Manufacturing Methods of Mono Crystalline and Poly Crystalline Silicon Cells
The main ingradient of PV cells is silicon. Pure Silicon is obtained by carbothermic reduction where silica is heated to 1700 ° C in the presence of carbon. On very slow rate of cooling it forms large sized crystals.
Mono crystalline silicon cells are made from pure crystalline silicon. Mono crystalline silicon is created by slowly pulling a monocrystalline silicon seed crystal out of melted monocrystalline silicon using Czochralski method to form a perfect crystal that has a solid or cylindrical shape.
A seed crystal is a small piece of silicon which is used as foundation for the molten molecule. This foundation helps the molten molecules to connect together faster to form an ingot. While the seed crystal is being withdrawn, it is rotated and cooled slowly. It helps to form the crystalline shape until it has the right diameter.
Another approach is string ribbon technique where two high temperature strings are pulled vertically through a shallow silicon melt and molten silicon expands and freezes between the strings. Monocrystalline silicon cells are continuous crystal lattice structure with almost no defects or impurities. These cells are costly to produce having high efficiency in the range of 14% -19%.
Polycrystalline silicon cells, also called Multi crystalline cells, a simpler method of production is used. In this, the molten silicon is just put into a cast into ingots of square or rectangular shapes which are allowed to cool with seed crystal. In this casting method the crystal surrounding the seed is not uniform and branches into many small crystals, thus it is polycrystalline. These ingots are then cut into very thin wafers and assembled into complete cells.
Polycrystalline cells are cheaper to produce compared to monocrystalline cells due to simpler manufacturing process. The efficiency of polycrystalline cells is between (11% -14%) while that of monocrystalline cells ranges between (14% -19%). Monocrystalline cells are more efficient in cold weather but as the temperature increases the electrical conversion efficiency goes down.
Amorphous Silicon Thin Film Cells :
Amorphous silicon is an alloy of silicon and hydrogen. It is represented by the symbol is manufactured by passing an electric discharge through the silane gas and hydrogen with dope under vacuum. The silane gas dissociates and a thin layer of amorphous silicon deposits on glass substrate. Dope used is boron to form p type semiconductor and phosphorous hydride (PH) for n type semiconductor. The efficiency of amorphous solar cells is about (8%-10%). It degrades with exposure to sunlight. But their cost of production is much lower compared to polycrystalline silicons.