The Production of the Solar Cells
After the silicon crystal had been grown and cut into wafers, the latter are used in the production of photo transformers — the solar cells. This process goes in several stages. This articles explains the conventional sequence of solar cells production process that is widely applied in contemporary solar industry.
The stages are as follows:
- Chemical milling of the surface (acid- or alkali-based)
- Diffusion — the creation of a working p-n-junction
- Texturing
- The construction of the antireflection layer
- Forming of an electric current collector (or contact lattice)
- Burning-in of the electric current collector (or contact lattice)
- Tests using the sun imitation lamp
Some of the contemporary production lines may have additional processes, but in general the technology involves the processes described above. Let’s begin with the first two stages.
Chemical etching
The surface of the starting wafer can have a lot of defects, microcracks and contaminations that emerged during the crystal cutting process. The presence of such defects increases the rate of recombination (endings) of the carriers and decreases the efficiency of photoelements. Thus, chemical milling is naturally the first step. In practice, it depends on the crystal type and the specifics of the production line, and can be done in both acid or alkali mortars.
Diffusion
After the wafer surface has been chemically cleaned, it is necessary to create a p-n-transition. Generally, diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential). In the process of diffusion, the admixtures move from the region of high concentration to the region of low concentration under the influence of high temperature. In this process the atoms of electrically active admixtures diffuse into the crystal lattice of silicon, creating the areas with p- (positive) or n-(negative) electrical conductivity. In the local diffusion defence masks are used that are made of dielectric (non-conducting) films.
This process is widely used to create the p-n-junction which serves as a kind of energetic barrier for the electric current carriers and lets them pass through in only one direction. Thus, the sunbeams that reach the surface of the solar cell release the carriers with different charge within the semiconductor material — holes (p) and electrons (n). This is the foundation of the electrical energy generation within the solar cells. The p-n junction functions as a “separator” that directs the movement of different types of carriers into different directions. Thus, chaotically moving carriers of charge (charged particles) reach different sides of the barrier and then are transmitted into the external circuit to create voltage.
Usually, the diffusion is carried out in a quartz tube placed into a special diffusion furnace with atmospheric pressure, under the temperature around 800–900 C. The admixtures used in the diffusion process include solid, liquid, gaseous, solid planar and surface sources. As a rule, the use of these sources of admixtures implies the application of the gas system. By regulating the timing, that is, for how long the silicon wafers are in the reactor, as well as the regulation of the temperature and the gas flows inside, professionals learned how to get the p-n-junction with the properties needed.
For example, in the process of phosphorus diffusion, a liquid substance is used as the source — namely, phosphorus oxychloride (POCI3); in the diffusion boron this role is played by boron nitride (BN). The diffusion process itself consist in putting the silicon wafer together with some admixture source into the quartz cassette, then moving it into the reaction area of the diffusion furnace for some time under fixed temperature. The gaseous oxide that evolves thereby diffuses to the silicon surface and interacts with it, which results into the formation of the glass layer from which the diffusion goes deeper into the semiconductor. Thus, on the surface of the silicon wafer emerges a layer with electrical n-conductivity which then takes part in the formation of the p-n-junction.
Nowadays the technique of ion bombardment is becoming increasingly popular. The advantages of this mechanism include a high degree of purity, as well as the possibility to control the quantity of the embedded atoms and the depth of their embeddedness.
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