polysilicon Solar cell Manufacturing process
As we all know, the use of solar energy has many advantages. Photovoltaic power generation will provide the main energy for human beings. But at present, to make solar power generation have a large market and be accepted by the majority of consumers, improving the photoelectric conversion efficiency of solar cells and reducing production costs should be our goal. From the current development process of international solar cells, we can see that their development trend is monocrystalline silicon , polycrystalline silicon, ribbon silicon, film materials (including microcrystalline silicon based films, compound based films and dye films). From the perspective of industrial development, the focus has shifted from single crystal to polycrystalline. The main reasons are: [1] There are fewer and fewer head and tail materials available for solar cells; [2] For solar cells, square substrates are more cost-effective. Polysilicon obtained by casting and direct solidification can directly obtain square materials; [3] The production process of polysilicon has made continuous progress. Every production cycle (50 hours) of the fully automatic casting furnace can produce more than 200kg of silicon ingots, and the grain size reaches centimeter level; [4] Due to the rapid research and development of monocrystalline silicon technology in the past decade, the technology has also been applied to the production of polycrystalline silicon batteries, such as the selection of corrosive emission junction, back surface field, corrosive textured surface, surface and body passivation, and fine metal gate electrode. The screen printing technology can reduce the width of the gate electrode to 50 microns and the height to more than 15 microns, The rapid thermal annealing technology used in the production of polysilicon can greatly shorten the process time. The thermal process time of a single chip can be completed in one minute. The conversion efficiency of the battery made on a 100 square centimeter polysilicon chip using this process exceeds 14%. It is reported that at present, the efficiency of the battery made on 50~60 μ m polysilicon substrate is more than 16%, the efficiency of the battery made on 100 square centimeters of polysilicon by mechanical grooving and screen printing technology is more than 17%, the efficiency of the battery made on the same area without mechanical grooving is 16%, and the efficiency of the battery made on 130 square centimeters of polysilicon by mechanical grooving with buried grid structure is 15.8%.
The process technology of polysilicon battery is discussed from two aspects:
1. Laboratory high-efficiency battery process
Laboratory technology usually does not consider the cost of battery production and whether it can be mass produced, but only studies the methods and ways to achieve high efficiency, and provides the limits that specific materials and processes can reach.
1.1 About light absorption
For light absorption:
(1) Reduce surface reflection;
(2) Changing the path of light in the battery body;
(3) Use back reflection.
For monocrystalline silicon, the pyramidal textured structure can be made on the (100) surface by anisotropic chemical etching to reduce the surface light reflection. However, the polycrystalline silicon crystal orientation deviates from the (100) surface, and the above method cannot be used to make uniform textured surface. At present, the following methods are used:
[1] Laser grooving
The inverted pyramid structure can be made on the surface of polysilicon by laser grooving. In the spectral range of 500 ～ 900nm, the reflectivity is 4 ～ 6%, which is equivalent to the double-layer antireflection coating made on the surface, while the reflectivity of chemical textured surface made on (100) face of monocrystalline silicon is 11%. The short circuit current of the battery with laser textured surface is about 4% higher than that of the battery with double-layer antireflection coating (ZnS/MgF2) on the smooth surface, which is mainly because the long wave light (wavelength greater than 800nm) slants into the battery. The problem of laser textured surface is that in the process of etching, the surface is damaged and some impurities are introduced, so the surface damage layer should be removed by chemical treatment. The short circuit current of solar cells made by this method is usually high, but the open circuit voltage is not too high. The main reason is that the composite current increases due to the increase of cell surface area.
[2] Chemical grooving
The mask (Si3N4 or SiO2) is used for isotropic etching. The etching solution can be acid etching solution, or sodium hydroxide or potassium hydroxide solution with high concentration. This method cannot form the tapered structure formed by anisotropic etching. It is reported that the textured surface formed by this method has obvious anti reflection effect on the spectral range of 700~1030 μ m. However, the mask layer is generally formed at a higher temperature, which causes the performance of polysilicon materials to decline. Especially for low-quality polysilicon materials, the minority carrier life is shortened. The conversion efficiency of the battery made by this process on 225 cm2 polysilicon reached 16.4%. The mask layer can also be formed by screen printing.
[3] Reactive Ion Etching (RIE)
This method is a maskless etching process, and the reflectivity of the textured surface formed by this method is particularly low, which can be less than 2% in the spectral range of 450~1000 μ m. From the optical point of view, it is an ideal method, but the problem is that the silicon surface is seriously damaged, and the open circuit voltage and filling factor of the battery decrease.
