Industrialization technology of crystalline silicon solar cells -Lithium - Ion Battery Equipment
With the continuous breakthroughs in silicon material technology, semiconductor industry equipment manufacturing technology and key manufacturing process technology of photovoltaic cells, the beautiful vision of "sunshine everywhere, electricity everywhere" is closer and closer to our real life! In the past 20 years, the research results of photovoltaic scientists and photovoltaic cell manufacturing process technicians have reduced the cost of solar photovoltaic power generation from a few dollars/KWh to less than 25 cents/KWh. This trend will further reduce the cost of solar photovoltaic (PV) power generation by developing new technology, developing more advanced supporting equipment, cheaper photovoltaic electronic materials and new high-efficiency solar cell structure, and will reduce to 4 cents/KWh by the middle of this century, which is better than the traditional cost of power generation.
Large-area, thin-film, high-efficiency and highly automated intensive production will be the development trend of photovoltaic silicon battery industry. The core competitiveness of PV companies in the future lies in reducing the silicon material cost of peak-watt cells, reducing the power generation cost of unit cells by improving the photoelectric conversion efficiency and extending its service life, reducing the manufacturing cost of unit cells by saving human resources through intensive production, establishing excellent technical teams through reasonable mechanisms, preventing the unreasonable flow of talents, and fully ensuring the continuous innovation of technology!
1. Development of solar cell industrialization technology
The development of crystalline silicon solar cells can be divided into three stages, and the improvement of efficiency in each stage is due to the introduction of new technologies.
In 1954, Chapin et al. of Bell Laboratories developed a monocrystalline silicon solar cell with an efficiency of 6%. The first stage of development was in 1960. The important technology that led to the efficiency improvement was the increasingly perfect preparation process of silicon materials, and the continuous improvement of the quality of silicon materials, which made the cell efficiency rise steadily. During this period, the cell efficiency was 15%. From 1972 to 1985, it was the second stage of development. Back-Electric Field Battery (BSF) [1] technology, shallow junction structure [2], suede technology, and dense grid metallization were the representative technologies in this stage. The battery efficiency increased to 17%, and the battery cost decreased significantly. After 1985, it was the third stage of battery development. Photovoltaic scientists explored a variety of new battery technologies, metallized materials and structures to improve the performance of the battery and improve its photoelectric conversion efficiency: surface and bulk passivation technology, Al/P gettering technology, selective emission area technology, double-layer antireflection film technology, etc. Many batteries with new structures and technologies have emerged in this stage, such as passive emitter with efficiency of 24.4% and back point contact (PERL) [3] batteries. At present, a considerable number of technologies, materials and equipment are gradually breaking through the limitations of laboratories and being applied to industrial production. At present, many domestic and foreign companies have announced that by the end of 2008, the conversion efficiency of their large-scale industrial production will reach 18% for single crystals and more than 17% for polycrystals.(Lithium - Ion Battery Equipment)
1.1 Surface texture
Reducing the incident optical loss is the most direct way to improve the efficiency of the battery. Chemical corrosion process is the most mature industrial production technology and the most widely used technology in the industry, with low process threshold and large output; However, the quality of suede is not easy to control, the defect rate is high, and the anti-reflection effect is limited (the reflectivity after corrosion is generally still more than 11%), and there are a large number of chemical waste liquid and acid and alkali gases, which is not environmentally friendly production mode. Reactive ion etching (RIE) is the most promising technology. It first forms a layer of MASK (mask) on the surface of silicon wafer, then develops the surface texture model, and then uses the reactive ion etching method to prepare the surface texture. The antireflective textured surface prepared by this method is very perfect, and the minimum surface reflectivity can be reduced to 0.4%. The single polycrystalline technology is unified, and the production process and equipment can be transplanted to the IC industry. If the production cost can be further reduced, it is expected to replace the chemical etching method and be used on a large scale. Kyocera's 17.2% - 17.7% polycrystalline silicon battery is a successful example of plasma etching technology.
