Overview of polysilicon solar cell fabrication process

As we all know, the use of solar energy has many advantages. Photovoltaic power generation will provide the main energy source for human beings. However, at present, solar power generation has a large market, which is accepted by the vast number of consumers, improving the photoelectric conversion efficiency of solar cells and reducing production. Cost should be the biggest goal we pursue. From the current development process of international solar cells, the development trend is that monocrystalline silicon, polycrystalline silicon, ribbon silicon, and thin film materials (including microcrystalline silicon-based films, compound-based films, and dye films). ).

From the perspective of industrialization, the center of gravity has evolved from single crystal to polycrystalline, the main reason is; [1] less and less tailpipes are available for solar cells; [2] for solar cells, square substrates are more In addition, the polycrystalline silicon obtained by the casting method and the direct solidification method can directly obtain a square material; [3] the production process of polycrystalline silicon is continuously progressing, and the fully automatic casting furnace can produce more than 200 kg of silicon ingot per production cycle (50 hours). The size of the crystal grains reaches the centimeter level; [4] due to the rapid research and development of the monocrystalline silicon process in the past decade, the process is also applied to the production of polycrystalline silicon cells, such as the selection of corrosion emission junctions, back surface fields, corroded suede Surface and bulk passivation, fine metal gate electrode, screen printing technology can reduce the width of the gate electrode to 50 microns, the height can reach more than 15 microns, rapid thermal annealing technology for polysilicon production can greatly shorten the process time, single The sheet thermal process time can be completed in less than one minute, and the cell conversion efficiency on the 100 square centimeter polycrystalline silicon wafer is more than 14%. According to reports, the current efficiency of cells fabricated on 50-60 micron polycrystalline silicon substrates exceeds 16%. Using mechanical grooved and screen printing technology, the efficiency is over 17% on 100 square centimeters of polycrystals, and the mechanical groove has an efficiency of 16% on the same area. The buried gate structure is used, and the mechanical groove is on the 130 square centimeter polycrystal. The battery efficiency reached 15.8%.

The following is a discussion of the process technology of polycrystalline silicon cells from two aspects.

2. Laboratory high efficiency battery process

Laboratory technology usually does not consider the cost of battery fabrication and whether it can be mass-produced. It only studies ways and means to achieve maximum efficiency, providing the limits that can be achieved with specific materials and processes.

2.1 About the absorption of light

The main reason for light absorption is:

(1) reducing surface reflection;

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

(3) Use back reflection.

For single crystal silicon, an anisotropic chemical etching method can be used to fabricate a pyramidal suede structure on the (100) surface to reduce surface light reflection. However, the polycrystalline silicon crystal orientation deviates from the (100) plane, and the above method cannot be used to make a uniform suede. The following methods are currently used:

[1] laser notching

The laser engraving method can be used to make an inverted pyramid structure on the surface of polycrystalline silicon, and the reflectivity is 4 to 6% in the spectral range of 500 to 900 nm, which is equivalent to the double-layer anti-reflection film on the surface. On the (100) plane, single crystal silicon chemically produced a suede with a reflectance of 11%. The short-circuit current of the laser-made suede is higher than that of the smooth-coated double-layer anti-reflection coating (ZnS/MgF2) battery by about 4%, which is mainly caused by long-wavelength light (wavelength greater than 800 nm) obliquely entering the battery. The problem with laser-made suede is that during etching, the surface is damaged and some impurities are introduced, and the surface damage layer is removed by chemical treatment. The solar cell made by this method usually has a high short-circuit current, but the open circuit voltage is not too high, mainly because the surface area of ​​the battery increases, causing the composite current to increase.

[2] Chemical groove

Isotropic etching using a mask (Si3N4 or SiO2). The etching solution can be an acidic etching solution or a sodium hydroxide or potassium hydroxide solution with a higher concentration. This method cannot form an anisotropic corrosion. Sharp-cone structure. It has been reported that the velvet formed by this method has a significant anti-reflection effect in the spectral range of 700 to 1030 microns. However, the mask layer is generally formed at a relatively high temperature, causing a decrease in the performance of the polysilicon material, particularly for a polycrystalline material having a lower quality, and a shorter lifetime of the minority. The conversion efficiency of the battery on the 225 cm2 polysilicon was 16.4%. The mask layer can also be formed by screen printing.

[3] Reactive ion etching (RIE)

The method is a maskless etching process, and the resulting suede reflectance is particularly low, and the reflectance in the spectral range of 450 to 1000 microns can be less than 2%. From an 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 the filling factor of the battery are degraded.

