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Crystal silicon cell manufacturing process and equipment How to detect silicon wafer
- Nov 23, 2018 -

In recent years, the production technology of solar cell chips has been continuously improved, the production cost has been continuously reduced, and the conversion efficiency has been continuously improved. The application of photovoltaic power generation has become increasingly popular and rapidly developed, and has gradually become an important source of power supply. A solar cell is an energy-converting photovoltaic element that converts light energy into electrical energy under the illumination of sunlight to achieve photovoltaic power generation. The process of producing cell sheets is complicated, and generally involves major steps such as wafer inspection, surface texturing, diffusion bonding, dephosphorization, plasma etching, anti-reflection coating, screen printing, rapid sintering, and inspection and packaging. . This article describes the general process and equipment for the production of crystalline silicon solar cells.

I. Silicon wafer inspection Silicon wafer is the carrier of solar cell wafer. The quality of silicon wafer directly determines the conversion efficiency of solar cell. Therefore, it is necessary to test the silicon wafer. This process is mainly used to measure some technical parameters of the silicon wafer. These parameters mainly include the unevenness of the surface of the silicon wafer, the minority lifetime, the resistivity, the P/N type and the microcrack. The equipment is divided into automatic loading and unloading, silicon wafer transmission, system integration and four detection modules. Among them, the photovoltaic wafer detector detects the unevenness of the surface of the silicon wafer, and simultaneously detects the appearance parameters such as the size and diagonal of the silicon wafer; the microcrack detection module is used to detect the internal microcrack of the silicon wafer; The detection module, one of the online test modules mainly tests the silicon wafer resistivity and the silicon type, and the other module is used to detect the minority lifetime of the silicon wafer. Before performing the minority carrier lifetime and resistivity detection, it is necessary to detect the diagonal and micro crack of the silicon wafer and automatically remove the damaged silicon wafer. The wafer inspection equipment is capable of automatically loading and unloading sheets, and is capable of placing defective products in a fixed position, thereby improving detection accuracy and efficiency.

Second, surface texturing Single crystal silicon suede is prepared by the anisotropic etching of silicon, forming millions of four-sided square pyramids per square centimeter of silicon surface, that is, pyramid structure. Due to the multiple reflections and refractions of the incident light on the surface, the absorption of light is increased, and the short-circuit current and conversion efficiency of the battery are improved. The anisotropic etching solution of silicon is usually a hot alkaline solution, and the usable bases are sodium hydroxide, potassium hydroxide, lithium hydroxide and ethylenediamine. Most of the low-temperature sodium hydroxide solution is used to prepare fumed silicon at a corrosion temperature of 70-85 ° C. In order to obtain uniform suede, alcohols such as ethanol and isopropanol should be added as a complexing agent in the solution to accelerate the corrosion of silicon. In the preparation of the velvet, the silicon wafer must first be subjected to preliminary surface corrosion, and etched with an alkaline or acidic etching solution to about 20 to 25 μm. After the velvet is eroded, general chemical cleaning is performed. Surface-prepared silicon wafers should not be stored in the water for a long time to prevent contamination and should be diffused as soon as possible.

Third, the diffusion of the solar cell requires a large area of the PN junction to achieve the conversion of light energy to electrical energy, and the diffusion furnace is a special equipment for the manufacture of solar cell PN junction. The tubular diffusion furnace is mainly composed of four parts: the upper download part of the quartz boat, the exhaust gas chamber, the furnace body part and the gas cabinet part. Diffusion generally uses a liquid source of phosphorus oxychloride as a diffusion source. P-type silicon wafer is placed in a quartz container of a tube diffusion furnace, and phosphorus oxychloride is introduced into a quartz container at a high temperature of 850---900 degrees Celsius, and reacted by phosphorus oxychloride and silicon wafer to obtain phosphorus. atom. After a certain period of time, the phosphorus atoms enter the surface layer of the silicon wafer from all sides, and diffuse into the inside of the silicon wafer through the gap between the silicon atoms, forming an interface between the N-type semiconductor and the P-type semiconductor, that is, the PN junction. The uniformity of the PN junction produced by this method is good, the unevenness of the sheet resistance is less than ten percent, and the lifetime of the minority carrier can be greater than 10 ms. Manufacturing PN junctions is the most basic and critical process in solar cell production. Because it is the formation of the PN junction, the electrons and holes do not return to the original position after flowing, thus forming a current, and the current is led out by the wire, which is direct current.

4. Dephosphorus-silicate glass This process is used in the production process of solar cell wafers. The chemical etching method is used to soak the silicon wafer in a hydrofluoric acid solution to produce a chemical reaction to form a soluble complex hexafluorosilicon. Acid to remove a layer of phosphosilicate glass formed on the surface of the silicon wafer after diffusion. During the diffusion process, POCL3 reacts with O2 to form P2O5 deposited on the surface of the silicon wafer. P2O5 reacts with Si to form SiO2 and phosphorus atoms, so that a layer of SiO2 containing phosphorus is formed on the surface of the silicon wafer, which is called phosphosilicate glass. The equipment for removing phosphorus silica glass is generally composed of a body, a cleaning tank, a servo drive system, a mechanical arm, an electric control system, and an automatic acid distribution system. The main power sources are hydrofluoric acid, nitrogen, compressed air, pure water, and hot exhaust. Wind and wastewater. Hydrofluoric acid is capable of dissolving silica because hydrofluoric acid reacts with silica to form a volatile silicon tetrafluoride gas. If the hydrofluoric acid is excessive, the silicon tetrafluoride formed by the reaction will further react with hydrofluoric acid to form a soluble complex hexafluorosilicic acid.

