Design of Solar Cell Sintering Furnace and Discussion on Its Key Performance

The mesh belt infrared heating fast sintering furnace is a key equipment for producing solar cells. Its performance index is directly related to the conversion rate, yield rate and production efficiency of the cells. This article focuses on several key performances and introduces the equipment structure to achieve these performances.

1. Introduction

Since the beginning of the 21st century, photovoltaic power generation has developed rapidly as an ideal renewable energy power generation technology. Driven by the market, by 2006, my country had formed a production capacity of 1,200MW. In the entire production process of solar cells, diffusion, coating and sintering are the most important processes, among which sintering is a crucial step to make the crystalline silicon substrate truly have the function of photoelectric conversion. Therefore, the performance of the sintering equipment directly affects the quality of the cell.

Solar cells currently use a co-firing process that only requires one sintering to form ohmic contacts between the upper and lower electrodes. After the silver paste, silver aluminum paste, and aluminum paste are printed on the silicon wafer, the organic solvent is completely volatilized after drying, and the film layer shrinks to become a solid and tightly adheres to the silicon wafer. At this time, it can be regarded as the metal electrode material layer and the silicon wafer are in contact with each other. When the electrode metal material and the semiconductor single crystal silicon are heated to the eutectic temperature, the single crystal silicon atoms are dissolved into the molten alloy electrode material in a certain proportion. The whole process of single crystal silicon atoms dissolving into the electrode metal is quite fast, generally only a few seconds. The number of dissolved single crystal silicon atoms depends on the alloy temperature and the volume of the electrode material. The higher the sintering alloy temperature and the larger the volume of the electrode metal material, the more silicon atoms are dissolved. The state at this time is called the alloy system of the crystal electrode metal. If the temperature is lowered at this time, the system begins to cool to form a recrystallization layer. At this time, the silicon atoms originally dissolved in the electrode metal material are crystallized again in a solid form, that is, an epitaxial layer grows on the contact interface between the metal and the crystal. If the epitaxial layer contains a sufficient amount of impurity components with the same conductivity type as the original crystal material, an ohmic contact is formed by the alloy process; if the crystal layer contains a sufficient amount of impurity components with a different conductivity type from the original crystal material, a P-N junction is formed by the alloy process.

The typical sintering curve of solar cells is shown in Figure 1. The whole process takes about 120 seconds and is divided into four stages: drying, pre-burning, sintering and cooling. The sintering stage is a rapid temperature rise from 500℃ to about 850℃, and then a sharp temperature drop to below 500℃, which takes about 10 seconds, and stays at 850℃ for only 2 to 3 seconds. This steep rise and fall process curve requires that the sintering equipment must have a special design in terms of heating elements, furnace structure, atmosphere layout, control principle, transmission system and cooling method.

2. Selection and fixing method of heating elements

Generally, mesh belt sintering furnaces use electric heating wires as heating elements, and heat the workpiece mainly through heat conduction, which cannot achieve rapid temperature rise. Only radiation or microwaves can quickly heat objects, and radiation heating has the advantages of economic use, safety and reliability, and easy replacement. Therefore, solar cell sintering furnaces currently basically use infrared quartz lamps as the main heating elements. Its design should pay attention to the following three issues:

1 Infrared radiation absorption spectrum

When infrared radiation energy is absorbed by the workpiece, the absorption spectrum of the material must match the emission spectrum in order to absorb the radiation energy with maximum efficiency in the shortest time. Therefore, the infrared quartz lamps used are different in different stages of sintering. In the drying stage, it is correct to use medium-wave tubes to assist hot air heating in order to quickly evaporate organic solvents and water; in the pre-burning stage, the substrate must be fully and evenly preheated, and the good infrared radiation, balanced absorption and penetration of medium-wave tubes just meet the requirements; in the sintering stage, the substrate must reach the eutectic temperature in a very short time, and only short-wave tubes can do this.

2. The structure of the heating tube

In order to achieve the temperature peak in the sintering section, sufficient heating power must be arranged in a very short furnace space. At present, there are two structures to choose from: short-wave twin tubes and short-wave single tubes, and their linear power density reaches 60kW/m2. Although the short-wave twin tube has a higher single-tube power (equivalent to two single tubes in parallel), due to its complex manufacturing process, it has higher quality requirements for the quartz glass tube, and the manufacturing cost is about 2.5 times that of a single tube. Therefore, in actual use, most single tubes are used.

