1. Source of silicon material
Silicon is the most widely distributed element in the earth’s crust, with a content of 25.8%. The silicon material is mainly derived from high-quality quartz sand, also known as silica sand. It is distributed in Shandong, Jiangsu, Hubei, Yunnan, Inner Mongolia, Hainan and other provinces in China. Converts silica sand into usable silicon material.
2. Sheet Silicon (with Silicon) Manufacturing
Monocrystalline silicon ingots and polycrystalline silicon chains are both bulk materials, which need to be cut to make solar cells, while sheet silicon (with silicon) has been attracting attention because it reduces cutting losses. In order to avoid the waste of materials caused by cutting, people have studied the method of growing band silicon directly from molten silicon liquid, and this method has been used in actual production. Using strip-shaped silicon without slicing as the substrate can increase the utilization rate of silicon material from 20% to 80%.
The main production methods of flake silicon are Edge defined Film-fed Growth (EFG), vine-like growth method,
Edge-sustained Pulling (Edge-sustained Pulling, SP), small-angle ribbon growth method, laser zone melting method and granular silicon ribbon method, etc. Among them, the EFG method has been industrialized and is considered to be the most mature technology with silicon. The principle is shown in Figure 1. The technique is to directly pull the octagonal silicon tube from the molten silicon using a suitable graphite die. The side length of the regular octagon is slightly longer than 10cm, and the total pipe diameter is about 30cm. The thickness of the pipe wall (the thickness of the silicon wafer) is related to the capillary shape of the graphite mold, the drawing temperature and the speed (several cm/min). The length of the tube drawn by this technology can reach 4~5m, and the efficiency of large-area (10cm ~ 10cm) EFG solar cells has reached 14.3%.
3. Amorphous or microcrystalline silicon film fabrication
Amorphous silicon films can be obtained by both chemical vapor deposition (CVD) and physical vapor deposition (PVD). When the substrate temperature is very high (600~800℃), the amorphous silicon film can be converted into a microcrystalline silicon film, or a microcrystalline silicon film can be obtained directly.
(1) Thermal chemical vapor deposition method
The principle of thermal chemical vapor deposition is to use the thermal decomposition of silane to obtain an amorphous silicon film. The schematic diagram of thermal chemical vapor deposition is shown in Figure 2, and its reaction equation is:
(2) Glow discharge method
The principle of the glow discharge method is that under the condition of high voltage AC or DC glow discharge, an amorphous silicon film can be obtained at a lower temperature. Because this method has the characteristics of simple equipment and easy doping, most of the current amorphous silicon cells use this method. The glow discharge vapor deposition process is shown in Figure 3, and its reaction equation is:
(3) Photochemical vapor deposition method
Photochemical vapor deposition is to irradiate the substrate with a laser beam that can just cut off the silicon-hydrogen bond. When silane passes through the surface of the substrate, silicon atoms are deposited on the substrate to form a high-quality amorphous silicon film.
Of course, the laser beam can also be replaced by a powerful special frequency microwave beam, as shown in Figure 4.
(4) Sputtering method
The sputtering method refers to the radio frequency or DC discharge in the low pressure gas, and the high-energy ionized particles (such as argon ionized into argon ions) continuously and violently hit the silicon, so that a part of the silicon atoms are separated from the silicon target and deposited on the substrate to form an amorphous silicon film. Its working principle is shown in Figure 5.
(5) Electron beam evaporation method
The electron beam evaporation method is to irradiate a silicon block with a high-energy electron beam to melt and evaporate it locally, and deposit it on the substrate to form an amorphous silicon film, as shown in Figure 6.
Among the above five methods, the first three belong to chemical vapor deposition method, and the latter two belong to physical vapor deposition method.