Impurity control in high performance multicrystalline silicon

In this project we have developed extensive competence on how impurities are transported and distributed into High Performance Multicrystalline Silicon (HPMC-Si), and how this influences the solar cell efficiency. The aim has been to develop methods to influence the different impurity sources and produce better and less expensive solar cells. HPMC-Si is the dominating solar cell material today. Compared to traditional multicrystalline silicon, it contains far more grain boundaries, but during the crystallization process, this leads to less development of a different class of defects, the dislocations. These defects are traditionally the limiting factor for the solar cell performance; we say they are "electrically active" because they capture electrons and prevent them from generating electric current. In HPMC-Si we postulate that the density of these defects is so low that the next important step to improve the solar cell performance is to reduce the contamination by metals such as Fe, Cr and Ni. There are primarily three sources of impurities: The silicon feedstock, the quartz crucible and a thin layer of silicon nitride "coating" between the crucible and the silicon, preventing direct chemical contact between the two. The latter two contain 1000 to 100000 as much of the important impurities as the feedstock does. However, little is known about the speed of diffusion through the crucible and coating, and this is what determines the final level of impurities in the silicon crystal. We have therefore performed experiments to investigate this, and for the important impurity element Fe, it turns out to be 1000 times higher than has been believed before. A possible action to reduce the contamination from the crucible is to use a thin layer of cleaner quartz on the inside of the crucible. In the crystal, the metals are distributed as single atoms in the lattice, but also as larger particles at dislocations and grain boundaries. These are less detrimental than single atoms but they are also more difficult to remove during the solar cell production process. It turns out that even though there are few dislocations in HPMC-Si, they are still the most potent sites for precipitation. It is therefore even more gain in growing crystals of low dislocation density. Removing impurities during the solar cell processing is called "gettering". It turns out that even though we remove metals during this process, the material does not improve, particularly the grain boundaries and dislocations become worse. We have performed measurements that indicate that the concentration of impurities on these defects in fact increase during the gettering process. The last stage in the cell process involves hydrogen that diffuses into the crystal and react with the metals to render them less electrically active. This is called "passivation" and when combined with gettering first, in sum the material quality improves over the original; however it is important that both processes are performed in succession. We have studied the influence of different types of grain boundaries on the material quality. It turns out that the grain boundaries characteristic of HPMC-Si, so called random grain boundaries where the crystals have grown independently of each other, are hardly electrically active in the finished cell. All grain boundaries that do remain active can be understood as either consisting of dislocations or being covered by dislocations, and they attract impurities in the same manner as dislocations do. An important part of the project has been to create mathematical models describing these processes. By varying different parameters in the models, we can investigate which efforts will have the biggest impact on impurities. This we have combined with lab experiments where we make crystals in the same way as the industry. Comparing experiment and model enables us to trust the results of the modelling, as well as explain phenomena observed in the experiments. The result of this work confirms that there is a large potential in improving the crucible insides. If this succeeds, it is actually probable that the quality of the silicon feedstock will determine the solar cell performance, particularly the new, low cost types of feedstocks that have been developed during the past decades. It is however still uncertain how much the impurity level in the different sources influence the quality of the solar cell since the diffusion speed through crucible and coating to a large extent still is unknown. Still it turns out that the most important factor to control in HPMC-Si is the density of dislocations. In the crystals we have made, there is a rather big difference in solar cell quality although the impurity level is the same, and this clearly correlates to the dislocation density.

Photovoltaic solar energy is recognized as one of the most promising future sustainable energy sources. Multicrystalline silicon solar cells represent the most cost effective alternative. In this type of solar cells crystal defects and impurities are pres ent; crystal defects are introduced during crystallization, and impurities are introduced from the feedstock, the crucibles or the coating. The impurities and defects interact to reduce the solar cell efficiency. Therefore research which aims to increase solar cell efficiency needs to address two factors: How to minimize the presence of crystal defects and impurities, and how to mitigate their effects. Recent developments in crystallization technology have shown that it is possible to produce silicon wit h particularly low defect density in a robust industrial manner. The underlying assumption for this project is therefore that future improvements can now most likely be reached by achieving a better control of contamination. The primary objective of the proposed project is therefore to develop knowledge about impurity transport processes and impurity-defect interaction throughout the process of producing high performance multicrystalline silicon solar cells. The final aim is to provide reliable specific ations for the main components in the crystal growth system, i.e. silicon feedstock, crucibles and coating, as well as best practice guidelines for the process. The project involves 4 industry and 3 research partners. The challenges will be approached th rough integrated use of experiments and mathematical modelling. The competence built within this project aims at answering technologically critical questions and providing a better general understanding of the impurity transport through the value chain. These results are of high importance for the partners as well as the global photovoltaic industry and research community.