| Multicrystalline silicon (mc-Si) is the most widely used material for solar cells. The highest efficiency of mc-Si solar cell is about 14%-15%, which is only 1.5-2% lower than that of Czochralski silicon (CZ-Si) solar cells. High density of defects in mc-Si, such as dislocations and grain boundaries (GBs), and transition metallic impurities, such as Fe, Cu, and Ni, play a crucial role on degrading the performance of mc-Si solar cells. The development of large-area and high-efficiency mc-Si has required the reduction of electrically active defects such as dislocations and GBs. Understanding the properties of these defects and their interaction with impurities helps us to adopt feasible impurity gettering and defect passivation processes, which can be adopted in the fabrication process to improve the quality of mc-Si ingots. On the other hand, in mc-Si there exist few systematic studies on the electrical properties of GBs as well as the influence of impurities on them.In this thesis, the GB structural character, the GB recombination activity, and the relation of GB character and impurity decoration were systematically studied by means of electron-beam-induced current (EBIC), electron back-scattered diffraction (EBSD), transmission electron microscope (TEM), and surface photovoltage (SPV) techniques. First, the character and intrinsic recombination activity of GBs were studied in as-grown mc-Si. Second, influence of metal (Fe, Cu, and Ni) contaminations on the recombination activity of different types of GBs was studied. At last, the effect of hydrogen passivation on mc-Si with different contamination level was compared.In as-grown mc-Si, SPV study revealed that the diffusion length of minority carriers varied with the ingot positions according to the impurity contamination level. The main impurity detected was iron. The concentration of iron in the central position was much lower than that of the bottom and top positions. EBSD study revealed that most GBs in mc-Si were large-angle GBs (θ, > 10~°). ∑3 and random GBs are the most frequently observed large angle GBs. By using the mc-Si cut from the central position with low contamination level, the intrinsic recombination activity of various GBs was found to be EBIC-inactive at 300 K. The EBIC contrasts of all the large-angle GBs were in the same range of 0-2% at 300 K and 2-4% at 100 K, indicating that therecombination activity of clean GBs is weak and the GB character has no significant effect on it. But the EBIC contrast of small-angle (SA) GBs was weak at 300 K but strong at 100 K, which may be related with the special dislocation structure of SA GBs.In the mc-Si with artificial metal contaminations, it was found that the recombination activity of GBs became stronger as the contamination level went up. Variation in the recombination activity related to the GB character was also observed in these specimens. Random or high-S GBs showed stronger EBIC contrast than low-2 GBs. Moreover, we found that at 300 K SA GBs showed strong EBIC contrast even in the weakly contaminated mc-Si. The average EBIC contrast of SA GBs was much higher than those of ￡ and R GBs. It is indicated that SA GBs are easily contaminated by metallic impurities and maybe one of the most detrimental electrically active defects in mc-Si. The above results suggest that the recombination activity of GBs is principally determined by the gettering abilities of GBs. Specifically, the SA GBs may have the strongest gettering ability, while S3 has the weakest. Not only the GB character but also the GB plane would affect on the recombination activity of GBs. It was found that 13 boundaries with different planes showed significant difference in the gettering of Fe. 13 {111} has the weakest gettering ability. Furthermore, EBIC results revealed that the recombination activity of GBs was greatly enhanced by Cu and Ni at even low annealing temperature (300-400 °C), whereas Fe contaminations showed no significant effect on it. The impact was in the order of Fe < Ni < Cu.The impacts of GB character and impurity contamination level on the hydrogen passivation of GBs in mc-Si were also studied by means of EBIC. In the mc-Si with low contamination of Fe, the 300 K EBIC contrast of all kinds of GBs in the H-passivated state was weak and similar to those in the as-grown state. The 100 K EBIC contrast of I (S=3, 9, and 27) GBs decreased about 75%-80%, whereas that of random and small-angle GBs decreased about 35%-40%<, Due to the different impurity gettering ability of different GBs, the variation in 100 K EBIC contrast has suggested that the effect of H-passivation depends on both the GB character and impurity contamination level. In the mc-Si with heavy contamination of Fe, at both 300 and 100 K, the EBIC contrast of both S (2=3) and random GBs decreased but the ratio was less than 40%, meaning that the H-passivation is mainly affected by the heavy contamination of Fe.
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