1.2 Emission zone diffusion
PN junction characteristics determine the performance of solar cells! The traditional process uniformly doped the surface of solar cells, and in order to reduce the contact resistance and improve the load capacity of the cells, the surface doping concentration is high. However, it was found that the high concentration of surface impurities led to the contraction of energy band in the diffusion zone, lattice distortion, new defects, obvious dead layer and poor short wave response of the battery. PN junction technology is an important technical gap between the world-class battery manufacturing companies and domestic battery companies. In order to improve the filling factor of the battery and prevent the dead layer on the surface, the selective diffusion emitter battery technology is the most promising low-cost revolutionary high-efficient battery technology for industrial production. Its technical principle is simple and has been realized in the laboratory through existing equipment, but how to reduce the manufacturing cost is an important challenge in the process of industrialization of this technology. At present, more than 17.6% of the high-efficiency batteries publicized by some large domestic companies come from this technology core. I believe that with the timely solution of supporting equipment and auxiliary materials, they will be rapidly popularized and promoted in the next two years.
At present, nitrogen carrying phosphorus oxychloride tubular high-temperature diffusion is the mainstream production technology in the manufacturing process, which is characterized by large output, mature process and simple operation. With the development of the battery towards large size and ultra-thin and low surface impurity concentration (surface square resistance 80~120Ω/port, uniformity± less than 3%), the advantages of reduced pressure diffusion technology (DOP) are very obvious. The low saturated vapor pressure of the impurity source in the process improves the molecular free path of the impurity, and its diffusion uniformity is still better than± for 156 size silicon wafers with 400 wafers per batch; 3% is the first choice for high-quality diffusion and environment-friendly production mode. The chain diffusion equipment not only adapts to the Inline automatic production mode, but also treats the silicon wafer with almost unlimited size and greatly reduced fragmentation rate, which is quickly paid attention to. Its process includes spraying phosphoric acid aqueous solution diffusion and screen printing phosphorus paste diffusion. In terms of chain diffusion technology, BTU, SCHMID and the 48th Institute of China Power Group have been studied and industrialized for a long time. As long as they can achieve breakthroughs in diffusion quality, they will replace the current tube diffusion as the mainstream production equipment and technology.
1.3 Edge removal technology
The industrialized peripheral PN junction removal method is plasma dry etching. This method has mature technology and large output, but there are over-etching, drilling and uneven phenomena, which not only affect the conversion efficiency of the battery, but also lead to the rise of the bad rate of the cell such as edge skipping, color difference and missing angle. The laser slotting isolation technology opens a physical isolation slot at the edge of the silicon wafer according to the PN junction depth, but contrary to the foreign situation, according to the domestic use situation, the battery efficiency is not as good as the plasma etching technology, so this method needs further research. At present, another technology emerging in the industry - chemical etching edge removal and back etching polishing technology integrates etching and PSG removal. The polishing of the back pile greatly reduces the transmission loss of incident light and improves the red light response of the battery. This method is simple in process, easy to realize inline automatic production, free of uneven drilling and etching, and relatively stable in process. Therefore, although the supporting equipment is expensive, it still attracts wide attention in the industry.
1.4 Surface antireflection film growth technology
In the early stage, TiO2 film or MgF2/ZnS mixed film was used to increase the absorption of incident light. However, this method requires the growth of a layer of 10-20 μ m SiO2 by thermal oxidation alone to make the surface of silicon wafer amorphous, and the effect on polycrystalline is not ideal.
