CR1225 battery.Polycrystalline silicon solar cell manufacturing process

　　As we all know, there are many advantages to using solar energy. Photovoltaic power generation will provide the main energy for mankind. However, at present, it is necessary to make solar power generation have a large market and be accepted by the majority of consumers, improve the photoelectric conversion efficiency of solar cells, and reduce production. Cost should be the biggest goal we pursue. From the current development process of international solar cells, it can be seen that the development trends are monocrystalline silicon, polycrystalline silicon, strip silicon, and thin film materials (including microcrystalline silicon-based films, compound-based films, and dye films). From the perspective of industrialization development, the focus has shifted from monocrystalline to polycrystalline. The main reasons are: [1] There are fewer and fewer tail materials that can be supplied for solar cells; [2] For solar cells, square substrates are more Cost-effective, polysilicon obtained through casting and direct solidification methods can be directly obtained into square materials; [3] The production process of polysilicon continues to make progress, and the fully automatic casting furnace can produce more than 200 kilograms of silicon ingots per production cycle (50 hours). The size of the crystal grains reaches centimeter level; [4] Due to the rapid research and development of single crystal silicon technology in the past decade, the technology has also been applied to the production of polycrystalline silicon cells, such as selective etching of emitter junctions, back surface fields, and etched textures. , surface and body passivation, fine metal gate electrode, the use of screen printing technology can reduce the width of the gate electrode to 50 microns, and the height can reach more than 15 microns. Rapid thermal annealing technology can greatly shorten the process time when used in the production of polysilicon. The wafer heating process can be completed within one minute, and the battery conversion efficiency achieved by this process on a 100 square centimeter polycrystalline silicon wafer exceeds 14%. According to reports, the efficiency of cells currently produced on 50-60 micron polysilicon substrates exceeds 16%. Using mechanical notching and screen printing technology, the efficiency exceeds 17% on 100 square centimeters of polycrystalline. The same area is achieved without mechanical notching. The efficiency reaches 16%. It adopts a buried gate structure and mechanically grooves on 130 square centimeters of polycrystalline. The cell efficiency reaches 15.8%. The process technology of polycrystalline silicon cells is discussed below from two aspects:

　　1. Laboratory high-efficiency battery technology

　　Laboratory technology usually does not consider the cost of battery production and whether it can be mass-produced. It only studies methods and approaches to achieve the highest efficiency and provides the limits that specific materials and processes can achieve.

　　1.1 About light absorption

　　The main ones for light absorption are:

　　(1) Reduce surface reflection;

　　(2) Change the path of light in the battery body;

　　(3) Use back reflection.

　　For single crystal silicon, anisotropic chemical etching method can be used to create a pyramid-shaped texture structure on the (100) surface to reduce surface light reflection. However, the crystal direction of polysilicon deviates from the (100) plane, and the above method cannot produce a uniform texture. Currently, the following method is used:

　　[1]Laser groove

　　The inverted pyramid structure can be made on the surface of polycrystalline silicon by laser grooving. In the 500-900nm spectral range, the reflectivity is 4-6%, which is equivalent to making a double-layer anti-reflection film on the surface. However, on the (100) surface of single crystal silicon The reflectivity of chemically produced suede is 11%. The short-circuit current of the battery using laser to make the textured surface is about 4% higher than that of the battery with double-layer anti-reflection coating (ZnS/MgF2) on the smooth surface. This is mainly due to the oblique incidence of long-wave light (wavelength greater than 800nm) into the battery. The problem with laser fabrication of textured surfaces is that during etching, the surface is damaged and some impurities are introduced, and the surface damage layer needs to be removed through chemical treatment. Solar cells made by this method usually have a higher short-circuit current, but the open-circuit voltage is not too high. The main reason is that the surface area of the battery increases, causing the recombination current to increase.

　　[2]Chemical groove

　　Use a mask (Si3N4 or SiO2) for isotropic etching. The etching liquid can be an acidic etching liquid or a sodium hydroxide or potassium hydroxide solution with a higher concentration. This method cannot form the kind of anisotropic corrosion that is formed. Pointed cone-like structure. According to reports, the texture formed by this method has obvious anti-reflective effect on the spectral range of 700 to 1030 microns. However, the mask layer is generally formed at a higher temperature, which causes the performance of the polysilicon material to decrease. Especially for lower-quality polycrystalline materials, the minority carrier lifetime is shortened. The conversion efficiency of the battery made using this process on 225cm2 polysilicon reached 16.4%. The mask layer can also be formed by screen printing.

　　[3] Reactive ion etching (RIE)

　　This method is a mask-less etching process, and the textured surface formed has a particularly low reflectance, with a reflectance of less than 2% in the spectral range of 450 to 1000 microns. From an optical point of view, it is an ideal method, but the problem is that the silicon surface is severely damaged, and the open circuit voltage and fill factor of the battery decrease.