[4] Make antireflection coating
For high-efficiency solar cells, * the common and * effective method is to evaporate and coat ZnS/MgF2 double-layer antireflection coating, whose * good thickness depends on the thickness of the underlying oxide layer and the characteristics of the battery surface, for example, whether the surface is smooth or textured. The antireflection process also includes evaporation of Ta2O5, PECVD deposition of Si3N3, etc. ZnO conductive film can also be used as antireflection material.
1.2 Metallization technology
In the production of high-efficiency battery, the metallized electrode must match the design parameters of the battery, such as surface doping concentration, PN junction depth, and metal materials. Laboratory batteries generally have a small area (less than 4cm2), so thin metal grid lines (less than 10 μ m) are required. The general methods are photolithography, electron beam evaporation, and electron plating. Electroplating process is also used in industrial production, but the combination of evaporation and photolithography is not a low-cost process technology.
[1] Electron beam evaporation and electroplating
In general, the positive gel stripping process is used to evaporate Ti/Pa/Ag multilayer metal electrodes. To reduce the series resistance caused by the metal electrode, the metal layer is usually thicker (8-10 μ m). The disadvantage is that the electron beam evaporation causes damage to the silicon surface/passivation layer interface, which improves the surface recombination. Therefore, in the process, the Ti/Pa layer is evaporated for a short time, and the silver layer is evaporated. Another problem is that when the metal silicon contact surface is large, it will inevitably lead to the increase of minority carrier recombination speed. In the process, the tunnel junction contact method is used to form a thin oxide layer (generally about 20 microns thick) between silicon and metal, and low work function metals (such as titanium) are applied A stable electron accumulation layer can be induced 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 μ m) on the passivation layer, and then deposit a wide metal gate line (usually 10 μ m) to form a Mushroom like electrode. The conversion efficiency of the battery on 4cm2 Mc Si by this method can reach 17.3%. At present, Shallow angle (abstract) technology is also used on the mechanical grooving surface.
1.3 Forming technology of PN junction
[1] Emission region formation and phosphorus gettering
For high-efficiency solar cells, the formation of the emission area generally adopts selective diffusion, which forms a heavy impurity area under the metal electrode and realizes shallow concentration diffusion between the electrodes. The shallow concentration diffusion in the emission area 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 corrosion process and buried diffusion process. At present, selective diffusion is adopted, and the conversion efficiency of 150mm × 150mm battery reaches 16.4%. The surface block resistance of n++and n+areas is 20 Ω and 80 Ω respectively.
For Mc Si materials, the influence of phosphorus diffusion and gettering on batteries has been widely studied. A long time of phosphorus gettering process (generally 3-4 hours) can increase the minority carrier diffusion length of some Mc Si materials by two orders of magnitude. In the study of the effect of substrate concentration on gettering, it was found that even for high concentration of lining materials, through gettering, large minority carrier diffusion length (more than 200 μ m) could be obtained, the open circuit voltage of the battery was more than 638mv, and the conversion efficiency was more than 17%.
[2] Formation of back surface field and aluminum gettering technology
In the Mc Si battery, the back p+p junction is formed by uniformly diffusing aluminum or boron. The boron source is generally BN, BBr, APCVD SiO2: B2O8, etc. The aluminum diffusion is evaporation or screen printed aluminum, and sintered at 800 degrees. A lot of research has also been carried out on the role of aluminum gettering. Unlike phosphorus diffusion gettering, aluminum gettering is carried out at relatively low temperatures. The bulk defects also participate in the dissolution and deposition of impurities. At higher temperatures, the deposited impurities are easy to dissolve into silicon, which has adverse effects on Mc Si. Up to now, the regional back field has been used in the monocrystalline silicon cell process, but in the polycrystalline silicon, the all aluminum back surface field structure is still used.
[3] Double sided Mc Si battery
The front side of the Mc Si double-sided battery is a conventional structure, and the back side is a cross structure of N+and P+. In this way, the photogenerated minority carrier generated by the front light but located near the back side can be effectively absorbed by the back electrode. As an effective complement to the front electrode, the back electrode also acts as an independent carrier collector to generate back light and scattered light. It is reported that under AM1.5 conditions, the conversion efficiency exceeds 19%.
1.4 Surface and volume passivation technology
For Mc Si, due to the existence of high grain boundary and point defects (vacancy, interstitial atoms, metal impurities, oxygen, nitrogen and their compounds), the passivation of defects on the material surface and in the body is particularly important. In addition to the impurity absorption technology mentioned above, there are many methods for the passivation process. It is a common method to saturate silicon dangling bonds through thermal oxidation, It can greatly reduce the recombination speed of the Si-SiO2 interface, and its passivation effect depends on the surface concentration of the emission region, the density of state at the interface, and the cross sections of electrons and holes. Annealing in hydrogen atmosphere can make the passivation effect more obvious. Deposition of silicon nitride by PECVD is very effective in the near future, because it has the effect of hydrogenation in the process of film formation. This process can also be used in large-scale production. The application of Remote PECVD Si3N4 can make the surface recombination speed less than 20cm/s.