The SixNy film not only slows down the corrosion of the glass body to silicon in the slurry and inhibits the diffusion rate of Ag, thus making the temperature range of the subsequent rapid firing process wider and easier to adjust, but also the dense SixNy film is a good barrier layer for harmful impurities. The hydrogen atom generated at the same time has the dual purpose of surface passivation and body passivation for silicon wafers, which can well repair dislocations and surface dangling bonds in silicon, improve the mobility of carriers in silicon wafers, and quickly become the mainstream technology of high-efficiency battery production. Double-layer SiN antireflection coating can achieve a reflectivity of 5.5% by controlling the enrichment rate of silicon in each layer; The reflectivity of the other kind of SiN and SiO mixed film is as low as 4.4%, and the best antireflection of the currently widely used single-layer SiN film is 10.4%.
It is a hot topic to grow a layer of 10-30nmSiN film on the back of the battery in order to maximize the passivation and defect repair of the battery and improve the efficiency of the battery. Because this technology involves the combination of screen printing technology, electrode paste technology and sintering technology, which is still in the experimental research stage, it is certainly a development trend in the future.
The customized antireflection film matching the refractive index of the packaging material to the spectrum to obtain the best practical use effect is the embodiment of the technical strength of the photovoltaic company! How to reduce the radiation damage of the PN junction on the surface of the battery caused by electromagnetic wave and the effective repair of the damage are the core technologies of this process. Poor treatment often leads to poor consistency of the battery efficiency. Equipment includes continuous indirect HF-PECVD and tubular direct LF-PECVD.
1.5 Screen printing and metal paste technology
Screen printing technology is the key technology for the industrialization of low-cost solar cells, and its important technological progress is closely linked with electrode paste and screen plate making technology. The technological progress of electrode slurry is the shortcut to improve the efficiency of battery, and also the key to the transformation of some laboratory technologies to industrialization. The development of corresponding pastes based on the diffusion thin layer square resistance, diffusion junction depth, and the thickness and density of the surface antireflection film on the surface of the battery has become a powerful force for the leading peers of world-class photovoltaic companies: for example, the P-doped silver paste realizes low-cost selective emitter technology; Add additives to the slurry to realize 80-100um fine grid technology; Low warpage aluminum paste with ultra-thin sheet.
As the thickness of silicon wafer continues to decrease and the battery area continues to increase, how to reduce the fragmentation rate and the warpage of the battery chip has become the focus of attention of equipment manufacturers and battery manufacturing companies. In terms of equipment, fully automatic printing equipment that can adapt to 120um thick silicon wafers has emerged.
2. Existing problems
Process: Although solar cell manufacturing is a short process, and photovoltaic technology and detection methods have also made considerable progress, the solar cell process cannot be fully controlled. We cannot accurately determine the specific problem from the unreasonable electrical parameters of the battery, and there is no completely effective detection method and means for the quality of each process. The online detection technology is far behind the development of process technology!
In terms of equipment: at present, there is no unified interface standard for the equipment of domestic and foreign manufacturers, which leads to the failure of effective connection between the previous and next processes, resulting in a large waste of time and resources! The equipment of new physical and chemical technology lags behind the development of the market!
In terms of raw materials: the raw material market, especially the quality of silicon wafers, is mixed. Many companies lack self-discipline, resulting in the unstable quality of photovoltaic products in China, and the lack of unified authoritative standards and access systems in the industry.
3. Development prospect
The theoretical limit efficiency of photovoltaic cell manufacturing technology based on silicon wafer is 29%. In recent years, due to a series of breakthroughs in new technologies, the conversion efficiency of silicon solar cells has reached 16% - 18% for single crystal and 15% - 17% for polycrystalline. According to the current road map of crystalline silicon cell efficiency and battery technology, it is very difficult to improve the efficiency. Therefore, some people predict the market life cycle of silicon battery, but the decisive factor of the product market vitality is its cost-performance ratio. Just like semiconductor integrated circuits, it is still inseparable from silicon for nearly a century. The status quo of crystalline silicon solar cells as an important material for photovoltaic power generation will not change, and the market leading position will continue to continue! It will be characterized by high efficiency, large size, ultra-thin and long service life. With the continuous deepening of our research on semiconductor materials and photovoltaic technology, some breakthrough technologies will continue to emerge to overcome the traditional, improve the efficiency of solar cells, reduce the cost of system power generation, and realize the leap of photovoltaic power generation from supplementary energy to mainstream energy! It is just that these technologies were developed by foreign companies and institutions before. It can be predicted that through the efforts of the majority of photovoltaic people in China, these revolutionary technological breakthroughs will appear in our local companies and scientific research institutions in the future!