[4] Making anti-reflection film

For high-efficiency solar cells, the most common and effective method is to vapor-deposit ZnS/MgF2 double-layer anti-reflection film. The optimum thickness depends on the thickness of the underlying oxide layer and the characteristics of the cell surface, for example, whether the surface is smooth or suede. The anti-reflection process also has vapor deposition of Ta2O5, PECVD deposition of Si3N3, and the like. The ZnO conductive film can also be used as an antireflection material.

2.2 Metallization technology

In the fabrication of high-efficiency batteries, the metallized electrodes must match the design parameters of the battery, such as surface doping concentration, PN junction depth, and metal material. Laboratory batteries generally have a small area (area less than 4 cm2), so a thin metal grid line (less than 10 microns) is required. The commonly used methods are photolithography, electron beam evaporation, and electron plating. Electroplating processes are also used in industrial production, but when combined with evaporation and lithography, they are not low-cost process technologies.

[1] Electron beam evaporation and electroplating

Usually, the positive stripping process is applied, and the Ti/Pa/Ag multilayer metal electrode is evaporated. To reduce the series resistance caused by the metal electrode, the metal layer is often required to be relatively thick (8 to 10 μm). 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, a short-time evaporation of the Ti/Pa layer is used in the process of evaporating the silver layer. Another problem is that when the contact surface between the metal and the silicon is large, the sub-composite speed will increase. In the process, a tunnel junction contact method is used to form a thin oxide layer between silicon and metal (typically about 20 microns thick). A metal with a lower work function (such as titanium) can induce a silicon surface. Stable electron accumulation layer (can also introduce a fixed positive charge deepening inversion). Another method is to open a small window (less than 2 microns) on the passivation layer, and then deposit a wider metal grid line (usually 10 microns) to form a mushroom-like electrode, using this method at 4cm2 Mc- The conversion efficiency of the battery on Si reached 17.3%. Currently, the Shallow angle (oblique) technique is also applied to the mechanical grooved surface.

2.3 PN junction formation technology

[1] Emitter formation and phosphorus gettering

For high-efficiency solar cells, the formation of the emitter region is generally selected diffusion, a heavy impurity region is formed under the metal electrode to achieve shallow concentration diffusion between the electrodes, and the shallow concentration diffusion of the emitter region enhances the response of the battery to blue light, and the silicon surface Easy to passivate. The diffusion method has a two-step diffusion process, a diffusion plus etching process, and a buried diffusion process. At present, selective diffusion is adopted, the conversion efficiency of the battery of 15×15cm2 is 16.4%, and the surface resistance of the n++ and n+ regions are 20Ω and 80Ω, respectively.

For the Mc-Si material, the effect of phosphorus diffusion and gettering on the battery has been extensively studied. The long-time phosphorus gettering process (generally 3 to 4 hours) can increase the diffusion length of some Mc-Si minority carriers by two orders of magnitude. In the study of the effect of the substrate concentration on the gettering effect, it is found that even for the high concentration of the lining material, a large minority diffusion length (greater than 200 μm) can be obtained by gettering, and the open circuit voltage of the battery is greater than 638 mv, and the conversion efficiency is obtained. 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, and the boron source is generally BN, BBr, APCVD SiO2: B2O8, etc., aluminum diffusion is evaporation or screen printing aluminum, and sintering is completed at 800 degrees. A lot of research has been carried out on the effect of aluminum gettering. Unlike phosphorus diffusion gettering, aluminum gettering is carried out at relatively low temperatures. The bulk defects are also involved in the dissolution and deposition of impurities, and at higher temperatures, the deposited impurities are easily dissolved into the silicon, which adversely affects Mc-Si. Up to now, the regional back field has been applied to the single crystal silicon cell process, but in polysilicon, the all aluminum back surface field structure is still applied.

[3] double-sided Mc-Si battery

The Mc-Si double-sided battery has a conventional structure on the front side and a structure in which the back side is a cross between N+ and P+, so that the photogenerated minority generated by the front side illumination but near the back side can be effectively absorbed by the back electrode. The back electrode acts as an effective complement to the front electrode and also acts as a separate carrier collector for backside illumination and scattered light. It is reported that the conversion efficiency exceeds 19% under AM1.5 conditions.

2.4 Surface and body passivation technology

For Mc-Si, the presence of higher grain boundaries, point defects (vacancies, interstitial atoms, metal impurities, oxygen, nitrogen, and their complexes) is particularly important for the passivation of material surface and in vivo defects, except as previously mentioned. In addition to the gettering technique, there are various methods for the passivation process. It is a common method to saturate the silicon dangling bonds by thermal oxidation, which can greatly reduce the recombination speed of the Si-SiO2 interface, and the passivation effect depends on the emissive region. Surface concentration, interface state density, and the emission cross section of electrons and holes. Annealing in a hydrogen atmosphere can make the passivation effect more obvious. The deposition of silicon nitride by PECVD has been very effective in the near future because of the hydrogenation effect during film formation. This process can also be applied to large-scale production. The surface recombination speed can be less than 20 cm/s by using Remote PECVD Si3N4.

