Application and research of silicon carbide ceramics in the field of photovoltaics - Vetek Semiconductor

With the increasing shortage of traditional energy sources such as oil and coal, new energy industries, led by solar photovoltaics, have developed rapidly in recent years. Since the 1990s, the world's photovoltaic installed capacity has increased 60 times. The global photovoltaic industry has taken off against the backdrop of energy structure transformation, and the industry scale and installed capacity growth rate have repeatedly set new records. In 2022, the global photovoltaic installed capacity will reach 239GW, accounting for 2/3 of all new renewable energy capacity. It is estimated that in 2023, the global photovoltaic installed capacity will be 411GW, a year-on-year increase of 59%. Despite the continued growth of photovoltaics, photovoltaics still only account for 4.5% of global power generation, and its strong growth momentum will continue until after 2024.

Silicon carbide ceramics have good mechanical strength, thermal stability, high temperature resistance, oxidation resistance, thermal shock resistance and chemical corrosion resistance, and are widely used in hot fields such as metallurgy, machinery, new energy and building materials and chemicals. In the photovoltaic field, it is mainly used in the diffusion of TOPcon cells, LPCVD (low pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition) and other thermal process links. Compared with traditional quartz materials, boat supports, boats, and pipe fittings made of silicon carbide ceramic materials have higher strength, better thermal stability, no deformation at high temperatures, and a lifespan of more than 5 times that of quartz materials, which can significantly reduce the cost of use and the loss of energy caused by maintenance and downtime, and have obvious cost advantages.

Ⅰ Advantages of silicon carbide ceramics in the photovoltaic field

The main products of silicon carbide ceramics in the photovoltaic cell field include silicon carbide boat supports, silicon carbide boats, silicon carbide furnace tubes, silicon carbide cantilever paddles, silicon carbide rods, silicon carbide protective tubes, etc. Among them, silicon carbide boat supports and silicon carbide boats replace the original quartz boat supports and boats. Due to their obvious advantages and rapid development, they have become a good choice for key carrier materials in the production process of photovoltaic cells, and their market demand is increasingly attracting attention from the industry.

Reaction Bonded Silicon Carbide (RBSC) ceramics are the most widely used silicon carbide ceramics in the field of photovoltaic cells. Its advantages are low sintering temperature, low production cost, and high material densification. In particular, there is almost no volume shrinkage during the reaction sintering process. It is particularly suitable for the preparation of large-sized and complex-shaped structural parts. Therefore, it is most suitable for the production of large-sized and complex products such as boat supports, small boats, cantilever paddles, furnace tubes, etc. The basic principle of the preparation of RBSC ceramics is: under the action of capillary force, reactive liquid silicon penetrates into the carbon-containing porous ceramic blank, reacts with the carbon source in the blank to generate secondary phase β-SiC, and at the same time, the secondary phase β-SiC is in situ combined with the α-SiC particles in the blank powder, and the remaining pores continue to be filled with free silicon, and finally the densification of RBSC ceramic materials is achieved. The various properties of RBSC ceramic products at home and abroad are shown in Table 1.

Table 1 Comparison of performance of reaction sintered SiC ceramic products in major countries

In the manufacturing process of solar photovoltaic cells, silicon wafers are placed on a boat, and the boat is placed on a boat holder for diffusion, LPCVD and other thermal processes. The silicon carbide cantilever paddle (rod) is a key loading component for moving the boat holder carrying silicon wafers into and out of the heating furnace. As shown in Figure 1, the silicon carbide cantilever paddle (rod) can ensure the concentricity of the silicon wafer and the furnace tube, thereby making the diffusion and passivation more uniform. At the same time, it is pollution-free and non-deformed at high temperatures, has good thermal shock resistance and large load capacity, and has been widely used in the field of photovoltaic cells.

