Casting The Study Of Oxygen Defects In The Polysilicon Material

Currently, cast multicrystalline silicon has replaced monocrystalline silicon as the main photovoltaic materials. However, the efficiency of cast multicrystalline silicon solar cell is lower than that of monocrystalline silicon due to a high density of defects and a high concentration of impurities, such as grain boundaries, dislocations, carbon and oxygen. As one of the main and detrimental impurities, the super-saturation of oxygen may form thermal donors, new donors, precipitates during the crystal growth or the cell fabrication processes. Therefore, it is very important to investigate the behavior of thermal donors and oxygen precipitation in cast muliticrystalline silicon. On the basis of reviewing previous work, the formation behavior of thermal donors and oxygen precipitation in cast mulitcrystalline silicon was systemically studied by means of four-proble resistance measurement, Fourier transform infrared spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and Optical Microscopy (OM).It was found that high concentration of thermal donors formed in the specimens with high concentration of oxygen, high density of dislocation and low concentration of carbon. The result above indicates that dislocations enhance the formation of thermal donors and grain boundaries hardly influence it, while carbon noticeably restrains thermal donors' formation. It was also found that the pre-annealing at 650℃ for 30 minutes could effectively eliminate the as-grown thermal donors, but couldn't affect the subsequent thermal donors' formation. Like the formation behavior of thermal donors in monocrystalline silicon, the temperature of the maximum concentration of thermal donors is about 450℃. In comparison with classical annealing, rapid thermal annealing could eliminate thermal donors much more quickly.The behavior of oxygen precipitation in cast multicrystalline silicon was similar to that in Czochralski Silicon. The concentration of oxygen precipitates in both materials annealed at 1050℃ for 32 hours is highest, but the concentration in mc-Si is much higher than that in Cz-Si. It was also found that the concentration of oxygen precipitates in Cz-Si with high density of dislocation was higher than that in dislocation-free Cz-Si, which indicates that dislocation in silicon can noticeably enhance oxygen precipitation. And the role of grain boundaries of mc-Si is similar to that of dislocation for oxygen precipitation, but the enhancement of grain boundariesto oxygen precipitation is weaker. The experiment results of two-step annealing reveal that oxygen precipitation is enhanced after high temperature annealing following low temperature annealing. Being different from oxygen precipitates in monocrystalline silicon, the defects induced by oxygen precipitates hardly occure in cast multicrystalline silicon, which indicate that interstitial silicon atoms are absorbed by dislocations and grain boundaries.