Solar cells employ a co-firing process requiring only one sintering step, simultaneously 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 solvents, causing the film to shrink into a solid layer that adheres tightly to the silicon wafer. At this point, the metal electrode material layer and the silicon wafer are considered to be in contact. When the electrode metal material and the semiconductor monocrystalline silicon are heated to the eutectic temperature, monocrystalline silicon atoms dissolve into the molten alloy electrode material in a specific proportion.
The entire process of single-crystal silicon atoms dissolving into the electrode metal is quite fast, generally taking only a few seconds. The number of dissolved single-crystal silicon atoms depends on the alloying temperature and the volume of the electrode material. The higher the sintering temperature and the larger the volume of the electrode metal material, the more silicon atoms will dissolve. This state is called an alloy system of crystalline electrode metals.
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 recrystallize in solid form, which means that an epitaxial layer is grown at the interface between the metal and the crystal.
If the epitaxial layer contains a sufficient amount of impurities with the same conductivity type as the original crystal material, an ohmic contact can be formed using an alloying process; if the crystalline layer contains a sufficient amount of impurities with a different conductivity type than the original crystal material, a PN junction can be formed using an alloying process.
Traditional mesh belt sintering furnaces use heating wires as heating elements, primarily heating the workpiece through heat conduction, which cannot achieve rapid temperature rise. Only radiation or microwaves can rapidly heat objects, and radiation heating has advantages such as being economical, safe, reliable, and easy to replace. Therefore, solar cell sintering furnaces generally use infrared quartz lamps as the main heating element. The design of solar cells requires attention to the following three issues:
1. Structural form of heating element
To achieve the temperature peaks in the sintering section, sufficient heating power needs to be arranged within a very short furnace space. Two structures are available: short-wave twin tubes and short-wave single tubes, both achieving a linear power density of 60 kW/m². Although short-wave twin tubes offer higher single-tube power (equivalent to two single tubes in parallel), their manufacturing process is more complex, requiring higher quality quartz glass tubes, resulting in a manufacturing cost approximately 2.5 times that of single tubes. Therefore, in practical applications, single tubes are mostly used.
2. Infrared radiation absorption spectrum
When infrared radiation energy is absorbed by the workpiece, the absorption spectrum unique to that material must match its emission spectrum to maximize the absorption efficiency in the shortest time. Therefore, different infrared quartz lamps are selected for different stages of sintering. In the drying stage, to allow organic solvents and moisture to evaporate quickly, using a medium-wave tube to assist hot air heating is appropriate. In the pre-sintering stage, to ensure the substrate receives sufficient and uniform preheating, the good infrared radiation, balanced absorption, and penetration capabilities of a medium-wave tube perfectly meet the requirements. In the sintering stage, the substrate must reach the eutectic temperature in an extremely short time, which only a short-wave tube can achieve.
3. Heating element fixing method
The peak temperature in the sintering section is around 850℃, at which point the surface temperature of the lamp tube will reach 1100℃, approaching the service limit of the quartz tube. Even slight overheating and the resulting pores will immediately burn out the lamp tube. Furthermore, at the lead wire section of the lamp tube, because the metal sheet of the soldered wire is sealed together with the quartz glass, their coefficients of thermal expansion are inconsistent. If the temperature here is too high, stress cracks will occur, causing the lamp tube to leak. Therefore, the method of installing and fixing the lamp tube in the furnace is extremely important.
