Precautions for solar cells

The solar cells adopt a co firing process that only requires one sintering, while forming Ohmic contacts between the upper and lower electrodes. Silicon wafers printed with silver paste, silver aluminum paste, and aluminum paste are dried to completely evaporate the organic solvent. The film layer shrinks into a solid and tightly adheres to the silicon wafer, which can be seen as the metal electrode material layer in contact with the silicon wafer. When the electrode metal material and semiconductor monocrystalline silicon are heated to the eutectic temperature, monocrystalline silicon atoms dissolve into the molten alloy electrode material in a certain proportion. The entire process of monocrystalline silicon atoms dissolving into the electrode metal is quite fast, usually taking only a few seconds. The number of monocrystalline silicon atoms dissolved depends on the alloy temperature and the volume of the electrode material. The higher the sintering alloy temperature, the larger the volume of the electrode metal material, and the more silicon atoms dissolved. This state is called the alloy system of crystalline electrode metal. If the temperature drops at this point, the system begins to cool and form a recrystallization layer. At this time, the silicon atoms that were originally dissolved in the electrode metal material will crystallize again in solid form, which is to grow an epitaxial layer at the interface between the metal and the crystal. If the epitaxial layer contains a sufficient amount of impurities of the same conductivity type as the original crystal material, it obtains the formation of Ohmic contact using the alloy method process; If there is a sufficient amount of impurity components in the crystalline layer that are different in conductivity type from the original crystalline material, it is possible to form P using the alloy process N-junction.

The general mesh belt sintering furnace uses electric heating wire as the heating element, mainly heating the workpiece through heat conduction, and cannot achieve rapid heating. Only radiation or microwaves can quickly heat objects, and radiation heating has the advantages of being economical, safe, reliable, and easy to replace. So solar cell sintering furnaces use infrared quartz tubes as the main heating element. Its design needs to pay attention to the following three issues:

1. The structural form of the heating tube

To achieve temperature spikes in the sintering section, sufficient heating power needs to be arranged within a short furnace space. There are two structures to choose from: shortwave twin tube and shortwave single tube, both of which have a linear power density of 60kW/m2. Although shortwave twin tubes have higher single power (equivalent to two single tubes in parallel), due to their complex manufacturing process, they require higher quality of quartz glass tubes, and the manufacturing cost is about 2.5 times that of a single tube. Therefore, in practical use, single tubes are mostly used.

2. Infrared radiation absorption spectrum

When the infrared radiation energy is absorbed by the workpiece, the unique absorption spectrum of the substance needs to match the emission spectrum in order to absorb the radiation energy with maximum efficiency in the shortest possible time. Therefore, at different stages of sintering, the selected infrared quartz lamps are also different. In the drying stage, it is correct to use a medium wave tube to assist in hot air heating in order to quickly evaporate organic solvents and moisture; In the pre firing stage, it is necessary to ensure sufficient and uniform preheating of the substrate. The medium wave tube has good infrared radiation, balanced absorption and penetration ability, which exactly meets the requirements; In the sintering stage, it is necessary to reach the eutectic temperature of the substrate in a very short time, which can only be achieved by shortwave tubes.

3. Fixation method of heating tube

The peak temperature of the sintering section is around 850 ℃, at which point the surface temperature of the lamp tube will reach 1100 ℃, close to the limit of the use of quartz tubes. A slight overheating that produces pores will immediately burn out the lamp tube. At the lead out position of the lamp tube, due to the sealing of the metal sheet and quartz glass used for welding the wire together, the thermal expansion coefficients of the two are not consistent. If the temperature is too high here, stress cracks will occur, causing the lamp tube to leak air. Therefore, the installation and fixing method of the lamp tube in the furnace is very important. Figure 2 shows a fixing method for infrared lamps in the furnace. This fixing method requires the cold end of the lamp tube to be at least 80mm away from the furnace wall, ensuring that the temperature at the lead out position is not too high; Moreover, the diameter of the installation hole on the furnace wall is 2-3mm larger than that of the lamp tube, and the lamp tube is suspended and clamped in the furnace through fixing fixtures on both sides.