Defects And Their Interaction With Metal Impurities In Crystalline Silicon Used For Solar Cells

Photovoltaic (PV) technology is currently one of the most effective solutions dealing with the energy crisis, and has achieved great development in recent years. Crystalline silicon solar cells occupy over 80% of the global PV market share. Low cost and high efficiency are the most mainstream strategies for the PV industry. Metal impurities and defects in crystalline silicon can remarkably degrade the quality of silicon and the performance of solar cells due to the introduction of deep levels in the silicon band gap. Therefore, it is of both theoretical and practical significance for the development of crystalline silicon solar cells to carry out research on metal impurities and defects.In this thesis, we investigate the properties of iron and nickel and the interactions between such metal impurities and other defects in crystalline silicon based on the previous researches, and taking use of lifetime measurement, scanning infrared microscopy and deep-level transient spectroscopy techniques. The obtained results are shown in the following:(1) Low temperature iron gettering in as-grown boron doped Czochralski silicon is first investigated. It is found that the decrease of interstitial iron concentration shows a double exponential dependence on annealing time, which suggests the existence of two sinks for interstitial iron. Meanwhile, the observed bulk defect densities and sizes show no difference in contaminated and as-grown samples. It implies that the grown-in defects could be the gettering sites in this process.(2) The origin of black center solar cells which show a black circle or ring in the center of the EL image is explored. The diffusion length of minority carrier degrades dramatically towards the cell center. QE scan comparison of black center and reference cells shows the response of long wave reduces in the center area, suggesting that the serious recombination is from the bulk. After Secco etching, the bulk defect density is observed to be high in the center and low at the edge. Dislocation which could form stacking fault after certain annealing is one of the main responsible defects, according to the SEM and TEM observations. Meanwhile, it is found that the iron concentration decreases towards the center, due to the gettering of the high density defects there. The black center solar cells are therefore mainly caused by decorations of metal (e.g. Fe) impurities at high density defects such as dislocations that grown in during crystal pulling.(3) The impact of nickel contaminants on the electrical properties of a direct wafer bonded (110)/(100) large angle grain boundary (GB) is studied using capacitance transient techniques. One can see that the density of GB states is increased and the energy levels at the GB go deeper due to nickel contamination, thus enhancing the recombination activity of the GB. After a 450℃ annealing, the GB states are reduced again since the nickel impurities form larger and sparser precipitates at the GB leading to the reduction of interaction area between bulk silicon and the nickel precipitates. Therefore, we find an approach to improve the electrical properties of a metal decorated GB by modulate the size and density distribution of the metal precipitates via an appropriate post-annealing.All the results above are important for better understanding the characteristics of metal impurities and defects in crystalline silicon, and controlling them to minimize their detrimental effect on wafer quality and cell performance.