Study Of Defects And Impurities In Multicrystalline Silicon For Solar Cells

Multicrystalline silicon (mc-Si) has been one of the most important materials for photovoltaic applications due to its economically-attractive compromise between solar cell efficiency and the cost of wafer production. However, the concentration of impurities and defects in mc-Si is significantly higher than that in monocrystalline silicon, and they will strongly degrades the minority carrier lifetime of the material. Especially in the border of mc-Si ingots, there are more defects and impurities, resulting in a layer of deteriorated region. Even after a border layer has been cut away, there still exists a layer of low minority carrier lifetime. Influenced by these deteriorated regions, the application of mc-Si wafers cut from the border block of mc-Si ingots is limited. Therefore, it is necessary to investigate how defects and impurities dominate the lifetime of materials in the deteriorated regions, whether the quality of wafers can be improved during solar cell fabrication processes, and whether wafers containing the deteriorated regions can be used for the fabrication of high efficiency solar cells or not.This thesis studies the properties of wafers cut from different silicon block of the same ingots by the use of Microwave Photo Conductive Decay (μ-PCD), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Fourier transform infrared spectroscopy (FTIR), and Optical Microscopy (OM) techniques. On the basis of minority carrier lifetime measurements and chemical analyses, the origin of the lower minority carrier lifetime in the border region of mc-Si ingots was investigated. It is found that the concentration of oxygen, carbon and other metal impurities in the border region are almost at the same level as that in the bulk. In contrast, the concentration of Fe-B pairs is significantly higher in the border region. The dislocation density in the border region is much lower than that in the adjacent part. The lower dislocation density in the border region leads to more iron existing in the form of Fe-B pairs, which should be responsible for the low lifetime.In addition, the behavior of solar cells prepared from the border of a mc-Si ingot, which contain deteriorated regions, has been investigated. It is found that the diffusion length of minority carriers in the cells is distributed uniformly. The efficiency of the solar cells is as high as normal cells (about 16%). Moreover, we did a systemic investigation of solar cell processes in which we find out the key processes that improve the quality of the deteriorated regions during the fabrication of solar cells significantly. It is indicated that phosphorus gettering and hydrogen passivation could significantly improve the quality of the deteriorated regions, leading to a uniform distribution of minority carrier lifetime in the samples, while aluminum gettering could not.The results presented here suggest that the quality of the deteriorated regions could be improved during the solar cell fabrication. The performance of solar cells fabricated with wafers containing deteriorated regions is as better as that of normal mc-Si solar cells. This will help to increase the utilization of mc-Si ingots effectively in photovoltaic industry.