2. Industrial battery process
The development path of solar cells is from laboratory to factory, and from experimental research to large-scale production. Therefore, the characteristics that can achieve industrial production should be:
[1] The production process of battery can meet the requirements of assembly line operation;
[2] Large scale and 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, there are 125mm × 125mm, 150mm × 150mm and even larger monolithic cells in the market. The thickness has been reduced from the original 300 microns to the current 250, 200 and 200 microns, and the efficiency has been greatly improved. The photoelectric conversion efficiency of Japan Kyocera Corporation's 150mm × 150mm batteries in small batch production reached 17.1%, and the company's production volume reached 25.4MW in 1998.
Screen printing and its related technologies
The screen printing process is widely used in the large-scale production of polysilicon batteries, which can be used for the printing of diffusion sources, front metal electrodes, back contact electrodes, antireflection coatings, etc. With the improvement of screen materials and process level, the screen printing process will be more widely used in the production of solar cells.
a. Formation of emission area
Screen printing is used to form PN junction instead of conventional diffusion process in tubular furnace. Generally, phosphorus containing slurry is printed on the front of polysilicon, and aluminum containing metal slurry is printed on the back. After printing, diffusion can be completed in the mesh belt furnace (usually at 900 ℃). In this way, printing, drying and diffusion can form continuous production. The emission area formed by screen printing diffusion technology usually has a high surface concentration, so the surface photogenerated carrier recombination is large. In order to overcome this shortcoming, the following process technology of selecting the emission area is adopted in the process, so that the conversion efficiency of the battery can be further improved.
b. Select launch area process
In the diffusion process of polysilicon battery, the emission zone technology can be divided into local corrosion or two-step diffusion method. Local corrosion is to use dry method (such as reactive ion corrosion) or chemical corrosion method to erode the heavy diffusion layer in the area between metal electrodes* At the beginning, Solarex applied the reactive ion etching method in the same equipment. First, the heavily doped layer between metal electrodes was etched off with high reaction power, and then a layer of silicon nitride film was deposited with low power. The film played a dual role of antireflection and battery surface passivation. A battery with a conversion efficiency of more than 13% is made on a 100cm2 polycrystalline. In the same area, the conversion efficiency can reach 16% without mechanical texturing by using the two-part diffusion method.
c. Formation of back surface field
The back PN junction is usually formed by screen printing A slurry and thermal annealing in a mesh belt furnace. While forming the back surface junction, this process has a good absorption effect on impurities in polysilicon. The aluminum impurity absorption process is generally completed in the high temperature section. The measurement results show that the impurity absorption effect can restore the decrease in minority carrier life of polysilicon caused by the previous high temperature process. A good back surface field can significantly increase the open circuit voltage of the battery.
d. Screen printed metal electrode
In large-scale production, the screen printing process has more advantages than vacuum evaporation, metal electroplating and other processes. In the current process, silver paste is generally used as the printing material on the front. The main reason is that silver has good conductivity, solderability and low diffusion in silicon. The conductivity of the metal layer formed by screen printing and annealing depends on the chemical composition of the slurry, the content of the glass body, the coarseness of the screen, the sintering conditions and the thickness of the screen plate. At the beginning of the 1980s, screen printing had some defects: i) if the grid line width was large, it was usually greater than 150 μ m; Ii) Large shading and low battery filling factor; Iii) Not suitable for surface passivation, mainly because the surface diffusion concentration is high, otherwise contact resistance Larger. At present, the grid line with line width up to 50 μ m, thickness over 15 μ m and block resistance of 2.5~4m Ω can be screen printed by advanced methods, which can meet the requirements of high-efficiency batteries. Someone compared the solar cells made by screen printing electrode and evaporation electrode on 150 mm × 150 mm Mc Si, and there was almost no difference in various parameters.
3. Conclusion
The manufacturing process of polysilicon battery is constantly developing, which ensures that the efficiency of the battery is constantly improved and the cost is reduced. With the deepening understanding of the physical and optical characteristics of materials and devices, the battery structure becomes more reasonable, and the distance between the laboratory level and industrial production continues to narrow. Screen printing and buried grid technology have played a major role in efficient and low-cost batteries. High efficiency Mc Si battery modules have entered the market in large numbers. At present, research is focusing on new film structures, batteries on cheap substrates, etc. For users, what we need to do is to achieve larger volume and low-cost production. We are willing to work harder to achieve this goal.
