The production process of solar cells is divided into wafer inspection - surface texturing - diffusion consolidation - dephosphorizing silicon glass - plasma etching - antireflection coating - screen printing - rapid sintering. The details are as follows:
1. Silicon wafer detection The silicon wafer is the carrier of the solar cell. The quality of the wafer directly determines the conversion efficiency of the solar cell. Therefore, the incoming silicon wafer needs to be inspected. This process is mainly used to perform on-line measurement of some technical parameters of the silicon wafer. These parameters mainly include the surface roughness, minority lifetime, resistivity, P/N type, and micro-cracks of the silicon wafer. This group of equipment is divided into automatic loading and unloading, silicon transmission, system integration and four detection modules. Among them, the photovoltaic wafer detector detects the unevenness of the silicon wafer surface, simultaneously detects the appearance parameters such as the size and diagonal of the silicon wafer; the micro-crack detection module is used to detect the internal micro-cracks of the silicon wafer; there are also two The inspection module, one of the on-line test modules mainly tests the silicon body resistivity and the silicon type, and the other module is used to detect the minority carrier lifetime of the silicon wafer. Before carrying out the detection of the minority lifetime and resistivity, it is necessary to detect the diagonal and micro-cracks of the silicon wafer and automatically remove the damaged wafer. The wafer inspection equipment can automatically load and unload the wafers, and can place defective products in a fixed position, thereby improving detection accuracy and efficiency.
2. The preparation of surface-texturing single-crystal silicon suede uses the anisotropic etching of silicon to form several million square pyramids, or pyramid structures, on each square centimeter of silicon surface. Due to multiple reflections and refractions of 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. Possible alkalis include sodium hydroxide, potassium hydroxide, lithium hydroxide and ethylenediamine. Most of the suede is prepared using an inexpensive dilute solution of sodium hydroxide having a concentration of about 1%, and the etching temperature is 70-85°C. In order to obtain a uniform suede, alcohols such as ethanol and isopropanol should also be added as complexing agents in the solution to accelerate the corrosion of silicon. Before preparing the cashmere, the silicon wafer must be first subjected to a preliminary surface corrosion, and then etched to about 20-25 μm with an alkaline or acidic etching solution. After etching the suede, general chemical cleaning is performed. Silicon wafers that have been surface-prepared should not be stored in water for a long time to prevent contamination and should be formed as soon as possible.
Third, the proliferation of solar cells to form a knot requires a large area of ​​PN junction to achieve the conversion of light energy to electricity, and the diffusion furnace is a special device for the manufacture of solar cells PN junction. The tube-type diffusion furnace is mainly composed of four parts: the upper part of the quartz boat, the exhaust gas chamber, the furnace body part and the gas cabinet part. Diffusion generally uses a phosphorus oxytrichloride liquid source as a diffusion source. The P-type silicon wafer is placed in a quartz vessel of a tubular diffusion furnace, and nitrogen phosphorus is introduced into the quartz vessel at a high temperature of 850 to 900 degrees Celsius to react phosphorus oxychloride with a silicon wafer to obtain phosphorus. atom. After a certain period of time, the phosphorus atoms enter the surface layer of the silicon wafer from the periphery and penetrate and diffuse into the silicon wafer through the gaps between the silicon atoms, forming an interface between the N-type semiconductor and the P-type semiconductor, that is, the PN junction. The PN junction made by this method has good uniformity, the non-uniformity of the sheet resistance is less than 10%, and the lifetime of the minority carrier can be greater than 10ms. Manufacturing PN junctions is the most basic and critical process for solar cell production. Because it is the formation of the PN junction, the electrons and holes are not returned to their original positions after flowing, so that a current is formed and the current is led out by the wire, that is, the direct current.
IV. Phosphorus-Silicate Silicate Glass This process is used in the production of solar cells. Through chemical etching, the silicon wafer is soaked in a hydrofluoric acid solution to produce a chemical reaction to form a soluble complex hexafluorosilicon. Acid, to remove the diffusion after the formation of a layer of phosphosilicate glass on the surface of the silicon wafer. During the diffusion process, POCl3 reacts with O2 to form P2O5 and deposits on the surface of the silicon wafer. P2O5 reacts with Si to generate 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 dephosphorizing silica glass is generally composed of a body, a cleaning tank, a servo drive system, a robot arm, an electrical control system, and an automatic acid-mixing system. The main power sources are hydrofluoric acid, nitrogen, compressed air, pure water, and hot water. Wind and wastewater. Hydrofluoric acid can dissolve silica because hydrofluoric acid reacts with silica to generate volatile silicon tetrafluoride gas. If the hydrofluoric acid is excessive, the silicon tetrafluoride produced by the reaction will further react with hydrofluoric acid to form the soluble hexafluorosilicic acid.
