observed by both OM combined with selective etching and SIRM. We found that the top region of mc-si ingots is heavily dislocated and corresponds to a local dislocation density of about 7× 106cm-2, as expected due to the rapid cooling that occurs there at the end of the casting process. Applying SIRM, we studied the configuration, size, as well as density of bulk defects in mc-si. The results show that the average density of bulk defects in top, middle and bottom parts of ingots are 2.8×108cm-3, 8.5×106cm-3, 1.6×108cm-3, respectively.
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