Low Defect Density Quasi-crystal Silicon Ingots With Large Percentage Of Single Crystal Fabrication And Its Surface Nano-texture Method Study

To grow quasi-silicon(quasi-Si) ingots by single-crystal Si seeded directional solidification technique is new and productive but with low-cost. The method is developed based on multi-crystal seeded directional solidification technique for the growth of multi-crystal Si ingots. The advantages of the technique are suitable for mass production in one large single furnace, low energy consumption per kilogram, high quality and high efficiency of the corresponding solar-cell wafers. However, there are some shortcomings for the current technique, such as low percentage of the single crystal relative to the multi-crystal part in the same ingots, high density of defects, difficulty to form the surface texture because both the single and multi-crystal grains coexist in the wafer surface, etc. In this thesis, we concentrated on how to get the quasi-crystal Si ingots with large percentage of single crystal, and trying to understand the corresponding growth mechanism. In addition, we try to solve the surface texture problem for the quasi-crystal Si wafers using a two-step metal-catalyzed chemical etching technique. We use the furnace in this study from American GT Company(G6-850) with an advanced double-electrical-source heating structure, which has a uniform thermal field. This furnace is suitable to mass production of large G6 Si ingots(one ingot can be cut into 36 sticks of 156×156mm2).We firstly optimized the fabrication processes of the growth of multi-crystal Si ingots by seeded directional solidification. The experimental results showed that the defect density distribution in the ingots from bottom to top can be divided into two zones: initial unchanged zone and subsequent quasi linear increasing zone. In the initial unchanged zone, the defect density of silicon ingot remained unchanged or slightly increased; in quasi linear increasing zone, the defect density of silicon ingot showed a linear increasing. With the decrease of cooling rate during fabrication, the defect density decreased and the unchanged zone extended, thus the quasi linear zone shortened. As a result, the percentage of the high efficient multi-crystal solar-cell wafers over 18% increased. To overcome local over-melting of the seeding layer, a new technique was developed to control the melting of seeding layer using a buffer loading layer. The buffer layer was over-coated on the seeding layer and composed of the crystal bricks layer above the layer of the small primary polycrystalline silicon grains layer. The buffer layer experiments showed: It can protect the seed layer from local over- melting by blocking the melt liquid flow down along the silicon grain gaps, resulting in the uniform melt of the seed layer, and extending the unchanged zone with low defect density, lowering the red zone(the zone where the life-time of minority carriers is less than 2 ?s) height, and improving the quality of silicon ingot. This strategy laid the foundation for the thin seed layer directional solidification process.Using the seeding of single-crystal Si, we explored the growth of quasi-crystal Si ingots, also based on the directional solidification by the G6 furnace. Similar to the growth of multi-crystal Si ingots, we found that: the defect density in the quasi-crystal Si ingots decreased with the decrease of cooling rate. And again the unchanged zone extended, while the quasi linear zone shortened. We obtained that the percentage of the single crystal in the quasi-crystal Si ingots reached 61%, when the cooling rate(defined as the ratio of decreased temperature during cooling and process time) was 0.467. However, it can be 80% when the cooling rate is 0.154. This percentage is valuable and acceptable for the mass production of quasi-crystal Si ingots by directional solidification.Based on the loading technique of buffer layer, we developed a new technique to fill the seeding layer with center dipped. The experiments showed that filling process can form a more flat melt-crystal interface which overcame the problem of the crucible center melting slower than the sides. This helped to form a larger percentage of single crystal in the ingot and it reached to 87.5%. This ratio is valuable for the mass production of quasi-Si ingots.The red zone in the ingot reduced the yield of Si wafers for solar cells. In this study, the formation mechanism of the red zone in Si ingot was studied by the solidification experiments of multi-crystal and quasi-Si ingots. The experiments showed that the height of the bottom red zone was effected by the Si melt temperature and the height of the red zone increased with the silicon melt temperature decreasing. The decrease of cooling rate lead to the decrease of the red zone height. The greater the temperature gradient, the higher the red zone at the bottom. The bottom red zone was consisting of mass small crystal grains due to larger temperature gradient at the very beginning of the solidification. And these small grains contained a large number of defects and grain boundaries which were the impurity adsorption centers which reduced the carrier lifetime. The buffer layer filling experiments showed that the buffer layer can avoid the partial over-melting of the seed layer, increase the melt temperature and lower the red zone height.The quasi-crystal Si wafer processes both single and multi-crystal grains on the surface, and therefore either acid or alkali etching is not fully successful for the surface texture during the solar cell fabrication. In this thesis, the quasi-crystal Si wafers have been nano-textured successfully by using a two-step metal-catalyzed chemical etching technique. The efficiencies of nano-textured cell are in a range from 18.4% to18.9% due to the quality diversity of wafers from the bottom to the top of a quasi-crystal Si ingot. A parallel sub cell model is proposed to explain the performance of quasi-crystal Si cells, which are mainly limited by the worst sub cell. Nevertheless, the results show that the performance of quasi-crystal Si solar cells is much superior to the traditional cast mc-Si solar cells(efficiency ~18.0%). However, it's still a great challenge to achieve efficient Si solar cells by not only increasing the proportion of sc-Si grains but also improving the quality and uniformity of mc-Si grains in the ingots.