Interaction Between Metallic Impurties And Structral Defects In Crystalline Silicon Used For Solar Cells

Cast multicrystalline silicon (mc-Si) occupies more and more market shares of solar cells because of its higher cost-effectiveness. However, since me silicon inevitably contains large quantities of metallic impurities and structural defects, solar cells based on such materials usually have lower photoelectric conversion efficiency than those based on single crystalline Czochralski (CZ) silicon. It is necessary to engineer the impurities and defects for improving the wafer quality and hence the cell performances.In this dissertation, based on some important problems associated with the practical solar cell manufacturing, we have studied the interactions between metallic impurities and structural defects in crystalline silicon and their impact on the performances of solar cells by combining microwave photocurrent decay, current/capacitance-voltage, optical microscope, and spreading resistance profiling techniques. Some innovative results have been achieved, which are illustrated below:(1) The causes of low minority carrier lifetime regions corresponding to the edge of casting me silicon ingots and its influence on the performance of solar cells have been explored. It is found that the distribution of interstitial iron exactly coincides with the minority carrier lifetime, indicating that iron contamination is mainly responsible for the lifetime degradation. After phosphorus diffusion gettering process, the low carrier lifetime region became narrower, and the concentration of interstitial iron was reduced by nearly one order of magnitude. After the celling process, the internal quantum efficiency of the edge zone has a lower response to the long wavelength light, in accordance with the minority carrier lifetime distribution. As a result, the solar cells based on edge zone wafers exhibit slightly lower efficiency than those conventional ones.(2) The effect of various kinds, forms and concentrations of metallic impurities on the electrical properties of a silicon grain-boundary (GB) formed by direct bonding technology has been investigated. It is found that compared to that of the clean GB, the density of interface states and their majority carrier capture cross-section for the contaminated GB are increased at different levels, dependent on the contaminated metal type. Meanwhile, the density of GB states in metal contaminated samples increases with the increase of metal content. The detrimental effect of a metal-contaminated GB on the electrical properties of me silicon solar cells can be minimized by engineering the metal precipitate density and size distribution. Compared to the clean GB, the energy distribution of interface states at the GB subjected to hydrogenation becomes shallower, and the carrier capture cross-section can be reduced by about two orders of magnitude, while the density of GB states varies slightly.(3) Influence of dislocations in silicon on solar cell performance has been studied. It is found that in high performance me silicon wafer, the wafer quality and the solar cell efficiency are both higher than normal me silicon solar cell, due to the lower dislocation density and the more uniform grain size distribution. Generally, the normal quasi-mono solar cell presents the best performance, as a result of low dislocation density and absence of grain boundary. However, the conversion efficiency of the defective quasi-mono solar cell is even lower than normal me cell, due to the existence of high density dislocations.(4) Effect of phosphorus diffusion gettering on metallic impurites has been studied. It is found that at a low level contamination, a single-step phosphorus gettering can be effective for most of metal impurities at the GB by extending the diffusion time. However, at a high level contamination, a two-step phosphorus gettering is more effective to getter out the gold impurities at the GB. After the effective phosphorus gettering for metallic impurities, the GB electrical characteristics could be recovered to the original level. However, during the phosphorus diffusion process, the present of SiP precipitates has strong influence on the performance of junction. By modifying phosphorus diffusion technology, the SiP precipitates can be eliminated, and therefore the conversion efficiency of solar cells gets improved.