The rapid development of solar cells has led to a tight supply of polysilicon raw materials, which makes the quality of raw materials entering the market different. Some fake and shoddy products flow into the market, and commercial disputes occur from time to time. As a result, the quality of products produced by downstream solar cell manufacturers according to standardization is unstable, the production cost is increased, and more importantly, the reputation is affected. Therefore, the establishment of solar grade polysilicon standards and testing methods is a problem that needs to be solved and must be solved. With this standard, we can effectively supervise the quality of solar grade polysilicon raw materials.
Relevant departments and institutions have held many meetings on the standard of solar grade polysilicon. On March 30, 2008, the national non ferrous metal Standardization Technical Committee organized a seminar on the national standard of solar energy level polysilicon (first draft) in Wuxi. The author also participated in the discussion. The delegates at the seminar conducted an in-depth analysis on the parameter setting, detection method and judgment basis of the national standard of solar energy level polysilicon (first draft), It is agreed that the theoretical basis and experimental basis for formulating the national standard of solar grade polysilicon are not sufficient, and the standard text is not yet mature.
The main issues under debate are as follows: what is solar grade polysilicon? What is the appropriate content of elements in solar grade polysilicon that mainly affect the photovoltaic conversion efficiency? Does the production method of solar grade polysilicon need to be distinguished?
The definition of solar grade polysilicon is a fundamental problem. Is the standard based on the cell efficiency or the content of impurities in polysilicon? In fact, the impurity content of polycrystalline silicon is closely related to the efficiency of solar cells. Electronic grade polycrystalline silicon with high purity (11 9, usually referred to as 11n) cannot be made into solar cells and must be mixed with impurities. This is why in the past, polycrystalline silicon of solar cell grade mostly used the head and tail materials with slightly lower purity of monocrystalline silicon rod or the leftover materials at the bottom of single crystal furnace for further smelting, doping The solar grade polycrystalline silicon is prepared by blending and melting the ingot again. It is generally believed that 6 ~ 7 9 polysilicon can be used as solar cells; If the impurity content increases, it will greatly affect the photovoltaic conversion efficiency. The research shows that the elements affecting the photovoltaic conversion efficiency include P, B, C, O, Fe, Cr, Ni, Cu, Zn, CA, Mg, Al, etc., and these elements also affect each other. Because the relationship between these interactions is very complex, so far, the research on its action mechanism and precise quantitative relationship is still not in-depth. Therefore, the time to formulate the standard of solar grade polysilicon is not very ripe.
At present, the photoelectric conversion efficiency of polycrystalline silicon solar cells made by cheap 5N physical method in China is 15%, which decays to 11% after two days. The purity of polycrystalline silicon by timminco physical method is 5N, boron 0.8×10-6 and phosphorus 5×10-6. The efficiency of polycrystalline silicon solar cells made by iron physical method in Kawasaki, Japan is 14.1%, and the efficiency of polycrystalline silicon solar cells made by French physical method is about 12.2%. Since polycrystalline (nano) silicon thin film solar cells with an efficiency of 6% ~ 8% can be used, the cells prepared by physical polycrystalline silicon should also be available from the development direction, but the stable efficiency is a little lower than that of Siemens polycrystalline silicon. The key is the cost and performance price ratio.
The minority carrier life directly affects the battery efficiency. For the polysilicon chip made by this method, the resistivity measured by the minority carrier life tester meets the production requirements, and the minority carrier life is not very uniform (see the figure below).
However, the solar cells prepared by physical polycrystalline silicon should consider the following issues:
① Stability issues. Because the purification technology of physical method is very different from the composition of raw material metal silicon, the stability of polysilicon chip is an important problem. As can be seen from the distribution diagram of minority carrier life in the figure above, the selection of raw materials and various purification processes should be accurately controlled.
② Improvement of solar cell production process. Because the polysilicon purified by physical method is different from that produced by chemical method, the production process of solar cell should also be different from the existing production process of solar cell, and a special impurity absorption process should be added. This process should reduce the requirements for materials and improve the efficiency of solar cells as much as possible, so as to produce solar cells with low cost and high relative efficiency.
③ Attenuation of solar cells. The attenuation of solar cells is an old problem. It was first found in amorphous silicon solar cells. In fact, there is attenuation problem in almost all silicon solar cells, but to varying degrees. The reason for the attenuation of the solar cell is not very clear. For the amorphous silicon solar cell, it is generally considered that the Si-H bond is unstable. Under the condition of light, it breaks into a hanging bond and forms a composite center, which attenuates the efficiency of the amorphous silicon solar cell. However, it can be stable after attenuation and can still be recovered after passivation. At present, the attenuation of solar cells prepared by physical polysilicon is serious, which must be related to the harmful impurities. It is a meaningful research direction to study the relationship between the attenuation of cell efficiency and harmful impurities and how to purify (or control) these harmful impurities.
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