3. Fixing method of heating tube

The temperature peak of the sintering section is about 850℃. At this time, the surface temperature of the lamp tube will reach 1100℃, which is close to the service limit of the quartz tube. If it is slightly overheated and produces pores, the lamp tube will be burned immediately. In the lead-out wire part of the lamp tube, since the metal sheet of the welding wire and the quartz glass are sealed together, the thermal expansion coefficients of the two are inconsistent. If the temperature here is too high, stress cracks will be generated, causing the lamp tube to leak. Therefore, the installation and fixing method of the lamp tube in the furnace is very important. Figure 2 shows a fixing method of the infrared lamp tube in the furnace. This fixing method requires that the cold end of the lamp tube is at least 80mm away from the furnace wall to ensure that the temperature of the lead-out wire part is not too high; and the diameter of the installation hole on the furnace wall is 2 to 3mm larger than the lamp tube, and the lamp tube is suspended and clamped in the furnace through the fixing clamps on both sides.

3. Furnace structure design and refractory material selection

1 Design of furnace structure

The sintering process of solar cells determines that the temperature difference between two adjacent temperature zones will be more than 300°C. Although radiation heating has good directionality, due to the presence of furnace atmosphere between the lamp tube and the substrate, some energy will be lost due to convection and air cooling. At the same time, the heat in the high-temperature zone tends to diffuse to the adjacent low-temperature zone. This heat diffusion will raise the actual temperature of the adjacent low-temperature zone and hinder the formation of temperature spikes. Therefore, how to reduce the heat loss of air and the heat diffusion of adjacent temperature zones is the focus of furnace structure design. Its longitudinal cross-sectional view is shown in Figure 3.

The entire furnace adopts a structure of a large heating chamber and a small channel. The large heating chamber can provide sufficient heat, reduce thermal shock, and improve the temperature uniformity of the furnace; the small channel can make the space of each temperature zone relatively independent, reduce convection, and reduce the interference of thermal diffusion between adjacent temperature zones. Since the thickness of the silicon substrate is only about 200 μm, the channel height can be very low in theory, but in actual design, it is necessary to consider the passage of the temperature curve measurement device through the furnace, and the effective height is generally about 30 mm.

2. Selection of refractory materials

For radiation heating, the selection of furnace refractory materials should not only consider the heat-resistance and heat-insulating properties, but also its radiation capacity. Ceramic fiber has extremely low thermal conductivity and specific heat capacity, and a higher blackness than general refractory materials. In the heating method dominated by radiation, the solar cell sintering furnace uses ceramic fiber to build the furnace, which improves the effect of radiation heating and saves energy. Based on the above analysis, in the future furnace design, calcium silicate materials and nano-insulation materials with better heat insulation and radiation capabilities are also worth considering.

4. Differences in atmosphere arrangement at different stages of sintering process

In the solar cell sintering process, the purpose of different stages is very clear. The drying stage completes the drying of the slurry, the pre-firing stage completes the preheating of the substrate, the sintering stage completes the sintering of the electrode, and the cooling stage completes the cooling of the substrate. Therefore, the atmosphere arrangement of different stages is completely different.

1. Atmosphere arrangement of drying section

Radiant heating can quickly dry the organic solvent and water in the slurry. In order to effectively discharge the waste gas, it is necessary to rely on a reasonable atmosphere flow direction. Figure 4 shows the use of hot air auxiliary heating in the drying section to increase air convection in the furnace to achieve this goal.

The cold air from the outside ① is sent to the heater ② by the blower, heated to about 200℃, and enters the furnace from the top, splitting into two streams on the left and right. The exhaust gas ③ generated during the drying process is carried to the exhaust gas collection pipe through convection, and discharged from the furnace under the action of the exhaust chimney ④. Through this forced hot air convection cycle, the exhaust gas in the furnace is discharged in a directional and orderly manner, reducing the pollution to the substrate.