5. Plasma Etching Because during the diffusion process, even with back-to-back diffusion, all surfaces of the wafer, including the edges, will inevitably diffuse phosphorus. The photogenerated electrons collected on the front side of the PN junction will flow along the edge of the phosphor-diffused region 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. Plasma etching techniques are commonly used to complete this process. The plasma etching is under the condition of low pressure, and the parent molecule of the reaction gas CF4 generates ionization and forms a plasma under the excitation of RF power. The plasma is composed of charged electrons and ions. The gas in the reaction chamber, under the impact of electrons, not only converts into ions, but also absorbs energy and forms a large number of active groups. Active reactive groups reach the surface of SiO2 due to diffusion or under the action of an electric field, where they chemically react with the surface of the material being etched, and form volatile reaction products that separate from the surface of the etched material and are extracted by the vacuum system.
6. The reflectivity of the polished silicon surface of the anti-reflection coating is 35%. In order to reduce the surface reflection and improve the conversion efficiency of the battery, a silicon nitride antireflective film needs to be deposited. PECVD equipment is often used in industrial production to prepare antireflection films. PECVD is plasma enhanced chemical vapor deposition. Its technical principle is to use a low-temperature plasma as an energy source, the sample is placed on the cathode of glow discharge at low pressure, the sample is heated to a predetermined temperature by glow discharge, and then an appropriate amount of reaction gases SiH4 and NH3 are introduced, and the gas After a series of chemical reactions and plasma reactions, a solid film, silicon nitride film, is formed on the surface of the sample. In general, films deposited using this plasma enhanced chemical vapor deposition method have a thickness of about 70 nm. Films of this thickness have optical functionality. 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.
Seven, screen printing solar cells after the process of texturing, diffusion and PECVD, has been made into PN junction, can generate current in the light, in order to export the generated current, need to make positive and negative electrodes on the surface of the battery . There are many methods for manufacturing electrodes, and screen printing is currently the most common production process for making solar cell electrodes. Screen printing is the use of imprinting the predetermined graphics printed on the substrate, the device from the back of the battery silver-aluminum paste printing, the back of the battery aluminum paste printing and the front of the battery silver paste printing three parts. Its working principle is: through the screen mesh part of the mesh through the slurry, with a blade in the screen of the slurry to apply a certain pressure, while moving toward the other end of the screen. In the movement, the ink is pressed by the doctor blade from the mesh portion of the pattern portion onto the substrate. Because the stickiness of the paste makes the imprinting fixed within a certain range, the squeegee in the printing is always in linear contact with the screen plate and the substrate, and the contact line moves with the squeegee to complete the printing stroke.
Eight, rapid sintering through the screen printing of silicon, can not be used directly, need to be sintered by the sintering furnace, the organic resin binder burned away, leaving almost pure, due to vitrification and close contact on the silicon The silver electrode. When the temperature of the silver electrode and the crystalline silicon reaches the eutectic temperature, the crystalline silicon atoms are incorporated into the molten silver electrode material at a certain ratio, thereby forming an ohmic contact between the upper and lower electrodes, and increasing the open circuit voltage and the fill factor of the cell. The key parameters make it have resistance characteristics to improve the conversion efficiency of the cell.
The sintering furnace is divided into three stages: pre-sintering, sintering and cooling and cooling. The purpose of the pre-sintering stage is to decompose and burn off the polymer binder in the slurry. The temperature gradually rises during this stage; during the sintering stage, various physical and chemical reactions are completed in the sintered body to form a resistive film structure, so as to truly have resistance characteristics. In this stage, the temperature reaches a peak value; in the cooling and cooling stage, the glass is cooled and hardened and solidified, so that the resistance film structure is fixedly adhered to the substrate.
Nine, peripheral equipment in the battery production process, but also need power, power, water supply, drainage, HVAC, vacuum, special steam and other peripheral facilities. Firefighting and environmental protection equipment are also particularly important for ensuring safety and sustainable development. A solar cell production line with an annual capacity of 50 MW can use only 1800 kW of electricity for the process and power equipment. The amount of process pure water is about 15 tons per hour, and the water quality requirements have reached the EW-1 level technical standard in China's electronic grade water GB/T11446.1-1997. The amount of process cooling water is also about 15 tons per hour. The particle size of water in the water should not be larger than 10 microns, and the water supply temperature should be 15-20°C. The vacuum displacement is about 300M3/H. At the same time, approximately 20 cubic meters of nitrogen storage tanks and 10 cubic meters of oxygen storage tanks are needed. Taking into account the safety factors of special gases such as silanes, it is also necessary to set up a special gas room separately to absolutely guarantee production safety. In addition, silane combustion towers, sewage treatment stations, etc. are also essential facilities for the production of solar cells.