5. Plasma Etching Due to the back-to-back diffusion during diffusion, all surfaces of the wafer, including the edges, will inevitably diffuse with phosphorus. The photogenerated electrons collected on the front side of the PN junction flow along the edge where the phosphorus diffuses to the back of the PN junction, causing a short circuit. Therefore, the doped silicon around the solar cell must be etched to remove the PN junction at the edge of the cell. This process is typically accomplished using plasma etching techniques. The plasma etching is performed under low voltage, and the parent molecule of the reaction gas CF4 is ionized and forms a plasma under the excitation of radio frequency power. The plasma is composed of charged electrons and ions. Under the impact of electrons, the gas in the reaction chamber absorbs energy and forms a large number of active groups in addition to being converted into ions. The reactive group reaches the surface of SiO2 due to diffusion or under the action of an electric field, where it chemically reacts with the surface of the material to be etched, and forms a volatile reaction product which is separated from the surface of the object to be etched and is evacuated to the cavity by the vacuum system.

6. Anti-reflection coating The reflectivity of the polished silicon surface is 35%. In order to reduce the surface reflection and improve the conversion efficiency of the battery, it is necessary to deposit a silicon nitride anti-reflection film. In the industrial production, PECVD equipment is often used to prepare anti-reflection coatings. PECVD is plasma enhanced chemical vapor deposition. Its technical principle is to use low temperature plasma as the energy source, the sample is placed on the cathode of the glow discharge under low pressure, the sample is heated to a predetermined temperature by glow discharge, and then the appropriate amount of reaction gases SiH4 and NH3 are introduced. A series of chemical reactions and plasma reactions form a solid film, ie, a silicon nitride film, on the surface of the sample. In general, the film deposited by this plasma enhanced chemical vapor deposition method has a thickness of about 70 nm. Films of this thickness have optical functionality. By using the principle of thin film interference, the reflection of light can be greatly reduced, the short-circuit current and output of the battery are greatly increased, and the efficiency is also considerably improved.

7. Screen printing solar cells have been made into PN junctions after being subjected to processes such as texturing, diffusion and PECVD. They can generate current under illumination. In order to derive the generated current, it is necessary to make positive and negative electrodes on the surface of the battery. . There are many ways to make electrodes, and screen printing is currently the most common production process for making solar cell electrodes. Screen printing is to print a predetermined pattern on a substrate by embossing. The device consists of three parts: silver aluminum paste printing on the back of the battery, aluminum paste printing on the back of the battery and silver paste printing on the front side of the battery. The working principle is as follows: the mesh is used to pass through the slurry in the mesh portion, and a certain pressure is applied to the slurry portion of the screen by the scraper while moving toward the other end of the screen. The ink is squeezed from the mesh of the pattern portion onto the substrate by the doctor blade during movement. Due to the viscous action of the slurry, the print is fixed within a certain range, and the squeegee is always in linear contact with the screen plate and the substrate during printing, and the contact line moves as the blade moves, thereby completing the printing stroke.

8. Rapid sintering of the silicon wafer after screen printing can not be used directly. It needs to be sintered quickly in the sintering furnace to burn off the organic resin binder, leaving almost pure, close to the silicon wafer due to the vitreous action. Silver electrode. When the silver electrode and the crystalline silicon reach the eutectic temperature at a temperature, the crystalline silicon atoms are incorporated into the molten silver electrode material in a certain ratio, thereby forming an ohmic contact of the upper and lower electrodes, and improving the open circuit voltage and the filling factor of the cell. The key parameters are such that they have resistive properties to improve the conversion efficiency of the cell. The sintering furnace is divided into three stages: pre-sintering, sintering, and cooling. The purpose of the pre-sintering stage is to decompose and burn off the polymer binder in the slurry. At this stage, the temperature rises slowly. In the sintering stage, various physical and chemical reactions are completed in the sintered body to form a resistive film structure, which makes it truly resistive. At this stage, the temperature reaches a peak; in the cooling and cooling stage, the glass is cooled and solidified, and the resistive film structure is fixedly adhered to the substrate.

Nine, peripheral equipment In the production process of the battery, it also needs power supply, power, water supply, drainage, HVAC, vacuum, special steam and other peripheral facilities. Fire and environmental protection equipment is also important to ensure safety and sustainable development. A solar cell production line with an annual capacity of 50MW, the power of only process and power equipment is about 1800KW. The amount of pure water used in process is about 15 tons per hour, and the water quality requirements meet the EW-1 technical standard of China Electronic Grade Water GB/T11446.1-1997. The amount of process cooling water is also about 15 tons per hour. The particle size of the water should not exceed 10 microns, and the water supply temperature should be 15-20 °C. The vacuum displacement is around 300M3/H. At the same time, about 30 cubic meters of nitrogen storage tank and 10 cubic meters of oxygen storage tank are needed. Considering the safety factors of special gases such as silane, it is also necessary to set up a special air room to ensure the safety of production. In addition, silane combustion towers, sewage treatment stations, etc. are also necessary facilities for the production of battery sheets.