2 Atmosphere arrangement of pre-burning section

While radiant heating rapidly increases the temperature of the substrate, it can also cause uneven heating of the substrate due to changes in the distance between the lamps, the on-off time of the current, and the angle of radiation. By introducing a certain amount of hot air into the furnace, the convection stirring effect is cleverly used to make the temperature distribution more uniform, eliminating the hidden danger of thermal stress on the substrate. In addition, during preheating, as the temperature rises, the aluminum ions in the slurry begin to diffuse and volatilize. In order to prevent cross-contamination of metal ions on the front and back of the substrate, how to form an atmosphere flow direction that does not interfere with each other in the upper and lower temperature zones is the focus of the design.

As shown in Figure 5, clean compressed air ① and ③ are introduced from the top and bottom of the furnace respectively. After being preheated by the refractory insulation layer, they are sprayed into the furnace evenly from small holes. Under the action of the Venturi effect of the exhaust chimneys ② and ④, independent upper and lower convection layers are formed, blowing through the upper and lower surfaces of the substrate respectively, and then discharged from the preheating part and the bottom, effectively suppressing the generation of pollution.

3 Atmosphere arrangement of sintering section

The surface temperature of the lamp tube in the sintering section is extremely high. The main purpose of the atmosphere arrangement here is to utilize the cooling effect of air to keep the lamp tube within a safe operating temperature. The specific structure is shown in Figure 6. Cold compressed air is introduced from the nozzles on both sides of the furnace and blown into the furnace from the gap between the lamp tube and the mounting hole. On the one hand, the cold air is gradually heated when passing through the refractory layer, and will not disturb the temperature of the temperature zone; on the other hand, the cold air forms a very thin cooling layer on the surface of the lamp tube to prevent the lamp tube from overburning and cool the cold end. Directly using water-cooled heating lamps produced by Heraeus of Germany, or arranging water-cooled walls on both sides of the furnace are thorough solutions, but the manufacturing cost of these equipment is relatively high.

4. Atmosphere arrangement in cooling section

The atmosphere arrangement of the cooling section is relatively simple, and its main function is to block the heat diffusion of the sintering section. Therefore, in the transition zone between the sintering section and the cooling section, several vertical compressed air curtains are arranged, and combined with the function of the water-cooled steel jacket, the heat energy is completely blocked in the sintering section.

5. Temperature control principle and the role of thermocouple

1. Choice of control method

The common load output modes of temperature control systems mainly include voltage regulation (variable conduction angle), cycle zero-crossing (duty cycle control) and cYc cycle zero-crossing (variable period). Their output principles are shown in Figure 7.

Voltage regulation, also known as phase shift control, refers to controlling the conduction angle of the thyristor to cut off a part of the sine wave of the power supply and retain a part. The retained waveform is the waveform of the current and voltage passing through the load. Changing the size of the retained waveform changes the power obtained by the load, thereby achieving the purpose of regulating power. Its advantages are small impact and high control accuracy; its disadvantages are low power factor, high harmonic pollution to the power grid, and high cost.

PWM mode refers to changing the power on the load by controlling the time ratio of the current on and off on the load within a fixed time period. Its advantages are simple control and low cost. The disadvantage is that it outputs full power when crossing zero, and the instantaneous current of the local line is too large, which brings impact to the heating element and power supply. In addition, its on-off interval is long, which is easy to cause temperature fluctuations, especially for the radiation heating of the lamp tube. The CYC mode distributes the output waveform as evenly as possible within a time period on the basis of PWM to avoid the impact of concentrated on and off on the power supply. It has the control advantages similar to the voltage regulation mode and overcomes the shortcomings of the PWM mode. The load heating power is more uniform, which is conducive to improving the adjustment quality of the PID instrument. The disadvantage is that when the output percentage is small, low-frequency flickering will occur, which will interfere with the power grid to a certain extent.

Taking all factors into consideration, the CYC frequency zero-crossing control method is more in line with the requirements of lamp radiation heating.