　　[4] Preparation of anti-reflection coating layer

　　For high-efficiency solar cells, the most common and effective method is to evaporate a ZnS/MgF2 double-layer anti-reflective film. The optimal thickness depends on the thickness of the underlying oxide layer and the characteristics of the cell surface, such as whether the surface is smooth or textured. , Anti-reflection processes also include evaporation Ta2O5, pECVD deposition Si3N3, etc. ZnO conductive film can also be used as an anti-reflection material.

　　1.2 Metalization technology

　　In the production of high-efficiency batteries, metallized electrodes must match the design parameters of the battery, such as surface doping concentration, pN junction depth, and metal materials. Laboratory batteries generally have a relatively small area (area less than 4cm2), so they require thin metal grid lines (less than 10 microns). The commonly used methods are photolithography, electron beam evaporation, and electron plating. The electroplating process is also used in industrial mass production, but when evaporation and photolithography are used together, it is not a low-cost process technology.

　　[1] Electron beam evaporation and electroplating

　　Usually, the positive glue stripping process is used to evaporate TI/pa/Ag multi-layer metal electrodes. To reduce the series resistance caused by the metal electrodes, the metal layer often needs to be thicker (8-10 microns). The disadvantage is that the metal layer is caused by electron beam evaporation. Damage to the silicon surface/passivation layer interface increases surface recombination. Therefore, in the process, the TI/pa layer is evaporated in a short time, and then the silver layer is evaporated. Another problem is that when the contact surface between metal and silicon is large, the minority carrier recombination speed will inevitably increase. In the process, a tunnel junction contact method is used to form a thin oxide layer between silicon and metal (generally a thickness of 20 About microns) The application of metals with lower work functions (such as titanium, etc.) can induce a stable electron accumulation layer on the silicon surface (fixed positive charges can also be introduced to deepen the inversion). Another method is to open a small window (less than 2 microns) in the passivation layer, and then deposit a wider metal gate line (usually 10 microns) to form a mushroom-like electrode. Using this method, 4cm2Mc-Si The conversion efficiency of the upper battery reaches 17.3%. Currently, Shallowangle (oblique) technology is also used on mechanically grooved surfaces.

　　1.3pN junction formation technology

　　[1] Emissive region formation and phosphorus gettering

　　For high-efficiency solar cells, selective diffusion is generally used to form the emission region. A heavy impurity region is formed under the metal electrode and shallow concentration diffusion is achieved between the electrodes. The shallow concentration diffusion in the emission region not only enhances the response of the cell to blue light, but also makes the silicon surface Easy to passivate. Diffusion methods include two-step diffusion process, diffusion plus etching process and buried diffusion process. Currently, selective diffusion is used. The conversion efficiency of 150mm×150mm battery reaches 16.4%. The surface sheet resistance of n++ and n+ areas are 20Ω and 80Ω respectively.

　　For Mc-Si materials, the impact of expanded phosphorus gettering on batteries has been extensively studied. A longer phosphorus gettering process (generally 3 to 4 hours) can increase the minority carrier diffusion length of some Mc-Si by two orders of magnitude. In the study of the gettering effect of substrate concentration, it was found that even for high-concentration substrate materials, a large minority carrier diffusion length (greater than 200 microns) can be obtained through gettering. The open circuit voltage of the battery is greater than 638mv, and the conversion efficiency More than 17%.

　　[2] Formation of back surface field and aluminum gettering technology

　　In Mc-Si batteries, the back p+p junction is formed by uniformly diffusing aluminum or boron. The boron source is generally BN, BBr, ApCVDSiO2:B2O8, etc. The aluminum diffusion is evaporated or screen-printed aluminum, and is completed by sintering at 800 degrees. Extensive research has also been carried out on the role of aluminum gettering, which, unlike phosphorus diffusion gettering, occurs at relatively low temperatures. The body defects also participate in the dissolution and deposition of impurities. At higher temperatures, the deposited impurities are easy to dissolve into silicon, which has an adverse effect on Mc-Si. So far, regional back fields have been used in monocrystalline silicon cell processes, but in polycrystalline silicon, all-aluminum back surface field structures are still used. [3] Double-sided Mc-Si battery

　　The Mc-Si bifacial cell has a conventional structure on the front side and a cross structure of N+ and p+ on the back side. In this way, the photogenerated minority carriers generated by illumination on the front side but located near the back side can be effectively absorbed by the back electrode. As an effective supplement to the front electrode, the back electrode also acts as an independent carrier collector for back illumination and scattered light. It is reported that under AM1.5 conditions, the conversion efficiency exceeds 19%.

　　1.4 Surface and bulk passivation technology

　　For Mc-Si, due to the presence of high grain boundaries and point defects (vacancies, interstitial atoms, metal impurities, oxygen, nitrogen and their complexes), the passivation of defects on the surface and body of the material is particularly important. In addition to the aforementioned In addition to the gettering technology, there are many methods of passivation process. Saturating the silicon dangling bonds through thermal oxidation is a relatively common method, which can greatly reduce the recombination speed of the Si-SiO2 interface. The passivation effect depends on the emission area. The surface concentration, interface state density and floating cross section of electrons and holes can be annealed in a hydrogen atmosphere to make the passivation effect more obvious. The use of pECVD to deposit silicon nitride on the front side has been very effective recently because it has a hydrogenation effect during the film formation process. This process can also be applied to large-scale production. The application of RemotepECVDSi3N4 can make the surface recombination speed less than 20cm/s.

