Solar cell processing regarding screen printing process?
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Answer:
Solar cells are manufactured from thin p-doped polycrystalline silicon wafers. They are produced by casting large ingots of high purity silicon. The ingots are wire-cut into wafers. The wire-cutting process generates damages on the surface of the wafers. This damage is removed by etching in a strong alkaline solution. Typically 40-50 microns are etched away creating a flat surface. To minimize reflection from the flat surface, the wafers are textured. This means creating a roughened surface so that incident light will have a larger probability of being adsorbed into the solar cell. This is performed by etching in a weak alkaline solution. To create the pn-junction, a thin n-type layer is made by in-diffusion of phosphorus (P) from the surface. A liquid glass containing P is uniformly distributed on one side of the wafer by spraying or spinning. This side now defines the front of the solar cell. The wafers are then heated in an oven to harden and evaporate the solvents in the liquid glass.To make the P-atoms diffuse into the silicon from the glass, the wafers are placed in a belt furnace and heated to about 900°C. Due to gas diffusion, some P-atoms are transported to the edges and the other side of the wafer, basically encapsulating the p-type silicon in a thin n-type layer. The glass which contained the P-atoms act as a thin insulating layer and is therefore removed by etching. To avoid a short circuit between the front and back of the solar cell, the n-type layer on the edges of the wafer is removed by plasma etching. To achive this, the diffused wafers are carefully stacked and placed in a dedicated plasma etching chamber. At this stage in the process, the solar cell has reflectivity of about 30%. To reduce losses due to surface reflection, a thin layer of silicon nitride (SiNxHy) is deposited on the front of the cell in a PECVD chamber. The silicon nitride works as an anti-reflective coating due to a combined effect of its favourable refractive index and the layer thickness. The solar cell front now has a dark blue colour.To extract current from the solar cell, metal contacts have to be made. These are applied by screen printing, in which a paste is pressed though a pre-defined pattern mask onto the wafer. While the back is totally covered by metal, the front of the wafer is covered in a finger-pattern allowing light to reach the surface of the silicon. The wafers are put in an oven to dry the metal pastes. As of now, the metal contacts in the front lie on top of the anti-reflection coating, whereas on the back the metal is in touch with the thin n-type layer caused by gas diffusion. In the sintering and firing process, the metallic contacts are rapidly heated up to about 900°C, allowing the front contact to etch through the anti-reflection coating. At the back-side, the metallic region depletes the thin n-type layer, and forms a heavily doped p-type layer referred to as a back surface field.
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