Figure 1 Schematic diagram of key battery loading components

In the traditional quartz boat and boat holder, in the soft landing diffusion process, the silicon wafer and the quartz boat holder need to be placed in the quartz tube in the diffusion furnace. In each diffusion process, the quartz boat holder filled with silicon wafers is placed on the silicon carbide paddle. After the silicon carbide paddle enters the quartz tube, the paddle automatically sinks to put down the quartz boat holder and silicon wafer, and then slowly runs back to the origin. After each process, the quartz boat holder needs to be removed from the silicon carbide paddle. Such frequent operation will cause the quartz boat support to wear out over a long period of time. Once the quartz boat support cracks and breaks, the entire quartz boat support will fall off the silicon carbide paddle, and then damage the quartz parts, silicon wafers and silicon carbide paddles below. Silicon carbide paddles are expensive and cannot be repaired. Once an accident occurs, it will cause huge property losses.

In the LPCVD process, not only will the above-mentioned thermal stress problems occur, but since the LPCVD process requires silane gas to pass through the silicon wafer, the long-term process will form a silicon coating on the boat support and the boat. Due to the inconsistency of the thermal expansion coefficients of the coated silicon and quartz, the boat support and the boat will crack, and the life span will be seriously reduced. The life span of ordinary quartz boats and boat supports in the LPCVD process is usually only 2 to 3 months. Therefore, it is particularly important to improve the boat support material to increase the strength and service life of the boat support to avoid such accidents.

Ⅱ Development trend of silicon carbide ceramic materials in the photovoltaic field

From the 13th Shanghai Photovoltaic Exhibition SNEC 2023, many photovoltaic companies in the country have begun to use silicon carbide boat supports, as shown in Figure 2, such as Longi Green Energy Technology Co., Ltd., JinkoSolar Co., Ltd., Yida New Energy Technology Co., Ltd. and other photovoltaic leading companies. Silicon carbide boat supports used for boron expansion, due to the high use temperature of boron expansion, usually at 1000 ~ 1050℃, the impurities in the boat support are easy to volatilize at high temperature to pollute the battery cell, thereby affecting the conversion efficiency of the battery cell, so there are higher requirements for the purity of the boat support material.

Figure 2 LPCVD silicon carbide boat support and boron expansion silicon carbide boat support

At present, the boat support used for boron expansion needs to be purified. First, the raw material silicon carbide powder is acid-washed and purified. The purity of lithium-grade silicon carbide powder raw materials is required to be above 99.5%. After acid washing and purification with sulfuric acid + hydrofluoric acid, the purity of the raw materials can reach above 99.9%. At the same time, the impurities introduced during the preparation of the boat support must be controlled. Therefore, the boron expansion boat holder is mostly formed by grouting to reduce the use of metal impurities. The grouting method is usually formed by secondary sintering. After re-sintering, the purity of the silicon carbide boat holder is improved to a certain extent.

In addition, during the sintering process of the boat holder, the sintering furnace must be purified in advance, and the graphite heat field in the furnace also needs to be purified. Usually, the purity of the silicon carbide boat holder used for boron expansion is about 3N.

The silicon carbide boat has a promising future. The silicon carbide boat is shown in Figure 3. Regardless of the LPCVD process or the boron expansion process, the life of the quartz boat is relatively low, and the thermal expansion coefficient of the quartz material is inconsistent with that of the silicon carbide material. Therefore, it is easy to have deviations in the process of matching with the silicon carbide boat at high temperature, which leads to shaking or even breaking of the boat.

The silicon carbide boat adopts an integrated molding and overall processing process route. Its shape and position tolerance requirements are high, and it cooperates better with the silicon carbide boat holder. In addition, silicon carbide has high strength, and the boat breakage caused by human collision is much less than that of the quartz boat. However, due to the high purity and processing precision requirements of silicon carbide boats, they are still in the small batch verification stage.

Since the silicon carbide boat is in direct contact with the battery cell, it must have a high purity even in the LPCVD process to prevent contamination of the silicon wafer.