Large-area, thin-film, high-efficiency and highly automated intensive production will be the development trend of photovoltaic silicon battery industry. The core competitiveness of PV companies in the future lies in reducing the silicon material cost of peak-watt cells, reducing the power generation cost of unit cells by improving the photoelectric conversion efficiency and extending its service life, reducing the manufacturing cost of unit cells by saving human resources through intensive production, establishing excellent technical teams through reasonable mechanisms, preventing the unreasonable flow of talents, and fully ensuring the continuous innovation of technology!
1. Development of solar cell industrialization technology
The development of crystalline silicon solar cells can be divided into three stages, and the improvement of efficiency in each stage is due to the introduction of new technologies.
In 1954, Chapin et al. of Bell Laboratories developed a monocrystalline silicon solar cell with an efficiency of 6%. The first stage of development was in 1960. The important technology that led to the efficiency improvement was the increasingly perfect preparation process of silicon materials, and the continuous improvement of the quality of silicon materials, which made the cell efficiency rise steadily. During this period, the cell efficiency was 15%. From 1972 to 1985, it was the second stage of development. Back-Electric Field Battery (BSF) [1] technology, shallow junction structure [2], suede technology, and dense grid metallization were the representative technologies in this stage. The battery efficiency increased to 17%, and the battery cost decreased significantly. After 1985, it was the third stage of battery development. Photovoltaic scientists explored a variety of new battery technologies, metallized materials and structures to improve the performance of the battery and improve its photoelectric conversion efficiency: surface and bulk passivation technology, Al/P gettering technology, selective emission area technology, double-layer antireflection film technology, etc. Many batteries with new structures and technologies have emerged in this stage, such as passive emitter with efficiency of 24.4% and back point contact (PERL) [3] batteries. At present, a considerable number of technologies, materials and equipment are gradually breaking through the limitations of laboratories and being applied to industrial production. At present, many domestic and foreign companies have announced that by the end of 2008, the conversion efficiency of their large-scale industrial production will reach 18% for single crystals and more than 17% for polycrystals.(Lithium - Ion Battery Equipment)
1.1 Surface texture
Reducing the incident optical loss is the most direct way to improve the efficiency of the battery. Chemical corrosion process is the most mature industrial production technology and the most widely used technology in the industry, with low process threshold and large output; However, the quality of suede is not easy to control, the defect rate is high, and the anti-reflection effect is limited (the reflectivity after corrosion is generally still more than 11%), and there are a large number of chemical waste liquid and acid and alkali gases, which is not environmentally friendly production mode. Reactive ion etching (RIE) is the most promising technology. It first forms a layer of MASK (mask) on the surface of silicon wafer, then develops the surface texture model, and then uses the reactive ion etching method to prepare the surface texture. The antireflective textured surface prepared by this method is very perfect, and the minimum surface reflectivity can be reduced to 0.4%. The single polycrystalline technology is unified, and the production process and equipment can be transplanted to the IC industry. If the production cost can be further reduced, it is expected to replace the chemical etching method and be used on a large scale. Kyocera's 17.2% - 17.7% polycrystalline silicon battery is a successful example of plasma etching technology.