2. Thermocouple selection

In mesh belt sintering furnaces below 1000℃, K-division thermocouples with ceramic protective tubes are currently used to detect temperature, which is highly accurate and low-cost. Since the thermocouple wire is protected in the ceramic protective tube, the thermal response time is relatively long, which has no effect on ordinary electric heating wire heating and low-speed kilns. However, solar cell sintering furnaces that adopt radiation heating and CYC control require that the response time of the thermocouple must be fast enough to control the temperature fluctuation of the temperature zone within the allowable range, otherwise the conversion rate of the sintered cell will change with the fluctuation of the furnace temperature, with large discreteness and inconsistent quality. In order to solve the problem of long thermocouple response time, the use of S-division thermocouples is a suitable choice.

6. Requirements of high-speed sintering on transmission system

At present, the sintering process of solar cells is required to reach 5000mm/min, while the operating speed of ordinary mesh belt furnace is generally only 200mm/min. High-speed sintering will make the running stability of the mesh belt worse, causing mesh patterns on the back electrode surface of the cell, affecting the appearance quality, and even causing fragmentation. In addition, how to quickly cool the mesh belt in a very short time during high-speed sintering to ensure the temperature of the cell out of the furnace is also a difficult problem that must be solved. At present, designers focus on overcoming it from the following four aspects.

1Try to use elastic tension

At present, all mesh belt sintering furnaces adopt friction transmission, that is, the mesh belt is pressed against the main drive wheel by a tensioning wheel, and moves with the driving wheel. Generally, mesh belt furnaces only apply tension at the main drive, which will cause the mesh belt to have loose and tight sides. It will not affect low-speed operation, but will cause shaking at the loose side at high-speed operation. For this reason, the full-range elastic tensioning method should be adopted. In this way, the springs applied to the tensioning wheels at different parts of the furnace body will automatically adjust the size of the friction force according to the different tension of the mesh belt, so that it matches the tension and minimizes the generation of shaking.

2 Structural requirements of the mesh belt itself

Since the battery cell itself is extremely thin (and there is a trend of further thinning), the heat brought out from the sintering section is mainly concentrated on the metal mesh belt. In theory, there are two ways to quickly cool down: one is to speed up heat exchange; the other is to reduce the heat source. When the cooling method of the equipment is determined, the only way is to reduce the heat storage of the mesh belt itself, that is, to reduce the amount of metal wire per unit area.

The main factors that affect the weight of the mesh belt are the mesh belt pitch p, mesh spacing t, mesh wire diameter d and wire threading diameter D, as shown in Figure 8. The larger the p and t, the thinner the d and D, the lighter the mesh belt per unit area, and vice versa. However, the mesh belt of the solar cell sintering furnace moves at high temperature and high speed, and requires long-term use without shaking or deformation. This is contradictory to the former, and the influence of p, t, d and D must be carefully analyzed and coordinated to solve. The advantage of the "human" structure is that the mesh wire is a spiral V-shaped structure with good rigidity. Therefore, increasing the mesh spacing t and reducing the mesh wire diameter d will have a smaller impact on the force of the mesh belt. Under the action of tension, all the points of force of the mesh belt are borne by the threading wire. Especially after d is increased, the rigidity of the threading wire is required to be higher, so the threading wire diameter D should be more than 1.5 times the mesh wire diameter d. When the pitch p is increased, a polygonal effect will be produced on the circumference of the transmission wheel, the contact surface will be reduced, resulting in insufficient friction, and when d becomes thinner, p is too large, which will cause the mesh wire to deform under the action of tension. Therefore, the pitch p is generally controlled to be 1.1 to 1.2 times the mesh spacing t. The mesh belt woven in this way has sufficient rigidity for long-term use, and its unit area weight is very light, and the heat storage is very small.

3. Requirements for mesh belt weaving and support methods

In order to meet the cooling needs, the mesh belt of the solar cell sintering furnace has larger mesh holes and finer wire diameters, which are more likely to cause the mesh belt to twist and deviate. Therefore, strict clamp positioning is required during weaving, especially to control the stress generated when welding the wire ends at both ends. In addition, the support structure of the entire transmission is also very important. The rolling structure should be used on the contact surface of the mesh belt as much as possible to reduce friction, thereby increasing the life of the mesh belt and reducing mesh belt vibration.

