502030 battery

Current status of industrialization technology of crystalline 502030 battery

With the continuous breakthroughs in silicon material technology, semiconductor industrial equipment manufacturing technology and key manufacturing process technology of photovoltaic cells, this beautiful vision of mankind is getting 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 the initial several dollars/KWh to less than 25 cents/KWh. And this trend will be further reduced by developing newer process technologies, developing more advanced supporting equipment, cheaper photovoltaic electronic materials and new high-efficiency solar cell structures. By the middle of this century, the cost of solar photovoltaic (PV) power generation will drop to 4 cents/KWh, which is better than the traditional power generation cost.

Large-area, thin-film, high-efficiency and highly automated intensive production will be the development trend of the photovoltaic silicon cell industry. The core competitiveness of photovoltaic companies in the future lies in reducing the silicon material cost of peak watt cells, improving the photoelectric conversion efficiency and extending its service life to reduce the power generation cost per cell, saving human resources through intensive production and reducing the manufacturing cost per cell, and establishing an excellent technical team through a reasonable mechanism, preventing the unreasonable flow of talents, and fully ensuring continuous technological innovation!

1. Development of solar cell industrialization technology

The development of crystalline 502030 battery can be divided into three stages. The improvement of efficiency in each stage is due to the introduction of new technologies.

In 1954, Chapin et al. of Bell Labs developed a single-crystal silicon solar cell with an efficiency of 6%. The first development stage was in 1960. The important technology that led to the improvement of efficiency was the increasingly perfect preparation process of silicon materials and the continuous improvement of the quality of silicon materials, which led to a steady increase in cell efficiency. During this period, the cell efficiency was 15%. The second development stage was from 1972 to 1985. Back-side electric field cell (BSF) [1] technology, shallow junction structure [2], velvet technology, and dense grid metallization were the representative technologies of this stage. The cell efficiency increased to 17% and the cell cost dropped significantly. After 1985, it was the third stage of battery development. Photovoltaic scientists explored various new battery technologies, metallization materials and structures to improve battery performance and increase its photoelectric conversion efficiency: surface and body passivation technology, Al/P doping technology, selective emission area technology, double-layer anti-reflection film technology, etc. Many batteries with new structures and technologies appeared in this stage, such as the passivated emitter and rear point contact (PERL) [3] battery with an efficiency of 24.4%. At present, a considerable number of technologies, materials and equipment are gradually breaking through the limitations of the laboratory 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 polycrystalline.

1.1 Surface texture

Reducing incident optical loss is the most direct way to improve battery efficiency. Chemical etching process is the most mature industrial production technology and the most widely used technology in the industry. It has low process threshold and large output. However, the quality of the velvet is difficult to control, the defect rate is high, and the anti-reflection effect is limited (the reflectivity after etching is generally still above 11%). A large amount of chemical waste liquid and acid-base gas appear, which is not an environmentally friendly production method. Reactive ion etching technology (RIE) is the most promising technology. It first forms a layer of MASK (mask) on the surface of the silicon wafer and then develops the surface texture model, and then uses the reactive ion etching method to prepare the surface texture. The anti-reflection velvet prepared by this method is very perfect, and the surface reflectivity can be reduced to 0.4% at the lowest. The single and 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 industrialized 17.2%~17.7% polycrystalline silicon cells are a successful example of using plasma etching technology.

1.2 Diffusion of emitter area

PN junction characteristics determine the performance of solar cells! Traditional processes uniformly dope the surface of solar cells, and the surface doping concentration is high in order to reduce contact resistance and improve the load capacity of the battery. However, studies have found that excessive surface impurity concentration leads to energy band shrinkage 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 world-class battery manufacturers and domestic battery companies. In order to prevent surface dead layer while improving the fill factor of the battery, selective diffusion emitter battery technology is the most promising low-cost revolutionary high-efficiency battery technology for industrial production. Its technical principle is simple and has been realized in the laboratory through existing equipment, but how to reduce manufacturing costs is an important challenge faced in the process of industrialization of this technology. At present, the technical core of high-efficiency batteries with a rate of more than 17.6% advertised by some large domestic companies comes from this. It is believed that with the timely solution of supporting equipment and auxiliary materials, it will be rapidly popularized and promoted in the next two years.

In the manufacturing process, the use of nitrogen-carrying phosphorus oxychloride tube-type high-temperature diffusion is the current mainstream production technology, which is characterized by large output, mature process and simple operation. As batteries develop towards larger sizes, ultra-thinness and lower surface impurity concentrations (surface sheet resistance 80-120Ω/口, uniformity within ±3%), the advantages of decompression diffusion technology (DOP) are very obvious. The low saturated vapor pressure of the impurity source in the process increases the molecular free path of the impurities. Its diffusion uniformity is still better than ±3% for 156-size silicon wafers with a batch output of 400 pieces. It is the first choice for high-quality diffusion and an environmentally friendly production method. Chain diffusion equipment is not only suitable for Inline automated production methods, but also has almost no restrictions on the size of silicon wafers, and the fragmentation rate is greatly reduced, which has quickly attracted attention. Its processes include spraying phosphoric acid aqueous solution diffusion and screen printing phosphorus slurry diffusion. In terms of chain diffusion technology, BTU, SCHMID and the 48th Institute of China Electric Power Group have all been conducting research and industrial applications for a long time. As long as a breakthrough can be made in diffusion quality, it will definitely replace the current tubular diffusion and become the mainstream production equipment and technology.