1. Silicon wafer detection The silicon wafer is the carrier of the solar cell. The quality of the wafer directly determines the conversion efficiency of the solar cell. Therefore, the incoming silicon wafer needs to be inspected. This process is mainly used to perform on-line measurement of some technical parameters of the silicon wafer. These parameters mainly include the surface roughness, minority lifetime, resistivity, P/N type, and micro-cracks of the silicon wafer. This group of equipment is divided into automatic loading and unloading, silicon transmission, system integration and four detection modules. Among them, the photovoltaic wafer detector detects the unevenness of the silicon wafer surface, simultaneously detects the appearance parameters such as the size and diagonal of the silicon wafer; the micro-crack detection module is used to detect the internal micro-cracks of the silicon wafer; there are also two The inspection module, one of the on-line test modules mainly tests the silicon body resistivity and the silicon type, and the other module is used to detect the minority carrier lifetime of the silicon wafer. Before carrying out the detection of the minority lifetime and resistivity, it is necessary to detect the diagonal and micro-cracks of the silicon wafer and automatically remove the damaged wafer. The wafer inspection equipment can automatically load and unload the wafers, and can place defective products in a fixed position, thereby improving detection accuracy and efficiency.
2. The preparation of surface-texturing single-crystal silicon suede uses the anisotropic etching of silicon to form several million square pyramids, or pyramid structures, on each square centimeter of silicon surface. Due to multiple reflections and refractions of 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. Possible alkalis include sodium hydroxide, potassium hydroxide, lithium hydroxide and ethylenediamine. Most of the suede is prepared using an inexpensive dilute solution of sodium hydroxide having a concentration of about 1%, and the etching temperature is 70-85°C. In order to obtain a uniform suede, alcohols such as ethanol and isopropanol should also be added as complexing agents in the solution to accelerate the corrosion of silicon. Before preparing the cashmere, the silicon wafer must be first subjected to a preliminary surface corrosion, and then etched to about 20-25 μm with an alkaline or acidic etching solution. After etching the suede, general chemical cleaning is performed. Silicon wafers that have been surface-prepared should not be stored in water for a long time to prevent contamination and should be formed as soon as possible.
Third, the proliferation of solar cells to form a knot requires a large area of ​​PN junction to achieve the conversion of light energy to electricity, and the diffusion furnace is a special device for the manufacture of solar cells PN junction. The tube-type diffusion furnace is mainly composed of four parts: the upper part of the quartz boat, the exhaust gas chamber, the furnace body part and the gas cabinet part. Diffusion generally uses a phosphorus oxytrichloride liquid source as a diffusion source. The P-type silicon wafer is placed in a quartz vessel of a tubular diffusion furnace, and nitrogen phosphorus is introduced into the quartz vessel at a high temperature of 850 to 900 degrees Celsius to react phosphorus oxychloride with a silicon wafer to obtain phosphorus. atom. After a certain period of time, the phosphorus atoms enter the surface layer of the silicon wafer from the periphery and penetrate and diffuse into the silicon wafer through the gaps between the silicon atoms, forming an interface between the N-type semiconductor and the P-type semiconductor, that is, the PN junction. The PN junction made by this method has good uniformity, the non-uniformity of the sheet resistance is less than 10%, and the lifetime of the minority carrier can be greater than 10ms. Manufacturing PN junctions is the most basic and critical process for solar cell production. Because it is the formation of the PN junction, the electrons and holes are not returned to their original positions after flowing, so that a current is formed and the current is led out by the wire, that is, the direct current.
IV. Phosphorus-Silicate Silicate Glass This process is used in the production of solar cells. Through chemical etching, the silicon wafer is soaked in a hydrofluoric acid solution to produce a chemical reaction to form a soluble complex hexafluorosilicon. Acid, to remove the diffusion after the formation of a layer of phosphosilicate glass on the surface of the silicon wafer. During the diffusion process, POCl3 reacts with O2 to form P2O5 and deposits on the surface of the silicon wafer. P2O5 reacts with Si to generate 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 dephosphorizing silica glass is generally composed of a body, a cleaning tank, a servo drive system, a robot arm, an electrical control system, and an automatic acid-mixing system. The main power sources are hydrofluoric acid, nitrogen, compressed air, pure water, and hot water. Wind and wastewater. Hydrofluoric acid can dissolve silica because hydrofluoric acid reacts with silica to generate volatile silicon tetrafluoride gas. If the hydrofluoric acid is excessive, the silicon tetrafluoride produced by the reaction will further react with hydrofluoric acid to form the soluble hexafluorosilicic acid.