4. Special requirements of new technology for transmission

At present, some manufacturers are testing the reverse firing process, that is, placing the back electrode of the cell on the mesh belt for sintering. Its advantages are that it is conducive to the formation of the back field, reducing the pollution of aluminum to the battery, and the conversion rate is high; the disadvantage is that the mesh belt is easy to scratch the front grid line. Therefore, the use of a mesh belt with protrusions or a chain transmission mechanism with a supporting short rod similar to a hot air reflow soldering machine will be the future research direction.

7. Methods for Rapidly Cooling Cells

As mentioned above, in addition to reducing the heat storage of the mesh belt, the cooling measures can also use efficient heat exchangers. Simple axial fan blowing or water-cooled steel jacket heat exchange cannot meet the requirements of rapid cooling within 1 minute. Air-cooled water-cooled heat exchangers are mature and reliable solutions. On this basis, two structures of internal circulation heat exchange and external convection heat exchange have been developed.

1Internal circulation heat exchange

The cooling chamber is closed relative to the external environment. The water-cooled heat exchanger is installed above and below the mesh belt. The axial flow fan fixed on the back of the heat exchanger forces the heat brought out by the mesh belt and the substrate to be drawn through the heat exchanger. The cool air after cooling is blown onto the mesh belt. In this way, the air circulates and cools down in the cooling chamber continuously, thereby reducing the temperature of the mesh belt and the substrate. Its advantages are internal heat circulation, less loss to the outside world, low energy consumption, little impact on the temperature of the workshop, and low cost; its disadvantages are poor heat exchange efficiency compared to external convection.

2External convection heat exchange

The cooling chamber is open to the outside environment. The water-cooled heat exchanger is installed above and below the mesh belt. The fan at the top of the cooling chamber draws in the filtered and purified air from the outside, cools it in the upper water-cooled heat exchanger, and blows it onto the mesh belt and substrate. The fan at the bottom of the cooling chamber extracts the hot air, cools it in the lower water-cooled heat exchanger, and discharges it outside the equipment. In this way, the external cold air is continuously used for cooling to achieve the purpose of cooling. Its advantage is that the heat exchange efficiency is higher than that of the internal circulation type; its disadvantage is that the exhausted gas affects the temperature of the workroom, the external loss is large, the energy consumption is high, and the cost is high. Therefore, for a short furnace body, rapid cooling, or strict requirements on the outlet temperature, external convection heat exchange can be used; for a long furnace body and a wide range of outlet temperature requirements, internal circulation heat exchange can be used.

3. The influence of air volume and flow direction on solar cells

When the water cooling temperature and flow rate are constant, the only thing that affects the cooling rate is the air volume. However, too much air volume will generate a lot of force on the battery cells, and poor control will cause fragmentation. In addition, the direction of the wind is also very important. If it is vertically downward, when it blows to the battery cells, the wind will spread to both sides, close to the sintering section, which will affect the temperature stability of the sintering temperature zone, thereby affecting the conversion rate. The use of a fan installation structure that is tilted toward the outlet can solve this problem well.

8. Conclusion

The special sintering process of solar cells determines that the design structure of sintering equipment is different from that of general mesh belt furnaces. Some key performance indicators directly affect the quality of cells. In 2005, we developed the first generation of mesh belt infrared heating rapid sintering furnaces, with a maximum belt speed of 3600mm/min and an average conversion rate of 14% to 15%. However, it has defects such as short lamp life, insufficient temperature peak, and excessive outlet temperature. After continuous exploration and research, and referring to the advantages and disadvantages of similar equipment at home and abroad, we summarized the above design experience and developed the second generation of equipment in 2006. Its overall structure is shown in Figure 9.

Its characteristics are that the sintering section uses imported infrared short-wave lamps, the rest uses domestic lamps, and the control adopts a computer distributed control system. It overcomes the shortcomings of the first generation, the maximum belt speed reaches 6700mm/min, the outlet temperature is controlled below 45℃, and the average conversion rate is very close to that of foreign equipment. After delivery and use, the user is very satisfied. The comparative test results (average value) of 10 groups of samples are shown in Table 1.

It can be seen that through in-depth analysis of the process curve, correct design structure and reasonable material selection, domestic equipment has approached the advanced level of foreign countries, and its huge price advantage is unmatched by the latter.