At present, most of the polycrystalline silicon materials used in solar cells are aggregates containing a large number of single crystal particles, or are made by melting and casting waste single crystal silicon materials and metallurgical grade silicon materials. The process is to select polycrystalline bulk material or monocrystalline silicon head and tail material with a resistivity of 100~300Ω·cm, crush it, use a 1:5 mixture of hydrofluoric acid and nitric acid for proper corrosion, and then use deionization Rinse with water to be neutral, and dry. Place the polysilicon material in a quartz crucible, add an appropriate amount of borosilicate, and put it into the casting furnace, and heat and melt it in a vacuum state. After melting, it should be kept for about 20 minutes, and then poured into a graphite mold. After it slowly solidifies and cools, it becomes a polycrystalline silicon ingot (Figure 1). This kind of silicon ingot can be cast into cubes for slicing and processing into square solar cells, which can improve material utilization and facilitate assembly. The production process of polycrystalline silicon solar cells is similar to that of monocrystalline silicon solar cells. The photoelectric conversion efficiency of ordinary mass-produced polycrystalline silicon modules is about 17.4%, which is slightly lower than that of monocrystalline silicon modules. Get large-scale development.
The emergence of polycrystalline silicon solar cells is mainly to reduce costs. Its advantage is that large-size square silicon ingots suitable for large-scale production can be directly prepared. The equipment is relatively simple, the manufacturing process is simple, power saving, and silicon materials are required. Low. The influence of grain boundaries and impurities can be improved by battery technology; the disadvantage is that the battery efficiency is low due to the influence of material and grain boundaries. The battery technology mainly adopts gettering, passivation, back field and other technologies.
In recent years, the gettering process has received attention again, including phosphorus oxychloride gettering and aluminum gettering process. The gettering process has also been applied in the process of microelectronic devices. It can be seen that it is also effective for monocrystalline silicon wafers with a certain level of purity. However, the conditions used may not be suitable for solar cells. Therefore, it is necessary to study the special gettering technology for solar cells. Craft. Studies have proved that in polycrystalline silicon solar cells, the gettering effects of different materials are different. Some scholars have proposed a phosphorus gettering model, that is, the rate of gettering is controlled by two steps: ①The release/diffusion of metal impurities determines the absorption. The lower limit of the impurity temperature; ②The segregation model controls the optimal temperature of the gettering. Other scholars have proposed that the self-interstitial current of silicon is the basic factor of the gettering mechanism when phosphorus is diffused.
The conventional aluminum gettering process is formed by sintering after the aluminum film is vapor-deposited on the back of the battery, and the back field of the battery can also be formed at the same time. The work on gettering in recent years has proved that it has a certain effect on both high-efficiency monocrystalline silicon solar cells and polycrystalline silicon solar cells.
Passivation is an effective method to improve the quality of polysilicon. One method is to use hydrogen passivation to passivate defects such as dangling bonds in the silicon body. The silicon material with more defects caused by stress during crystal growth, the better the effect of hydrogen passivation. Hydrogen passivation can use ion implantation or plasma treatment. A layer of silicon nitride anti-reflection film is plated on the surface of polycrystalline silicon solar cells by PECVD method. Since hydrogen ions are generated when silane is decomposed, the polycrystalline silicon can have a hydrogen passivation effect.
Surface oxygen passivation technology is often used in high-efficiency solar cells to improve the efficiency of solar cells. In recent years, it has also been used on photovoltaic-grade crystalline silicon materials to have obvious effects, especially the use of thermal oxidation. The PECVD method is used to oxidize the surface at a lower temperature, and it has also been used in recent years, which has a certain effect.
Due to the presence of multiple crystal orientations on the surface of polycrystalline silicon solar cells, it is not as good as monocrystalline silicon that can be corroded to obtain an ideal texture structure. Therefore, various treatments on the surface to achieve anti-reflection effects are also a recent research goal. Among them, the multi-knife grinding wheel is used for surface grooving, and the process time for silicon wafers with an area of 10cm×10cm can be reduced to 30 seconds, which has a certain practical potential.
Since the production cost of polycrystalline silicon materials is lower than that of monocrystalline silicon materials, polycrystalline silicon modules have greater potential to reduce costs than monocrystalline silicon modules, and research work to improve the efficiency of polycrystalline silicon cells has also received widespread attention.