　　2.Industrial battery technology

　　The development path of solar cells is from research laboratories to factories, and experimental research to large-scale production. Therefore, the characteristics that can achieve industrial production should be:

　　[1] The battery manufacturing process can meet the needs of assembly line operations;

　　[2] Capable of large-scale, modern production;

　　[3] Achieve high efficiency and low cost.

　　Of course, its main goal is to reduce the production cost of solar cells. At present, the main development direction of polysilicon cells is towards large areas and thin substrates. For example, 125mm×125mm, 150mm×150mm or even larger single-chip cells can be seen on the market. , the thickness has been reduced from the original 300 microns to the current 250, 200 and below 200 microns, and the efficiency has been greatly improved. The photoelectric conversion efficiency of 150mm×150mm cells produced in small batches by Japan's Kyocera Company reached 17.1%. The company's production volume in 1998 reached 25.4MW.

　　Screen printing and related technologies

　　The screen printing process is widely used in the large-scale production of polycrystalline silicon cells. This process can be used for printing diffusion sources, front metal electrodes, back contact electrodes, anti-reflection coatings, etc. With the improvement of screen materials and the improvement of process levels, , the screen printing process will be more commonly used in the production of solar cells.

　　a. Formation of emission area

　　Screen printing is used to form pN junctions, replacing the conventional tube furnace diffusion process. Generally, phosphorus-containing slurry is printed on the front side of polysilicon and aluminum-containing metal slurry is printed on the reverse side. After printing is completed, diffusion can be completed in a mesh belt furnace (usually the temperature is 900 degrees). In this way, printing, drying, and diffusion Can form continuous production. The emitter area formed by screen printing diffusion technology usually has a relatively high surface concentration, so the photogenerated carriers recombine on the surface are larger. In order to overcome this shortcoming, the following selective emitter area process technology is used to achieve the highest conversion efficiency of the battery. further improvement.

　　b. Select the emission area process

　　In the diffusion process of polycrystalline silicon cells, the selective emission region technology is divided into local etching or two-step diffusion methods. Local corrosion is to use dry methods (such as reactive ion etching) or chemical etching methods to etch away the re-diffusion layer in the area between the metal electrodes. Initially, Solarex applied the reactive ion etching method in the same equipment. It first used high reaction power to etch away the heavily doped layer between the metal electrodes, and then used low power to deposit a silicon nitride film. This film exerted anti-reflection and The dual role of battery surface passivation. A cell with a conversion efficiency exceeding 13% is made on 100cm2 polycrystalline. On the same area, using the two-part diffusion method, the conversion efficiency reaches 16% without mechanical texture.

　　c. Formation of back surface field

　　The back pN junction is usually formed by screen printing A paste and thermal annealing in a mesh belt furnace. This process not only forms the back surface junction, but also has a good gettering effect on impurities in polysilicon. The aluminum gettering process is generally The high-temperature section is completed, and the measurement results show that the gettering effect can restore the decrease in polysilicon minority carrier lifetime caused by the previous high-temperature process. A good back surface field can significantly increase the open circuit voltage of the battery.

　　d. Screen printing metal electrodes

　　In large-scale production, the screen printing process has more advantages than vacuum evaporation, metal plating and other processes. In the current process, silver-containing slurries are generally used as front printing materials. The main reason is that silver has good properties. conductivity, solderability and low diffusion in silicon. The electrical conductivity of the metal layer formed by screen printing and annealing depends on the chemical composition of the slurry, the content of glass body, the roughness of the screen, the sintering conditions and the thickness of the screen plate. In the early 1980s, screen printing had some drawbacks:

　　ⅰ) If the gate line width is large, usually greater than 150 microns;

　　ⅱ) Resulting in greater light shading and lower battery filling factor;

　　ⅲ) It is not suitable for surface passivation, mainly because the surface diffusion concentration is high, otherwise the contact resistance is large. At present, advanced methods can be used to screen-print grid lines with a line width of 50 microns, a thickness of more than 15 microns, and a sheet resistance of 2.5 to 4mΩ. This parameter can meet the requirements of high-efficiency batteries. Someone compared solar cells made with screen-printed electrodes and evaporated electrodes on 150mm×150mm Mc-Si, and there was almost no difference in parameters.

　　3.Conclusion

　　The manufacturing process of polycrystalline silicon cells continues to advance, ensuring that the efficiency of the cells continues to increase and the cost decreases. With the deepening of understanding of the physical and optical properties of materials and devices, the structure of batteries has become more reasonable, and the distance between laboratory level and industrial mass production has continued to shrink. Screen printing and buried gate processes have played a major role in high-efficiency and low-cost batteries. High-efficiency Mc-Si battery components have entered the market in large quantities. Current research is focusing on new thin film structures, batteries on cheap substrates, etc. Facing the Users, what we need to do is to achieve larger batches and low-cost production. We hope that we will work harder to achieve this goal.

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