1.2 Emission zone diffusion
PN junction characteristics determine the performance of solar cells! The traditional process uniformly doped the surface of solar cells, and in order to reduce the contact resistance and improve the load capacity of the cells, the surface doping concentration is high. However, it was found that the high concentration of surface impurities led to the contraction of energy band in the diffusion zone, lattice distortion, new defects, obvious dead layer and poor short wave response of the battery. PN junction technology is an important technical gap between the world-class battery manufacturing companies and domestic battery companies. In order to improve the filling factor of the battery and prevent the dead layer on the surface, the selective diffusion emitter battery technology is the most promising low-cost revolutionary high-efficient battery technology for industrial production. Its technical principle is simple and has been realized in the laboratory through existing equipment, but how to reduce the manufacturing cost is an important challenge in the process of industrialization of this technology. At present, more than 17.6% of the high-efficiency batteries publicized by some large domestic companies come from this technology core. I believe that with the timely solution of supporting equipment and auxiliary materials, they will be rapidly popularized and promoted in the next two years.
At present, nitrogen carrying phosphorus oxychloride tubular high-temperature diffusion is the mainstream production technology in the manufacturing process, which is characterized by large output, mature process and simple operation. With the development of the battery towards large size and ultra-thin and low surface impurity concentration (surface square resistance 80~120Ω/port, uniformity± less than 3%), the advantages of reduced pressure diffusion technology (DOP) are very obvious. The low saturated vapor pressure of the impurity source in the process improves the molecular free path of the impurity, and its diffusion uniformity is still better than± for 156 size silicon wafers with 400 wafers per batch; 3% is the first choice for high-quality diffusion and environment-friendly production mode. The chain diffusion equipment not only adapts to the Inline automatic production mode, but also treats the silicon wafer with almost unlimited size and greatly reduced fragmentation rate, which is quickly paid attention to. Its process includes spraying phosphoric acid aqueous solution diffusion and screen printing phosphorus paste diffusion. In terms of chain diffusion technology, BTU, SCHMID and the 48th Institute of China Power Group have been studied and industrialized for a long time. As long as they can achieve breakthroughs in diffusion quality, they will replace the current tube diffusion as the mainstream production equipment and technology.
1.3 Edge removal technology
The industrialized peripheral PN junction removal method is plasma dry etching. This method has mature technology and large output, but there are over-etching, drilling and uneven phenomena, which not only affect the conversion efficiency of the battery, but also lead to the rise of the bad rate of the cell such as edge skipping, color difference and missing angle. The laser slotting isolation technology opens a physical isolation slot at the edge of the silicon wafer according to the PN junction depth, but contrary to the foreign situation, according to the domestic use situation, the battery efficiency is not as good as the plasma etching technology, so this method needs further research. At present, another technology emerging in the industry - chemical etching edge removal and back etching polishing technology integrates etching and PSG removal. The polishing of the back pile greatly reduces the transmission loss of incident light and improves the red light response of the battery. This method is simple in process, easy to realize inline automatic production, free of uneven drilling and etching, and relatively stable in process. Therefore, although the supporting equipment is expensive, it still attracts wide attention in the industry.
1.4 Surface antireflection film growth technology
In the early stage, TiO2 film or MgF2/ZnS mixed film was used to increase the absorption of incident light. However, this method requires the growth of a layer of 10-20 μ m SiO2 by thermal oxidation alone to make the surface of silicon wafer amorphous, and the effect on polycrystalline is not ideal.
The SixNy film not only slows down the corrosion of the glass body to silicon in the slurry and inhibits the diffusion rate of Ag, thus making the temperature range of the subsequent rapid firing process wider and easier to adjust, but also the dense SixNy film is a good barrier layer for harmful impurities. The hydrogen atom generated at the same time has the dual purpose of surface passivation and body passivation for silicon wafers, which can well repair dislocations and surface dangling bonds in silicon, improve the mobility of carriers in silicon wafers, and quickly become the mainstream technology of high-efficiency battery production. Double-layer SiN antireflection coating can achieve a reflectivity of 5.5% by controlling the enrichment rate of silicon in each layer; The reflectivity of the other kind of SiN and SiO mixed film is as low as 4.4%, and the best antireflection of the currently widely used single-layer SiN film is 10.4%.