3 Industrial battery technology

Solar cells move from the research laboratory to the factory, and the experimental research to scale production is the road to its development, so the characteristics that can achieve industrial production should be:

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

[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 polycrystalline silicon cells is toward large-area, thin substrates. For example, a single-chip battery of 125 x 125 mm2, 150 x 150 mm2 or even larger is available on the market, and the thickness is reduced from the original 300 micrometers to the current 250, 200 and 200 micrometers. The efficiency has been greatly improved. Japan's Kyocera's 150×150 battery has a photoelectric conversion efficiency of 17.1% in small batch production, and the company's production in 1998 reached 25.4MW.

(1) Screen printing and related technologies

The screen printing process is widely used in the large-scale production of polycrystalline silicon batteries, which can be used for printing of diffusion sources, front metal electrodes, back contact electrodes, anti-reflection coatings, etc., with the improvement of the screen material and the improvement of the process level. The screen printing process will be more widely used in the production of solar cells.

a. Formation of the launching area

A PN junction is formed by screen printing instead of the conventional tube furnace diffusion process. Generally, a phosphorus-containing slurry is printed on the front side of the polycrystalline silicon, and an aluminum-containing metal paste is printed on the reverse side. After printing is complete, the diffusion can be done in a mesh belt furnace (typically at a temperature of 900 degrees) so that printing, drying, and diffusion can form a continuous production. The emission area formed by screen printing diffusion technology usually has a relatively high surface concentration, and the surface photogenerated carrier recombination is large. In order to overcome this shortcoming, the following selective emission area process technology is adopted in the process, so that the conversion efficiency of the battery is obtained. Further improvement.

b. Select the launch area process

In the diffusion process of polycrystalline silicon cells, the selective emitter region technique is divided into localized etching or two-step diffusion. Localized etching is a method of dry (such as reactive ion etching) or chemical etching to etch away the re-diffusion layer in the region between the metal electrodes. Initially, Solarex applied a method of reactive ion etching in the same equipment, first etching the heavily doped layer between the metal electrodes with a large reaction power, and then depositing a silicon nitride film with a small power, the film exerted anti-reflection and The dual role of battery surface passivation. A battery having a conversion efficiency of more than 13% was made on a 100 cm2 polycrystal. In the same area, two diffusion methods were applied, and the conversion efficiency reached 16% without mechanical suede.

c. Formation of the back surface field

The back PN junction is usually formed by screen printing A paste and thermally annealed in a mesh belt furnace. The process has a good absorption effect on impurities in the polysilicon while forming the back surface junction, and 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 of the polysilicon lifetime of the polysilicon caused by the high temperature process in the front. A good back surface field can significantly increase the open circuit voltage of the battery.

d. Screen printing metal electrode

In the large-scale production, the screen printing process is more advantageous than the vacuum evaporation, metal plating and other processes. In the current process, the positive printing materials generally use silver-containing paste, the main reason is that silver has good Conductivity, solderability and low diffusion properties 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 the glass body, the roughness of the screen, the sintering conditions, and the thickness of the screen plate. At the beginning of eighty years, screen printing has some defects, i) such as the width of the gate line is large, usually greater than 150 microns; ii) causing greater shading, lower cell fill factor; iii) not suitable for surface passivation, mainly surface The diffusion concentration is higher, otherwise the contact resistance is larger. At present, an advanced method can be used to screen a gate line with a line width of 50 μm, a thickness of more than 15 μm, and a sheet resistance of 2.5 to 4 mΩ. This parameter can meet the requirements of high-efficiency batteries. The solar cells of the screen-printed electrode and the evaporated electrode were compared on a 15×15 cm 2 Mc-Si, and there were almost no differences in the parameters.

4 Conclusion

The manufacturing process of polycrystalline silicon cells continues to advance, ensuring the continuous improvement of the efficiency of the battery and the cost reduction. With the deepening of the understanding of the physical and optical properties of materials and devices, the structure of the battery is more reasonable, and the laboratory level and industrialization are large. The distance is constantly shrinking. Screen printing and buried gate technology play a major role in high-efficiency and low-cost batteries. High-efficiency Mc-Si battery modules have entered the market in large quantities. The current research is working on new thin film structures, batteries on inexpensive substrates, etc. Users, what we need to do is to achieve larger batch, low-cost production, and we hope to work harder to achieve this goal.

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