The biggest difficulty of silicon carbide boats lies in machining. As we all know, silicon carbide ceramics are typical hard and brittle materials that are difficult to process, and the shape and position tolerance requirements of the boat are very strict. It is difficult to process silicon carbide boats with traditional processing technology. At present, the silicon carbide boat is mostly processed by diamond tool grinding, and then polished, pickled and other treatments are performed.

Figure 3 Silicon carbide boat

Compared with quartz furnace tubes, silicon carbide furnace tubes have good thermal conductivity, uniform heating, and good thermal stability, and their lifespan is more than 5 times that of quartz tubes. The furnace tube is the main heat transfer component of the furnace, which plays a role in sealing and uniform heat transfer. The manufacturing difficulty of silicon carbide furnace tubes is very high, and the yield rate is also very low. First, due to the huge size of the furnace tube and the wall thickness usually between 5 and 8 mm, it is very easy to deform, collapse or even crack during the process of blank forming.

During sintering, due to the huge size of the furnace tube, it is also difficult to ensure that it will not deform during the sintering process. The uniformity of silicon content is poor, and it is easy to have local non-siliconization, collapse, cracking, etc., and the production cycle of silicon carbide furnace tubes is very long, and the production cycle of a single furnace tube exceeds 50 days. Therefore, silicon carbide furnace tubes are still in the research and development state and have not yet been mass-produced.

The main cost of silicon carbide ceramic materials used in the photovoltaic field comes from high-purity silicon carbide powder raw materials, high-purity polycrystalline silicon, and reaction sintering costs.

With the continuous development of silicon carbide powder purification technology, the purity of silicon carbide powder continues to increase through magnetic separation, pickling and other technologies, and the impurity content gradually decreases from 1% to 0.1%. With the continuous increase in silicon carbide powder production capacity, the cost of high-purity silicon carbide powder is also decreasing.

Since the second half of 2020, polysilicon companies have successively announced expansions. Currently, there are more than 17 domestic polysilicon production companies, and the annual output is estimated to exceed 1.45 million tons in 2023. The overcapacity of polysilicon has led to a continuous decline in prices, which in turn has reduced the cost of silicon carbide ceramics.

In terms of reaction sintering, the size of the reaction sintering furnace is also increasing, and the loading capacity of a single furnace is also increasing. The latest large-size reaction sintering furnace can load more than 40 pieces at a time, which is much larger than the existing reaction sintering furnace loading capacity of 4 to 6 pieces. Therefore, the sintering cost will also drop significantly.

On the whole, silicon carbide ceramic materials in the photovoltaic field are mainly developing towards higher purity, stronger carrying capacity, higher loading capacity, and lower cost.

Ⅲ The significance of silicon carbide ceramic materials in the photovoltaic field

At present, the high-purity quartz sand required for quartz materials used in the domestic photovoltaic field is still mainly dependent on imports, while the quantity and specifications of high-purity quartz sand exported from foreign countries to China are strictly controlled. The tight supply of high-purity quartz sand materials has not been alleviated and has restricted the development of the photovoltaic industry. At the same time, due to the low life of quartz materials and easy damage leading to downtime, the development of battery technology has been seriously restricted. Therefore, it is of great significance for my country to get rid of foreign technological blockades by conducting research on the gradual replacement of quartz materials with silicon carbide ceramic materials.

In a comprehensive comparison, whether it is product performance or use cost, the application of silicon carbide ceramic materials in the field of solar cells is more advantageous than quartz materials. The application of silicon carbide ceramic materials in the photovoltaic industry has great help for photovoltaic companies to reduce the investment cost of auxiliary materials and improve product quality and competitiveness. In the future, with the large-scale application of large-size silicon carbide furnace tubes, high-purity silicon carbide boats and boat supports and the continuous reduction of costs, the application of silicon carbide ceramic materials in the field of photovoltaic cells will become a key factor in improving the efficiency of light energy conversion and reducing industry costs in the field of photovoltaic power generation, and will have an important impact on the development of photovoltaic new energy.