It is a hot topic to grow a layer of 10-30nmSiN film on the back of the battery in order to maximize the passivation and defect repair of the battery and improve the efficiency of the battery. Because this technology involves the combination of screen printing technology, electrode paste technology and sintering technology, which is still in the experimental research stage, it is certainly a development trend in the future.
The customized antireflection film matching the refractive index of the packaging material to the spectrum to obtain the best practical use effect is the embodiment of the technical strength of the photovoltaic company! How to reduce the radiation damage of the PN junction on the surface of the battery caused by electromagnetic wave and the effective repair of the damage are the core technologies of this process. Poor treatment often leads to poor consistency of the battery efficiency. Equipment includes continuous indirect HF-PECVD and tubular direct LF-PECVD.
1.5 Screen printing and metal paste technology
Screen printing technology is the key technology for the industrialization of low-cost solar cells, and its important technological progress is closely linked with electrode paste and screen plate making technology. The technological progress of electrode slurry is the shortcut to improve the efficiency of battery, and also the key to the transformation of some laboratory technologies to industrialization. The development of corresponding pastes based on the diffusion thin layer square resistance, diffusion junction depth, and the thickness and density of the surface antireflection film on the surface of the battery has become a powerful force for the leading peers of world-class photovoltaic companies: for example, the P-doped silver paste realizes low-cost selective emitter technology; Add additives to the slurry to realize 80-100um fine grid technology; Low warpage aluminum paste with ultra-thin sheet.
As the thickness of silicon wafer continues to decrease and the battery area continues to increase, how to reduce the fragmentation rate and the warpage of the battery chip has become the focus of attention of equipment manufacturers and battery manufacturing companies. In terms of equipment, fully automatic printing equipment that can adapt to 120um thick silicon wafers has emerged.
2. Existing problems
Process: Although solar cell manufacturing is a short process, and photovoltaic technology and detection methods have also made considerable progress, the solar cell process cannot be fully controlled. We cannot accurately determine the specific problem from the unreasonable electrical parameters of the battery, and there is no completely effective detection method and means for the quality of each process. The online detection technology is far behind the development of process technology!
In terms of equipment: at present, there is no unified interface standard for the equipment of domestic and foreign manufacturers, which leads to the failure of effective connection between the previous and next processes, resulting in a large waste of time and resources! The equipment of new physical and chemical technology lags behind the development of the market!
In terms of raw materials: the raw material market, especially the quality of silicon wafers, is mixed. Many companies lack self-discipline, resulting in the unstable quality of photovoltaic products in China, and the lack of unified authoritative standards and access systems in the industry.
3. Development prospect
The theoretical limit efficiency of photovoltaic cell manufacturing technology based on silicon wafer is 29%. In recent years, due to a series of breakthroughs in new technologies, the conversion efficiency of silicon solar cells has reached 16% - 18% for single crystal and 15% - 17% for polycrystalline. According to the current road map of crystalline silicon cell efficiency and battery technology, it is very difficult to improve the efficiency. Therefore, some people predict the market life cycle of silicon battery, but the decisive factor of the product market vitality is its cost-performance ratio. Just like semiconductor integrated circuits, it is still inseparable from silicon for nearly a century. The status quo of crystalline silicon solar cells as an important material for photovoltaic power generation will not change, and the market leading position will continue to continue! It will be characterized by high efficiency, large size, ultra-thin and long service life. With the continuous deepening of our research on semiconductor materials and photovoltaic technology, some breakthrough technologies will continue to emerge to overcome the traditional, improve the efficiency of solar cells, reduce the cost of system power generation, and realize the leap of photovoltaic power generation from supplementary energy to mainstream energy! It is just that these technologies were developed by foreign companies and institutions before. It can be predicted that through the efforts of the majority of photovoltaic people in China, these revolutionary technological breakthroughs will appear in our local companies and scientific research institutions in the future!