1.3 Edge removal technology

The industrialized method of removing the peripheral PN junction is plasma dry etching. This method is mature and has a large output, but there are over-engraving, drilling and unevenness, which not only affects the conversion efficiency of the battery, but also leads to an increase in the defective rate of battery cell edge jumping, color difference and missing corners. Laser slotting isolation technology opens a physical isolation groove at the edge of the silicon wafer according to the depth of the PN junction, but contrary to the situation abroad, according to domestic usage, the battery efficiency is not as good as plasma etching technology, so this method needs further research. Another technology currently emerging in the industry - chemical corrosion edge removal and back corrosion polishing technology integrates etching and PSG removal. The polishing of the back velvet greatly reduces the transmission loss of the incident light and improves the red light response of the battery. This method has a simple process and is easy to realize inline automated production. There is no uneven drilling and etching, and the process is relatively stable. Therefore, although the supporting equipment is expensive, it still attracts widespread attention in the industry.

1.4 Surface anti-reflection film growth technology

TiO2 film or MgF2/ZnS mixed film was used in the early stage to add absorption of incident light, but this method requires the use of thermal oxidation method to grow a layer of 10-20um SiO2 to make the silicon wafer surface amorphous, and the effect on polycrystalline is not ideal.

The SixNy film layer not only slows down the corrosion of silicon by the glass in the slurry and inhibits the diffusion rate of Ag, making the subsequent fast-firing process temperature range wider and easier to adjust, but also the dense SixNy film layer is a good barrier layer for harmful impurities. The hydrogen atoms generated at the same time have the dual purpose of surface passivation and body passivation for silicon wafers, which can well repair dislocations and surface hanging bonds in silicon, and improve the mobility of carriers in silicon wafers, thus quickly becoming the mainstream technology for high-efficiency battery production. The double-layer SiN anti-reflection film achieves a reflectivity of 5.5% by controlling the enrichment rate of silicon in each film layer; and the reflectivity of another SiN and SiO mixed film is as low as 4.4%. The currently widely used single-layer SiN film has an optimal anti-reflectivity of 10.4%.

Growing a 10-30nm SiN film on the back of the battery to maximize the passivation of the battery and repair defects to improve the efficiency of the battery is a hot topic at present. Since this technology involves the cooperation with the subsequent screen printing technology, electrode slurry technology and sintering process, it is still in the experimental research stage, but it is definitely a development trend in the future.

Matching the refractive index of the packaging material to the spectrum to customize the anti-reflection film to obtain the best practical use effect is a reflection of the technical strength of the photovoltaic company! How to reduce the radiation damage of electromagnetic waves to the PN junction on the surface of the battery and the effective repair of the damage is the core technology of this process. If it is not handled properly, it often leads to poor consistency of battery efficiency. Equipment includes continuous indirect HF-PECVD and tubular direct LF-PECVD.

1.5 Screen printing and metal slurry technology

Screen printing technology is a key technology for the industrial production of low-cost solar cells. Its important technological progress is closely related to electrode slurry and screen plate making technology. The advancement of electrode slurry technology is a shortcut to improve battery efficiency and is also the key to the transformation of some laboratory technologies to industrialization. Developing corresponding pastes according to the surface diffusion thin layer square resistance, diffusion junction depth, and surface anti-reflection film thickness and density of the battery has become a powerful way for international first-class photovoltaic companies to lead their peers: such as P-doped front silver paste to achieve low-cost selective emitter technology; adding additives to the paste to achieve 80-100um fine grid technology; low-warpage back aluminum paste with ultra-thin slices, etc.

As the thickness of silicon wafers continues to decrease and the area of batteries continues to increase, how to reduce the fragmentation rate and the warpage of the battery has become a focus of common concern for equipment manufacturers and battery manufacturers. In terms of equipment, fully automatic printing equipment that can adapt to 120um thick silicon wafers has appeared.

2. Existing problems

In terms of process: Although solar cell manufacturing is a short process production process, photovoltaic technology and detection methods have also made great progress, the solar cell process is still not in a fully controlled state. We cannot accurately judge 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 lags far behind the development of process technology!

Equipment: At present, the equipment of various manufacturers at home and abroad lacks a unified interface standard, which leads to the inability to effectively connect the upper and lower processes, resulting in a large waste of time and resources! The equipment of the new physical and chemical process lags behind the development of the market!

Raw materials: The quality of the raw material market, especially silicon wafers, is uneven, and many companies lack self-discipline, resulting in the instability of the quality of my country's photovoltaic products, and the industry lacks a unified authoritative standard and access system.

3. Development Outlook

The theoretical limit efficiency of photovoltaic cell manufacturing technology based on silicon wafers is 29%. In recent years, due to a series of new technology breakthroughs, the industrialization level of silicon solar cell conversion efficiency is 16% to 18% for single crystals and 15% to 17% for polycrystalline. According to the current crystalline silicon cell efficiency roadmap and cell technology, it is very difficult to improve efficiency. Therefore, some people predict the market life cycle of silicon batteries, but the determining factor of the vitality of the product market is its cost performance, just like semiconductor integrated circuits, which have been inseparable from silicon for nearly a century. The status quo of crystalline 502030 battery as important materials for photovoltaic power generation will not change, and their market dominance will continue! Its characteristics will be high efficiency, large size, ultra-thinness, and long life. With our continuous deepening of research on semiconductor materials and photovoltaic technology, there will be some breakthrough technologies that will subvert tradition, improve the efficiency of solar cells, reduce the cost of system power generation, and realize the leap from supplementary energy to mainstream energy for photovoltaic power generation! It’s just that these technologies were previously developed by foreign companies and institutions. It can be foreseen that through the efforts of the vast number of photovoltaic people in my country, these revolutionary technological breakthroughs will appear in our local companies and scientific research institutions in the future!

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