5. Plasma Etching Because during the diffusion process, even with back-to-back diffusion, all surfaces of the wafer, including the edges, will inevitably diffuse phosphorus. The photogenerated electrons collected on the front side of the PN junction will flow along the edge of the phosphor-diffused region 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. Plasma etching techniques are commonly used to complete this process. The plasma etching is under the condition of low pressure, and the parent molecule of the reaction gas CF4 generates ionization and forms a plasma under the excitation of RF power. The plasma is composed of charged electrons and ions. The gas in the reaction chamber, under the impact of electrons, not only converts into ions, but also absorbs energy and forms a large number of active groups. Active reactive groups reach the surface of SiO2 due to diffusion or under the action of an electric field, where they chemically react with the surface of the material being etched, and form volatile reaction products that separate from the surface of the etched material and are extracted by the vacuum system.
6. The reflectivity of the polished silicon surface of the anti-reflection coating is 35%. In order to reduce the surface reflection and improve the conversion efficiency of the battery, a silicon nitride antireflective film needs to be deposited. PECVD equipment is often used in industrial production to prepare antireflection films. PECVD is plasma enhanced chemical vapor deposition. Its technical principle is to use a low-temperature plasma as an energy source, the sample is placed on the cathode of glow discharge at low pressure, the sample is heated to a predetermined temperature by glow discharge, and then an appropriate amount of reaction gases SiH4 and NH3 are introduced, and the gas After a series of chemical reactions and plasma reactions, a solid film, silicon nitride film, is formed on the surface of the sample. In general, films deposited using this plasma enhanced chemical vapor deposition method have a thickness of about 70 nm. Films of this thickness have optical functionality. 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.
Seven, screen printing solar cells after the process of texturing, diffusion and PECVD, has been made into PN junction, can generate current in the light, in order to export the generated current, need to make positive and negative electrodes on the surface of the battery . There are many methods for manufacturing electrodes, and screen printing is currently the most common production process for making solar cell electrodes. Screen printing is the use of imprinting the predetermined graphics printed on the substrate, the device from the back of the battery silver-aluminum paste printing, the back of the battery aluminum paste printing and the front of the battery silver paste printing three parts. Its working principle is: through the screen mesh part of the mesh through the slurry, with a blade in the screen of the slurry to apply a certain pressure, while moving toward the other end of the screen. In the movement, the ink is pressed by the doctor blade from the mesh portion of the pattern portion onto the substrate. Because the stickiness of the paste makes the imprinting fixed within a certain range, the squeegee in the printing is always in linear contact with the screen plate and the substrate, and the contact line moves with the squeegee to complete the printing stroke.
Eight, rapid sintering through the screen printing of silicon, can not be used directly, need to be sintered by the sintering furnace, the organic resin binder burned away, leaving almost pure, due to vitrification and close contact on the silicon The silver electrode. When the temperature of the silver electrode and the crystalline silicon reaches the eutectic temperature, the crystalline silicon atoms are incorporated into the molten silver electrode material at a certain ratio, thereby forming an ohmic contact between the upper and lower electrodes, and increasing the open circuit voltage and the fill factor of the cell. The key parameters make it have resistance characteristics to improve the conversion efficiency of the cell.
The sintering furnace is divided into three stages: pre-sintering, sintering and cooling and cooling. The purpose of the pre-sintering stage is to decompose and burn off the polymer binder in the slurry. The temperature gradually rises during this stage; during the sintering stage, various physical and chemical reactions are completed in the sintered body to form a resistive film structure, so as to truly have resistance characteristics. In this stage, the temperature reaches a peak value; in the cooling and cooling stage, the glass is cooled and hardened and solidified, so that the resistance film structure is fixedly adhered to the substrate.
Nine, peripheral equipment in the battery production process, but also need power, power, water supply, drainage, HVAC, vacuum, special steam and other peripheral facilities. Firefighting and environmental protection equipment are also particularly important for ensuring safety and sustainable development. A solar cell production line with an annual capacity of 50 MW can use only 1800 kW of electricity for the process and power equipment. The amount of process pure water is about 15 tons per hour, and the water quality requirements have reached the EW-1 level technical standard in China's electronic grade water GB/T11446.1-1997. The amount of process cooling water is also about 15 tons per hour. The particle size of water in the water should not be larger than 10 microns, and the water supply temperature should be 15-20°C. The vacuum displacement is about 300M3/H. At the same time, approximately 20 cubic meters of nitrogen storage tanks and 10 cubic meters of oxygen storage tanks are needed. Taking into account the safety factors of special gases such as silanes, it is also necessary to set up a special gas room separately to absolutely guarantee production safety. In addition, silane combustion towers, sewage treatment stations, etc. are also essential facilities for the production of solar cells.
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